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Non­destructive Testing

Table of Contents

Chapter No:

Name of the Chapter Page No

1 Course daily schedule 1

2 Course Contents 2

3 Introduction NDT processes & their Uses 3 ­ 11

4 Identification of weld Discontinuities 12 ­ 20

5 Penetrant Testing 21­ 30

6 Magnetic Particle Testing 31 – 48

7 Ultrasonic Testing 49 ­60

8 Radiographic Testing 61 ­ 77

9 Eddy Current Testing 78 ­ 80

10 Comparison and Selection of NDT Methods 81

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Chapter I

INTRODUCTION

Nondestructive Testing

The field of Nondestructive Testing (NDT) is a very broad, that plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause serious accidents such as, planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of troubling events.

These tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and materials to be inspected and evaluated without damaging them. Because it allows inspection without interfering with a product's final use, NDT provides an excellent balance between quality control and cost­effectiveness.

Nondestructive Evaluation

Nondestructive Evaluation (NDE) is a term that is often used interchangeably with NDT. However, technically, NDE is used to describe measurements that are more quantitative in nature. For example, a NDE method would not only locate a defect, but it would also be used to measure something about that defect such as its size, shape, and orientation. NDE may be used to determine material properties such as fracture toughness, ductility, conductivity and other physical characteristics.

Uses of NDE

• Flaw Detection and Evaluation • Leak Detection, Location Determination • Dimensional Measurements • Structure and Microstructure Characterization • Estimation of Mechanical and Physical Properties • Stress (Strain) and Dynamic Response Measurements • Material Sorting and Chemical Composition Determination

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Background on Nondestructive Testing (NDT)

Nondestructive testing has been practiced for many decades. One of the earliest applications was the detection of surface cracks in railcar wheels and axles. The parts were dipped in oil, then cleaned and dusted with a powder. When a crack was present, the oil would seep from the defect and wet the oil providing visual indication indicating that the component was flawed. This eventually led to oils that were specifically formulated for performing these and other inspections and these inspection techniques are now called penetrant testing.

X­rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845­1923) who was a Professor at Wuerzburg University in Germany. Soon after his discovery, Roentgen produced the first industrial radiograph when he imaged a set of weights in a box to show his colleagues. Other electronic inspection techniques such as ultrasonic and eddy current testing started with the initial rapid developments in instrumentation spurred by technological advances and subsequent defense and space efforts following World War II. In the early days, the primary purpose was the detection of defects. Critical parts were produced with a "safe life" design, and were intended to be defect free during their useful life. The detection of defects was automatically a cause for removal of the component from service.

The continued improvement of inspection technology, in particular the ability to detect smaller and smaller flaws, led to more and more parts being rejected. At this time the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a particular material property or fracture toughness, were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for a new design philosophy called "damage tolerant designs." Components having known defects could continue to be used as long as it could be established that those defects would not grow to a critical size that would result in catastrophic failure. A new challenge was thus presented to the nondestructive testing community.

Mere detection of flaws was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics calculations to predict the remaining life of a component. These needs, led to the creation of a number of research programs around the world and the emergence of nondestructive evaluation (NDE) as a new discipline.

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NDT/NDE Methods

The list of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying physics and other scientific disciplines to develop better NDT methods. However, there are six NDT methods that are used most often. These methods are Visual Inspection, Penetrant Testing, Magnetic Particle Testing, Electromagnetic or Eddy Current Testing, Radiography, and Ultrasonic Testing.

Visual and Optical Testing (VT)

Visual inspection involves using an inspector's eyes to look for defects. The inspector may also use special tools such as magnifying glasses, mirrors, or borescopes to gain access and more closely inspect the subject area. Visual examiners follow procedures that range fm simple to very complex.

Penetrant Testing (PT)

Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface. With visible dyes, vivid color contrasts between the penetrant and developer make "bleedout" easy to see. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.

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Magnetic Particle Testing (MT)

This method is accomplished by inducing a magnetic field in a ferromagnetic material and then dusting the surface with iron particles (either dry or suspended in liquid). Surface and near­surface imperfections distort the magnetic field and concentrate iron particles near imperfections, previewing a visual indication of the flaw.

Electromagnetic Testing (ET) or Eddy Current Testing

Electrical currents are generated in a conductive material by an induced alternating magnetic field This electrical currents is called eddy currents because they flow in circles at and just below the surface of the material. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material's conductive and permeability properties, are detected.

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Radiography (RT)

Radiography involves the use of penetrating gamma or X­radiation to examine parts and products for imperfections. An X­ray generator or radioactive isotope is used as a source of radiation. Radiation is directed through a part and onto film or other imaging media. The resulting radiograph shows the dimensional features of the part. Possible imperfections are indicated as density changes on the film in the same manner as a medical X­ray shows broken bones.

Ultrasonic Testing (UT)

Ultrasonics use transmission of high­frequency sound waves into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, wherein sound is introduced into a test object and reflections (echoes) are returned to a receiver from internal imperfections or from the part's geometrical surfaces

.

crack

0 2 4 6 8 1 0

Initial pulse

Crack echo

Back surface echo

Sound waves

X­ray film

Source

Rays

Object with defect Film

Defect Image Film with image

Probe

Couplant

Plate Screen

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Acoustic Emission Testing (AE)

When a solid material is stressed, imperfections within the material emit short bursts of acoustic energy called "emissions." As in ultrasonic testing, acoustic emissions can be detected by special receivers. Emission sources can be evaluated through the study of their intensity, rate, and location.

Leak Testing (LT)

Several techniques are used to detect and locate leaks in pressure containment parts, pressure vessels, and structures. Leaks can be detected by using electronic listening devices, pressure gauge measurements, liquid and gas penetrant techniques, and/or a simple soap­bubble test.

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Test Method

UT X­ray Eddy Current

MPI LPT

Capital cost Medium to high

High Low to medium

Medium Low

Consumable cost

Very low High Low Medium Medium

Time of results

Immediate Delayed Immediate Short delay

Short delay

Effect of geometry

Important Important Important Not too Important

Not too Important

Access problems

Important Important Important Important Important

Type of defect

Internal Most External External Near Surface

Surface breaking

Relative sensitivity

High Medium High Low Low

Operator skill

High High Medium Low Low

Operator training

Important Important Important Important Not Important

Training needs

High High Medium Low Low

Portability of equipment

High Low High to medium

High to medium

High

Capabilities Thickness gauging, composition testing

Thickness gauging

Thickness gauging, grade sorting

Defects only

Defects only

The Relative Uses and Merits of Various NDT Methods

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Table 1 ­ Reference Guide to Major Methods for the Nondestructive Examination of Welds Inspection Method

Equipment Required

Enables Detectiort of

Advantages Limitations Remarks

Visual Magnifying glass Weld sizing gauge Pocket rule Straight edge Workmanship standards

Surface flaws ­ cracks, porosity, unfilled craters, slag inclusions Warpage, underwelding, overwelding, poorly formed beads, misalignments, improper fitup

Low cost. Can be applied while work is in process, permitting correction of faults. Gives indication of incorrect procedures.

Applicable to surface defects only. Provides no permanent record.

Should always be the primary method of inspection, no matter what other techniques are required. Is the only "productive" type of inspection. Is the necessary function of everyone who in any way contributes to the making of the weld.

Radiographic Commercial X­ray or gamma units made especially for inspecting welds, castings and forgings. Film and processing facilities. Fluoroscopic viewing equipment.

Interior macroscopic flaws ­ cracks, porosity, blow holes, nonmetallic inclusions, incomplete root penetration, undercutting, icicles, and burnthrough.

When the indications are recorded on film, gives a permanent record. When viewed on a fluoroscopic screen, a low­ cost method of internal inspection

Requires skill in choosing angles of exposure, operating equipment, and interpreting indications. Requires safety precautions. Not generally suitable for fillet weld inspection.

X­ray inspection is required by many codes and specifications. Useful in qualification of welders and welding processes. Because of cost, its use should be limited to those areas where other methods will not provide the assurance required.

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Magnetic Particle

Special commercial equipment. Magnetic powders ­ dry or wet form; may be fluorescent for viewing under ultraviolet light.

Excellent for detecting surface discontinuities ­ especially surface cracks.

Simpler to use than radiographic inspection. Permits controlled sensitivity. Relatively low­cost method.

Applicable to ferromagnetic materials only. Requires skill in interpretation of indications and recognition of irrelevant patterns. Difficult to use on rough surfaces.

Elongated defects parallel to the magnetic field may not give pattern; for this reason the field should be applied from two directions at or near right angles to each other.

Liquid Penetrant

Commercial kits containing fluorescent or dye penetrants and developers. Application equipment for the developer. A source of ultraviolet light ­ if fluorescent method is used.

Surface cracks not readily visible to the unaided eye. Excellent for locating leaks in weldments.

Applicable to magnetic and nonmagnetic materials. Easy to use. Low cost.

Only surface defects are detectable. Cannot be used effectively on hot assemblies.

In thin­walled vessels will reveal leaks not ordinarily located by usual air tests. irrelevant surface conditions (smoke, slag) may give misleading indications.

Ultrasonic Special commercial equipment, either of the pulse­echo or transmission type. Standard reference patterns for interpretation of RF or video patterns.

Surface and subsurface flaws including those too small to be detected by other methods. Especially for detecting subsurface lamination­like defects.

Very sensitive. Permits probing of joints inaccessible to radiography.

Requires high degree of skill in interpreting pulse­echo patterns. Permanent record is not readily obtained.

Pulse­echo equipment is highly developed for weld inspection purposes. The transmission­ type equipment simplifies pattern interpretation where it is applicable.

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Chapter II

IDENTIFICATION OF WELD DISCONTINUITIES

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specification used to invoke and control an inspection, are referred to as defects.

