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Non Destructive Testing Techniques

Introduction..............................................................................................................2

List of NDT techniques............................................................................................3

Radiography.............................................................................................................4

Ultrasonic Inspection...............................................................................................4

Manual Pulse-Echo Ultrasound .............................................................................5

TODF Ultrasound....................................................................................................6

Acoustic Emission Technology ...............................................................................7

Acousto-Ultrasonics.................................................................................................7

Dye Penetrant testing ..............................................................................................8

In-Situ Metallography.............................................................................................8

Electromagnetics......................................................................................................9

Fluorescent Particle testing...................................................................................10

Magnetic Particle Testing .....................................................................................10

Portable Hardness .................................................................................................11

Replication..............................................................................................................11

Metal etch ...............................................................................................................12

Blue Etch Anodizing..............................................................................................12

Transient Thermography......................................................................................12

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Introduction

NDT (Non-destructive testing ) includes those test methods used to examine an object, material or

system without impairing its future usefulness.

Non-destructive testing methods or technology used by providers of non-destructive testing

services include acoustic emission, beta gauge, eddy current or electromagnetic, magnetic

(induction / Barkhausen), magnetic particle system, optical (shearography / holography / magneto

optical), Penetrant testing systems, radiographic / X-ray imaging, ultrasonic, and other specialized

techniques. Depending on the method and measurement requirements, non-destructive testing can

be used off-line in laboratory environments as well as in continuous production line or field

monitoring applications.

Acoustic emission measures the specific acoustic or vibration response of flaws or features within a

mechanical system. If a break, deformation or other failure occurs in a piece of equipment,

sensitive acoustic emission sensors can detect the high frequency burst given off during the event.

In a Beta gauge non-destructive testing, the absorption of Beta particles is used to measure the

thickness of materials or coatings.

Eddy current, penetrating radar and other electromagnetic techniques are used to detect or measureflaws, bond or weld integrity, thickness, electrical conductivity, detect the presence of rebar or

metals.

The eddy current method is the most widely applied electromagnetic NDT technique.

Several different magnetic techniques are used in non destructive testing including Hall effect and

induction.

In a magnetic particle non-destructive testing system current flow or an external magnet magnetizes

the part. Magnetic poles created at flaws, cracks or other discontinuities attract magnetic particles.

Optical based non-destructive testing instruments using methods such as laser shearography,

magneto-optical, holographic interferometry or other optical techniques to detect flaws, residualstress or measure thickness.

In penetrant testing, penetrant is applied to the part by spray or immersion. The penetrant is pulled

into surface flaws by capillary action.

Radiographic or x-ray equipment uses penetrating X-rays or gamma rays to capture images of the

internal structure or a part or finished product. The density and composition of the internal

features will alter the intensity or density of these features in the X-ray image.

Ultrasonic (UT) inspection techniques are used to detect surface and subsurface flaws or to measure

thickness. Choices for form factor include bench or rack or cabinet, computer board, portable orhandheld or mobile, and monitoring system. Beams of high frequency acoustic energy are

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Tandem

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introduced into the material and subsequently retrieved.

Areas of application for non-destructive testing (NDT) services include aerial or crane, automotive,

aviation or aerospace, coatings or platings, piping or pressure vessels, welding or fabrication, and

structural or construction.

List of NDT techniques

Radiography

Compton Backscatter

Gamma Ray Film

Gamma Ray Real TimeNeutron

Neutron Backscatter

X-Ray Film

X-Ray Real-Time

X-Ray Tomography

Ultrasound

Automated Pulse-Echo

Chime

C-Scan Imaging

Lamb Waves

Manual Pulse-EchoPitch-Catch

Remote Access

Self Tandem

Surface Waves

Thickness Gauge

TOFD

Eddy Current

AC Potential Drop

Conventional

Low FrequencyMulti Sensor Coil

Pulsed

Remote Field

Penetrant Inspection

Automated

Manual

Thermography

Passive

Transient

Visual Inspection

Closed Circuit TV

Endoscopy

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Unaided

Magnetic

Flux Leakage

Particle Inspection

Squid Flux Leakage

Stress Measurement

Acoustic Emission

Crack Detection

Impact Testing

Leak Detection

Optical

HolographyInterferometry

Profilometry

Shearography

Triangulation

Vibration Interferometry

Radiography

Radiography is the creation of radiographs, made by exposing a photographic film or plate to X-

rays. Since X-rays penetrate solid objects, but are slightly attenuated by them, the picture resulting

from the exposure reveals the internal structure of the object. The most common use of radiography

is in the medical field (where it is known as medical imaging, but veterinarians and engineersalsouse radiography.

