basic principle of each test

8/9/2019 Basic Principle of Each Test

1/18

2010-ME181

Compression Test

Compression tests are used to determine how a product or material reacts when it is

compressed, squashed, crushed or flattened by measuring fundamental parameters that

determine the specimen behavior under a compressive load. Compression tests can beundertaken as part of the design process, in the production environment or in the quality

control laboratory, and can be used to:

1. Assess the strength of components e.g. automotive and aeronautical control switches,

compression springs, bellows, keypads, package seals, PET containers, PVC / ABS pipes,

solenoids etc.

2. Characterize the compressive properties of materials e.g. foam, metal, PET and other

plastics and rubber

3. Assess the performance of products e.g. the expression force of a syringe or the load-

displacement characteristics of a tennis ball

Types of Compression Testing:

Types of compression testing include:

Flexure/Bend

Spring Testing

Top-load/Crush

Advantages:

Compression testing provides data on the integrity and safety of materials, components and

products, helping manufacturers ensure that their finished products are fit-for-purpose and

manufactured to the highest quality.

The data produced in a compression test can be used in many ways including:

To determine batch quality

To determine consistency in manufacture

To aid in the design process

To reduce material costs and achieve lean manufacturing goals

To ensure compliance with international and industry standards

Materials under Compression

Certain materials subjected to a compressive force show initially a linear relationship between

stress and strain. This is the physical manifestation of Hooke's Law, which states:

http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616


2/18

2010-ME181

E = Stress (s) / Strain (e)

where E is known as Young's Modulus for compression. This value represents how much the

material will deform under applied compressive loading before plastic deformation occurs. A

material's ability to return to its original shape after deformation has occurred is referred to as

its elasticity. Vulcanized rubber, for instance, is said to be very elastic, as it will revert back to itsoriginal shape after considerable compressive force has been applied.

Once a certain force or stress threshold has been achieved, permanent or plastic deformation

will occur and is shown on graphs as the point where linear behavior stops. This threshold is

known as the proportional limit and the force at which the material begins exhibiting this

behavior is called the yield point or yield strength. A specimen will then exhibit one of two

types of behavior; it will either continue to deform until it eventually breaks, or it will distort

until flat. In either case a maximum stress or force will be evident, providing its ultimate

compressive strength value.

Each of these parameters offers useful information relating to the physical characteristics of the

material in question.

Some materials, such as a PET bottle, distort during a compression test and are measured by

the degree of distortion, whereas other materials such as ceramics fracture produce a definitive

compressive strength value.

Applications of Compression Testing:

Compression testing is used to guarantee the quality of components, materials and finished

products within wide range industries. Typical applications of compression testing arehighlighted in the following sections on:

Construction Industry

Cosmetics Industry

Electrical and Electronic Industry

Medical Device Industry

Packaging Industry

Paper and Board Industry

Plastics, Rubber and Elastomers Industry

http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616http://www.azom.com/ads/abmc.aspx?b=7616


3/18

2010-ME181

Tensile Testing

Tensile testing, also known as tension testing, is a

fundamental materials science test. In which a sample is subjected to a

controlled tension until failure. The results from the test are commonly

used to select a material for an application, for quality control, and to

predict how a material will react under other types of forces.

Properties that are directly measured via a tensile test are ultimate

tensile strength, maximum elongation and reduction in area. From

these measurements the following properties can also be

determined: Young's modulus, Poisson's ratio, yield strength,

and strain-hardening characteristics.

Tensile Test Specimen:

A tensile specimen is a standardized sample cross-section. It has two shoulders and a gauge(section) in between. The shoulders are large so they can be readily gripped, whereas the gauge

section has a smaller cross-section so that the deformation and failure can occur in this area.

Hooke's Law:

For most tensile testing of materials, you will notice that in the initial portion of the test, the

relationship between the applied force and the elongation the specimen exhibits is linear. In

this linear region, the line obeys the relationship defined as "Hooke's Law" where the ratio of

stress to strain is a constant and E is the slope of the line in this region where stress ( σ) is

proportional to strain (ε) and is called the "Modulus of Elasticity" or "Young's Modulus".