General Welding Discontinuities The following discontinuities are typical of all types of welding. Cracks: Crack is tight linear separations of metal that can be very short to very long indications. Cracks are grouped as hot or cold cracks. Hot cracks usually occur as the metal solidifies at elevated temperatures. Cold cracks occur after the metal has cooled to ambient temperatures ( delayed cracks). Cracks can be detected in a radiograph only when they are propagating in a direction that produces a change in thickness that is parallel to the x-ray beam. Cracks will appear as jagged and often very faint irregular lines. Cracks can sometimes appear as "tails" on inclusions or porosity.

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Lack of Fusion: Lack of fusion (Cold Lap) is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (inter pass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into base material without bonding.

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Porosity: Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters or rows. Sometimes porosity is elongated and may have the appearance of having a tail This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material it will have a radiographic density more than the surrounding area.

Cluster porosity: Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into gases when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

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Slag inclusions: Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions.

Incomplete penetration (IP): Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

Root concavity:

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Root or Internal concavity or suck back is condition where the weld metal has contracted as it cools and has been drawn up into the root of the weld. On a radiograph it looks similar to lack of penetration but the line has irregular edges and it is often quite wide in the center of the weld image.

Internal or root undercut: Internal or root undercut is an erosion of the base metal next to the root of the weld. In the radiographic image it appears as a dark irregular line offset from the centerline of the weldment. Undercutting is not as straight edged as LOP because it does not follow a ground edge.

External or crown undercut:

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External or crown undercut is an erosion of the base metal next to the crown of the weld. In the radiograph, it appears as a dark irregular line along the outside edge of the weld area.

Offset or mismatch: Offset or mismatch are terms associated with a condition where two pieces being welded together are not properly aligned. The radiographic image is a noticeable difference in density between the two pieces. The difference in density is caused by the difference in material thickness. The dark, straight line is caused by failure of the weld metal to fuse with the land area.

Inadequate weld reinforcement:

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Inadequate weld reinforcement is an area of a weld where the thickness of weld metal deposited is less than the thickness of the base material. It is very easy to determine by radiograph if the weld has inadequate reinforcement, because the image density in the area of suspected inadequacy will be more (darker) than the image density of the surrounding base material.

Excess weld reinforcement : Excess weld reinforcement is an area of a weld that has weld metal added in excess of that specified by engineering drawings and codes. The appearance on a radiograph is a localized, lighter area in the weld. A visual inspection will easily determine if the weld reinforcement is in excess of that specified by the engineering requirements.

Discontinuities in TIG welds

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The following discontinuities are peculiar to the TIG welding process. These discontinuities occur in most metals welded by the process including aluminum and stainless steels. The TIG method of welding produces a clean homogeneous weld which when radiographed is easily interpreted.

Tungsten inclusions. Tungsten is a brittle and inherently dense material used in the electrode in tungsten inert gas ( TIG ) welding. If improper welding procedures are used, tungsten may be entrapped in the weld. Radiographically, tungsten is denser than aluminum or steel; therefore, it shows as a lighter area with a distinct outline on the radiograph.

Oxide inclusions: Oxide inclusions are usually visible on the surface of material being welded (especially aluminum). Oxide inclusions are less dense than the surrounding materials and, therefore, appear as dark irregularly shaped discontinuities in the radiograph.

Discontinuities in Gas Metal Arc Welds (GMAW) The following discontinuities are most commonly found in GMAW welds.

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Whiskers: Whiskers are short lengths of weld electrode wire, visible on the top or bottom surface of the weld or contained within the weld. On a radiograph they appear as light, "wire like" indications.

Burn-Through: Burn-Through results when too much heat causes excessive weld metal to penetrate the weld zone. Often lumps of metal sag through the weld creating a thick globular condition on the back of the weld. These globs of metal are referred to as icicles. On a radiograph, burn through appears as dark spots, which are often surrounded by light globular areas (icicles).

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Chapter III

PENETRANT INSPECTION

Introduction

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer is applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence.

Colored (contrast) penetrants require good white light while fluorescent penetrants need to be viwed in darkened conditions with an ultraviolet "black light".

A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method. In this method, heavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated and stroked with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century through to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for the ferromagnetic iron and steels. Penetrant Inspection Improves the Detect ability of Flaws

The advantage that a liquid penetrant inspection (LPI) offers over an unaided visual inspection is that it makes defects easier to see for the inspector. There are basically two ways that a penetrant inspection process makes flaws more easily seen. First, LPI produces a flaw indication that is much larger and easier for the eye to detect than the flaw itself. Many flaws are so small or narrow that they are undetectable by the unaided eye.

The second way that LPI improves the detectability of a flaw is that it produces a flaw indication with a high level of contrast between the indication and the background which also helps to make the indication more easily seen. When a

visible dye penetrant inspection is performed, the penetrant materials are formulated using a bright red dye that provides for a high level of contrast

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between the white developer that serves as a background as well as to pull the trapped penetrant from the flaw. When a fluorescent penetrant inspection is performed, the penetrant materials are formulated to glow brightly and to give off light at a wavelength that the eye is most sensitive to under dim lighting conditions.

Basic Processing Steps of a Liquid Penetrant Inspection

1. Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied on the surface by spraying, brushing, or immersing the parts in a penetrant bath.

3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the

penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer

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penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and is often very specific to a particular application.

4­ Excess Penetrant Removal: This is a most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water .

5­ Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).

6­ Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

7­ Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws that may be present.

8­ Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Penetrant Testing Materials

The penetrant materials used today are much more sophisticated than the kerosene and whiting first used by railroad inspectors near the turn of the 20th century. Today's penetrants are carefully formulated to produce the level of sensitivity desired by the inspector.

1­ Penetrant: Penetrant materials are classified in the various industry and government specifications by their physical characteristics and their performance Penetrant materials come in two basic types. These types are listed below:

• Type 1 - Fluorescent Penetrants • Type 2 ­ Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation. Visible penetrants contain a red dye that provides high contrast against the white developer background. Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication. However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection. Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid that can significantly reduce the strength of a fluorescent indication.

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Penetrants are then classified by the method used to remove the excess penetrant from the part. The four methods are listed below:

• Method A ­ Water Washable • Method B ­ Post Emulsifiable, Lipophilic • Method C ­ Solvent Removable • Method D ­ Post Emulsifiable, Hydrophilic

Water washable (Method A) penetrants can be removed from the part by rinsing with water alone. These penetrants contain some emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self­emulsifying systems. Post emulsifiable penetrants come in two varieties, lipophilic and hydrophilic. In post emulsifiers, lipophilic systems (Method B), the penetrant is oil soluble and interacts with the oil­based emulsifier to make removal possible. Post emulsifiable, hydrophilic systems (Method D), use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash. Solvent removable penetrants require the use of a solvent to remove the penetrant from the part.

Properties of good Penetrant To perform well, a penetrant must possess following important characteristics.

• spread easily over the surface of the material being inspected to provide complete and even coverage.

• be drawn into surface breaking defects by capillary action. • remain in the defect but remove easily from the surface of the part. • remain fluid so it can be drawn back to the surface of the part through the drying

and developing steps. • be highly visible or fluoresce brightly to produce easy to see indications. • must not be harmful to the material being tested or the inspector.

2­ Emulsifiers: When removal of the penetrant from the defect due to over­washing of the part is a concern, a post emulsifiable penetrant system can be used. Post emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water washable. Most penetrant inspection specifications classify penetrant systems into four methods of excess penetrant removal. These are listed below:

1. Method A: Water­Washable 2. Method B: Post Emulsifiable, Lipophilic

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3. Method C: Solvent Removable 4. Method D: Post Emulsifiable, Hydrophilic

Method C relies on a solvent cleaner to remove the penetrant from the part being inspected. Method A has emulsifiers built into the penetrant liquid that makes it possible to remove the excess penetrant with a simple water wash. Method B and D penetrants require an additional processing step where a separate emulsification agent is applied to make the excess penetrant more removable with a water wash. Lipophilic emulsification systems are oil­based materials that are supplied in ready­to­use form. Hydrophilic systems are water­based and supplied as a concentrate that must be diluted with water prior to use .Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a water spray. Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical action of the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and lift newly exposed penetrant from the surface.

The hydrophilic post emulsifiable method (Method D) was introduced in the mid 1970's and since it is more sensitive than the lipophilic post emulsifiable method it has made the later method virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is used. However, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used.

3­ Developers The role of the developer is to pull the trapped penetrant material out of defects and to spread the developer out on the surface of the part so it can be seen by an inspector. The fine developer particles both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluorescence. The developer also allows more light to be emitted through the same mechanism. This is why indications are brighter than the penetrant itself under UV light. Another function that some developers performs is to create a white background so there is a greater degree of contrast between the indication and the surrounding background.

Developer Forms

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The AMS 2644 and Mil­I­25135 classify developers into six standard forms. These forms are listed below:

1. Form a ­ Dry Powder 2. Form b ­ Water Soluble 3. Form c ­ Water Suspendible 4. Form d ­ Nonaqueous Type 1 Fluorescent (Solvent Based) 5. Form e ­ Nonaqueous Type 2 Visible Dye (Solvent Based)

The developer classifications are based on the method that the developer is applied. The developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Each of the developer forms has advantages and disadvantages.

A)­ Dry Powder Dry powder developer is generally considered to be the least sensitive but it is inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways. The developer can be applied by dipping parts in a container of developer, or by using a puffer to dust parts with the developer. Parts can also be placed in a dust cabinet where the developer is blown around and allowed to settle on the part. Electrostatic powder spray guns are also available to apply the developer. The goal is to allow the developer to come in contact with the whole inspection area.