Radiography is a non-destructive method of inspecting materials for hidden flaws by utilising the

ability of electromagnetic radiation of short wavelength to penetrate various materials. The value of 

this ability lays in the fact the material to a degree dependent upon its composition and thickness

absorbs penetrating radiation. 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 wave length less about 10 Angstromunits (10 to the power -

10 m).

The type of electromagnetic radiation of most interest to radiography is gamma radiation 

Tthis radiation is much more energetic than the more familiar types such as radio waves and visiblelight. It is this relatively high energy, which makes gamma rays useful in radiography and potential

hazards in radiation protection 

They are produced by X-ray tubes, high energy X-ray equipment, and natural radioactive elements,

such as Radium and Radon and artificially produced radioactive isotopes of elements, such as

Cobalt192.

Ultrasonic Inspection

Ultrasonic methods of NDT use beams of sound waves (vibrations) of short wavelength and high

frequency, transmitted from a probe and detected by the same or other probes. Usually, pulsed

beams of ultrasound are used and in the simplest instruments a single probe, hand held, is placed on

the specimen surface.

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With manual pulse-echo ultrasonic inspection, various standard steel blocks are used to calibrate

the probe and the flaw detector. The blocks contain side-drilled holes or curved surfaces at

precisely known ranges. With these blocks, the operator can measure:

• The probe beam angle in steel

• The probe index point (point on the probe from which the ultrasound beam appears to

radiate)

• The relation between signal arrival time and target range ("time base expansion")

• Sensitivity of the equipment, so that "reporting levels" can be applied to an inspection

(usually expressed in terms of screen height in relation to the signal from a side-drilled hole

at the same range)

•

DAC curve which represents the amplitude of the standard calibration reflector (e.g. side-drilled hole) at different ranges from the probe.

Various different ultrasonic probes are used in manual ultrasonic testing. The main probe types are

as follows:

• 0° compression-wave probe: for measuring wall thickness, and detecting pores, inclusion

and shrinkage cracks, and horizontal laminations in plates

• 45° shear-wave probe: for many applications including inspection for root cracks ("corner-

trap" signal) and volumetric defects in welds.

• 60° shear-wave probe: for detection of fusion face defects in welds

• 70° shear-wave probe: for detection of fusion face defects in welds and through-wall cracksin the weld material.

• 70° compression-wave probe: for detection of front-wall braking cracks

The manual pulse-echo technique can be used both for defect detection and defect sizing. Sizing

methods include those based on probe movement – the so-called 6 dB and 20 dB drop methods. and

also the so-called Max Amp method.

However, if accurate defect sizing is needed another ultrasonic method is often preferred.

TODF Ultrasound

In this ultrasonic method, two probes are used either side of the flaw, and the time of arrival of pulses diffracted from the extremities of the flaw are used to detect and size it.

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Principle of the time-of-flight diffraction (TOFD) technique. 

Generally the probe beam widths are very broad in TOFD, so that a wide range of depths can be

covered with a single probe separation, although in practice a number of different

transmitter/receiver probe separations are needed to cover the full range of depths from the

inspection surface to the backwall.

Signals indicating the front surface (known as the "lateral" wave) and the backwall are also

generally present, and can be used to calibrate the measured arrival times of the signals from

defects.

Unlike pulse-echo methods, in TOFD there is a non-linear relation between signal arrival time, t,

and defect depth, d:

t = (S

2

+ 4d

2

)

1/2

 /cwhere S is the probe separation and c is the ultrasound velocity.

TOFD inspection is generally carried out in the form of line scans of the two probes, made with

constant separation. A computer-based ultrasonic recording and imaging system is used to digitise

and store the unrectified RF waveform data, which is then displayed in B-scan or D-scan format.