http://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Tension_(physics)http://en.wikipedia.org/wiki/Quality_controlhttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Ultimate_tensile_strengthhttp://en.wikipedia.org/wiki/Ultimate_tensile_strengthhttp://en.wikipedia.org/wiki/Elongation_(materials_science)http://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Poisson%27s_ratiohttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Strain-hardeninghttp://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=99http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=99http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=185http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=185http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=99http://en.wikipedia.org/wiki/Strain-hardeninghttp://en.wikipedia.org/wiki/Yield_strengthhttp://en.wikipedia.org/wiki/Poisson%27s_ratiohttp://en.wikipedia.org/wiki/Young%27s_modulushttp://en.wikipedia.org/wiki/Elongation_(materials_science)http://en.wikipedia.org/wiki/Ultimate_tensile_strengthhttp://en.wikipedia.org/wiki/Ultimate_tensile_strengthhttp://en.wikipedia.org/wiki/Forcehttp://en.wikipedia.org/wiki/Quality_controlhttp://en.wikipedia.org/wiki/Tension_(physics)http://en.wikipedia.org/wiki/Materials_science


4/18

2010-ME181

Modulus of Elasticity:

The modulus of elasticity is a measure of the stiffness of the material, but it only applies in the

linear region of the curve. If a specimen is loaded within this linear region, the material will

return to its exact same condition if the load is removed. At the point that the curve is no

longer linear and deviates from the straight-line relationship, Hooke's Law no longer applies

and some permanent deformation occurs in the specimen. This point is called the "elastic,

or proportional, limit". From this point on in the tensile test, the material reacts plastically to

any further increase in load or stress. It will not return to its original, unstressed condition if the

load were removed.

Yield Strength:

A value called "yield strength" of a material is defined as the stress applied to the material at

which plastic deformation starts to occur while the material is loaded.

Strain:

You will also be able to find the amount of stretch or elongation the specimen undergoes

during tensile testing. This can be expressed as an absolute measurement in the change in

length or as a relative measurement called "strain". Strain itself can be expressed in two

different ways, as "engineering strain" and "true strain". Engineering strain is probably the

easiest and the most common expression of strain used. It is the ratio of the change in length to

the original length.

Whereas, the true strain is similar but based on the instantaneous length of the specimen as

the test progresses, where Li is the instantaneous length and L0 the initial length.

Ultimate Tensile Strength:

One of the properties you can determine about a material is its ultimate tensile strength (UTS).

This is the maximum load the specimen sustains during the test. The UTS may or may not

equate to the strength at break. This all depends on what type of material you are testing

brittle, ductile, or a substance that even exhibits both properties. And sometimes a material

may be ductile when tested in a lab, but, when placed in service and exposed to extreme cold

temperatures; it may transition to brittle behavior.

http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=118http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=182http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=175http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=178http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=43http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=43http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=178http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=175http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=182http://www.instron.us/wa/resourcecenter/glossaryterm.aspx?ID=118


5/18

2010-ME181

Bending Test

Bend testing determines the ductility or the strength of a material by bending the material over

a given radius. Following the bend, the sample is inspected for cracks on the outer surface.

Bend testing provides insight into the modulus of elasticity and the bending strength or amaterial. Metallurgical offers three and four point bend setups with interchangeable rollers for

a variety of test configurations.

Specimens are often cut into rectangular bars or tested as whole. We routinely test machined

coupons, fasteners, wire, cable, tubes, pipes, plates, structural beams, or bars. Our machine

shop and qualified machinists will machine your raw materials to standard coupons if

necessary. We utilize an extensive variety of fixtures to provide the tensile setup needed to

complete your test.

Advantages:

Bend tests for ductility provide a simple way to evaluate the quality of materials by

their ability to resist cracking or other surface irregularities during one continuous

bend. No reversal of the bend force shall be employed when conducting these tests.

The type of bend test used determines the location of the forces and constraints on the

bent portion of the specimen, ranging from no direct contact to continuous contact.

The test can terminate at a given angle of bend over a specified radius or continue until

the specimen legs are in contact. The bend angle can be measured while the specimen

is under the bending force (usually when the semi-guided bend test is employed), or

after removal of the force as when performing a free-bend test. Product requirements

for the material being tested determine the method used.

Materials with an as-fabricated cross section of rectangular, round, hexagonal, or

similar defined shape can be tested in full section to evaluate their bend properties by

using the procedures outlined in these test methods, in which case relative width and

thickness requirements do not apply.


