15 questions

AE 2352EXPERIMENTAL STRESS ANALYSIS

Mr. HASTON AMIT KUMAR (ASSISTANT PROFESSOR)

DEPARTMENT OF AERONAUTICAL ENGINEERING

JEPPIAAR ENGINEERING COLLEGE

JEPPIAAR ENGINEERING COLLEGE DEPARTMENT OF AERONAUTICAL ENGINEERING

AE 2352 EXPERIMENTAL STRESS ANALYSIS

QUESTION BANK

UNIT - I

1. Explain in detail the Principles of Measurements. 2. What are the types of Mechanical Extensometers? Explain with aid of neat sketch

Huggenberger extensometer.

3. What is Strain Gauge? Explain with a neat sketch the working principle of Acoustical strain gage and measurements of strain using this gage. Also mention its advantages

and disadvantages.

UNIT - II

1. What are the different types of electrical strain gauges? Derive the expression of strain experienced by electrical resistance type strain gages.

(ii) Explain how the electrical resistance type gage can be used to determine

modulus of Elasticity and Poissons ratio of engineering materials 2. Determine the principal stresses and principal strains with the help of a delta rosette

mounted on an aluminum specimen with values of A= 400 m/m; B= 200 m/m; C= 100 m/m, E= 70 Gpa, = 0.3. Also determine the principal directions and shear stress.

3. (i) Explain Wheat stone Bridge and obtain the balance condition. (ii) Derive an expression for the output voltage measured from wheat stone bridge

circuit.

UNIT III

1. Sketch a circular polariscope. Explain the effects of a stressed model and the fringes obtained in it

2. Describe in detail how fringe sharpening is obtained using partial mirrors. 3. Explain any two compensation techniques used in photo elasticity. Why Tardys

compensation method is preferred over other methods?

UNIT IV

1. Explain the brittle coating method of stress analysis. Drive expression for brittle stress 2. What are fiber-optic sensors? What is their application in experimental mechanics? 3. Write notes on (a) moir method of strain analysis,

(b) Holography





UNIT V

1. State the uses and advantages of non-destructive testing procedures. Explain in detail any one NDT procedure of evaluating a given specimen.

2. Explain in detail the principle of ultrasonic pulse echo testing and also explain the sequence of test and its application.

3. Explain any four (a)Eddy current testing

(b)Acoustic emission technique

(c)Radiography

(d) Fluorescent penetrant technique

(e)Magnetic particle inspection

(f)Resonance test





UNIT - I

1. Explain in detail the Principles of Measurements. DEFINITION:

The process of obtaining the magnitude of a quantity such as length or mass

relative to a unit of measurement such as meters or kilogram.

The act of measuring or the process of being measured [used]

The system of measuring

PRINCIPLES OF MEASUREMENT:

The techniques of measurement are of immense importance in most

facets of scientific research & human civilization.

Computation with decimals frequently involves the addition or

subtractions of numbers do not have the same number of decimal places.

1. ESTIMATION:

Estimation is the calculated approximation of a result which is usable

even if input data may be incomplete or uncertain. It can be computed

precisely.

2. PRECISION:

The Measurement of a precision depends upon how precisely the

instrument is marked. It is important to realize that precision refers to the size

of the smallest division on the scale.

Simply we can say, that one instrument is more precise than another does

not imply that the less precise instrument is poorly manufactured.

The precision of measurement system also called reproducibility or repeatability

It is degree to which repeated measurement under unchanged conditions show

the same results.





3. REPRODUCIBILITY:

It is one of the main principles of the scientific method & refers to the

ability of a test or experiment to be accurately reproduced.

4. REPEATABILITY:

It is the variation in measurement taken by a single, person or instrument

on the same item & under the same conditions.

5. ACCURACY:

The accuracy of measurement depends upon the relative size of the

probable error.

The Accuracy of a measurement system is the degree of closeness of

measurements of a quantity to its actual [true] value.

The measurement system is valid if it is both accurate & precise.

No of true positives + no of true negativesACCURACY =

no of true positives & false positives + false negatives + true negatives

No of true positives

PrecisionNo of true positives false positives

Accuracy = (Sensitivity) (prevalence) + specificity [1-prevalency]

Accuracy may be determined from sensitivity & specificity provided

prevalence.

6. RESOLUTION:

7. SENSITIVITY:

No of true positives

sensitivityNo of true positives + no of false negatives

8. SPECIFICITY:

Specificity= No of true negatives

No of true negatives + no of false positives

Example:

True positives (TP) sick people correctly diagnose as sick False positives (FP) _ Healthy as sick

True Negatives (TN) _ Healthy correctly identified as healthy

False negatives (FN)_ Sick people incorrectly identified as healthy

False positives & False negatives also called as Type I & Type II error

TP condition present + positive result





FP condition absent + positive result

FN condition present + Negative result

TN condition absent + Negative result Example:

1) 3.72 inches or 2417 feet

We can say 3.72 inches is more precise 2417 feet is more accurate

2) 30 seconds or 28 seconds

30 second is more accurate & precise.

2. What are the types of Mechanical Extensometers? Explain with aid of neat sketch Huggenberger extensometer.

The mechanical devices are generally known as extensometers and are used to

measure strain under static or gradually varying loading conditions.

CLASSIFICATION:

1. Wedge and screw magnification (Howard Strain gauge)

2. Simple mechanical lever magnification (Capps multiplying divider) 3. Compound magnification system (Berry strain gauge/ Tinius olsen strain

gauage)

4. Compound lever magnification (Huggenberger strain gauge/Porter Lipp

strain gauge)

5. Magnification by rack and pinion (Dial gauge indicator)

6. Combined lever, rack and pinion magnification. (Whitlemore strain gauge)

These mechanical devices are generally known as extensometers and are

used to measure strain under static or gradually varying loading conditions. An

extensometer is usually provided with two knife edges which are clamped

firmly in contact with the test component at a specific distance or gauge length

apart. When the test component is strained, the two knife edges undergo a

small relative displacement. This is amplified through a mechanical linkage

and the magnified displacement or strain is displayed on a calibrated scale.

