Introduction to Nondestructive Testing NDT

Outline

• Introduction to NDT• Overview of Six Most

Common NDT Methods

• Selected Applications

Introduction of NDT

• The testing of a material or component for flaws using physical means (energy) without impairing the usefulness of the material or component.

As an NDT technologist you are a service provider.

• As a service provider you shall follow written and verbal instructions. You will be testing life critical equipment and that equipment may carry or involve your own family. When you miss a flaw, they may bear the brunt of your incompetence.

• Your responsibility is to find flaws or fault with what other people manufactured. You are essentially the “speed cop” of industry. Most companies only want a signed piece of paper that proves that they are not to blame when something goes wrong. But when something goes wrong and eventually it does. YOU will get the blame. Because you have been “certified competent” by your employer and his Level III, therefore the blame will be squarely placed on your shoulders.

• Choosing a career in NDT is a life long commitment and should not be seen as job.

The use of noninvasive techniques to determine the integrity of a material, component or structure

or quantitatively measuresome characteristic ofan object.

i.e. Inspect or measure without doing harm.

Definition of NDT

Notable events in early industrial NDT.

• 1854 Hartford, Connecticut: a boiler at the Fales and Gray Car works explodes, killing 21 people and seriously injuring 50. Within a decade, the State of Connecticut passes a law requiring annual inspection (in this case visual) of boilers.

• 1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In his first paper he discusses the possibility of flaw detection.

• 1880 - 1920 The "Oil and Whiting" (Paraffin and Chalk powder) method of crack detection is used in the railroad industry to find cracks in heavy steel parts.

•  

• The part is soaked in thinned oil, and then painted with a white coating that dries to a powder. Oil seeping out from cracks turns the white powder brown, allowing the cracks to be seen. This was the precursor to modern liquid Penetrant materials.

• 1924. H. H. Lester uses radiography to examine castings to be installed in a Boston Edison Company steam pressure power plant.

• 1926 The first electromagnetic eddy current instrument is available to measure material thicknesses.

• 1927 - 1928 Magnetic induction system to detect flaws in railroad track developed by Dr. Elmer Sperry and H.C. Drake.

• 1929 Magnetic particle methods and equipment pioneered (A.V. DeForest and F.B. Doane.)

• 1930s Robert F. Mehl demonstrates radiographic imaging using gamma radiation from Radium, which can examine thicker components than the low-energy X-ray machines available at the time.

• 1935 - 1940 Liquid Penetrant tests developed (Betz, Doane, and DeForest)

• 1935 - 1940s Eddy current instruments developed (H.C. Knerr, C. Farrow, Theo Zuschlag, and Fr. F. Foerster).

• 1940 - 1944 Ultrasonic test method developed in USA by Dr. Floyd Firestone.

• 1950 J. Kaiser introduces acoustic emission as an NDT method.

Popular terms used in NDT.Fatigue

•In materials science, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material.

•Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture. The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure.

Popular terms used in NDT.• Indication. 

The response or evidence from an examination. (Bleed-out, Flux Leakage or a blip on screen ...)

• Interpretation. 

Determining if an indication is relevant or nonrelevant. Relevant indications shall be investigated. Nonrelevant indications are considered harmless.

• Flaw. 

A type of discontinuity that must be evaluated to see if it is rejectable.

• Evaluation.

Determining if a flaw is rejectable in accordance with acceptance criteria.

• Discontinuity.

Disruption in the physical structure of a component or material.

• Defect. 

A discontinuity / flaw that is rejectable — does not meet acceptance criteria. Defects are removed or repaired.

NDT standards:

• Laws and Regulations:

Laws and regulations are issued by the authorities and are normally written in general terms. In some cases NDT programs may be specified.

 

• Standards and Codes:

A standard is a document prepared by international or national standardization organizations. Examples are ISO (International Standardization Organization) and ANSI (American National Standards Institute). The term code may indicate the same level of recognition as a standard. Examples are the ASME Pressure Vessel Code, EURO Norms (EN).

 

• Guidelines and Recommendations:

Different international or national societies, organizations or bodies may issue guidelines, recommendations etc. concerning NDT. Guidelines are publications giving practical information on specific items like for instance ‘Ultrasonic Inspection of Weld Connections’.

 

• Specification:

A specification is a precise statement of a set of requirements to be satisfied by a material, product, system or service.

