Non destructive testing

 NDT:
Non-destructive testing or Non-destructive testing (NDT) is a wide group of
analysis techniques used in science and industry to evaluate the properties of a
material, component or system without causing damage.
 TYPES OF NDT METHODS:
there is no. of NDT method used to inspect the work piece.
 Acoustic emission testing (AE or AT)
 Blue Etch Anodize (BEA)
 Dye penetrant inspection or Liquid penetrant Testing (PT or LPI)
 Electromagnetic testing (ET) or Electromagnetic Inspection (commonly known as
"EMI")
 Alternating current field measurement (ACFM)
 Alternating current potential drop measurement (ACPD)
 Barkhausen testing
 Direct current potential drop measurement (DCPD)
 Eddy-current testing (ECT)
 Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope
 Magnetic-particle inspection (MT or MPI)
 Magneto vision
 Remote field testing (RFT)
 Ellipsometry
 Endoscope inspection
 Guided wave testing (GWT)
 Hardness testing
 Impulse excitation technique (IET)
 Terahertz non-destructive evaluation(THz)
 Infrared and thermal testing (IR)
 Thermographic inspection
 Infrared thermal microscopy
 Laser testing

 Electronic speckle pattern interferometry
 Holographic interferometry
 Low coherence interferometry
 Profilometry
 Stereography
 Leak testing (LT) or Leak detection
 Absolute pressure leak testing (pressure change)
 Bubble testing
 Halogen diode leak testing
 Hydrogen leak testing
 Mass spectrometer leak testing
 Tracer-gas leak testing method Helium, Hydrogen and refrigerant gases
 Magnetic resonance imaging (MRI) and NMR spectroscopy
 Metallographic replicas [12][13]
 Near-infrared spectroscopy (NIRS)
 Optical microscopy
 Positive Material Identification (PMI)
 Radiographic testing (RT) (see also Industrial radiography and Radiography)
 Computed radiography
 Digital radiography (real-time)
 Neutron Imaging
 SCAR (Small Controlled Area Radiography)
 X-ray computed tomography (CT)
 Resonant Inspection
 Resonant Acoustic Method (RAM) [14]
 Scanning electron microscopy
 Surface Temper Etch (Nital Etch)
 Ultrasonic testing (UT)
 ART (Acoustic Resonance Technology)
 Angle beam testing
 Electro Magnetic Acoustic Transducer (EMAT) (non-contact)
 Laser ultrasonic (LUT)
 Internal rotary inspection system (IRIS) ultrasonic for tubes
 Phased array ultrasonic
 Thickness measurement
 Time of flight diffraction ultrasonic (TOFD)
 Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)
 Vibration Analysis

 Visual inspection (VT)
 Pipeline video inspection
 Weight and load testing of structures
 Corroscan/C-scan
 3D Computed Tomography
 Industrial CT scanning
 Heat Exchanger Life Assessment System
 RTJ Flange Special Ultrasonic Testing
 COMMANLY USED NDT METHOD:

This are the widely used method of NDT to inspect the W/P.
1. LPT (liquid penetration test)
2. MPT (magnetic particle test)
3. UT (ultrasonic testing)
4. ED (eddy current testing)
5. RT (radiography testing)
- X-ray
- Gamma ray

1.LPT (liquid penetration test):
Dye penetrant inspection (DPI), also called liquid penetrant
inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost
inspection method used to locate surface-breaking defects in all non-
porous materials (metals, plastics, or ceramics). The penetrant may be applied to all
non-ferrous materials and ferrous materials, although for ferrous
components magnetic-particle inspection is often used instead for its subsurface
detection capability. LPI is used to detect casting, forging and welding surface
defects such as hairline cracks, surface porosity, leaks in new products, and fatigue
cracks on in-service components.
Principles:
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
component by 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 non-fluorescent (visible).
Inspection steps:
Below are the main steps of Liquid Penetrant Inspection:
1. Pre-cleaning:
The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale
that could either keep penetrant out of a defect, or cause irrelevant or false
indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor
degreasing, or media blasting. The end goal of this step is a clean surface where any
defects present are open to the surface, dry, and free of contamination. Note that if