HUGGENBERGER TENSOMETER:

Function: This is a device used to measure very small displacements between

its two tips. It is especially used for determining the changes in the length of

materials due to strain, temperature and other factors.





It has a system of levers and gears connected to a needle that amplifies the angle

between its two arms by factors of more than one thousand, and uses this

amplified needle movement to point the displacement on a scale.

In the Huggenberger extensometer a set of compound levers is used to

magnify the displacement of the knife edges. The extensometer is highly

accurate, reliable, light-weight and self-contained. The movable knife edge (f)

rotates the lever c about the lower pivot. The lever c in turn rotates the pointer

through the link d. The magnification ratio is given by (1112) / (a1a2)

Extensometers with this ratio varying between 300 and 2000 and with gauge

lengths in the range 6.5 to 100 mm are available. The sensitivity of these

extensometers could be as high as 10 micro strain. It is well suited for

applications where its unusually large height does not pose problems of

instability in mounting.

Advantages: Light Weight High amplification Self contianed Sensitivity is 10 strains

Dis-advantages: Cannot measure dynamic strains Huge size





The Berry strain gauge uses a system of a lever and dial gauge to magnify

the small displacement between the knife edges. It can Measure strains down

to 10 microstrain over a 50 mm gauge length. The mechanical amplifying

element in the CEJ extensometer is a twisted metal strip or torsion tape

stretched between the knife edges.

Figure: Berry Strain Gauge

Figure: Johansson extensometer

Half the length of this strip is twisted in one direction while the other half

is twisted in the opposite direction. A pointer is attached at the centre. The

displacement of the knife edges, i.e. starching of the torsion tape is converted

into a highly amplified rotational movement of the pointer. The CEJ

extensometer can measure strain with a sensitivity of 5 micro strain over a

gauge length of 50 mm.





3. What is Strain Gauge? Explain with a neat sketch the working principle of Acoustical strain gage and measurements of strain

using this gage. Also mention its advantages and disadvantages.

ACOUSTICAL STRAIN GAUGE:

In these gauges the measurement and magnification are done optically. A

system of mirrors may be used to produce large displacement on scale. It is

suitable for measuring dynamics strains with a photographic recording system.

It is difficult to handle and is a heavy instrument.

The vibrating wire or acoustical gauge consists essentially of a steel wire

tensioned between two supports a predetermined distance apart. Variation of the

distance alters the natural frequency of vibration of the wire and this change in

frequency may be correlated with the change in strains causing it. An electro-

magnet adjacent to the wire may be used to set the wire in vibration and this

wire movement will then generate an oscillating electrical signal. The signal

may be compared with the pitch of an adjustable standard wire, the degree of

adjustment necessary to match the two signal frequencies being provided by a

tensioning screw on the standard wire. Calibration of this screw allows a direct

determination of the change of length of a measuring gauge to be made once the

standard gauge has been tuned to match the frequency of the measuring wire.

The visual display produced or a CRO renders adjustment easier. Tuning

is now more usually accomplished by feeding the two signals into the two pairs

of plates of an oscillograph and making use of the Lissajous figure formation to

balance the frequencies. Matching of the tones is simplified and made more

accurate by tuning out the beats which results when the vibration frequencies of

two wires are nearly the same, which can be compared by using earphones.

The fundamental frequency of a stretched wire may be estimated from the

expression.

/1 1

2 2

E L LPf A

L m L m





Figure: Acoustical Strain Gauge

Where A = cross sectional area of vibrating wire E = Youngs modulus of wire material L = length of vibrating wire

m = mass per unit length of the wire

P = tensioning force in the wire

L = increment in length of the vibrating wire.

Figure: Shows an acoustical gauge developed by Dr. O. Schaefer about 1933.

The sensitivity of this gauge is very high, with possible determinations of

displacement of the order of 0.25 cm. The range is limited to about 1/1000 of

the wire length. The gauge is temperature sensitive unless the thermal

coefficients of expansion of the base and wire are closely matched over the

temperature range encountered during a test.





UNIT - II

1. What are the different types of electrical strain gauges? Derive the expression of

strain experienced by electrical resistance type strain gages.

TYPES OF ELECTRICAL STRAIN GAUGES:

1. Inductance or magnetic strain gauge 2. Capacitance strain gauge 3. Electrical Resistance strain gauge

In electrical resistance strain gauge the displacement or strain is measured as a

function of resistance change produced by the displacement in the gauging circuit.

.

When the conductor is stretched, its length will increase and area of cress section will decrease this will result in change in resistance.

Change in resistance per unit strain is defined as Gauge Factor. Gauge factor indicates the sensitivity of the strain gauge

As the wire is stretched along with the specimen, the wire's electrical resistance R

changes both because its length L is increased and its cross-sectional area A is

reduced

TYPES OF ELECTRICAL RESISTANCE

STRAIN GAUGES





(ii) Explain how the electrical resistance type gage can be used to determine

modulus of Elasticity and Poissons ratio of engineering materials 2. Determine the principal stresses and principal strains with the help of a delta rosette

mounted on an aluminum specimen with values of A= 400 m/m; B= 200 m/m; C= 100 m/m, E= 70 Gpa, = 0.3. Also determine the principal directions and shear stress.

3. (i) Explain Wheat stone Bridge and obtain the balance condition. (ii) Derive an expression for the output voltage measured from wheat stone bridge

circuit.

WHEAT STONE BRIDGE

The Wheatstone bridge (or resistance bridge) circuit can be used in a number of

applications and today, with modern Operational Amplifiers we can use the Wheatstone

Bridge Circuit to interface various transducers and sensors to these amplifier circuits.





The Wheatstone Bridge circuit is nothing more than two simple series-parallel

arrangements of resistances connected between a voltage supply terminal and ground

producing zero voltage difference between the two parallel branches when balanced. A

Wheatstone bridge circuit has two input terminals and two output terminals consisting of four

resistors configured in a diamond-like arrangement as shown.