• ASME:

American Society of Mechanical Engineers has issued a ‘Boiler and Pressure Vessel Code’ containing 11 sections. The relevant sections are:

• Section V, Non-destructive Examination, which describes in detail the performance of NDT.

• Section VIII ‘Pressure Vessels’ describing NDT and acceptance criteria for such vessels.

• The ASME-code is extensively used throughout the world.

• ASTM:

American Society for Testing and Materials Standards are often referred to for radiography of steel castings. Corresponding standards exist for castings of aluminium, magnesium …

• IIW:

International Institute of Welding has established, as a recommendation, collections of reference radiographs of welds in steel and aluminium.

5 Main NDT Technique/Method

• Electromagnetic Radiation

• Sound

• X-Ray and radiation

• Microscopy

• Visual with dyes flourescent.

• NDT methods may rely upon use of electromagnetic radiation, sound, and inherent properties of materials to examine samples. This includes some kinds of microscopy to examine external surfaces in detail, although sample preparation techniques for metallography, optical microscopy and electron microscopy are generally destructive as the surfaces must be made smooth through polishing or the sample must be electron transparent in thickness.

Electromagnetic radiation• Electromagnetic radiation (often abbreviated E-M radiation or

EMR) is a phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It comprises electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified into several types according to the frequency of its wave; these types include (in order of increasing frequency and decreasing wavelength): radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. A small and somewhat variable window of frequencies is sensed by the eyes of various organisms; this is what is called the visible spectrum. The photon is the quantum of the electromagnetic interaction and the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force.

• EM radiation carries energy and momentum that may be imparted to matter with which it interacts.

Sound

• Sound is a travelling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.

Microscopy• Microscopy is the technical field of using microscopes to view

samples or objects that cannot be seen with the unaided eye(objects that are not within the resolution range of the normal eye). There are three well-known branches of microscopy, optical, electron, and scanning probe microscopy.

• Optical and electron microscopy involve the diffraction, reflection, or refraction of electromagnetic radiation/electron beams interacting with the subject of study, and the subsequent collection of this scattered radiation in order to build up an image. This process may be carried out by wide-field irradiation of the sample (for example standard light microscopy and transmission electron microscopy) or by scanning of a fine beam over the sample (for example confocal laser scanning microscopy and scanning electron microscopy). Scanning probe microscopy involves the interaction of a scanning probe with the surface or object of interest. The development of microscopy revolutionized biology and remains an essential tool in that science, along with many others including materials science and forensic engineering where microstructure and surface details such as trace evidence are

important for understanding properties or the causes of accidents.

X-ray• X-radiation (composed of X-rays) is a form of

electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × 1016 Hz to 3 × 1019 Hz) and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma rays. In many languages, X-radiation is called Röntgen radiation, after Wilhelm Conrad Röntgen, who is generally credited as their discoverer, and who had named them X-rays to signify an unknown type of radiation.[1]:1-2

• X-rays from about 0.12 to 12 keV (10 to 0.10 nm wavelength) are classified as "soft" X-rays, and from about 12 to 120 keV (0.01 to 0.10 nm wavelength) as "hard" X-rays, due to their penetrating abilities.

• Hard X-rays can penetrate solid objects, and their largest use is to take images of the inside of objects in diagnostic radiography and crystallography. As a result, the term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. By contrast, soft X-rays can hardly be said to penetrate matter at all; for instance, the attenuation length of 600 eV (~ 2 nm) x-rays in water is less than 1 micrometer.[4] X-rays are a form of ionizing radiation, and exposure to them can be a health hazard

• The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays).[2] So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10−11 m, defined as gamma rays.[3] However, as shorter wavelength continuous spectrum "X-ray" sources such as linear accelerators and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.[

Visual with Dyes

• One method (liquid penetrant testing) involves using dyes, fluorescent or non-fluorescing, in fluids for non-magnetic materials, usually metals.

Methods of NDT

Visual

Liquid Penetrant

Magnetic Particle

Eddy Current

Ultrasonic

X-ray

Microwave

Acoustic Emission

Thermography

Laser Interferometry

Replication

Flux Leakage

Acoustic Microscopy

Magnetic Measurements

Tap Testing

• NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.