Unless the part is electrostatically charged, the powder will only adhere to areas where trapped penetrant has wet the surface of the part. The penetrant will try to wet the surface of the penetrant particle and fill the voids between the particles, which brings more penetrant to the surface of the part where it can be seen. Since dry powder developers only stick to the part where penetrant is present, the dry developer does not provide a uniform white background as the other forms of developers do. Having a uniform light background is very important for a visible inspection to be effective and since dry developers do not provide one, they are seldom used for visible inspections. When a dry developer is used, indications tend to stay bright and sharp since the penetrant has a limited amount of room to spread.

B) - Water Soluble As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. Aqueous developers contain wetting agents that cause the solution to function much like dilute hydrophilic emulsifier and can lead to additional removal of entrapped penetrant. Drying is achieved by placing the wet but well drained

part in a recalculating warm air dryer with the temperature held between 70 and 75°F. If the parts are not dried quickly, the indications will will be blurred and indistinct. Properly developed parts will have an even, pale white coating over the entire surface.

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C) ­ Water Suspendible Water suspendible developers consist of insoluble developer particles suspended in water. Water suspendible developers require frequent stirring or agitation to keep the particles from settling out of suspension. Water suspendible developers are applied to parts in the same manner as water soluble developers. Parts coated with a water suspendible developer must be forced dried just as parts coated with a water soluble developer are forced dried. The surface of a part coated with a water suspendible developer will have a slightly translucent white coating.

B) ­ Nonaqueous Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a spray gun. Nonaqueous developers are commonly distributed in aerosol spray cans for portability. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required. A nonaqueous developer should be applied to a thoroughly dried part to form a slightly translucent white coating.

Preparation of Part

One of the most critical steps in the penetrant inspection process is preparing the part for inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be removed to ensure that defects are open the surface of the part. If the parts have been machined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. It is even possible for metal smearing to occur as a result of cleaning operations such as grit or vapor blasting. This layer of metal smearing must be removed before inspection.

Contaminants

Coatings, such as paint, are much more elastic than metal and will not fracture even though a large defect may be present just below the coating. The part must be thoroughly cleaned as surface contaminates can prevent the penetrant from entering a defect. Surface contaminants can also lead to a higher level of background noise since the excess penetrant may be more difficult to remove.

Common coatings and contaminates that must be removed include: paint, dirt, flux, scale, varnish, oil, etchant, smut, plating, grease, oxide, wax, decals, machining fluid, rust, and residue from previous penetrant inspections.

Some of these contaminants would obviously prevent penetrant from entering defects and it is, therefore, clear that they must be removed. However, the impact of other contaminants such as the residue from previous penetrant inspections is less clear, but they can have a disastrous affect on the inspection. Take the link below to review some

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of the research that has been done to evaluate the effects of contaminants on LPI sensitivity.

A good cleaning procedure will remove all contamination from the part and not leave any residue that may interfere with the inspection process. It has been found that some alkaline cleaners can be detrimental to the penetrant inspection process if they have silicates in concentrations above 0.5 percent. Sodium metasilicate, sodium silicate, and related compounds can adhere to the surface of parts and form a coating that prevents penetrant entry into cracks. Researchers in Russia have also found that some domestic soaps and commercial detergents can clog flaw cavities and reduce the wettability of the metal surface, thus, reducing the sensitivity of the penetrant. Conrad and Caudill found that media from plastic media blasting was partially responsible for loss of LPI indication strength. Microphotographs of cracks after plastic media blasting showed media entrapment in addition to metal smearing.

It is very important that the material being inspected has not been smeared across its own surface during machining or cleaning operations. It is well recognized that machining, honing, lapping, hand sanding, hand scraping, shot peening, grit blasting, tumble deburring, and peening operations can cause a small amount of the material to smear on the surface of some materials. It is perhaps less recognized that some cleaning operations, such as steam cleaning, can also cause metal smearing in the softer materials. Take the link below to learn more about metal smearing and its affects on LPI.

Common Uses of Liquid Penetrant Inspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using LPI include the following:

• Metals (aluminum, copper, steel, titanium, etc.) • Glass • Many ceramic materials • Rubber • Plastics

LPI offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components.

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Penetrant material can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. At right, visible dye penetrant being locally applied to a highly loaded connecting point to check for fatigue cracking.

Penetrant inspection systems have been developed to inspect some very large components. In this picture, DC­10 banjo fittings are being moved into a penetrant inspection system at what used to be the Douglas Aircraft Company's Long Beach, California facility. These large machined aluminum forgings are used to support the number 3 engine in the tail of a DC­10 aircraft.

Liquid penetrant inspection is used to inspect of flaws that break the surface of the sample. Some of these flaws are listed below:

• Fatigue cracks • Quench cracks • Grinding cracks • Overload and impact fractures • Porosity • Laps • Seams • Pin holes in welds • Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface.

Advantages and Disadvantages of Penetrant Testing Like all nondestructive inspection methods, liquid penetrant inspection has both advantages and disadvantages. The primary advantages and disadvantages when compared to other NDE methods are summarized below. Primary Advantages

• The method has high sensitive to small surface discontinuities. • The method has few material limitations, i.e. metallic and nonmetallic, magnetic

and nonmagnetic, and conductive and nonconductive materials may be inspected.

• Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

• Parts with complex geometric shapes are routinely inspected. • Indications are produced directly on the surface of the part and constitute a visual

representation of the flaw. • Penetrant materials and associated equipment are relatively inexpensive.

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Primary Disadvantages

• Only surface breaking defects can be detected. • Only materials with a relative nonporous surface can be inspected. • Precleaning is critical as contaminants can mask defects. • Metal smearing from machining, grinding, and grit or vapor blasting must be

removed prior to LPI. • The inspector must have direct access to the surface being inspected. • Surface finish and roughness can affect inspection sensitivity. • Multiple process operations must be performed and controlled. • Post cleaning of acceptable parts or materials is required. • Chemical handling and proper disposal is require

Chapter IV

Magnetic Particle Inspection

Introduction:

Magnetic particle inspection is a nondestructive testing method used for surface and near surface defect detection. MPI is a fast and relatively easy to apply and surface preparation is not as critical as it is for some other NDT methods. These characteristics make MPI one of the most widely utilized nondestructive testing methods.

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MPI uses magnetic fields and small magnetic particles, such as iron filings to detect flaws in components. The only requirement is that the component being inspected must be made of a ferromagnetic material such iron, nickel, cobalt, or some of their alloys.

Ferromagnetic materials are materials that can be magnetized to a level that will allow the inspection to be effective.

The method is used to inspect a variety of product forms such as castings, forgings, and weldments. Many different industries use magnetic particle inspection for determining a component's fitness­for­use. Some examples of industries that use magnetic particle inspection are the structural steel, automotive, petrochemical, power generation, and aerospace industries. Underwater inspection is another area where magnetic particle inspection may be used to test items such as offshore structures and underwater pipelines.

Basic Principles

In theory, magnetic particle inspection (MPI) is a relatively simple concept. Consider a bar magnet. It has a magnetic field in and around the magnet. Any place that a magnetic line of force exits or enters the magnet is called a pole. A pole where a magnetic line of force exits the magnet is called a north pole and a pole where a line of force enters the magnet is called a south pole.

When a bar magnet is broken in the center of its length, two complete bar magnets with magnetic poles on each end of each piece will result. If the magnet is just cracked but not broken completely in two, a north and south pole will form at each edge of the crack. The magnetic field exits the north pole and reenters the at the south pole. The magnetic field spreads out when it encounter the small air gap created by the crack because the air cannot support as much magnetic field per unit volume as the magnet can. When the field spreads out, it appears to leak out of the material and, thus, it is called a flux leakage field.

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If iron particles are sprinkled on a cracked magnet, the particles will be attracted to and cluster not only at the poles at the ends of the magnet but also at the poles at the edges of the crack. This cluster of particles is much easier to see than the actual crack and this is the basis for magnetic particle inspection.

The first step in a magnetic particle inspection is to magnetize the component that is to be inspected. If any defects on or near the surface are present, the defects will create a leakage field. After the component has been magnetized, iron particles, either in a dry or wet suspended form, are applied to the surface of the magnetized part. The particles will be attracted and cluster at the flux leakage fields, thus forming a visible indication that the inspector can detect.

History of Magnetic Particle Inspection

Magnetism is the ability of matter to attract other matter. The ancient Greeks were the first to discover this phenomenon in a mineral they named magnetite. Later on Bergmann, Becquerel, and Faraday discovered that all matter including liquids and gasses were affected by magnetism, but only a few responded to a noticeable extent.

The earliest known magnetic inspection an object took place as early as 1868. Cannon barrels were checked for defects by magnetizing the barrel then sliding a magnetic compass along the barrel's length. These early inspectors were able to locate flaws in the barrels by monitoring the needle of the compass.

In the early 1920’s, William Hoke realized that magnetic particles could be used with magnetism as a means of locating defects. Hoke discovered that a surface or subsurface flaw in a magnetized material caused the magnetic field to distort and extend beyond the part. This discovery was brought to his attention in the machine shop. He noticed that the metallic grindings from hard steel parts, which were being held by a magnetic chuck while being ground, formed patterns on the face of the parts

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which corresponded to the cracks in the surface. Applying a fine ferromagnetic powder to the parts caused a build up of powder over flaws and formed a visible indication.

Today, the MPI inspection method is used extensively to check for flaws in a large variety of manufactured materials and components. MPI is used to check materials such as steel bar stock for seams and other flaws prior to investing machining time during the manufacturing of a component. Critical automotive components are inspected for flaws after fabrication to ensure that defective parts are not placed into service. MPI is used to inspect some highly loaded components that have been in­service for a period of time. For example, many components of high performance race cars are inspected whenever the engine, drive train and other systems are overhauled. MPI is also used to evaluate the integrity of structural welds on bridges, storage tanks, pipelines and other critical structures.