WHEAT STONE BRUDGE CIRCUIT

Wheatstone bridge circuit can be used to compare an unknown resistance RXwith

others of a known value, for example, R1 and R2, have fixed values, and R3 could be

variable. If we connected a voltmeter, ammeter or classically a galvanometer between

points C and D, and then varied resistor, R3 until the meters read zero, would result in the

two arms being balanced and the value of RX, (substituting R4) known as shown.

By replacing R4 above with a resistance of known or unknown value in the sensing

arm of the Wheatstone bridge corresponding to RX and adjusting the opposing resistor, R3 to

balance the bridge network, will result in a zero voltage output. Then we can see that balance occurs when:

The Wheatstone Bridge equation required to give the value of the unknown

resistance, RX at balance is given as:





EXPRESSION FOR OUTPUT VOLTAGE:





UNIT III PHOTOELASTICITY

1. Sketch a circular polariscope. Explain the effects of a stressed model and the fringes obtained in it

In a circular polariscope setup two quarter-wave plates are added to the

experimental setup of the plane polariscope. The first quarter-wave plate is

placed in between the polariser and the specimen and the second quarter-wave

plate is placed between the specimen and the analyser.

The effect of adding the quarter-wave plates is that we get circularly

polarised light.

The basic advantage of a circular polariscope over a plane polariscope is that in

a circular polariscope setup we only get the isochromatics and not the isoclinics.

This eliminates the problem of differentiating between the isoclinics and the

isochromatics.

Experimental Setup of the Circular Polariscope





2. Describe in detail how fringe sharpening is obtained using partial mirrors.





3. Explain any two compensation techniques used in photo elasticity. Why Tardys compensation method is preferred over other methods?

In compensation, we add to the birefringence of the model, at

some given point, an equal and opposite birefringence such as to

bring the total relative retardation to zero. This is done by placing an

instrument a compensator in the optical path adjacent to the model, as shown in Fig. 6.12. The compensator is adjusted to bring

the center of a zero-order fringe to some given point on the

photoelastic model and the fringe order contributed by the

compensator is read off the instrument. Of course, this corresponds

to the fringe order in the model, too.

The active element of the compensator is usually a wedge of a

permanently birefringen crystal which is moved across any given

point in the model until compensation, or zero relative retardation is

achieved: alternatively, a photoelastic model with a simple

predetermined stress system is used as the active element. The

simple wedge, the Babinet compensator and the Babinet-Soleil

compensator are in this category.





UNIT IV

1. Explain the brittle coating method of stress analysis. Drive expression for brittle stress

BRITTLE-COATING METHOD

Introduction

The brittle-coating method, servo the following broad objectives:

1. Diagnosis or failure-analysis of in-service failure of components, and

2. Determination of location and orientation of strain-sensors such as strain-gauges required

for further strain or stress analysis.

Tests with brittle coatings can be carried out with case and at lesser cost, as compared to

the cost of other surface strain measurement techniques.

Brittle coating is any thin surface coating applied on the surface of a model or a

component under test and which fractures or cracks in response to the strain applied to the

model on which it is coated, indicating quantitatively the direction in magnitude of surface

strains in the model. If these surface strains are within the elastic limit of the material of the

models, the resulting crack pattern provides an overall graphical picture of the distribution,

sequence and direction of surface strains. If the coatings are carefully calibrated, he crack

pattern also provides quantitative values for the magnitudes of principal surface strains. The

state of strain in the coating thus indicates, both qualitatively and quantitatively, the state of

strain in the model, and hence the state of stress.

Advantages:

1. The determination of stress concentrations in components under the influence of static, dynamic and impact loads.

2. It is also used for the measurement of thermal and residual stresses. The models or proto-type on which brittle coatings are applied can be made of any material plastics,

wood, paper, rubber, glass, bone and metals.

Disadvantages:

1. The brittle-coating method is the loss in the accuracy of results if elaborate precautions are not taken in evaluating the sensitivity of the coating and the hazardous

nature of the chemicals involved in the application of this technique.

In order to understand the steps involved in a typical brittle-coating application, consider

a flat tension model as shown in fig. The width of the specimen varies along the length of the

model, the minimum width being at the middle of the model. The model is first coated with a

thin layer of brittle coating, 0.125 to 0.25 mm thick, which is later dried at room temperature

and cured at an elevated temperature. The details of the technique of applying brittle coating

and curing are discussed in later sections. The coated model is loaded in incremental steps.

When the strain or stress in the coating at point A exceeds a critical value a crack develops in

the coating. On further loading, this crack gets extended and, in addition, new cracks are also

grating is bonded onto which one can obtain the state of strain and hence the state of stress in

the model.

MOIRE TECHNIQUES FOR INPLANE PROBLEMS:

Moire fringes, i.e., the isothetics, in general, are obtained by optical interference

between a specimen grating which is either printed or bonded, and closely placed master

grating. Its diagram indicates a general set-up used for this purpose. The specimen grating is

either rigidly bonded to the specimen or chemically etched or printed by a photographic

process directly onto the transparent specimen. The master grating is placed very close to the





specimen grating. The primary directions of both the specimen and master gratings are

aligned to coincide with the imaginary x-axis in the specimen. Collimated light passes the

gratings. When the specimen is strained, the specimen grating undergoes deformation which

interference with the unstrained maser grating resulting in moir fringes. /this is

photographed by a camera. The fringe pattern gives the uy -displacement field. The specimen

grating is removed from the specimen and another specimen grating is bonded onto it such

that its primary direction coincides with the y-axis of the coordinate system. Alternatively, a

new specimen similar to the earlier on with the primary direction of the specimen grating

bonded to it coinciding with the y-axis can be used. The master grating is now aligned with

the specimen grating. The fringe pattern now obtained gives the ux displacement field. From the ux and uy displacement fields, derivatives of displacements are evaluated and the strain

components at the points of interest are determined using these equations. These diagram

indicates the moire patterns for a diametrally compressed disk for ux and uy displacement

fields.

2. What are fiber-optic sensors? What is their application in experimental mechanics?

FIBER OPTIC SENSORS:

A fiber optic sensor is a sensor that uses optical fiber either as the sensing element

("intrinsic sensors"), or as a means of relaying signals from a remote sensor to the

electronics that process the signals ("extrinsic sensors").