6/18

2010-ME181

Torsion Test

A torsion test can be conducted on most materials to determine the torsion properties of the

material. These properties include but are not limited to:

Modulus of elasticity in shear

Yield shear strength

Ultimate shear strength

Modulus of rupture in shear

Ductility

While they are not the same, they are analogous to properties that can be determined during a

tensile test. In fact, the "torque versus angle" diagram looks very similar to a "stress versus

strain" curve that might be generated by a tensile test.

Types of Torsion Tests:

Torsion tests can be performed by applying only a rotational motion or by applying both axial

(tension or compression) and torsion forces. Types of torsion testing vary from product to

product but can usually be classified as failure, proof, or product operation testing.

Torsion Only: Applying only torsion loads to the test specimen.

Axial-Torsion: Applying both axial (tension or compression) and torsional forces to the

test specimen.

Failure Testing: Twisting the product, component, or specimen until failure. Failure can

be classified as either a physical break or a kink/defect in the specimen. Proof Testing: Applying a torsion load and holding this torque load for a fixed amount of

time.

Operational Testing: Testing complete assemblies or products such as bottle caps,

switches, dial pens, or steering columns to verify that the product performs as expected

under torsion loads.

Hydrostatic Test

A hydrostatic test is a way in which pressure vessels such as pipelines, plumbing, gas

cylinders, boilers and fuel tanks can be tested for strength and leaks. The test involves filling thevessel or pipe system with a liquid, usually water, which may be dyed to aid in visual leak

detection, and pressurization of the vessel to the specified test pressure. Pressure tightness can

be tested by shutting off the supply valve and observing whether there is a pressure loss. The

location of a leak can be visually identified more easily if the water contains a colorant. Strength

is usually tested by measuring permanent deformation of the container. Hydrostatic testing is

the most common method employed for testing pipes and pressure vessels. Using this test

http://www.instron.us/wa/glossary/Shear-Modulus-of-Elasticity.aspxhttp://www.instron.us/wa/glossary/Yield-Strength.aspxhttp://www.instron.us/wa/glossary/Ultimate-Strength.aspxhttp://www.instron.us/wa/glossary/Modulus-of-Rupture.aspxhttp://www.instron.us/wa/glossary/Ductility.aspxhttp://en.wikipedia.org/wiki/Pressure_vesselhttp://en.wikipedia.org/wiki/Pipeline_transporthttp://en.wikipedia.org/wiki/Plumbinghttp://en.wikipedia.org/wiki/Gas_cylinderhttp://en.wikipedia.org/wiki/Gas_cylinderhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Boilerhttp://en.wikipedia.org/wiki/Gas_cylinderhttp://en.wikipedia.org/wiki/Gas_cylinderhttp://en.wikipedia.org/wiki/Plumbinghttp://en.wikipedia.org/wiki/Pipeline_transporthttp://en.wikipedia.org/wiki/Pressure_vesselhttp://www.instron.us/wa/glossary/Ductility.aspxhttp://www.instron.us/wa/glossary/Modulus-of-Rupture.aspxhttp://www.instron.us/wa/glossary/Ultimate-Strength.aspxhttp://www.instron.us/wa/glossary/Yield-Strength.aspxhttp://www.instron.us/wa/glossary/Shear-Modulus-of-Elasticity.aspx


7/18

2010-ME181

helps maintain safety standards and durability of a vessel over time. Newly manufactured pieces

are initially qualified using the hydrostatic test.

Hardness Test

1.

Brinell hardness test: The oldest of the hardness test methods in common use today,the Brinell test is frequently used to determine the hardness of forgings and castings that

have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are

frequently done on large parts. By varying the test force and ball size, nearly all metals can

be tested using a Brinell test. Brinell values are considered test force independent as long as

the ball size/test force relationship is the same. All Brinell tests use a carbide ball indenter.

The test procedure is as follows:

The indenter is pressed into the sample by an accurately controlled test force.

The force is maintained for a specific dwell time, normally 10 - 15 seconds.

After the dwell time is complete, the indenter is removed leaving a round indent in thesample.

The size of the indent is determined optically by measuring two diagonals of the round

indent using either a portable microscope or one that is integrated with the load

application device.

The Brinell hardness number is a function of the test force divided by the curved surface

area of the indent. The indentation is considered to be spherical with a radius equal to

half the diameter of the ball. The average of the two diagonals is used in the following

formula to calculate the Brinell hardness.