Magnetism

Magnets are very common items in the workplace and household. Uses of magnets range from holding pictures on the refrigerator to causing torque in electric motors. The term "magnetic field" simply describes a volume of space where there is a change in energy within that volume. This change in energy can be detected and measured. The location where a magnetic field can be detected exiting or entering a material is called a magnetic pole. Magnetic poles have never been detected in isolation but always occur in pairs and, thus, the name dipole.

A bar magnet can be considered a dipole with a north pole at one end and South Pole at the other. A magnetic field can be measured leaving the dipole at the North Pole and returning the magnet at the South Pole. If a magnet is cut in two, two magnets or dipoles are created out of one. This sectioning and creation of dipoles can continue to the atomic level. Therefore, the source of magnetism lies in the basic building block of all matter...the atom.

The Source of Magnetism

All matter is composed of atoms, and atoms are composed of protons, neutrons and electrons. The protons and neutrons are located in the atom's nucleus and the electrons are in constant motion around the nucleus. Electrons carry a negative electrical charge and produce a magnetic field as they move through space. A magnetic field is produced whenever an electrical charge is in motion. The strength of this field is called the magnetic moment.

consider electric current flowing through a conductor. When the electrons (electric current) are flowing through the conductor, a magnetic field forms around the conductor.

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The magnetic field can be detected using a compass. The magnetic field will place a force on the compass needle.

Since all matter is comprised of atoms, all materials are affected in some way by a magnetic field. However, not all materials react the same way.

Diamagnetic, Paramagnetic, and Ferromagnetic Materials In most atoms, electrons occur in pairs. Each electron in a pair spins in the opposite direction. So when electrons are paired together, there opposite spins cause their magnetic fields to cancel each other. Therefore, no net magnetic field exists. Alternately, materials with some unpaired electrons will have a net magnetic field and will react more to an external field. Most materials can be classified as ferromagnetic, diamagnetic or paramagnetic.

Diamagnetic metals have a very weak and negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Paramagnetic metals have a small and positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.

Ferromagnetic materials have a large and positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atoms moments (10 12 to 10 15 ) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnitized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains become aligned to produce a strong magnetic field within the part. Iron, nickel, and cobalt are examples of ferromagnetic materials.

Magnetic Domains

Ferromagnetic materials get their magnetic properties because the material is made up of small regions known as magnetic domains. In each domain, all of the atomic dipoles are coupled together in a preferential direction. This alignment develops as the material develops its crystalline structure during solidification from the molten state.

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During solidification a trillion or more atom moments are aligned parallel so that the magnetic force within the domain is strong in one direction. Even though the domains are magnetically saturated, the bulk material may not show any signs of magnetism because the domains develop themselves are randomly oriented relative to each other.

Ferromagnetic materials become magnetized when the magnetic domains within the material are aligned. This can be done by placing the material in a strong external magnetic field or by passing electrical current through the material. Some or all of the domains can become aligned. The more domains are aligned, the stronger the magnetic field in the material. When all of the domains are aligned, the material is magnetically saturated and additional amount of external magnetization force will not cause any increase in its internal level of magnetization.

Unmagnetized Material Magnetized Material

Magnetic Field Characteristics

Magnetic lines of force have a number of important properties, which include:

• They seek the path of least resistance between opposite magnetic poles. In a single bar magnet as shown to the right, they attempt to form closed loop from pole to pole.

• They never cross one another. • They all have the same strength. • Their density decreases (they spread out) when they move from an area of

higher permeability to an area of lower permeability. • Their density decreases with increasing distance from the poles. • They are considered to have direction as if flowing, though no actual movement

occurs. They flow from the south pole to the north pole within the material and north pole to south pole in air.

Electromagnetic Fields

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In most conductors, the magnetic field exists only as long as the current is flowing

the direction of the magnetic field is dependent on the direction of the electrical current in the wire. A three­dimensional representation of the magnetic field is shown above. There is a simple rule for remembering the direction of the magnetic field around a conductor. It is called the right­hand rule. If a person grasps a conductor in ones right hand with the thumb pointing in the direction of the current, the fingers will circle the conductor in the direction of the magnetic field.

Magnetic Field Produced by a Coil

When a current carrying conductor is formed into a loop or several loops to form a coil, a magnetic field develops that flows through the center of the loop or coil along longitudinal axis and circles back around the outside of the loop or coil. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil. A loosely wound coil is illustrated below to show the interaction of the magnetic field. The magnetic field is essentially uniform down the length of the coil when it is wound tighter.

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The strength of a coil's magnetic field increases not only with increasing current but also with each loop that is added to the coil. A long straight coil of wire is called a solenoid and can be used to generate a nearly uniform magnetic field similar to that of a bar magnet. The concentrated magnetic field inside a coil is very useful in magnetizing ferromagnetic materials for inspection using the magnetic particle testing method. Please be aware that the field outside the coil is weak and is not suitable for magnetize ferromagnetic materials.

The Hysteresis Loop and Magnetic Properties

A great deal of information can be learned about the magnetic properties of a material by studying its hysteresis loop. A hysteresis loop shows the relationship between the induced magnetic flux density B and the magnetizing force H. It is often referred to as the B­H loop. An example hysteresis loop is shown below.

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Plotting the change in magnetic flux B induced a ferromagnetic material while the magnetizing force H is changed generates the hysteresis loop. A ferromagnetic material that has never been previously magnetized or has been thoroughly demagnetized will follow the dashed line as H is increased. As the line demonstrates, the greater the amount of current applied (H+), the stronger the magnetic field in the component (B+). At point "a" almost all of the magnetic domains are aligned and an additional increase in the magnetizing force will produce very little increase in magnetic flux. The material has reached the point of magnetic saturation.

When H is reduced back down to zero, the curve will move from point "a" to point "b." At this point, it can be seen that some magnetic flux remains in the material even though the magnetizing force is zero, this is referred to as the point of retentivity on the graph and indicates the remanence or level of residual magnetism in the material. (Some of the magnetic domains remain aligned but some have lost there alignment.) As the magnetizing force is reversed, the curve

moves to point "c", where the flux has been reduced to zero. This is called the point of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.) The force required to remove the residual magnetism from the material, is called the coercive force or coercivity of the material.

As the magnetizing force is increased in the negative direction, the material will again become magnetically saturated but in the opposite direction (point "d"). Reducing H to

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zero brings the curve to point "e." It will have a level of residual magnetism equal to that achieved in the other direction. Increasing H back in the positive direction will return B to zero. Notice that the curve did not return to the origin of the graph because some force is required to remove the residual magnetism. The curve will take a different path from point "f" back the saturation point where it with complete the loop.

From the hysteresis loop, a number of primary magnetic properties of a material can be determined.

Retentivity ­ A measure of the residual flux density corresponding to the saturation induction of a magnetic material. In other words, it is a material's ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation. (The value of B at point B on the hysteresis curve.)

Residual Magnetism or Residual Flux ­ the magnetic flux density that remains in a material when the magnetizing force is zero. Note that residual magnetism and retentivity are the same when the material has been magnetized to the saturation point. However, the level of residual magnetism may be lower than the retentivity value when the magnetizing force did not reach the saturation level.

Coercive Force ­ The amount of reverse magnetic field which must be applied to a magnetic material to make the magnetic flux return to zero. (The value of H at point C on the hysteresis curve.)

Permeability ­ A property of a material that describes the ease with which a magnetic flux is established in the component.

Reluctance ­ Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is analogous to the resistance in an electrical circuit.

The shape of the hysteresis loop tells a great deal about the material being magnetized. The hysteresis curves of two different materials are shown in the graph.

Magnetic Field Orientation and Flaw Detectability

To properly inspect a component for cracks or other defects, it is important to understand that orientation between the magnetic lines of force and the flaw is very important. There are two general types of magnetic fields that can be established within a component.

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A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. Longitudinal magnetization of a component can be accomplished using the longitudinal field set up by a coil or solenoid. It can also be accomplished using permanent or electromagnets.

A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. A circular magnetic field is induced in an article by either passing current through the component or by passing current through a conductor surrounded by the component.

To magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect, if the magnetic field is parallel to the defect, the field will see little disruption and no flux leakage field will be produced.

An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. Since defects may occur in various directions, each part is normally magnetized in two directions at right angles to each other. To determine most of the defects.

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Demagnetization

After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component

Remanent magnetic fields can:

• Affect machining by causing cuttings to cling to a component. • Interfere with electronic equipment such as a compass. • Create a condition known as "ark blow" in the welding process. Arc blow may

cause the weld arc to wonder or filler metal to be repelled from the weld. • Cause abrasive particle to cling to bearing or faying surfaces and increase wear.

Magnetizing Equipment for Magnetic Particle Inspection

To properly inspect a part for cracks or other defects, it is important to become familiar with the different types of magnetic fields and the equipment used to generate them. As discussed previously, one of the primary requirements for detection of a defect in a ferromagnetic material is that the magnetic field induced in the part must intercept the defect at a 45 to 90 degrees angle. Flaws that are normal (90 degrees) to the magnetic field will produce the strongest indications because they disrupt more of the magnet flux.

A variety of equipment exist to establish the magnetic field for MPI. Some equipment is designed to be portable so that inspections can be made in the field and some is designed to be stationary for ease of inspection in the laboratory or manufacturing facility.

Permanent magnets

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Permanent magnets are sometimes used for magnetic particle inspection as the source of magnetism. The two primary types of permanent magnets are bar magnets and horseshoe (yoke) magnets. These industrial magnets are usually very strong and may require significant strength to remove them from a piece of metal. Some permanent magnets require over 50 pounds of force to remove them from the surface. Because it is difficult to remove the magnets from the component being inspected, and sometimes difficult and dangerous to place the magnets, their use is not particularly popular. However, a diver for inspection in an underwater environment or other areas sometimes uses permanent magnets, such as in an explosive environment, where electromagnets cannot be used. Permanent magnets can also be made small enough to fit into tight areas where electromagnets might not fit

Electromagnets Today, most of the equipment used to create the magnetic field used in MPI is based on electromagnetism. That is, using an electrical current to produce the magnetic field. An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. It is basically made by wrapping an electrical coil around a piece of soft ferromagnetic steel. A switch is included in the electrical circuit so that the current and, therefore, also the magnetic field can be turn on and off. They can be powered with alternating current from a wall socket or by direct current from a battery pack. This type of magnet generates a

very strong magnetic field in a local area where the poles of magnet touch the part to be inspected. Some yokes can lift weights in excess of 40 pounds.