Fibers have many uses in remote sensing.

Depending on the application, fiber may be used because of its small size, or because

no electrical power is needed at the remote location, or because many sensors can

be multiplexed along the length of a fiber by using different wavelengths of light for

each sensor, or by sensing the time delay as light passes along the fiber through each

sensor. Time delay can be determined using a device such as an optical time-domain

reflectometer.

Fiber optic sensors are also immune to electromagnetic interference, and do not

conduct electricity so they can be used in places where there is high voltage electricity

or inflammable material such as jet fuel. Fiber optic sensors can be designed to

withstand high temperatures as well.

WHY OPTICAL SENSORS

ELECTROMAGNETIC IMMUNITY

ELECTRICAL ISOLATION

COMPACT AND LIGHT

BOTH POINT AND DISTRIBUTED CONFIGURATION

WIDE DYNAMIC RANGE

AMENABLE TO MULTIPLEXING





WORKING PRINCIPLE:

LIGHT BEAM CHANGES BY THE PHENOMENA THAT IS BEING MEASURED

LIGHT MAY CHANGE IN ITS FIVE OPTICAL PROPERTIES i.e INTENSITY,

PHASE, POLARIZATION,WAVELENGTH AND SPECTRAL DISTRIBUTION

EP(t)cos[t+(t)] INTENSITY BASED SENSORS EP (t)

FREQUENCY VARYING SENSORS - P(t)

PHASE MODULATING SENSING- (t)

POLARIZATION MODULATING FIBER SENSING

CLASSIFICATION:

1. INTRINSIC SENSORS





Optical fibers can be used as sensors to measure strain, temperature, pressure and other

quantities by modifying a fiber so that the quantity to be measured modulates the intensity,

phase, polarization, wavelength or transit time of light in the fiber. Sensors that vary the

intensity of light are the simplest, since only a simple source and detector are required. A

particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide

distributed sensing over very large distances

Intrinsic sensors are different in that the light beam does not leave the optical fiber but is

changed whilst still contained within it.

2. EXTRINSIC SENSORS

Where the light leaves the feed or transmitting fiber to be changed before it continues to the

detector by means of the return or receiving fiber

Extrinsic fiber optic sensors use an optical fiber cable, normally a multimode one, to

transmit modulated light from either a non-fiber optical sensor, or an electronic sensor

connected to an optical transmitter. A major benefit of extrinsic sensors is their ability to

reach places which are otherwise inaccessible. An example is the measurement of

temperature inside aircraft jet by using a fiber to transmit radiation into a

radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same

way to measure the internal temperature of electrical transformers, where the

extreme electromagnetic fields present make other measurement techniques impossible.

Extrinsic fiber optic sensors provide excellent protection of measurement signals against

noise corruption. Unfortunately, many conventional sensors produce electrical output which

must be converted into an optical signal for use with fiber. For example, in the case of

a platinum resistance thermometer, the temperature changes are translated into resistance

changes. The PRT must therefore have an electrical power supply. The modulated voltage

level at the output of the PRT can then be injected into the optical fiber via the usual type of

transmitter. This complicates the measurement process and means that low-voltage power

cables must be routed to the transducer.

Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration,

torque, and twisting.





EXTRINSIC INTRINSIC

APPLICATIONS-

TEMPERATURE, PRESSURE,

LIQUID LEVEL AND FLOW.

LESS SENSITIVE

EASILY MULTIPLEXED

INGRESS/ EGRESS

CONNECTION PROBLEMS

EASIER TO USE

LESS EXPENSIVE

APPLICATIONS- ROTATION,

ACCELERATION, STRAIN,

ACOUSTIC PRESSURE AND

VIBRATION.

MORE SENSITIVE

TOUGHER TO MULTIPLEX

REDUCES CONNECTION

PROBLEMS

MORE ELABORATE SIGNAL

DEMODULATION

MORE EXPENSIVE

APPLICATIONS: MILITARY AND LAW ENFORCEMENT

NIGHT VISION CAMERA

BIOMETRICS

IMAGE CAPTURE

IMAGE PROCESSING

FEATURE EXTRACTION

FEATURE COMPARISON





3. Write notes on (a) moir method of strain analysis, (b) Holography

(a) moir method of strain analysis

MOIRE PHENOMENON:

Two arrays of alternately placed transparent and opaque lines or dots, when

moved relative to each other, result in fringe patterns consisting of alternately placed bright

and dark bands which are termed moir fringes. An ensemble of equispaced opaque lines

separated by transparent slits or lines, which are used to obtain moir fringes is called a

grating. In parallel - line grating shown in figure which are most commonly used for the

moir method of strain analysis, the opaque and transparent lines are perfectly parallel and

equi spaced. In a radial-line grating opaque and transparent lines are alternate radial lines and

in a circular-line grating these form circles of varying radii. The opaque and transparent lines

in these line gratings can either be equal or unequal.

Figure shows a parallel line grating. The distance between corresponding points

in a grating is called the pitch and is denoted by p. The density of the grating, which

represents the number of lines per unit length is denoted by d. The direction perpendicular to

the lines in the plane of grating is called the primary direction, while the direction parallel to

the lines is called the secondary direction.





FUNDAMENTAL PROPERTIES OF THE MOIRES FRINGES: The arrays used to produce Moires fringes may be a series of straight parallel lines, a series of radial lines emanating from a point, a series of concentric circles, or a pattern

of dots. Arrays consisting of straight parallel lines having opaque bars with transparent

interspaces of equal width are the most commonly used for experimental work. Such arrays

are called grids, grading, or grills. Coarse arrays having upto about 4 lines per cm arre

generally called grids and arrays having from 20 to 400 lines per cm are called gratings. Two

mutually perpendicular line arrays are termed as cross grating. Gratings having upto 200 lines

per cm are most commonly used.

b) Holography is a method that uses the wave character of light, which depicts an exact

description that goes beyond the options of the classic photography.