• Acoustic emission testing (AE or AT)

• Dye penetrant inspection Liquid penetrant testing (PT or LPI)

• Electromagnetic testing (ET) Alternating current field measurement (ACFM)

Alternating current potential drop measurement (ACPD)

Barkhausen testing

Direct current potential drop measurement (DCPD)

Eddy-current testing (ECT)

Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope

Magnetic-particle inspection (MT or MPI)

Remote field testing (RFT)

• Ellipsometry

• Guided wave testing (GWT)

• Hardness testing

• Impulse excitation technique (IET)

• Infrared and thermal testing (IR) Thermographic inspection

• Laser testing Electronic speckle pattern interferometry

Holographic interferometry

Profilometry

Shearography

• Leak testing (LT) or Leak detection Absolute pressure leak testing (pressure change)

Bubble testing

Halogen diode leak testing

Hydrogen leak testing

Mass spectrometer leak testing

Tracer-gas leak testing method Helium, Hydrogen and refrigerant gases

• Magnetic resonance imaging (MRI) and NMR spectroscopy

• Optical microscopy• Positive Material Identification (PMI)• Radiographic testing (RT) (see also Industrial radiography

and Radiography) Computed radiographyDigital radiography (real-time)Neutron radiographic testing (NR)SCAR (Small Controlled Area Radiography)X-ray computed tomography (CT)

• Scanning electron microscopy• Ultrasonic testing (UT)

Electro Magnetic Acoustic Transducer (EMAT) (non-contact)Laser ultrasonics (LUT)Internal rotary inspection system (IRIS) ultrasonics for tubesPhased array ultrasonicsTime of flight diffraction ultrasonics (TOFD)Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)

• Visual inspection (VT) • Pipeline video inspection

What are Some Uses of NDE Methods?

• 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

Fluorescent penetrant indication

When are NDE Methods Used?

– To assist in product development

– To screen or sort incoming materials

– To monitor, improve or control manufacturing processes

– To verify proper processing such as heat treating

– To verify proper assembly

– To inspect for in-service damage

There are NDE application at almost any stage in the production or life cycle of a component.

There are NDE application at almost any stage in the production or life cycle of a component.

Six Most Common NDT Methods

• Visual• Liquid Penetrant • Magnetic • Ultrasonic• Eddy Current• X-ray

Most basic and common inspection method.

Tools include fiberscopes, borescopes, magnifying glasses and mirrors.

Robotic crawlers permit observation in hazardous or tight areas, such as air ducts, reactors, pipelines.

Portable video inspection unit with zoom allows

inspection of large tanks and vessels, railroad tank

cars, sewer lines.

Visual Inspection

• A liquid with high surface wetting characteristics is applied to the surface of the part and allowed time to seep into surface breaking defects.

• The excess liquid is removed from the surface of the part.

• A developer (powder) is applied to pull the trapped penetrant out the defect and spread it on the surface where it can be seen.

• Visual inspection is the final step in the process. The penetrant used is often loaded with a fluorescent dye and the inspection is done under UV light to increase test sensitivity.

Liquid Penetrant Inspection

Magnetic Particle Inspection

The part is magnetized. Finely milled iron particles coated with a dye pigment are then applied to the specimen. These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.

Magnetic Particle Crack Indications

RadiographyThe radiation used in radiography testing is a higher energy (shorter wavelength) version of the electromagnetic waves that we see as visible light. The radiation can come from an X-ray generator or a radioactive source.

High Electrical Potential

Electrons

-+

X-ray Generator or Radioactive Source Creates

Radiation

Exposure Recording Device

Radiation Penetrate the Sample

Film Radiography

Top view of developed film

X-ray film

The part is placed between the radiation source and a piece of film. The part will stop some of the radiation. Thicker and more dense area will stop more of the radiation.

= more exposure

= less exposure

The film darkness (density) will vary with the amount of radiation reaching the film through the test object.

Radiographic Images

Conductive material

CoilCoil's magnetic field

Eddy currents

Eddy current's magnetic field

Eddy Current Testing

Eddy Current TestingEddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements. Here a small surface probe is scanned over the part surface in an attempt to detect a crack.

Eddy current testing is particularly well suited for detecting surface cracks but can also be used to make electrical conductivity and coating thickness measurements. Here a small surface probe is scanned over the part surface in an attempt to detect a crack.

High frequency sound waves are introduced into a material and they are reflected back from surfaces or flaws.

Reflected sound energy is displayed versus time, and inspector can visualize a cross section of the specimen showing the depth of features that reflect sound.

f

plate

crack

0 2 4 6 8 10

initial pulse

crack echo

back surface echo

Oscilloscope, or flaw detector screen

Ultrasonic Inspection (Pulse-Echo)

Ultrasonic Imaging

Gray scale image produced using the sound reflected from the front surface of the coin

Gray scale image produced using the sound reflected from the back surface of the coin (inspected from “heads” side)

High resolution images can be produced by plotting signal strength or time-of-flight using a computer-controlled scanning system.