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Portable yoke with battery pack Portable magnetic particle kit

Prods Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current through the metal. The current passing between the prods creates a circular magnetic field around the prods that is can be used in magnetic particle inspection. Prods are typically made from copper and have an insulated handle to help protect the operator. One of the prods has a trigger switch so that the current can be quickly and easily turned on and off. Sometimes the the two prods are connected by any insulator as shown in the image to facilitate one hand operation. This is referred to as a dual prod and is commonly used for weld inspections.

If proper contact is not maintained between the prods and the component surface, electrical arcing can occur and cause damage to the component. For this reason, the use of prods are not allowed when inspecting aerospace and other critical components. To help to prevent arcing, the prod tips should be

inspected frequently to ensure that they are not oxidized, covered with scale or other contaminant, or damaged.

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Portable Coils and Conductive Cables Coils and conductive cables are used to establish a longitudinal magnetic field within a component. When a preformed coil is used, the component is placed against the inside surface on the coil. Coils typically have three or five turns of a copper cable within the molded frame. A foot switch is often used to energize the coil. Conductive cables are wrapped around the component. The cable used is typically 00 extra flexible or 0000 extra flexible. The number of wraps is determined by the magnetizing force needed and, of course, the length of the cable. Normally the wraps are kept as close together as possible. When using a coil or cable wrapped into a coil, amperage is usually expressed in ampere­turns. Ampere­turns is the amperage shown on the amp meter times the number of turns in the coil.

Portable coil Conductive Cable

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central conductor.

This type of a setup is used to inspect parts that are hollow such as gears, tubes, and other ring­shaped objects. A central conductor is an electrically conductive bar that is usually made of copper or aluminum. The bar is inserted through the center of the hollow part and the bar is then clamped between the contact pads. When current is passed through the central conductor, a circular magnetic field flows around the bar and enters into the part or parts being inspected.

Lights for Magnetic Particle Inspection

Magnetic particle inspection can be performed using particles that are highly visible under white lighting conditions or particles that are highly visible under ultraviolet lighting conditions. When an inspection is being performed using the visible color contrast particles, no special lighting is required as long as the area of inspection is well lit. A light intensity of at least 1000 lux (100 fc) is recommended when a visible particles are used, but a variety of light sources can be used.

When fluorescent particles are used, special ultraviolet light must be used. Fluorescence is defined as the property of emitting radiation as a result of and during exposure to radiation. Particles used in fluorescent magnetic particle inspections are coated with a material that produces light in the visible spectrum when exposed to the near­ultraviolet light. This "particle glow" provides high contrast indications on the component anywhere particles collect. Particles that fluoresce yellow­green are most common because this color matches the peak sensitivity of the human eye under dark conditions. However, particles that fluoresce red, blue, yellow, and green colors are available.

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Ultraviolet Light

Ultraviolet light or "black light" is light in the 1,000 to 4,000 Angstroms (100 to 400 nm) wavelength range in the electromagnetic spectrum. It is a very energetic form of light that is invisible to the human eye. Wavelengths above 4,000 Angstroms fall into the visible light spectrum and are seen as the color violet. UV is separated according to wavelength into three classes: A, B, and C. The shorter the wavelength, the more energy that is carried in the light and the more dangerous it is to the human cells.

Class UV­A UV­B UV­C

Wavelength Range 3,200–4,000 Angstroms 2,800–3,200 Angstroms 2,800–1,000 Angstroms

The desired wavelength range for use in nondestructive testing is between 3,500 and 3,800 Angstroms with a peak wavelength at about 3,650 A. This wavelength range is used because it is in the UV­A range, which is the safest to work with. UV­B will do an effective job of causing substances to fluoresce, however, it should not be used because harmful effects such as skin burns, and eye damage can occur. This wavelength of radiation is found in the arc created during the welding process. UV­C (1,000 to 2,800) is even more dangerous to living cells and is used to kill bacteria in industrial and medical settings.

The desired wavelength range for use in NDT is obtained by filtering the ultraviolet light generated by the light bulb. The output of a UV bulb spans a wide range of wavelengths. The short wave lengths of 3,120 A to 3,340 A are produced in low levels. A peak wavelength of 3650 A is produced at a very high intensity. Wavelengths in the visible violet range (4050 A to 4350 A), green­yellow (5460 A), yellow (6220 A) and orange (6770 A) are also usually produced. The filter allows only radiation in the range of 3200 to 4000 angstroms and a little visible dark purple to pass.

Magnetic Particles

As mentioned previously, the particles that are used for magnetic particle inspection are a key ingredient as they form the indications that alert the inspector to defects. Particles start out as tiny milled (a machining process) pieces of iron or iron oxide. A pigment (somewhat like paint) is bonded to their surfaces to give the particles color. The metal used for the particles has high magnetic permeability and low retentivity. High magnetic permeability is important because it makes the particles attract easily to small magnetic leakage fields from discontinuities, such as flaws. Low retentivity is important because the particles themselves never become strongly magnetized so they do not stick to each other or the surface of the part. Particles are available in a dry mix or a wet solution.

Dry Magnetic Particles

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Dry magnetic particles can typically be purchased in red, black, gray, yellow and several other colors so that a high level of contrast between the particles and the part being inspected can be achieved. The size of the magnetic particles is also very important. Dry magnetic particle products are produced to include a range of particle sizes. The fine particles are around 50 m (0.002 inch) in size are about three times smaller in diameter and more than 20 times lighter than the coarse particles (150 m or 0.006 inch), which make them more sensitive to the leakage fields from very small discontinuities. However, dry testing particles cannot be made exclusively of the fine particles. Coarser particles are needed to bridge large discontinuities and to reduce the powder's dusty nature. Additionally, small particles easily adhere to surface contamination, such as remanent dirt or moisture, and get trapped in surface roughness features producing a high level of background. It should also be recognized that finer particles will be more easily blown away by the wind and, therefore, windy conditions can reduce the sensitivity of an inspection. Also, reclaiming the dry particles is not recommended because the small particle are less likely to be recaptured and the "once used" mix will result in less sensitive inspections.

The particle shape is also important. Long, slender particles tend align themselves along the lines of magnetic force. However, research has shown that if dry powder consists only of long, slender particles, the application process would be less than desirable. Elongated particles come from the dispenser in clumps and lack the ability to flow freely and form the desired "cloud" of particles floating on the component. Therefore, globular particles are added that are shorter. The mix of globular and elongated particles result in a dry powder that flows well and maintain good sensitivity. Most dry particle mixes have particle with L/D ratios between one and two.

Wet Magnetic Particles Magnetic particles are also supplied in a wet suspension such as water or oil. The wet magnetic particle testing method is generally more sensitive than the dry because the suspension provides the particles with more mobility and makes it possible for smaller particles to be used since dust and adherence to surface contamination is reduced or

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eliminated. The wet method also makes it easy to apply the particles uniformly to a relatively large area.

Wet method magnetic particles products differ from dry powder products in a number of ways. One way is that both visible and fluorescent particle are available. Most nonfluorescent particles are ferromagnetic iron oxides, which are either black or brown in color. Fluorescent particles are coated with pigments that fluoresce when exposed to ultraviolet light. Particles that fluoresce green­yellow are most common to take advantage of the peak color sensitivity of the eye but other fluorescent colors are also available. (For more information on the color sensitivity of the eye, see the penetrant inspection material.)

The particles used with the wet method are smaller in size than those used in the dry method for the reasons mentioned above. The particles are typically 10 m (0.0004 inch) and smaller and the synthetic iron oxides have particle diameters around 0.1 m (0.000004 inch). This very small size is a result of the process used to form the particles and is not particularly desirable, as the particles are almost too fine to settle out of suspension. However, due to their slight residual magnetism, the oxide particles are present mostly in clusters that settle out of suspension much faster than the individual particles. This makes it possible to see and measure the concentration of the particles for process control purposes. Wet particles are also a mix of long slender and globular particles. The carrier solutions can be water­ or oil­based. Water­based carriers form quicker indications, are generally less expensive, present little or no fire hazard, give off no petrochemical fumes, and are easier to clean from the part. Water­based solutions are usually formulated with a corrosion inhibitor to offer some corrosion protection. However, oil­based carrier solutions offer superior corrosion and hydrogen embrittlement protection to those materials that are prone to attack by these mechanisms.

Chapter IV

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Ultrasonic Testing

Basic Principles of Ultrasonic Testing

Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, dimensional measurements, material characterization, and more.

The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface. The reflected wave signal is transformed into electrical signal by the transducer and is displayed on a screen.

SCREEN

Ultrasonic Inspection is a very useful and versatile NDT method for detecting both surface and subsurface volumetric defects and is widely used in pipeline, oil and gas and processing industry.

Plate Crack

0 2 4 6 8 10

Initial pulse

Crack echo

Back surface echo

Oscilloscope, or flaw detector screen

Probe

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Advantages of Ultrasonic Inspection

Some of the advantages of ultrasonic inspection that are often cited include:

• It is sensitive to both surface and subsurface discontinuities. • The depth of penetration for flaw detection or measurement is superior to

other NDT methods. • Only single­sided access is needed when the pulse­echo technique is

used. • It is high accuracy in determining reflector position and estimating size and

shape. • Minimal part preparation required. • Electronic equipment provides instantaneous results. • Detailed images can be produced with automated systems. • It has other uses such as thickness measurements, in addition to flaw

detection.