media blasting is used, it may "work over" small discontinuities in the part, and an
etching bath is recommended as a post-blasting treatment.
2. Application of Penetrant:
The penetrant is then applied to the surface of the item being tested. The penetrant
is allowed "dwell time" to soak into any flaws (generally 5 to 30 minutes). The dwell
time mainly depends upon the penetrant being used, material being tested and the
size of flaws sought. As expected, smaller flaws require a longer penetration time.
Due to their incompatible nature one must be careful not to apply solvent-based
penetrant to a surface which is to be inspected with a water-washable penetrant.
3. Excess Penetrant Removal:
The excess penetrant is then removed from the surface. The removal method is
controlled by the type of penetrant used. Water-washable, solvent-
removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the
common choices. Emulsifiers represent the highest sensitivity level, and chemically
interact with the oily penetrant to make it removable with a water spray. When using
solvent remover and lint-free cloth it is important to not spray the solvent on the test
surface directly, because this can remove the penetrant from the flaws. If excess
penetrant is not properly removed, once the developer is applied, it may leave a
background in the developed area that can mask indications or defects. In addition,
this may also produce false indications severely hindering your ability to do a proper
inspection. Also, the removal of excessive penetrant is done towards one direction
either vertically or horizontally as the case may be.
4. Application of Developer:
After excess penetrant has been removed, a white developer is applied to the
sample. Several developer types are available, including: non-aqueous wet
developer, dry powder, water-suspendable, and water-soluble. Choice of developer
is governed by penetrant compatibility (one can't use water-soluble or -suspendable
developer with water-washable penetrant), and by inspection conditions. When using
non-aqueous wet developer (NAWD) or dry powder, the sample must be dried prior
to application, while soluble and suspendable developers are applied with the part
still wet from the previous step. NAWD is commercially available in aerosol spray
cans, and may employ acetone, isopropyl alcohol, or a propellant that is a
combination of the two. Developer should form a semi-transparent, even coating on
the surface.
The developer draws penetrant from defects out onto the surface to form a visible
indication, commonly known as bleed-out. Any areas that bleed out can indicate the
location, orientation and possible types of defects on the surface. Interpreting the
results and characterizing defects from the indications found may require some
training and/or experience [the indication size is not the actual size of the defect].

5. Inspection:
The inspector will use visible light with adequate intensity (100 foot-candles or
1100 lux is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate
intensity (1,000 micro-watts per centimetre squared is common), along with low
ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations.
Inspection of the test surface should take place after 10- to 30-minute development
time, depends of product kind. This time delay allows the blotting action to occur.
The inspector may observe the sample for indication formation when using visible
dye. It is also good practice to observe indications as they form because the
characteristics of the bleed out are a significant part of interpretation characterization
of flaws.
6. Post Cleaning:
The test surface is often cleaned after inspection and recording of defects, especially
if post-inspection coating processes are scheduled.
Advantages and disadvantages:
The main advantages of DPI are the speed of the test and the low cost.
Disadvantages include the detection of only surface flaws, skin irritation, and the
inspection should be on a smooth clean surface where excessive penetrant can be
removed prior to being developed. Conducting the test on rough surfaces, such-as
"as-welded" welds, will make it difficult to remove any excessive penetrant and could
result in false indications. Water-washable penetrant should be considered here if no
other option is available. Also, on certain surfaces a great enough colour contrast
cannot be achieved or the dye will stain the work piece.
Limited training is required for the operator — although experience is quite
valuable. Proper cleaning is necessary to assure that surface contaminants have
been removed and any defects present are clean and dry. Some cleaning methods
have been shown to be detrimental to test sensitivity, so acid etching to remove
metal smearing and re-open the defect may be necessary.

2. MPT (magnetic particle testing)
Magnetic particle Inspection (MPI) is a non-destructive testing (NDT)
process for detecting surface and slightly subsurface discontinuities in ferromagnetic
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, since air cannot support as much magnetic field per
unit volume as metals. Ferrous particles are then 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.
Inspection
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.
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 while the operator looks for indications of defects that are 0
to ±45 degrees from path the current flowed through the part. Indications only
appear 45 to 90 degrees of the magnetic field applied. 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 the 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 a crankshaft, 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 degrees back to 45 degrees
to 0 then -45 to -90 to -45 to 0 and this is repeated for each crankpin. Thus, it
can be time consuming to find indications that are only 45 to 90 degrees from
the magnetic field.
5. The part is either accepted or rejected, based on pre-defined 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 indications that cannot be detected
from steps 3 to 5. The most common way to change magnetic field orientation
is to use a "coil shot". In Fig 1 a 36-inch coil can be seen then steps 4, 5, and
6 are repeated.

3.UT (Ultrasonic testing):
Ultrasonic testing (UT) is a family of non-destructive testing techniques
based on the propagation of ultrasonic waves in the object or material tested. In
most common UT applications, very short ultrasonic pulse-waves with centre
frequencies ranging from 0.1-15 MHz, and occasionally up to 50 MHz, are
transmitted 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
used in many industries including steel and aluminium construction, metallurgy,
manufacturing, aerospace, automotive and other transportation sectors.
How it works
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. However, when ultrasonic testing is conducted with
an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.
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.
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. In many cases only one surface needs to be accessible.
4. Greater accuracy than other non-destructive 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. Some capability of estimating the structure of alloys of components with
different acoustic properties
7. Non-hazardous to operations or to nearby personnel and has no effect on
equipment and materials in the vicinity.
8. Capable of portable or highly automated operation.
9. Results are immediate. Hence on the spot decisions can be made.
Disadvantages
1. Manual operation requires careful attention by experienced technicians. The
transducers alert to both normal structure of some materials, tolerable
anomalies of other specimens (both termed “noise”) and to faults therein
severe enough to compromise specimen integrity. These signals must be
distinguished by a skilled technician, possibly requiring follow up with other
non-destructive testing methods.
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.