Acoustic Emission Technology

Acoustic Emission (AE) testing is a powerful method for examining the behavior of materials

deforming under stress. Acoustic Emission may be defined as a transient elastic wave generated by

the rapid release of energy within a material. Materials "talk" when they are in trouble: with

Acoustic Emission equipment you can "listen" to the sounds of cracks growing, fibers breaking andmany other modes of active damage in the stressed material.

Acousto-Ultrasonics

The AU technology consists of sending low frequency acoustic pulses at a predetermined angle of 

incidence into a material under inspection. These acoustic pulses travel through the material and are

reflected by the different interfaces inside the sample. If a discontinuity (delamination, debond etc.)

is present inside the material, the reflected acoustic energy changes,

revealing the presence of the discontinuity.

The AU technology can be used in the inspection of critical composite

structures.In order to determine the optimal inspection parameters for a particular

composite structure (incidence angle, frequency, and pulse length), a

wave propagation model for multi-layered structures is used. This model

is based on a plane wave propagation model using the Thomson-Haskell

transfer matrix for multilayered media. The characteristics of the

composite material such as layout and material properties are used as

input data to the model.

The system can be used with traditional wedge sensors or adapted for use with rolling sensors,

which eliminate the need for extra coupling between the sensor and the piece under inspection.

The system can also be used with a free-motion, wireless position tracking manual scanner.

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Duplex stainless steel weld Pitting and stress corrosion cracking in a weldedstainless steel tube

Electromagnetics

The research and development of electromagnetic (EM) technology is related to the EM induction

of fields in a conducting material to generate an eddy current or an acoustic emission (AE) for

nondestructive testing. While the EM induced eddy current is a mature technology, the EM induced

AE is a newer concept developed to solve the problem of detection and characterization of metal

casting with complex structures, weldments, and weld repair defects.

It is known that an electrical current passing through a plate of material containing a defect (such as

porosity, cracks, or inclusions) is concentrated at the tips of the defect. This effect of electric field

distortion by a defect or flaw can be used for Nondestructive Evaluation (NDE) of the structure.The induced high current density in conducting materials containing flaws (electric field

concentrators) will load the defect faces with

electromagnetic forces, which will tend to open

them (due to current flowing in opposite

directions along the defect faces). In addition,

the tips of the defect act like single turn s

coils, which concentrate the magnetic flux and

produce localized Lorentz forces along with

local thermal stresses from Joule heating. The

effective current density at the defect tips may

be higher than in all of the surrounding materby one order of magnitude (depending on stress

or field intensity factor). If the current density is

high enough, the generated heat may be

sufficient to melt the material at the defect tips

a phenomenon that has also been used to arrest

fast-growing cracks. However, if the effectiv

current density has the right value, the effect will be the generation of AE without an increase o

the flaw size. Finite element analysis can predict the values of the electric current density arou

defect embedded in a metallic sample, as shown in Figure 1.

olenoid

ial

,

e

n

nd a

The EM induced AE may also be enhanced by the application of an external magnetic field normal

to the defect plane. The presence of this field increases the Lorentz stresses at the defect tips by the

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creation of an additional interaction between the dynamic electric current and a static magnetic

field. This mechanism of coupling the electrical and acoustic energy is similar to the one used in to

electromagnetic-acoustic transducers (EMAT).

By using the EM stimulation method to produce AE, specific areas may be inspected without

loading the whole structure since EM induction produces local loading. Structures may be tested in-

situ and there is no need for disassembly and fabrication of special test fixtures used to apply

mechanical loading. EM induction loading may also permit AE "inspection on demand ", which

could reduce the need for long term AE monitoring.

Fluorescent Particle testing

Fluorescent Particle Inspection methods (a.k.a., FPI and PT) are typically used to detect cracks,micro-shrinkage, or other discontinuities that are open or otherwise connected to the surface of a

part being inspected.

In general, FPI can be applied at any point during the manufacture and/or in-service use of an

applicable part. Applicable parts are those made of ferromagnetic or non-ferromagnetic material,

including some plastics and ceramics. FPI cannot be used on porous materials or parts with

interfering coatings or contaminants. Surface defects as small as .015" (.38mm) can be reliably

detected with FPI.

In aerospace applications for example, FPI is commonly used on aluminum and magnesium alloys,

stainless steel, brass, and copper as well as graphite-epoxy composite materials.