Common Application of NDT

• Inspection of Raw Products

• Inspection Following Secondary Processing

• In-Services Damage Inspection

NDT is used in a variety of settings that covers a wide range of industrial activity.

Automotive Engine partsFrame

Aviation / Aerospace AirframesSpaceframesPowerplants PropellersReciprocating EnginesGas turbine enginesRocketry

Construction StructuresBridgesCover Meter

Maintenance, repair and operations Bridges

Manufacturing Machine partsCastings and Forgings

• Industrial plants such as Nuclear, Petrochemical, Power, Refineries, Pulp and Paper, Fabrication shops, Mine processing and their Risk Based Inspection programmes.

Pressure vesselsStorage tanksWeldsBoilersHeat exchangersTurbine boresIn-plant Piping

• Miscellaneous Pipelines In-line Inspection using "pigs“Pipeline integrity managementLeak Detection

Railways Rail InspectionWheel Inspection

Tubular NDT, for Tubing materialCorrosion Under Insulation (CUI)Amusement park ridesSubmarines and other Naval warshipsMedical imaging applications (see also Medical physics)

Inspection of Raw Products

• Forgings,• Castings,• Extrusions,• etc.

• Machining• Welding• Grinding• Heat treating• Plating• etc.

Inspection Following Secondary Processing

• Cracking

• Corrosion

• Erosion/Wear

• Heat Damage

• etc.

Inspection For In-Service Damage

Power Plant Inspection

Probe

Signals produced by

various amounts of corrosion

thinning.

Periodically, power plants are shutdown for inspection. Inspectors feed eddy current probes into heat exchanger tubes to check for corrosion damage.

Pipe with damage

Wire Rope InspectionElectromagnetic devices and visual inspections are used to find broken wires and other damage to the wire rope that is used in chairlifts, cranes and other lifting devices.

Storage Tank Inspection

Robotic crawlers use ultrasound to inspect the walls of large above ground tanks for signs of thinning due to corrosion.

Cameras on long articulating arms are used to inspect underground storage tanks for damage.

Aircraft Inspection• Nondestructive testing is used

extensively during the manufacturing of aircraft.

• NDT is also used to find cracks and corrosion damage during operation of the aircraft.

• A fatigue crack that started at the site of a lightning strike is shown below.

Jet Engine Inspection• Aircraft engines are overhauled

after being in service for a period of time.

• They are completely disassembled, cleaned, inspected and then reassembled.

• Fluorescent penetrant inspection is used to check many of the parts for cracking.

Sioux City, Iowa, July 19, 1989A defect that went undetected in an engine disk was responsible for the crash of United Flight 232.

Crash of United Flight 232

Pressure Vessel InspectionThe failure of a pressure vessel can result in the rapid release of a large amount of energy. To protect against this dangerous event, the tanks are inspected using radiography and ultrasonic testing.

Rail Inspection

Special cars are used to inspect thousands of miles of rail to find cracks that could lead to a derailment.

Bridge Inspection• The US has 578,000

highway bridges.

• Corrosion, cracking and other damage can all affect a bridge’s performance.

• The collapse of the Silver Bridge in 1967 resulted in loss of 47 lives.

• Bridges get a visual inspection about every 2 years.

• Some bridges are fitted with acoustic emission sensors that “listen” for sounds of cracks growing.

NDT is used to inspect pipelines to prevent leaks that could damage the environment. Visual inspection, radiography and electromagnetic testing are some of the NDT methods used.

Remote visual inspection using a robotic crawler.

Radiography of weld joints.

Magnetic flux leakage inspection. This device, known as a pig, is placed in the pipeline and collects data on the condition of the pipe as it is pushed along by whatever is being transported.

Pipeline Inspection

Special MeasurementsBoeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at various points to evaluate its uniformity.

Reliability and statistics• Defect detection tests are among the more

commonly employed of non-destructive tests. The evaluation of NDT reliability commonly contains two statistical errors.

• First, most tests fail to define the objects that are called "sampling units" in statistics; it follows that the reliability of the tests cannot be established.

• Second, the literature usually misuses statistical terms in such a way as to make it sound as though sampling units are defined. These two errors may lead to incorrect estimates of probability of detection.