4.ET (eddy current testing):
Eddy-current testing (also commonly seen as eddy current
testing and ECT) is one of many electromagnetic testing methods used in non-
destructive (NDT) making use of electromagnetic induction to detect and
characterize surface and sub-surface flaws in conductive materials.
ECT principle
In its most basic form — the single-element ECT probe — a coil of conductive
wire is excited with an alternating electrical current. This wire coil produces an
alternating magnetic field around itself in the direction ascertained by the right-hand
rule. The magnetic field oscillates at the same frequency as the current running
through the coil. When the coil approaches a conductive material, currents opposed
to the ones in the coil are induced in the material — eddy currents.
Variations in the electrical conductivity and magnetic permeability of the test
object, and the presence of defects causes a change in eddy current and a
corresponding change in phase and amplitude that can be detected by measuring
the impedance changes in the coil, which is a tell-tale sign of the presence of
defects. This is the basis of standard (pancake coil) ECT.
ECT has a very wide range of applications. Because ECT is electrical in
nature, it is limited to conductive material. There are also physical limits to
generating eddy currents and depth of penetration (skin depth).

Applications
The two major applications of eddy current testing are surface inspection and
tubing inspections. Surface inspection is used extensively in the aerospace industry,
but also in the petrochemical. The technique is very sensitive and can detect tight
cracks. Surface inspection can be performed both on ferromagnetic and non-
ferromagnetic materials.
Tubing inspection is generally limited to non-ferromagnetic tubing and is
known as conventional eddy current testing. Conventional ECT is used for inspecting
steam generator tubing in nuclear plants and heat exchangers tubing in power and
petrochemical industries. The technique is very sensitive to detect and size pits. Wall
loss or corrosion can be detected but sizing is not accurate.
A variation of conventional ECT for partially magnetic materials is full
saturation ECT. In this technique, permeability variations are suppressed by applying
a magnetic field. The saturation probes contain conventional eddy current coils and
magnets. This inspection is used on partially ferromagnetic materials such as nickel
alloys, duplex alloys, and thin-ferromagnetic materials such as ferritic chromium
molybdenum stainless steel. The application of a saturation eddy current technique
depends on the permeability of the material, tube thickness, and diameter.
A method used for carbon steel tubing is remote field eddy current testing.
This method is sensitive to general wall loss and not sensitive to small pits and
cracks.

5.RT (Radiographic testing).
Radiographic Testing (RT), or industrial radiography, is a non-
destructive (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 tomography machine 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. Most industries are moving from film based
radiography to a digital sensor based radiography much the same way that
traditional photography has made this move.
Inspection of welds
The beam of radiation must be directed to the middle of the section under
examination and must be normal to the material surface at that point, except in
special techniques where known defects are best revealed by a different alignment
of the beam. The length of weld under examination for each exposure shall be such
that the thickness of the material at the diagnostic extremities, measured in the
direction of the incident beam, does exceed the actual thickness at that point by
more than 6%. The specimen to be inspected is placed between the source of
radiation and the detecting device, usually the film in a light tight holder or cassette,
and the radiation is allowed to penetrate the part for the required length of time to be
adequately recorded.
The result is a two-dimensional projection of the part onto the film, producing
a latent image of varying densities according to the amount of radiation reaching
each area. It is known as a radio graph, as distinct from a photograph produced by
light. Because film is cumulative in its response (the exposure increasing as it
absorbs more radiation), relatively weak radiation can be detected by prolonging the

exposure until the film can record an image that will be visible after development.
The radiograph is examined as a negative, without printing as a positive as in
photography. This is because, in printing, some of the detail is always lost and no
useful purpose is served.
Before commencing a radiographic examination, it is always advisable to
examine the component with one's own eyes, to eliminate any possible external
defects. If the surface of a weld is too irregular, it may be desirable to grind it to
obtain a smooth finish, but this is likely to be limited to those cases in which the
surface irregularities (which will be visible on the radio graph) may make detecting
internal defects difficult.
After this visual examination, the operator will have a clear idea of the
possibilities of access to the two faces of the weld, which is important both for the
setting up of the equipment and for the choice of the most appropriate technique.
Defects such as delamination and planar cracks are difficult to detect using
radiography, particularly to the untrained eye.
Without overlooking the negatives of radiographic inspection, Radiography
does hold many significant benefits over ultrasonic, particularly insomuch that as a
'picture' is produced, more accurate identification of the defect can be made, and by
more interpreters. Very important as most construction standards permit some level
of defect acceptance, depending on the type and size of the defect.
To the trained Radiographer, subtle variations in visible film density provide
the technician the ability to not only accurately locate a defect, but identify its type,
size and location; an interpretation that can be physically reviewed and confirmed by
others, possibly eliminating the need for expensive and unnecessary repairs.
Safety
Industrial radiography appears to have one of the worst safety profiles of the
radiation professions. Possibly because there are many operators using
strong gamma sources (> 2 Ci) in remote sites with little supervision when
compared. With workers within the nuclear industry or within hospitals.

Non destructive testing

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Non destructive testing