Holography is a technique that enables a light field, which is generally the product of a light

source scattered off of objects, to be recorded and later reconstructed when the original light

field is no longer present, due to the absence of the original objects. Holography can be

thought of as somewhat similar to sound recording, whereby a sound field created by

vibrating matter like musical instruments or vocal cords, is encoded in such a way that it can

be reproduced later, without the presence of the original vibrating matter.

Apparatus

A hologram can be made by shining part of the light beam directly onto the recording

medium, and the other part onto the object in such a way that some of the scattered light falls

onto the recording medium.

A more flexible arrangement for recording a hologram requires the laser beam to be aimed

through a series of elements that change it in different ways. The first element is a beam

splitter that divides the beam into two identical beams, each aimed in different directions:

One beam (known as the illumination or object beam) is spread using lenses and directed

onto the scene using mirrors. Some of the light scattered (reflected) from the scene then

falls onto the recording medium.

The second beam (known as the reference beam) is also spread through the use of lenses,

but is directed so that it doesn't come in contact with the scene, and instead travels

directly onto the recording medium.

Several different materials can be used as the recording medium. One of the most common is

a film very similar to photographic film (silver halide photographic emulsion), but with a

much higher concentration of light-reactive grains, making it capable of the much

higher resolution that holograms require. A layer of this recording medium (film, etc.) is

attached to a transparent substrate, which is commonly glass, but may also be plastic.

Process

When the two laser beams reach the recording medium, their light waves intersect

and interfere with each other. It is this interference pattern that is imprinted on the recording

medium. The pattern itself is seemingly random; as it represents the way in which the scene's

light interfered with the original light source but not the original light source itself. The

interference pattern can be said to be an encoded version of the scene, requiring a particular

key that is, the original light source in order to view its contents.





This missing key is provided later by shining a laser, identical to the one used to record the

hologram, onto the developed film. When this beam illuminates the hologram, it

is diffracted by the hologram's surface pattern. This produces a light field that is identical to

the one originally produced by the scene and scattered onto the hologram. The image this

effect produces in a person's retina is known as a virtual image

RECORDING A HOLOGRAM

RECONSTRUCTING A HOLOGRAM





UNIT V

1. State the uses and advantages of non-destructive testing procedures. Explain in detail

any one NDT procedure of evaluating a given specimen.

Nondestructive testing (NDT) are noninvasive techniques to determine the integrity of a

material, component or structure or quantitatively measure some characteristic of an object.

In contrast to destructive testing, NDT is an assessment without doing harm, stress or

destroying the test object. The destruction of the test object usually makes destructive testing

more costly and it is also inappropriate in many circumstances.

NDT plays a crucial role in ensuring cost effective operation, safety and reliability of plant,

with resultant benefit to the community. NDT is used in a wide range of industrial areas and

is used at almost any stage in the production or life cycle of many components. The

mainstream applications are in aerospace, power generation, automotive, railway,

petrochemical and pipeline markets. NDT of welds is one of the most used applications. It is

very difficult to weld or mold a solid object that has no risk of breaking in service, so testing

at manufacture and during use is often essential.

While originally NDT was applied only for safety reasons it is today widely accepted as cost

saving technique in the quality assurance process. Unfortunately NDT is still not used in

many areas where human life or ecology is in danger. Some may prefer to pay the lower costs

of claims after an accident than applying of NDT. That is a form of unacceptable risk

management. Disasters like the railway accident in Eschede Germany in 1998 is only one

example, there are many others.

For implementation of NDT it is important to describe what shall be found and what to reject.

A completely flawless production is almost never possible. For this reason testing

specifications are indispensable. Nowadays there exists a great number of standards and

acceptance regulations. They describe the limit between good and bad conditions, but also

often which specific NDT method has to be used.

The reliability of an NDT Method is an essential issue. But a comparison of methods is only

significant if it is referring to the same task. Each NDT method has its own set of advantages

and disadvantages and, therefore, some are better suited than others for a particular

application. By use of artificial flaws, the threshold of the sensitivity of a testing system has

to be determined. If the the sensitivity is to low defective test objects are not always

recognized. If the sensitivity is too high parts with smaller flaws are rejected which would

have been of no consequence to the serviceability of the component. With statistical methods

it is possible to look closer into the field of uncertainly. Methods such as Probability of

Detection (POD) or the ROC-method "Relative Operating Characteristics" are examples of

the statistical analysis methods. Also the aspect of human errors has to be taken into account

when determining the overall reliability.

Personnel Qualification is an important aspect of non-destructive evaluation. NDT techniques

rely heavily on human skill and knowledge for the correct assessment and interpretation of

test results. Proper and adequate training and certification of NDT personnel is therefore a

must to ensure that the capabilities of the techniques are fully exploited. There are a number

of published international and regional standards covering the certification of competence of





personnel. The EN 473 (Qualification and certification of NDT personnel - General

Principles) was developed specifically for the European Union for which the SNT-TC-1A is

the American equivalent.

The nine most common NDT Methods are shown in the main index of this encyclopedia. In

order of most used, they are: Ultrasonic Testing (UT), Radiographic Testing (RT),

Electromagnetic Testing (ET) in which Eddy Current Testing (ECT) is well know and

Acoustic Emission (AE or AET). Besides the main NDT methods a lot of other NDT

techniques are available, such as Shearography Holography, Microwave and many more and

new methods are being constantly researched and developed.

Nondestructive Testing

The field of Nondestructive Testing (NDT) is a very broad, interdisciplinary field 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 planes

to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of less visible, but

equally 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 measured without damaging them. Because it allows inspection without

interfering with a product's final use, NDT provides an excellent balance between quality

control and cost-effectiveness. Generally speaking, NDT applies to industrial inspections.

While technologies are used in NDT that are similar to those used in the medical industry,

typically nonliving objects are the subjects of the inspections.

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, formability, and other

physical characteristics.

EDDY CURRENT TESTING

Eddy-current testing uses electromagnetic induction to detect flaws

in conductive materials. There are several limitations, among them: only conductive materials

can be tested, the surface of the material must be accessible, the finish of the material may

cause bad readings, the depth of penetration into the material is limited by the materials'

conductivity, and flaws that lie parallel to the probe may be undetectable.