8/18

2010-ME181

Applications:

Because of the wide test force range the Brinell test can be used on almost any metallic

material. The part size is only limited by the testing instrument's capacity.

Strengths

i. One scale covers the entire hardness range, although comparable results can only be

obtained if the ball size and test force relationship is the same.

ii. A wide range of test forces and ball sizes to suit every application.

iii. Nondestructive, sample can normally be reused.

Weaknesses

i. The main drawback of the Brinell test is the need to optically measure the indent size.

This requires that the test point be finished well enough to make an accuratemeasurement.

ii. Slow. Testing can take 30 seconds not counting the sample preparation time.

2. Rockwell test: There are two types of Rockwell tests:

i. Rockwell: the minor load is 10 kgf, the major load is 60, 100, or 150 kgf.

ii. Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30, or 45 kgf.

In both tests, the indenter may be either a diamond cone or steel ball, depending upon thecharacteristics of the material being tested.

Rockwell Scales

Rockwell hardness values are expressed as a combination of a hardness number and a scale

symbol representing the indenter and the minor and major loads. The hardness number is

expressed by the symbol HR and the scale designation.

There are 30 different scales. The majority of applications are covered by the Rockwell C and B

scales for testing steel, brass, and other metals. However, the increasing use of materials other

than steel and brass as well as thin materials necessitates a basic knowledge of the factors that

must be considered in choosing the correct scale to ensure an accurate Rockwell test. The

choice is not only between the regular hardness test and superficial hardness test, with three

different major loads for each, but also between the diamond indenter and the 1/16, 1/8, 1/4

and 1/2 in. diameter steel ball indenters.


9/18

2010-ME181

If no specification exists or there is doubt about the suitability of the specified scale, an analysis

should be made of the following factors that control scale selection:

Type of material

Specimen thickness

Test location Scale limitations

Principal of the Rockwell Test:

1. Select image to enlarge The indenter moves down into position on

the part surface

2. A minor load is applied and a zero reference position is established

3. The major load is applied for a specified time period (dwell time)

beyond zero

4.

The major load is released leaving the minor load applied

The resulting Rockwell number represents the difference in depth from the zero reference

position as a result of the application of the major load.

3. Vickers hardness: The Vickers (HV) test was developed in England is 1925 and wasformally known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has two

distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg), to cover all testing

requirements. The indenter is the same for both ranges therefore Vickers hardness values

are continuous over the total range of hardness for metals (typically HV100 to HV1000).

With the exception of test forces below 200g, Vickers values are generally considered test

force independent. In other words, if the material tested is uniform, the Vickers values will

be the same if tested using a 500g force or a 50kg force. Below 200g, caution must be used

when trying to compare results.

Vickers Test Method

All Vickers ranges use a 136° pyramidal diamond indenter that forms a square indent.

The indenter is pressed into the sample by an

accurately controlled test force.

The force is maintained for a specific dwelltime, normally 10 – 15 seconds.

After the dwell time is complete, the indenter

is removed leaving an indent in the sample that

appears square shaped on the surface.

The size of the indent is determined optically

by measuring the two diagonals of the square

indent.

http://%20void%20gopop%28%27%27%2C%27474%27%2C%27341%27%2C%2725%27%2C%2725%27%2C%27no%27%2C%27no%27%2C%27no%27%2C%27no%27%2C%27no%27%2C%27images/rockwell_test_principle_z.gif','Principle%20of%20the%20Rockwell%20Test','','yes');


10/18

2010-ME181

The Vickers hardness number is a function of the test force divided by the surface area of

the indent. The average of the two diagonals is used in the following formula to calculate

the Vickers hardness.

HV = Constant x test force / indent diagonal squared

The constant is a function of the indenter geometry and the units of force and diagonal. The

Vickers number, which normally ranges from HV 100 to HV1000 for metals, will increase as

the sample gets harder. Tables are available to make the calculation simple, while all digital

test instruments do it automatically.

Applications

Because of the wide test force range, the Vickers test can be used on almost any metallic

material. The part size is only limited by the testing instrument's capacity.

Strengths

1.

One scale covers the entire hardness range.

2. A wide range of test forces to suit every application.

3. Nondestructive, sample can normally be used.

Weaknesses

1. The main drawback of the Vickers test is the need to optically measure the indent size. This

requires that the test point be highly finished to be able to see the indent well enough tomake an accurate measurement.