Disadvantages of Ultrasonic Inspection

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

16Hz 20 kHz 200 kHz 15 MHz

256 Hz 70 kHz

Audible range

Ultrasonic testing range

1­5 MHz

Usual steel testing range

Sound Spectrum

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• Surface must be accessible to transmit ultrasound. • Skill and training is more extensive than with some other methods. • It normally requires a coupling medium to promote transfer of sound

energy into test specimen. • Materials that are rough, irregular in shape, very small, exceptionally thin

or not homogeneous are difficult to inspect. • Cast iron and other coarse grained materials are difficult to inspect due to

low sound transmission and high signal noise. • Linear defects oriented parallel to the sound beam may go undetected. • Reference standards are required for both equipment calibration, and

characterization of flaws.

Properties of sound wave

Wave Propagation Ultrasonic testing is based on time­varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibrational motion about their equilibrium positions.

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as longitudinal waves, shear waves, surface waves, and in thin materials as plate waves. Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

Longitudinal waves: In longitudinal waves the oscillations occur in the longitudinal direction of the direction of wave propagation. Since compressional forces are active in these waves, they are also called compressional waves. Compression waves can be

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generated in liquids, as well as solids because the energy travels through the atomic structure by a series of comparison and expansion (rarefaction) movements.

Transverse or shear wave: In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation and, therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves

Surface or Rayleigh waves : Surface or Rayleigh waves travel on the surface of a relative thick solid material penetrating to a depth of one wavelength. The particle movement has an elliptical orbit. Raleigh waves are useful because they are very sensitive to surface defects and since they will follow the surface around curves, therefore can be used to inspect areas that other waves might have difficulty in reaching.

Plate waves: Plate waves can be propagated only in very thin metals. Lamb waves are the most commonly used plate waves in NDT. Lamb waves are a complex vibrational wave that travels through the entire thickness of a material. Propagation of Lamb waves depends on density, elastic, and material properties of a component, and they are influenced by a great deal by selected frequency and material thickness.

Velocity: How quickly a sound wave will travel

Frequency: How many vibrations per second

Wave length:

How far a sound wave will advance in completing one cycle The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation.

1 Second A 2H

1 Second B=5H

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A change in frequency will result in a change in wavelength. In ultrasonic testing, the shorter wavelength resulting from an increase in frequency will help in the detection of smaller discontinuities.

Sensitivity: Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths).

Resolution: Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases.

Velocity of sound traveling through materials: Velocity of sound varies with the material in which it is traveling. Material Compression

Velocity m\sec Shear Velocity m\sec

Steel 5960 3245

Water 1490 NA

Air 344 NA

Copper 4700 2330

Attenuation of Sound Waves When sound travels through a medium, its intensity diminishes with distance. This weakening results from two basic causes, which are scattering and absorption. The combined effect of scattering and absorption is called attenuation.

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Refraction and Snell's Law

When an ultrasound wave passes through an interface between two materials, it produces both reflected and refracted waves. Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials. The velocity of sound in each material is determined by the material properties (elastic modules and density) for that material.

Snell's Law describes the relationship between the angles and the velocities of the waves. Snell's law equates the ratio of material velocities v1 and v2 to the ratio of the sine's of incident ( ) and refraction ( ) angles, as shown in the following equation.

Where:

VL1 is the longitudinal wave velocity in material 1.

VL2 is the longitudinal wave velocity in material 2.

Ultrasonic Probes

The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for

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ultrasonic testing. The active element is the Probe. It converts the electrical energy to acoustic energy, and vice versa.

Characteristics of Probes

The probe is a very important part of the ultrasonic instrumentation system. The probe converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). Many factors, including material, mechanical and electrical construction, and the external mechanical and electrical load conditions, influence the behavior a transducer. Mechanical construction includes parameters such as radiation surface area, mechanical damping, housing, connector type

.

Types of Probes

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Ultrasonic transducers are manufactured for a variety of application and can be custom fabricated when necessary. Careful attention must be paid to selecting the proper transducer for the application It is important to choose transducers that have the desired frequency, bandwidth, and focusing to optimize inspection capability. Most often the transducer is chosen either to enhance sensitivity or resolution of the system.

Transducers are classified into groups according to the application.

• Contact transducers are used for direct contact inspections, and are generally hand manipulated. They have elements protected in a rugged casing to withstand sliding contact with a variety of materials. These transducers are designed so that they are easy to grip and move along a surface. They also often have replaceable wear plates to lengthen their useful life. Coupling materials of water, grease, oils, or commercial materials are used to remove the air gap between the transducer and the component inspected. Contact probes are classified as.

• Single crystal probe • Twin crystal probe • Normal beam or zero degree probe • Angle beam probe

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Contact transducers are available in a variety of configurations to improve their usefulness for a variety of applications.

Single crystal probe normal probe:

The flat contact transducer shown above is used normal beam inspections of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may need to be added to the face of the transducer.

Twin crystal normal probe:

contain two independently operating elements in a single housing. One of the elements transmits and the other receives. Active elements can be chosen for their sending and receiving capabilities providing a transducer with a cleaner signal, and transducers for special applications, such as inspection of course grain material. Dual element transducers are especially well suited for making measurements in applications where reflectors are very near the transducer since this design eliminates the ring down effect that single­element transducers experience. (When single­element transducers are operating in pulse echo mode, the element can not start receiving reflected signals until the element has stopped ringing from it transmit function.) Dual element transducers are very useful when making thickness measurements of thin materials and when inspecting for near surface defects. The two elements are angled towards each other to create a crossed­beam sound path in the test material.

Angle beam transducers:

Angle beams are typically used to introduce a refracted shear wave into the test material.In the fixed angle versions, the angle of refraction that is marked on the transducer is only accurate for a particular material, which is usually steel. The angled sound path allows the sound beam to be reflected from the back wall to improve detectability of flaws in and around welded areas. They are also used to generate surface waves for use in detecting defects on the surface of a component.

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Couplant

A couplant is a material (usually liquid) that facilitates the transmission of ultrasonic energy from the transducer into the test specimen. Couplant is generally necessary because the acoustic impedance mismatch between air and solids

Calibration Blocks

Standard blocks are used to calibrate the instrument and to calculate different features of probe and the instrument. These blocks consists accurately cut and fine polished surfaces, holes ,angles etc.

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Inspection of Welded Joints

The most commonly occurring defects in welded joints are porosity, slag inclusions, lack of side­wall fusion, lack of inter­run fusion, lack of root penetration, undercutting, and longitudinal or transverse cracks.

Ultrasonic weld inspections are typically performed using a straight beam probe in conjunction with an angle beam probe A straight beam probe, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat­affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

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Chapter VI

RADIORGAPHIC TESTING

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Introduction:

In this method of Non­destructive testing the penetration property of X­ray and Gamma rays to detect the discontinuities. The object to be inspected is placed between the radiation source and a piece of film. X­rays or gamma rays pass through the object. The object will stop some of the radiation. Thicker and denser area will stop more of the radiation and show on the film lighter than thinner or less dense area. Most weld defects will show on the film darker than the surrounding area.

Nature of Penetrating Radiation

X­rays and gamma rays are part of the electromagnetic spectrum. They are waveforms as are light rays, microwaves, and radio wave, but x­rays and gamma rays cannot been seen, felt, or heard. They possess no charge and no mass and, therefore, are not influenced by electrical and magnetic fields and will always travel in straight lines. They can be characterized by frequency, wavelength, and velocity

The Electromagnetic Spectrum

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The International System (SI) unit for activity is the Becquerel (Bq), w

Wavelengths of Electro Magnetic Spectrum

Electro Magnetic Radiation Type Wave length in nm

Visible Light 700-400

Ultraviolet light 400-100

X-Rays

Gamma -Rays

1 nm =10­9 Meters

Advantage of Radiography 1.Gives a permanent record 2.Detects internal Flaws 3.Detects volumetric flaws readily 4.Can be used on most materials 5.Can check for correct assembly

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6.Gives direct Images 7.Real time Image is possible

Disadvantages of Radiography

1­Radiation Health 2­Can be sensitive to defect orientation and could miss planar flaws 3­Has limited ability to detect fine cracks 4­Access is required to both sides of the object 5­Limited thickness of the material can be penetrated 6­Skilled radiographic interpretation is required 7­Require high capital cost 8­Relatively slow process 9­Require high capital cost 10­Require high running cost

Properties of X­rays and gamma rays

1.They have no effect on the human senses 2.They have adverse effect on the body tissues and blood 3.They penetrate matter 4.They move in straight line 5.They are part of electromagnetic spectrum 6.They travel at the speed of light 7.They obey the inverse square law 8.They ionize gases 9.They may be scattered 10.They make certain materials fluoresce 11.They may be refracted, diffracted and polarized

X­ray Tube

High Electrical Potential Electrons

­ + X­ray Generator or Radioactive Source

Creates Radiation

Exposure Recording Device

Radiation Penetrate the Sample

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Properties of X­rays

1. Potential difference of around 300 kv is used 2. Approximately around 97­ 99% heat & 1­3 % of x­ rays are generated

3. Anode is made up of cupper to carry out the heat. Additional cooling using oil, air or water is also used

4­ Target is made up of tungsten 5. Area of the target struck by the electrons is called as focal spot 6. Focal spot size should be big to absorb more heat but to produce

good quality radiograph this size should be the smallest 7. Important control points of the x­ray machine are timer, Amperage

control and Voltage control 8. More time more radiation more exposure 9. Amperage controls the intensity or quality of X­ray, 6­12 Amp are usually used 10.More voltage generates the shorter wave length or quality of x­

rays more penetrating power 11.Increase in voltage increases the speed of the electrons, therefore

high kinetic energy and high penetration

Gamma­rays:

Gamma­rays are electromagnetic radiation emitted by the disintegration of a radioactive isotope and have energy from about 100 keV to well over 1 MeV. The most useful gamma­emitting radioactive isotopes for radiological purposes are found to be cobalt (Co60),iridium (Ir192),cesium (Cs137),ytterbium (Yb169), and thulium (Tm170).