In a standard eddy current testing a circular coil carrying current is placed in proximity to the

test specimen (which must be electrically conductive).The alternating current in the coil

generates changing magnetic field which interacts with test specimen and generates eddy

current. Variations in the phase and magnitude of these eddy currents can be monitored using

a second 'receiver' coil, or by measuring changes to the current flowing in the primary

'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test

object, or the presence of any flaws, will cause a change in eddy current and a corresponding





change in the phase and amplitude of the measured current. This is the basis of standard (flat

coil) eddy current inspection, the most widely used eddy current technique.

However, eddy-current testing can detect very small cracks in or near the surface of the

material, the surfaces need minimal preparation, and physically complex geometries can be

investigated. It is also useful for making electrical conductivity and coating thickness

measurements.

Some of the advantages of eddy current inspection include:

Sensitive to small cracks and other defects

Detects surface and near surface defects

Inspection gives immediate results

Equipment is very portable

Method can be used for much more than flaw detection

Minimum part preparation is required

Test probe does not need to contact the part

Inspects complex shapes and sizes of conductive materials

Some of the limitations of eddy current inspection include:

Only conductive materials can be inspected





Surface must be accessible to the probe

Skill and training required is more extensive than other techniques

Surface finish and roughness may interfere

Reference standards needed for setup

Depth of penetration is limited

Flaws such as de-laminations that lie parallel to the probe coil winding and

probe scan direction are undetectable

2. Explain in detail the principle of ultrasonic pulse echo testing and also explain the sequence of test and its application.

ULTRASONIC TESTING (UT)

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center

frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are launched

into materials to detect internal flaws or to characterize materials. A common

example is ultrasonic thickness measurement, which tests the thickness of the test

object, for example, to monitor pipework corrosion.

Ultrasonic testing is often performed on steel and other metals and alloys, though it

can also be used on concrete, wood and composites, albeit with less resolution. It is

a form of non-destructive testing used in many industries including aerospace,

automotive and other transportation sectors.

In ultrasonic testing, an ultrasound transducer connected to a diagnostic

machine is passed over the object being inspected. The transducer is typically

separated from the test object by a couplant (such as oil) or by water, as in

immersion testing.





There are two methods of receiving the ultrasound waveform, reflection and

attenuation. In reflection (or pulse-echo) mode, the transducer performs both the

sending and the receiving of the pulsed waves as the "sound" is reflected back to

the device. Reflected ultrasound comes from an interface, such as the back wall of

the object or from an imperfection within the object. The diagnostic machine

displays these results in the form of a signal with an amplitude representing the

intensity of the reflection and the distance, representing the arrival time of the

reflection. In attenuation (or through-transmission) mode, a transmitter sends

ultrasound through one surface, and a separate receiver detects the amount that has

reached it on another surface after traveling through the medium. Imperfections or

other conditions in the space between the transmitter and receiver reduce the

amount of sound transmitted, thus revealing their presence. Using the couplant

increases the efficiency of the process by reducing the losses in the ultrasonic wave

energy due to separation between the surfaces.

At a construction site, a technician tests a pipeline weld for defects using an

ultrasonic phased array instrument. The scanner, which consists of a frame with





magnetic wheels, holds the probe in contact with the pipe by a spring. The wet area

is the ultrasonic couplant that allows the sound to pass into the pipe wall.

Non-destructive testing of a swing shaft showing spline cracking

Advantages

1. High penetrating power, which allows the detection of flaws deep in the part.

2. High sensitivity, permitting the detection of extremely small flaws.

3. Only one surface need be accessible.

4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces.

5. Some capability of estimating the size, orientation, shape and nature of defects.

6. Nonhazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity.

7. Capable of portable or highly automated operation.

Disadvantages

1. Manual operation requires careful attention by experienced technicians 2. Extensive technical knowledge is required for the development of inspection

procedures.

3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect.

4. Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that is properly bonded to a surface need not be removed.

5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact

technique is used. Non-contact techniques include Laser and Electro

Magnetic Acoustic Transducers (EMAT).

6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.

3.Explain any four

(a)Eddy current testing





(b)Acoustic emission technique

(c)Radiography

(d) Fluorescent penetrant technique

(e)Magnetic particle inspection

(f)Resonance test

(a)EDDY CURRENT TESTING

Eddy-current testing uses electromagnetic induction to detect flaws

in conductive materials. There are several limitations, among them: only conductive materials

can be tested, the surface of the material must be accessible, the finish of the material may

cause bad readings, the depth of penetration into the material is limited by the materials'

conductivity, and flaws that lie parallel to the probe may be undetectable.

In a standard eddy current testing a circular coil carrying current is placed in proximity to the

test specimen (which must be electrically conductive).The alternating current in the coil

generates changing magnetic field which interacts with test specimen and generates eddy

current. Variations in the phase and magnitude of these eddy currents can be monitored using

a second 'receiver' coil, or by measuring changes to the current flowing in the primary

'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test

object, or the presence of any flaws, will cause a change in eddy current and a corresponding

change in the phase and amplitude of the measured current. This is the basis of standard (flat

coil) eddy current inspection, the most widely used eddy current technique.

However, eddy-current testing can detect very small cracks in or near the surface of the

material, the surfaces need minimal preparation, and physically complex geometries can be

investigated. It is also useful for making electrical conductivity and coating thickness

measurements.





Some of the advantages of eddy current inspection include:

Sensitive to small cracks and other defects

Detects surface and near surface defects

Inspection gives immediate results

Equipment is very portable

Method can be used for much more than flaw detection

Minimum part preparation is required

Test probe does not need to contact the part

Inspects complex shapes and sizes of conductive materials

Some of the limitations of eddy current inspection include:

Only conductive materials can be inspected

Surface must be accessible to the probe

Skill and training required is more extensive than other techniques

Surface finish and roughness may interfere

Reference standards needed for setup

Depth of penetration is limited

Flaws such as de-laminations that lie parallel to the probe coil winding and

probe scan direction are undetectable





(b)ACOUSTIC EMISSION TECHNIQUE

Acoustic emission (AE) is the phenomenon of radiation of acoustic (elastic)

waves in solids that occurs when a material undergoes irreversible changes in

its internal structure, for example as a result of crack formation or plastic

deformation due to aging, temperature gradients or external mechanical forces.