2. Slow. Testing can take 30 seconds not counting the sample preparation time.

Impact Test

Impact testing is testing an object's ability to resist high-rate loading. An impact test is a test for

determining the energy absorbed in fracturing a test piece at high velocity. Most of us think ofit as one object striking another object at a relatively high speed. Impact resistance is one of the

most important properties for a part designer to consider, and without question, the most

difficult to quantify. The impact resistance of a part is, in many applications, a critical measure

of service life. More importantly these days, it involves the perplexing problem of product

safety and liability.


11/18

2010-ME181

One must determine:

1. the impact energies the part can be expected to see in its

lifetime,

2. the type of impact that will deliver that energy, and then

3.

Select a material that will resist such assaults over theprojected life span.

Molded-in stresses, polymer orientation, weak spots (e.g. weld

lines or gate areas), and part geometry will affect impact

performance. Impact properties also change when additives, e.g.

coloring agents, are added to plastics.

Method: The apparatus consists of a pendulum of known mass and length that is droppedfrom a known height to impact a notched specimen of material. The energy transferred to the

material can be inferred by comparing the difference in the height of the hammer before andafter the fracture (energy absorbed by the fracture event).

The notch in the sample affects the results of the impact test, thus it is necessary for the notch

to be of regular dimensions and geometry. The size of the sample can also affect results, since

the dimensions determine whether or not the material is in plane strain. This difference can

greatly affect conclusions made.

Fatigue test

A method for determining the behavior of materials under fluctuating loads. A specified mean

load (which may be zero) and an alternating load are applied to a specimen and the number ofcycles required to produce failure (fatigue life) is recorded. Generally, the test is repeated with

identical specimens and various fluctuating loads. Loads may be applied axially, in torsion, or in

flexure. Depending on amplitude of the mean and

cyclic load, net stress in the specimen may be in one

direction through the loading cycle, or may reverse

direction. Data from fatigue testing often are

presented in an S-N diagram which is a plot of the

number of cycles required to cause failure in a

specimen against the amplitude of the cyclical stress

developed. The cyclical stress represented may bestress amplitude, maximum stress or minimum stress.

Each curve in the diagram represents a constant

mean stress. Most fatigue tests are conducted in

flexure, rotating beam, or vibratory type machines.

http://en.wikipedia.org/wiki/Pendulumhttp://en.wikipedia.org/wiki/Inferhttp://en.wikipedia.org/wiki/Inferhttp://en.wikipedia.org/wiki/Pendulum


12/18

2010-ME181

Creep Test

Methods for determining creep or stress relaxation behavior. To determine creep properties,

material is subjected to prolonged constant tension or compression loading at constant

temperature. Deformation is recorded at specified time intervals and a creep vs. time diagramis plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates test and time

for rupture is recorded. If specimen does not fracture within test period, creep recovery may

be measured. To determine stress relaxation of material, specimen is deformed a given

amount and decrease in stress over prolonged period of exposure at constant temperature is

recorded.

Creep occurs in three stages: Primary or

Stage I; Secondary, or Stage II: and Tertiary,

or Stage III. Stage I, or Primary creep occurs

at the beginning of the tests, and creep is

mostly transiently, not at a steady rate.Resistance to creep increases until Stage II

is reached. In Stage II, or Secondary creep,

The rate of creep becomes roughly steady.

This stage is often referred to as steady

state creep. In Stage III, or tertiary creep,

the creep rate begins to accelerate as the

cross sectional area of the specimen

decreases due to necking or internal voiding

decreases the effective area of the

specimen. If stage III is allowed to proceed, fracture will occur.

X-rays Test

Radiography is one of the most versatile non destructive testing methods. Radiography can

determine the internal soundness of a material (for example cracks, inclusions, voids) without

destroying it. Radiography records the amount of radiation that penetrates a sample. The

magnitude (intensity) of the radiation that penetrates the sample indicates the attenuation of

the radiation. Areas of differential attenuation can be identified in the images generated in

either film or real-time radiography. These variations are caused by differences in density,material thickness, and material composition. Since flaws, such as voids, inclusions, cracks, etc.,

constitute density variations, they can be identified in the image.


13/18

2010-ME181

The radiography process used to inspect industrial products is very similar to the more familiar

X-ray used in the medical industry. Industrial radiography uses X-rays to penetrate test items

made of solid materials so hidden defects, inconsistencies or features can be revealed. An

image of the X-rayed location appearing as a negative is produced on film for evaluation by

certified NDT inspectors and to create a permanent record of the internal condition of the test

piece.