Magnetic Particle TestingMagnetic Particle Inspection processes are used for the detection of defects in Ferrous materials,

the detection of defects by magnetism depends on the magnetic susceptibility of a fault is clearly

poorer to that of the surrounding material of the specimen. Basically, the magnetic resistance of the

defect is greater than the sound material.

The flaw acts in the same way as a discontinuity in the path of the magnetic fluctuation caused to

flow in the specimen in such a direction it crosses the main plane of the defect at right –angles,

causing the flux to bend around the defect in an alternative direction in the material which

surrounds it. This deformation of the flux is not limited to the immediate locality of the defect but

extends, in a deteriorating degree, for a considerable distance around it, and out through the surface

into the surrounding air if the intensity of the magnetism is high enough.

It is the external field effect, which makes Magnetic Particle Inspection possible. The simplest of these, and the main one, which is generally accepted, is the method, which finely divided iron or

magnetic iron oxide, held in suspension in kerosene or other suitable liquid is applied to the

specimen. (This ‘Ink’ is usually black in colour and a White Lacquer background should be applied

to ensure any defects can be observed. Also the method using a ‘Black Light’ and Fluorescent Ink 

can used)

The ‘Ink’, which is the term used for the above, can be sprayed or painted over the magnetised

specimen. The magnetic particles are attracted by the surface field in the area of the defect and hold

on to the edges of the defect to reveal it as a build up of particles.

The Magnetic Particle Inspection method of Non-Destructive testing is a method for locating

surface and sub-surface discontinuities in ferromagnetic material. It depends for its operation on the

face that when the material or part under test is magnetized, discontinuities that lie in a direction

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generally transverse to the direction of the magnetic field, will cause a leakage field, and therefore,

the presence of the discontinuity, is detected by use of finely divided ferromagnetic particles

applied over the surface, some of these particles being gathered and held by the leakage field, this

magnetically held collection of particles forms an outline of the discontinuity and indicates its

location, size, shape and extent.

The typical use of magnetic particle inspection methods (a.k.a., MPI and MT) are to detect cracks,

inclusions, seams, laps, and other discontinuities at or near the surface of ferromagnetic materials.

Typical materials that fall into this category include "plain carbon steels" (i.e., ferrite matrix phase -

those containing < .8% carbon), low grade stainless steel, cast iron, etc. [Note that a quick and easy

test to determine the suitability of MPI is to place a magnet on the part. If the part is not magnetic,

MPI is not an applicable test method.]

In general, MPI can be applied at any point during the manufacture and/or in-service use of aferromagnetic part. MPI cannot be used on non-ferromagnetic materials or on parts with interfering

coatings. Surface defects as small as .015" (.38mm) can be reliably detected with MPI.

Visual Inspection (NDT technique)

Visual inspection also known as VT, commonly refers to inspecting a part or material without the

use of magnification (i.e., 1X power). VT can be applied alone or in conjunction with other NDT

methods.

As a stand alone, VT is done to detect macro manufacturing defects or damage. In conjunction with

other NDT methods, it is essentially applied at every stage involving the human eye (i.e., reading

instrument screens, printouts, images, examining surfaces after applying etch, FPI, and/or MPI.

Non-Destructive visual inspections can be preformed on-site or at the laboratory facility, and arebased upon the requirements of the client or specification. Industries utilizing this service include

Fabrication, Construction, Automotive, Power Generation and Transportation. Inspections can be

performed at the laboratory facility or onsite. These inspections are performed to IS, BS, ASTM,

AWS, ASME (American Society for Mechanical Engineers) and many others.

Portable Hardness

Per ASTM E110, this testing is normally used for on-site applications or on very large samples.

The TCR portable hardness unit performs the hardness testing by applying a 5 kg. Vickers load

indenter and electronically converts the values in the preferred scale.

Replication

Replication is a follow-on inspection technique to assist in further evaluating surface indications. It

involves polishing an indication, application and development of a wetted film, and subsequent

visual examination under high magnification. Replication essentially transfers the part's surface

topography to the film for examination under a microscope.

It is commonly used as a follow-up to Blue Etch Anodize (BEA) on titanium to enhance a material

segregation indication. It can also be used after any etch process.