In particular, AE is occurring during the processes of mechanical loading of

materials and structures accompanied by structural changes that generate local

sources of elastic waves. This results in small surface displacements of a

material produced by elastic or stress waves generated when the accumulated

elastic energy in a material or on its surface is released rapidly. The waves

generated by sources of AE are of practical interest in methods used to capture

AE in a controlled fashion, for study and/or use for inspection of structural

integrity, quality control, system feedback, process monitoring, and others.

The application of acoustic emission to non-destructive testing of materials,

typically takes place between 100 kHz and 1 MHz. Unlike conventional

ultrasonic testing, AE tools are designed for monitoring acoustic emissions

produced within the material during failure or stress, rather than actively

transmitting waves, then collecting them after they have traveled through the

material. Part failure can be documented during unattended monitoring. The

monitoring of the level of AE activity during multiple load cycles forms the

basis for many AE safety inspection methods, that allow the parts undergoing

inspection to remain in service





(c)RADIOGRAPHY

Radiographic Testing (RT), or industrial radiography, is a nondestructive

testing (NDT) method of inspecting materials for hidden flaws by using the

ability of short wavelength electromagnetic radiation (high energy photons)

to penetrate various materials.

Either an X-ray machine or a radioactive source, like Ir-192, Co-60, or in

rarer cases Cs-137 are used in a X-ray computed tomographymachine as a

source of photons. Neutron radiographic testing (NR) is a variant of

radiographic testing which uses neutrons instead of photons to penetrate

materials. This can see very different things from X-rays, because neutrons

can pass with ease through lead and steel but are stopped by plastics, water

and oils.

Since the amount of radiation emerging from the opposite side of the

material can be detected and measured, variations in this amount (or

intensity) of radiation are used to determine thickness or composition of





material. Penetrating radiations are those restricted to that part of the

electromagnetic spectrum of wavelength less than about 10 nanometres.





(d) FLUORESCENT PENETRANT TECHNIQUE

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, simple process, FPI is used widely in a

variety of industries.

There are many types of dye used in penetrant inspections. FPI operations

use a dye much more sensitive to smaller flaws than penetrants used in other DPI

procedures. This is because of the nature of the fluorescent penetrant that is

applied. With its brilliant yellow glow caused by its reaction with ultraviolet

radiation, FPI dye sharply contrasts with the dark background. A vivid reference to

even minute flaws is easily observed by a skilled inspector.

Because of its sensitivity to such small defects, FPI is ideal for most metals which

tend to have small, tight pores and smooth surfaces. Defects can vary but are

typically tiny cracks caused by processes used to shape and form the metal. It is

not unusual for a part to be inspected several times before it is finished (an

inspection often follows each significant forming operation).





Selection of inspection type is, of course, largely based on the material in

question. FPI is a nondestructive inspection process which means that the part is

not in any way damaged by the test process. Thus, it is of great importance that a

dye and process are selected that ensure the part is not subjected to anything that

may cause damage or staining.

Inspection Steps

See the following main steps in a Fluorescent Penetrant Inspection:

1. Initial Cleaning:

Before the dye 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. Methods such as sand blasting or

chemical etching can be used to rid the surface of undesired contaminates and

ensure good penetration when the dye is applied. Even if the part has already been

through a previous DPI operation it is imperative that it is cleaned again. Most

dyes are not compatible and therefore will thwart any attempt to identify defects

that are already penetrated by any other dye. This process of cleaning is critical

because if the surface of the part is not properly prepared to receive the dye,

defective product may be moved on for further processing. This can cause lost time

and money in reworking, overprocessing, 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. Time varies by material and the size of the flaws

that are intended to be identified but is generally around 30 minutes. It requires

much less time to penetrate larger defects because the dye is able to soak in much

faster. The opposite is true for smaller flaws.





3. Excess Dye Removal:

Penetrant on the outer surface of the material is next removed. This highly

controlled process is necessary in order to ensure that the dye is removed only

from the surface of the material and not from 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 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.

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 defects 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 either shipped, moved on to another process, or deemed defective and

reworked or scrapped. Note that a flawed part may never be cleaned if it is

considered not to be cost effective.

Advantages

Highly sensitive fluorescent penetrant is ideal for even the smallest

imperfections

Little training is needed for the operator/ inspector

Low cost and potentially high volume

Potential Disadvantages

Requires adequate cleaning (neglect of this step can have costly repercussions)

Test materials can be damaged if compatibility is not ensured

Dyes stain clothe and skin and must be treated with care





Penetrant dyes stain cloth, skin and other porous surfaces brought into contact. One

should verify compatibility on the test material, especially when considering the

testing of plastic components. Further information on inspection steps may be

found in industry standards

(e)MAGNETIC PARTICLE INSPECTION

Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for

detecting surface and slightly subsurface discontinuities in ferroelectric

materials such as iron, nickel, cobalt, and some of their alloys. The process puts a

magnetic field into the part. The piece can be magnetized by direct or indirect

magnetization. Direct magnetization occurs when the electric current is passed

through the test object and a magnetic field is formed in the material. Indirect

magnetization occurs when no electric current is passed through the test object, but

a magnetic field is applied from an outside source. The magnetic lines of force are

perpendicular to the direction of the electric current which may be

either alternating current (AC) or some form of direct current (DC) (rectified AC).

The presence of a surface or subsurface discontinuity in the material allows

the magnetic flux to leak. Ferrous iron particles are applied to the part. The

particles may be dry or in a wet suspension. If an area of flux leakage is present the

particles will be attracted to this area. The particles will build up at the area of

leakage and form what is known as an indication. The indication can then be

evaluated to determine what it is, what may have caused it, and what action should

be taken, if any.