Magnetic Particle Inspection

It is used for finding surface/near surface defects in ferromagnetic materials, Magnetic Particle

testing (MT) is a versatile inspection method used for field and shop applications. Magnetic

particle testing works by magnetizing a ferromagnetic specimen using a magnet or special

magnetizing equipment. If the specimen has discontinuity, the magnetic field flowing through

the specimen is interrupted and leakage field occurs. Finely milled iron particles coated with a

dye pigment are applied to the specimen. These are attracted to leakage fields and cluster to

form an indication directly over the discontinuity. The indication is visually detected under

proper lighting conditions.

The basic procedure that is followed to perform magnetic particle testingconsist of the

following:

1. Pre-cleaning of component

2. Introduction of Magnetic field

3. Application of magnetic media

4.

Interpretation of magnetic particle indications

It is essential for the particles to have an unimpeded path for migration to both strong and

weak leakage fields. Therefore, the component in question should be clean and dry before

beginning the inspection process. The presence of oil, grease or scale may compromise the

inspection.

The introduction of the magnetic field can be introduced a number of ways including use of a

permanent magnet, flowing of electrical current through the specimen or flowing an electrical

current through a coil of wire around the part or through a central conductor running near thepart. Two types of magnetic fields can be established within the specimen. These are a

longitudinal magnetic field that runs parallel to the long axis of the part or a circular magnetic

field that runs circumferentially around the perimeter. Longitudinal magnetic fields are

produced using a magnetic coil or a permanent magnet called a magnetic particle yoke. Circular

magnetic fields are produced by passing current through the part or by placing the part in a

strong circular magnetic field.


14/18

2010-ME181

Magnetic particle inspection can use either wet or dry magnetic media. The dry method is more

portable, while the wet method is generally more sensitive since the liquid carrier gives the

magnetic particles additional mobility.

Indications that are formed after applying the magnetic field must be interpreted by a skilled

inspector. This requires the individual to distinguish between relevant and irrelevant

indications.

Advantages

The following are the advantages of magnetic particle inspection:

Can detect both surface and near subsurface indications

Can inspect parts with irregular shapes easily

Pre-cleaning is not as critical as for some other inspection methods

Fast method of inspection and indications are visible directly on the specimen surface Considered low cost compared to many other NDT techniques

Very portable inspection especially when used with battery powered equipment

Dye Penetrant Test

Liquid penetrant testing is a non-destructive method used to detect surface breaking defects in

any nonporous material. Liquid penetrant is applied to the surface and is drawn into defects by

capillary action. Once a preset dwell time has passed, excess penetrant is removed and

developer applied to draw out penetrant from defects. Visual inspection is then performed.

Visible and Fluorescent Liquid Penetrant Examinations are Non-Destructive methods ofrevealing discontinuities that are open to the surfaces of solid and essentially non-porous

materials, ferrous or non-ferrous.

Strong dye sprayed onto surface drawn into cracks and pores by capillary action

Surplus is wiped off

Developer (e.g. chalk powder suspension) sprayed on to reveal defects

Dye can be UV active so viewing under UV illumination reveals cracks

Principle: DPI is based upon capillary action, where low surface tension fluid, penetrates into

clean and dry surface-breaking discontinuities. Penetrant may be applied to the test componentby dipping, spraying, or brushing. After adequate penetration time has been allowed, the

excess penetrant is removed and a developer is applied. The developer helps to draw penetrant

out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is

performed under ultraviolet or white light, depending on the type of dye used - fluorescent or

nonfluorescent (visible).

http://en.wikipedia.org/wiki/Capillary_actionhttp://en.wikipedia.org/wiki/Fluorescenthttp://en.wikipedia.org/wiki/Fluorescenthttp://en.wikipedia.org/wiki/Capillary_action


15/18

2010-ME181

Applications:

Grinding cracks

Heat affect zone cracks

Poor weld penetration

Heat treatment cracks Fatigue cracks

Hydrogen cracks

Inclusions

Laminations

Micro shrinkage

Gas porosity

Hot tears

Cold shuts

Stress corrosion cracks

Inter-granular corrosion

Ultrasonic test

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. To illustrate the general inspection

principle, a typical pulse/echo inspection configuration as illustrated below will be used.