In general, replication as an NDT method can be applied at any point during the manufacture

and/or in-service use of an applicable part. Replication can be applied to nearly any material.

Surface defects as small as grains can be reliably detected with replication and subsequent

magnification.

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Metal etch

Metal etch methods, both chemical and anodic, are typically used to detect grain structure, grain

segregation, cracks, micro-shrinkage, or other discontinuities that are open or otherwise connected

to the surface of a part being inspected. In general, etching as an NDT method can be applied at any

point during the manufacture and/or in-service use of an applicable part. Surface defects as small as

.015" (.38mm) can be reliably detected with metal etch methods.

In aerospace applications for example, etching is commonly used on aluminum, carbon steel,

stainless steel, nickel and titanium.

Blue Etch Anodizing

Blue Etch Anodizing (BEA) is a highly sensitive nondestructive testing (NDT) technique to detectsurface discontinuities such as laps, cracks, material segregations, heat-treating imperfections, and

abnormalities caused by machining. Under normal conditions, surface defects as small as .015" can

be reliably detected.

BEA is specifically used to detect discontinuities in titanium materials such as those involved in the

manufacture of critical rotating parts for the aircraft and power generation industries. One of the

most critical applications of BEA is to detect material segregations known as "Hard Alpha

Inclusions" (HAI), and/or High Aluminum Defects (HAD's).

Transient Thermography

Transient thermography involves the rapid application of a short ‘pulse’ of heat to the component.The temperature of the component’s surface is then monitored for a period of time after the

application of the transient heat source. The heat source can be applied to the same surface as that

monitored by the infra-red camera (single sided or reflective method) or the far component surface

(double sided or transmission method).

Flaws in the component disrupt the flow of heat caused by the transient heat source, and cause

corresponding hot or cold spots on the adjacent component surface.

Principle of reflective (single-sided) transient thermography

For poor thermal conductors such as composite materials (e.g. CFRP, GRP), the time-scales overwhich the defects appear after application of the heat pulse are generally several seconds, or even a

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few minutes. For good conductors, such as metals, the timescales are often much shorter, typically

a second or less.

In general, the timescales for single-sided (reflective) thermography are less than those for double-

sided (transmission) thermography.

The rate of heating should be short compared to the response time of the material, which may be

calculated.

Because heat diffuses sideways as well a through the specimen, the worst spatial resolution of the

method will be of the same order as the specimen thickness. Flaws at different depths will have

different resolutions however, and the resolution will be optimum when the flaw is close to the

surface being scanned.

Small-scale damage is detectable long before failure, so AE can be used as a non-destructive

technique to find defects during structural proof tests and plant operation. AE also offers unique

capabilities for materials research and development in the laboratory. Finally, AE equipment is

adaptable to many forms of production QC testing, including weld monitoring and leak detection.

Some typical applications of the Acoustic Emission principle in testing materials are as follows:

Behavior of materials: metals, ceramics, composites, rocks, concrete: 

• Crack propagation

• Yielding

• Fatigue

• Corrosion, Stress corrosion

• Creep

• Fiber fracture, delamination

Nondestructive testing during manufacturing processes: 

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• Material processing

• Phase transformation in metals and alloys (martensitic transformation)

• Detection of defects such as pores, quenching cracks, inclusions, etc.

• Fabrication

• Deforming processes rolling, forging, extruding

• Welding and brazing detects detection (inclusions, cracks, lack of penetration)

• TIG, MIG, spot, electron beam, etc.

• Weld monitoring for process control

Monitoring structures: 

•

Continuous monitoring (metallic structures, mines, etc.)• Periodic testing (pressure vessels, pipelines, bridges, cables)

• Loose Part Detection

• Leak Detection

Special applications: 

• Petrochemical and chemical: storage tanks, reactor vessels, offshore platforms, drill pipe,

pipelines, valves, hydro-treaters

• Electric utilities: nuclear reactor vessels, piping, steam generators, ceramic insulators,

transformers, aerial devices

• Aircraft and aerospace: fatigue cracks, corrosion, composite structures, etc.• Electronics: loose particles in electronic components, bonding, substrate cracking.

Credits to:

- Nicholas J. Carino 

- Metal Testing Company 

- Physical Acoustics Corporation