Alternating current (AC) is commonly used to detect surface discontinuities.

Using AC to detect subsurface discontinuities is limited due to what is known

as the skin effect, where the current runs along the surface of the part. Because

the current alternates in polarity at 50 to 60 cycles per second it does not

penetrate much past the surface of the test object. This means the magnetic

domains will only be aligned equal to the distance AC current penetration into

the part. The frequency of the alternating current determines how deep the

penetration.

Direct current (DC, full wave DC) is used to detect subsurface discontinuities

where AC can not penetrate deep enough to magnetize the part at the depth

needed. The amount of magnetic penetration depends on the amount of current

through the part.[1] DC is also limited on very large cross-sectional parts how

effective it will magnetize the part.

Half wave DC (HWDC, pulsating DC) work similar to full wave DC, but

allows for detection of surface breaking indications. HWDC is advantageous

for inspection process because it actually helps move the magnetic particles

over the test object so that they have the opportunity to come in contact with

areas of magnetic flux leakage. The increase in particle mobility is caused by

the pulsating current, which vibrates the test piece and particles.





Each method of magnetizing has its pros and cons. AC is generally always best for

discontinuities open to the surface and some form of DC for subsurface.

A wet horizontal MPI machine is the most commonly used mass production

inspection machine. The machine has a head and tail stock where the part is

placed to magnetize it. In between the head and tail stock is typically an

induction coil, which is used to change the orientation of the magnetic field by

90 from head stock. Most of the equipment is customized and built for a

specific application.

Mobile power packs are custom-built magnetizing power supplies used in wire

wrapping applications.

Magnetic yoke is a hand-held devices that induces a magnetic field between

two poles. Common applications are for outdoor use, remote locations,

andweld inspection. The draw back of magnetic yokes is that they only induce

a magnetic field between the poles so large-scale inspections using the device

can be time-consuming. For proper inspection the yoke needs to be rotated 90

degrees for every inspection area to detect horizontal and vertical

discontinuities. Yokes subsurface detection is limited. These systems used dry

magnetic powders, wet powders, or aerosol cans

AC demagnetizing

Pull through AC demagnetizing coils: seen in Fig 3 are AC powered

devices that generate a high magnetic field where the part is slowly

pulled through by hand or on a conveyor. The act of pulling the part

through and away from coil's magnetic field slows drops the magnetic

field in the part. Note many AC demagnetizing coils have power

cycles of several seconds so the part must be passed through the coil

and be several feet (meters) away before the demagnetizing cycle

finishes or the part will have residue magnetism.

AC step down demagnetizing: This is built in only a few MPI

equipment, the process is where the part is subjected to equal or

greater AC current, the current is reduced by X amps in several

sequential pulses till zero current is reached. The number of steps

required to demagnetizing a part is a function of amount current to

magnetize the part.

Reversing DC demagnetizing: The simply reverses the current flow of

magnetizing pulse canceling the magnetic flow. Note: This is built in the

MPI equipment by the manufacturer.

Wet system particle range in size from less than 0.5 to 10 micrometres for

use with water or oil carriers. Particles used in wet systems have pigments

applied that fluoresce at 365 nm (ultraviolet A) requiring 1000 W/cm2 (10

W/m2) at the surface of the part for proper inspection. If the particles do not





have the correct light applied in a darkroom the particles can not be

detected/seen. It is industry practice to use UV goggles/glasses to filter the

UV light and amplify the visible light spectrum (normally green and yellow)

created by the fluorescing particles. Green and yellow fluorescence was

chosen because the human eye reacts best to these colors.

Dry particle powders range in size from 5 to 170 micrometres, designed to

be seen in white light conditions. The particles are not designed to be used

in wet environments. Dry powders are normally applied using hand operated

air powder applicators.

Aerosol applied particles are similar to wet systems, sold in premixed

aerosol cans similar to hair spray.

The following are general steps for inspecting on a wet horizontal machine:

1. Part is cleaned of oil and other contaminants

2. Necessary calculations done to know the amount of current required to magnetize the part. See ASTM E1444-05 for formulas.

3. The magnetizing pulse is applied for 0.5 seconds during which the operator washes the part with the particle, stopping before the magnetic pulse is

completed. Failure to Stop prior to end of the magnetic pulse will wash

away indications.

4. UV light is applied the operator looks for indications of defects that are 0 to +/- 45 degrees from path the current flowed through the part. Defects only

appear that are 45 to 90 degrees the magnetic field. The easiest way to

quickly figure out which way the magnetic field is running is grab the part

with either hand between the head stocks laying your thumb against the part

(do not wrap your thumb around the part) this is called either left or right

thumb rule or right hand grip rule. The direction thumb points tell us the

direction current is flowing, the Magnetic field will be running 90 degrees

from the current path. On complex geometry like an engine crank the

operator needs to visualize the changing direction of the current and

magnetic field created. The current starts at 0 degrees then 45 degrees to 90

degree back to 45 degrees to 0 then -45 to -90 to -45 to 0 and repeats this

for crankpin. So inspection can be time consuming to carefully look for

indications that are only 45 to 90 degrees from the magnetic field.

5. The part is either accepted or rejected based on pre-defined accept and reject criteria

6. The part is demagnetized

7. Depending on requirements the orientation of the magnetic field may need to be changed 90 degrees to inspect for defects that can not be detected

from steps 3 to 5. The most common way is change magnetic field

orientation is to a use Coil Shot. in Fig 1 a 36 inch Coil can be seen then

steps 4, 5, and 6 are repeated





Advantages:

Cannot inspect non-ferrous materials such as aluminum, magnesium or most

stainless steels.

Inspection of large parts may require use of equipment with special power

requirements.

Some parts may require removal of coating or plating to achieve desired

inspection sensitivity.

Limited subsurface discontinuity detection capabilities. Maximum depth

sensitivity is approximately 0.6 (under ideal conditions).

Post cleaning, and post demagnetization is often necessary.

Alignment between magnetic flux and defect is important

15 questions

Documents

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shear stress

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principles of measurements