A typical UT inspection system consists of several functional units, such as the pulser/receiver,

transducer, and display devices. A pulser/receiver is an electronic device that can produce highvoltage electrical pulses. Driven by the pulser, the transducer generates high frequency

ultrasonic energy. 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 an electrical signal by the transducer and is displayed on a screen. In the applet below, the

reflected signal strength is displayed versus the time from signal generation to when a echo

was received. Signal travel time can be directly related to the distance that the signal traveled.

From the signal, information about the reflector location, size, orientation and other features

can sometimes be gained.

Advantages:

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of

ultrasonic inspection that are often cited include:


16/18

2010-ME181

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 single-sided access is needed when the pulse-echo technique is used.

It is highly accurate in determining reflector position and estimating size and shape.

Minimal part preparation is required. Electronic equipment provides instantaneous results.

Detailed images can be produced with automated systems.

It has other uses, such as thickness measurement, in addition to flaw detection.

Limitations:

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

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 the transfer of sound energy into

the 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 the

characterization of flaws.

Fluorescent Test

Fluorescent penetrant inspection (FPI) is a type of dye penetrant inspection in which a

fluorescent dye is applied to the surface of a non-porous material in order to detect defects

that may compromise the integrity or quality of the part in question. Noted for its low cost and

simple process, FPI is used widely in a variety of industries.

Inspection Steps:

1. Initial Cleaning: Before the penetrant can be applied to the surface of the material in question one must ensure

that the surface is free of any contamination such as paint, oil, dirt, or scale that may fill a

defect or falsely indicate a flaw. Chemical etching can be used to rid the surface of undesired

contaminates and ensure good penetration when the penetrant is applied. Sandblasting to

remove paint from a surface prior to the FPI process may mask (smear material over) cracks

making the penetrant not effective. Even if the part has already been through a previous FPI

http://en.wikipedia.org/wiki/Dye_penetrant_inspectionhttp://en.wikipedia.org/wiki/Dye_penetrant_inspection


17/18

2010-ME181

operation it is imperative that it is cleaned again. Most penetrants are not compatible and

therefore will thwart any attempt to identify defects that are already penetrated by any other

penetrant. This process of cleaning is critical because if the surface of the part is not properly

prepared to receive the penetrant, defective product may be moved on for further processing.

This can cause lost time and money in reworking, over processing, or even scrapping a finished

part at final inspection.

2. Penetrant Application:

The fluorescent penetrant is applied to the surface and allowed time to seep into flaws or

defects in the material. The process of waiting for the penetrant to seep into flaws is called

Dwell Time. Dwell time varies by material and the size of the indications that are intended to be

identified but is generally around 30 minutes. It requires much less time to penetrate larger

flaws because the penetrant is able to soak in much faster. The opposite is true for smaller

flaws.

3. Excess Penetrant Removal:

After the identified dwell time has passed, penetrant on the outer surface of the material is

then removed. This highly controlled process is necessary in order to ensure that the penetrant

is removed only from the surface of the material and not from inside any identified flaws.

Various chemicals can be used for such a process and vary by specific penetrant types.

Typically, the cleaner is applied to a lint-free cloth that is used to carefully clean the surface.

4. Developer Application:

Having removed excess penetrant a contrasting developer may be applied to the surface. This

serves as a background against which flaws can more readily be detected. The developer also

causes penetrant that is still in any defects to surface and bleed. These two attributes allow

defects to be easily detected upon inspection. Dwell time is then allowed for the developer to

achieve desired results before inspection.

5. Inspection:

In the case of fluorescent inspection, the inspector will use ultraviolet radiation with an

intensity appropriate to the intent of the inspection operation. This must take place in a dark

room to ensure good contrast between the glow emitted by the penetrant in the defected

areas and the unlit surface of the material. The inspector carefully examines all surfaces in

question and records any concerns. Areas in question may be marked so that location of

indications can be identified easily without the use of the UV lighting. The inspection should

occur at a given point in time after the application of the developer. Too short a time and the

flaws may not be fully blotted, too long and the blotting may make proper interpretation

difficult.

6. Final Cleaning:

Upon successful inspection of the product, it is returned for a final cleaning before it is shipped,

moved on to another process, or deemed defective and reworked or scrapped. Note that a


18/18

2010-ME181

flawed part may not go through the final cleaning process if it is considered not to be cost

effective.

basic principle of each test

Documents