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Properties of Gamma­rays

1.Gamma rays are emitted from artificial radio active isotope 2.Radio active isotope is an unstable state of element which has different number of neutrons to the normal state of the same element

3.The mass number of Radio active Isotope will be different from same element 4.The radio active isotope disintegrate continuously releasing electromagnetic energy (gamma rays)

5­Gamma ray sources are usually disc,cylindrical or spherical shape 6­The discs: 3.0 mm diameter and 1 mm thick, stacked together 7­Cylindrical: Typically upto 4 mm in length 8­Spherical: 0.6 – 3.0 mm diameter 9­Sources are encapsulated in the capsules of 316 \ S12 grade Stainless steel

Isotope Decay Rate (Decay of the Gamma Source)

Loss of activity of a radioactive nuclease due to Disintegration

Half Life of Gamma source:

Time taken for a radio active Isotope to reduce its out put by half

Source Half­life Penetration range steel

60Cobalt 26 Years 75­ 150 mm

192Iridium 74 days 20 – 45 mm

Ytterbium 169 31 days 1­15 mm

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Advantages of Gamma rays over X­rays

1.No electrical or water supply are needed

2.Gamma equipment is usually smaller and lighter and therefore more portable

3.The equipment is more simple

4.Places inaccessible to x­ray equipment are accessible to gamma equipment

5.Because of high energy there is less scatter

6.Gamma equipment is less expensive than x­ray equipment

7.Greater penetrating power than x­rays

Disadvantages of gamma rays over x­rays 1.Due to the higher energy, poorer contrast and definition

2.Exposure times are longer

3.Sources need replacing at regular intervals

4.The radiation cannot be switched off

5.SFD is shorter, resulting in poorer geometric unsharpness

6.Remote handling is necessary

Radiographic Techniques

1) SWSI : ( Film Inside Source Outside ) 2) SWSI : ( Film Outside Source Inside ) 3) DWSI : ( Film Outside Source Outside ) 4) DWDI : (Film Outside Source Outside

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Radiographic Contrast

Radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector or film contrast.

Subject contrast is determined by the following variables: ­ Absorption differences in the specimen ­ Wavelength of the primary radiation ­ Scatter or secondary radiation

Film contrast is determined by the following: ­ Grain size or type of film ­ Chemistry of film processing chemicals ­ Concentrations of film processing chemicals ­ Time of development ­ Temperature of development ­ Degree of mechanical agitation (physical motion)

Exposing the film to produce higher film densities will generally increase contrast. In other words, darker areas will increase in density faster than lighter areas because in any given period of time more x­rays are reaching the darker areas.

Reasons for low contrast Radiation wave length too short Over exposure Prolonged development Too cold developer

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Insufficient fixing Fog on the film

Reasons for High contrast

Radiation wave length too long Incorrect developer Under exposure

Definition

Radiographic definition is the abruptness of change in going from one density to another. There are a number of geometric factors of the X­ray equipment and the radiographic setup that have an effect on definition. These geometric factors include: ­ Focal spot size, which is the area of origin of the radiation.The focal spot size should be as close to a point source as possible to produce the most definition. ­ Source to film distance, which is the distance from the source to the part. Definition increases as the source to film distance increase. ­ Specimen to detector (film) distance, which is the distance between the specimen and the detector. For optimal definition, the specimen and detector should be as close together as possible. . ­ Abrupt changes in specimen thickness may cause distortion on the radiograph. ­ Movement of the specimen during the exposure will produce distortion on the radiograph. ­ Film graininess, and screen mottling will decrease definition.

The grain size of the film will affect the definition of the radiograph. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased development of the film will increase the apparent graininess of the radiograph.

Radiographic Density

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Degree of blackening of a radiograph. Density is measured by a densitometer.High density area is a dark area and low density area is a light area. A high density area absorbs more light than the low density area Density is the log of the intensity of light incident on the film to the intensity of light transmitted through the film. A density reading of 2.0 is the result of only 1 percent of the transmitted light reaching the sensor. Density required in the area of interest should be between 1.5 and 2.5.Radiographs with very low density and with very high density are not acceptable.

Reasons for low density • Under exposure to radiation • Insufficient development time • Development temperature too low • Incorrect developer

Reasons for Excessive density . Over exposure to Radiation • Excessive development time • Development temperature too high • Incorrect Developer

sensitivity The ability of the radiographic technique to detect the smallest possible defect.Sensitivity is measured by using Image Quality Indicators ( IQI ),also called as Penetrameters. Sensitivity depends on Radiographic contrast and Density.

Controlling Radiographic Quality One of the methods of controlling the quality of a radiograph is through the use of image quality indicators (IQI). IQIs provide a means of visually informing the film interpreter of the contrast sensitivity and definition of the radiograph. The IQI indicates that a specified amount of material thickness change will be detectable

in the radiograph, and that the radiograph has a certain level of definition so that the density changes are not lost due to unsharpness. Without such a reference

point, consistency and quality could not be maintained and defects could go undetected.

Image quality indicators take many shapes and forms due to the various codes or standards that invoke their use. In the United States two IQI styles are prevalent; the placard, or hole­type and the wire IQI. IQIs comes in a variety of material

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types so that one with radiation absorption characteristics similar to the material being radiographed can be used.

Hole­Type IQIs ASTM Standard E1025 gives detailed requirements for the design and material group classification of hole­type image quality indicators. E1025 designates eight groups of shims based on their radiation absorption characteristics. A notching system is incorporated into the requirements allowing the radiographer to easily determine if the penetrameter is the correct material type for the product. The thickness in thousands of an inch is noted on each pentameter by a lead number 0.250 to 0.375 inch wide depending on the thickness of the shim. Military or Government standards require a similar penetrameter but use lead letters to indicate the material type rather than notching system as shown on the left in the image above.

Image quality levels are typically designated using a two part expression such as 2­2T. The first term refers to the IQI thickness expressed as a percentage of the region of interest of the part being inspected. The second term in the expression refers to the diameter of the hole that must be revealed and it is expressed as a multiple of the IQI thickness. Therefore, a 2­2T call­out would mean that the shim thickness should be two percent of material thickness and that a hole that is twice the IQI thickness must be detectable on the radiograph. This presentation of a 2­2T IQI in the radiograph verifies that the radiographic technique is capable of showing a material loss of 2% in the area of interest.

It should be noted that even if 2­2T sensitivity is indicated on a radiograph, a defect of the same diameter and material loss may not be visible. The holes in the penetrameter represent sharp boundaries, and a small thickness change. Discontinues within the part may contain gradual changes, and are often less visible. The penetrameter is used to indicate quality of the radiographic technique and not intended to be used as a measure of size of cavity that can be located on the radiograph.

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Wire­Type IQIs

ASTM Standard E747 covers the radiographic examination of materials using wire penetrameters (IQIs) to control image quality. Wire IQIs consist of a set of six wires arranged in order of increasing diameter and encapsulated between two sheets of clear plastic. E747 specifies four wire IQIs sets, which control the wire diameters. The set letter (A, B, C or D) is shown in the lower right corner of the IQI. The number in the lower left corner indicates the material group. The same image quality levels and expressions (i.e. 2­2T) used for hole­type IQIs are typically also used for wire IQIs. The wire sizes that correspond to various hole­ type quality levels can be found in a table in E747 F = 0.79 (constant form factor forwire)

Placement of IQIs IQIs should be placed on the source side of the part over a section with a material thickness equivalent to the region of interest. If this is not possible, the IQI may be placed on a block of similar material and thickness to the region of interest. When a block is used, the IQI should the same distance from the film as it would be if placed directly on the part in the region of interest. The IQI should also be placed slightly away from the edge of the part so that atleast three of its edges are visible in the radiograph.

Secondary (Scatter) Radiation

Secondary or scatter radiation must often be taken into consideration when producing a radiograph. The scattered radiation create a loss of contrast and definition. Often secondary radiation is thought of as radiation striking the film

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reflected from an object in the immediate area, such as a wall, or from the table or floor where the part is resting. Side scatter originates from walls, or objects on the source side of the film. Control of side scatter can be achieved by moving objects in the room away from the film, moving the x­ray tube to the center of the vault, or placing a collimator at the exit port thus reducing the diverging radiation surrounding the central beam.

It is often called backscatter when it comes from objects behind the film. Industry codes and standards often require that a lead letter "B" be placed on the back of the cassette to verify the control of back scatter. If the letter "B" shows as a "ghost" image on the film the letter has absorbed the back scatter radiation indicating a significant amount of radiation reaching the film. Control of back scatter radiation is achieved by backing the film in the cassette with sheets of lead typically 0.010 inch thick. It is a common practice in industry to place 0.005 lead screen in front and 0.010 backing the film.

Lead screens in the thickness range of 0.004 to 0.015 inch typically reduce scatter radiation at energy levels below 150, 000 volts. Above this point they will emit electrons to provide more exposure of the film to ionizing radiation thus increasing the density of the radiograph.

Undercut

Another condition that must often be controlled when producing a radiograph is called undercut. Parts with holes, hollow areas, or abrupt thickness changes are

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likely to suffer from undercut if controls are not put in place. Undercut appears as lightening of the radiograph in the area of the thickness transition. This results in a loss of resolution or blurring at the transition area.

Undercut occurs due to scattering within the film. At the edges of a part or areas where the part transitions from thick to thin, the intensity of the radiation reaching the film is much greater than in the thicker areas of the part. The high level of radiation intensity reaching the film results in a high level of scattering within the film. It should also be noted that the faster the film speed, the more undercut that is likely to occur. Scattering from within the walls of the part also contributed some to undercut but research has shown that scattering within the film is the primary cause. Masks are used to control undercut. Sheets of lead cut to fill holes or surround the part and metallic shot and liquid absorbers are often used as masks.

Filters in Radiography

At x­ray energies, filters consist of material placed in the useful beam to absorb, preferentially, radiations based on energy level or to modify the spatial distribution of the beam. Filtration is required to absorb the lower­energy x­ray photons emitted by the tube before they reach the target. The use of filters produce a cleaner image by absorbing the lower energy x­ray photons that tend to scatter more.

Radiographic Film

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X­ray films for general radiography consist of an emulsion­gelatin containing a radiation sensitive silver halide and a flexible, transparent, blue­tinted base. Usually, the emulsion is coated on both sides of the base in layers about 0.0005 inch thick. Putting emulsion on both sides of the base doubles the amount of radiation­sensitive silver halide, and thus increases the film speed. The emulsion layers are thin enough so developing, fixing, and drying can be accomplished in a reasonable time.

Film Selection

The selection of a film when radiographing any particular component depends on a number of different factors. Listed below are some of the factors that must be considered when selected a film and developing a radiographic technique.

1. the composition, shape, and size of the part being examined and, in some cases, its weight and location.

2. the type of radiation used, whether x­rays from an x­ray generator or gamma rays from a radioactive source.

3. the kilovoltages available with the x­ray equipment or the intensity of the gamma radiation.

4. the relative importance of high radiographic detail or quick and economical results.

Film Processing

Processing film is a strict science governed by rigid rules of chemical concentration, temperature, time, and physical movement. Whether processing is done by hand or automatically by machine, excellent radiographs require the highest possible degree of consistency and quality control.

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Manual Processing & Darkrooms

Manual processing begins with the darkroom. An area dry and free of dust and dirt should be used to load and unload the film. While another area, the wet side, will be used to process the film. Thus protecting the film from any water or chemicals that may be located on the surface of the wet side.

Each of step in film processing must be excited properly to develop the image, wash out residual processing chemicals, and to provide adequate shelf life of the radiograph.A radiograph may be retrieved after 5 or even 20 years in storage.

Viewing Radiographs

Radiographs (developed film exposed to x­ray or gamma radiation) are generally viewed on a light­box. Proper viewing conditions are very important when interpreting a radiograph. The viewing conditions can enhance or degrade the subtle details of radiographs.

Before beginning the evaluation of a radiograph, the viewing equipment and area should be considered. The area should be clean and free of distracting materials. Magnifying aids, masking aids, and film markers should be close at hand. Thin cotton gloves should be available and worn to prevent fingerprints on the radiograph. Ambient light levels should be low. Ambient light levels of less than 2 fc are often recommended, but subdued lighting, rather than total darkness, is preferable in the viewing room. The brightness of the surroundings should be about the same as the area of interest in the radiograph. Room illumination must be arranged so that there are no reflections from the surface of the film under examination.

Film viewers should be clean and in good working condition.Film viewers should provide a source of defused, adjustable, and relativity cool light as heat from viewers can cause distortion of the radiograph. A film having a measured density of 2.0 will allow only 1.0 percent of the incident light to pass. A film containing a density of 4.0 will allow only 0.01 percent of the incident light to pass. With such low levels of light passing through the radiograph the delivery of a good light source is important.

Radiographic film quality and acceptability, as required by the procedure, should first be determined. It should be verified that the radiograph was produced to the correct density on the required film type, and that it contains the correct identification information. It should also be verified that the proper image quality indicator was used and that the required sensitivity level was met. Next, the

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radiograph should be checked to ensure that it does not contain processing and handling artifacts that could mask discontinuities or other details of interest.

Once a radiograph passes these initial checks it is ready for interpretation. Radiographic film interpretation is an acquired skill combining, visual acuity with knowledge of materials, manufacturing processes, and their associated discontinues. If the component is inspected while in service, an understanding of applied loads and history of the component is helpful.

A process for viewing radiographs, left to right top to bottom etc., is helpful and will prevent overlooking an area on the radiograph. One part of the interpretation process, sometimes overlooked, is rest. The mind as well as the eyes needs to occasionally rest when interpreting radiographs.

When viewing a particular region of interest, techniques such as using a small light source and moving the radiograph over the small light source, or changing the intensity of the light source will help the radiographer identify relevant indications. Magnifying tools should also be used when appropriate to help identify and evaluate indications. Viewing the actual component being inspected is very often helpful in developing an understanding of the details seen in a radiograph.

Interpretation of radiographs is an acquired skill that is perfected over time. By using the proper equipment and developing consistent evaluation processes, the interpreter will increase his or her probability of detecting defects.

Spurious Indications on the films or ( Artefacts )

Spurious Indications are caused by incorrect processing or careless handling of the film

The common artefacts are

1­ Radiation Fogging: This occurs when the film is stored too close to a source of radiation or when the film is accidentally left in the exposure area during the exposure of another film

2­ Light Fog: Due to the storage of the film in a faulty storage storage box where white light is leaks into the film OR due to the wrong type of safe light in the dark room

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3­ Pressure Markings: Due to careless handling of the film during loading and unloading of the film

4­ Static Marks:This has the appearance of the dark branched and jagged fine lines. It is due to rapid removal of the film from the wrapper

5 ­ Scratch Marks: Usually caused by a finger nail or abrasive material on the screens or during handling

6­ High or Low Density Marks: Caused when handling the films with greasy or chemically – stained fingers

7­ Low density Patches or smears : Due to splashes of water or fixing solution on the film

8­ High density Patches or smears : Due to splashes of Developer on the film

9­ Film Mottle: Due to the use of old films

10­ Light Spots: Caused by dust particles between the film and the intensifying Screen

11­ Screen Marks: Due to contamination of the intensifying screens with chemicals OR due to the defects on the screen such as cracking or buckling

12­Air bells: These are shown discs of lower density caused by air traped on the surface of the emulsion, due to insufficient agitation

13­ Patches or Streaks: Due to insufficient agitation during developer or in the rinse bath

14­ Reticulation:This is the appearance of leather grain and due to rupture of the emulsion caused by great differences between succesive processing solutions

15­ Drying Marks: Due to drops of water remaining on the surface of the film, often occur due to rapid drying the film in the high temperature cabinet.

Chapter VII

Eddy Current Testing Introduction

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A varying electric current flowing in a coil gives rise to a varying magnetic field. A nearby conductor resists this magnetic field and this produces an eddy current flowing in the surface layer of the conductor and flowing in the direction opposite to the current in the coil. An induced eddy current in a conductor produces a magnetic field that opposes the magnetic field produced by the coil, resulting in a change of impedance. It is this impedance change that is to be detected with a high degree of accuracy by the measuring equipment.

Cracks and other surface conditions modify the eddy currents generated in the conductor and give rise to a local brief change in the impedance. This change is accurately monitored.

Eddy currents always flow parallel to the plane of the winding of the test coil producing them ,A discontinuity parallel to the eddy current can be missed by this method

Eddy current testing method can be successfully used to detects surface breaking and near surface discontinuities such as

• Cracks

Conductive Material

Coil Coil's

magnetic field

Eddy

currents

Eddy current's

magnetic field

Basic Principle

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• Inclusions • Dents • Holes • Scratches

Eddy current testing method can also identify the material properties such as • Alloy composition • Heat treatment • Hardness • Grain size • Magnetic Permeability

Eddy current testing method can also be used to monitor the surface condition such as • Surface coating • Corrosion • Specimen Temperature

Eddy Current Testing Depends on

• Electrical conductivity of the material

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• Nature of discontinuities in the material • Magnetic permeability of the material • Dimensions and shape of the specimen • Current frequency • Coil size • Number of turns in the coil • Metal condition

Advantages of Eddy Current Testing Method

• Instantaneous results • Sensitive to a range of physical properties • Firm contact between inspection coil and specimen not required • Equipment is small and self­contained • Can detect very small discontinuities • Defects in tubes and other circular parts can be detected using special probes • Internal surface of cylinders can be using special probes

Disadvantages of Eddy Current Testing Method

• This method can be used on electrical conductors only • Depth of penetration is restricted • Interpretation needs skill • Defects parallel to coil surface can be missed • Ends of the parts can not be tested

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Chapter VIII

Selection of NDT Methods

Defect to be detected Most Suitable Method

Comments

Surface Cracks ( i ) Grinding cracks ( ii ) Heat treat cracks ( iii ) Solidification Cracks ( iv ) Crater Cracks ( v ) Cold Cracks

PT, MPI & Eddy Current

PT for all metals MPI for Ferrous Metals ET for conductive materials

Inclusions UT, RT & MPI UT & RT for deep defects MPI for near surface defects

Lack of penetration Lack of root fusion Concavity

RT,UT, MPI, DPI & ET

RT&PT for all metals MPI for Ferrous Metals ET for conductive materials UT if the rear side of the weld is not visible

(i) Lack of sidewall fusion ( ii ) Lack of inter run fusion

RT & UT RT is sensitive for vertical or inclined defects UT is sensitive for horizontal or inclined defects

Laminations UT UT is very sensitive Surface Porosity VT, MPI, PT PT is most sensitive Internal Porosity RT,UT RT is most sensitive Undercut VT, MPI, PT,RT RT for root undercut, if not visible Mismatch VT,RT RT if other side is not visible Inadequate weld reinforcement VT,RT RT if weld face is not visible Burn­Through VT,RT RT if weld root is not visible Whiskers UT,RT RT gives clear image

Underfill, Underflush VT,UT,RT VT & RT are better

Root concavity VT,RT RT if weld root is not visible

Convexity VT Welder Gauge is used to measure