CONCEPTS OF NDT

CONCEPTS OF NDT UNIT - I
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UNIT – I
CONCEPTS OF NDT
(Relative merits and limitations of NDT Vs. Conventional testing –Visual
inspection, thermal inspection methods. Liquid penetrate Inspection)

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Nondestructive evaluation (NDE) is a term that is often used
interchangeably with NDT. However, technically, NDE is used to describe
measurements that are quantitative in nature. NDE may be used to
determine material properties, such as fracture toughness, formability,
and other physical characteristics.
1.0 INTRODUCTION:
There are various testing methods those somehow destruct the test
specimens. These were, tensile testing, hardness testing, etc. In certain
applications, the evaluation of engineering materials or structures without
impairing their properties is very important, such as the quality control of
the products, failure analysis or prevention of the engineered systems in
service.
This kind of evaluations can be carried out with Non destructive test
(NDT) methods. It is possible to inspect and/or measure the materials or
structures without destroying their surface texture, product integrity and
future usefulness.
The field of NDT is a very broad, interdisciplinary field that plays a
critical role in inspecting that structural component and systems perform
their function in a reliable fashion. Certain standards has been also
implemented to assure the reliability of the NDT tests and prevent certain
errors due to either the fault in the equipment used, the miss-application
of the methods or the skill and the knowledge of the inspectors.
Successful NDT tests allow locating and characterizing material
conditions and flaws that might otherwise cause planes to crash, reactors
to fail, trains to derail, pipelines to burst, and variety of less visible, but

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equally troubling events. However, these techniques generally require
considerable operator skill and interpreting test results accurately may be
difficult because the results can be subjective.
These methods can be performed on metals, plastics, ceramics,
composites, cermets, and coatings in order to detect cracks, internal
voids, surface cavities, delamination, incomplete c defective welds and
any type of flaw that could lead to premature failure.
1.0.1 CONVENTIONAL TESTING (DESTRUCTIVE TESTING):
Destructive testing are the tests, which are carried out to the
specimens failure. Destructive testing includes methods where material is
broken down to evaluate the mechanical properties, such as strength,
toughness and hardness. Destructive tests in turn indicate how and when
the objects are in danger of breaking down or failing.
For example, finding the quality of a weld is good enough to
withstand extreme pressure and to verify the properties of a material.
1.0.2 NON-DESTRUCTIVE TESTING (NDT):
Nondestructive testing (NDT) is the process of inspecting, testing,
or evaluating materials, components or assemblies for discontinuities, or
differences in characteristics without destroying the serviceability of the
part or system. In addition, when the inspection or test is completed the
part can still be used.
The use of noninvasive techniques to determine the integrity of a
material, component or structure or quantitatively measure some
characteristic of an object. i.e. Inspect or measure without doing harm.

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1.1 RELATIVE MERITS AND LIMITATIONS OF NDT VS.
CONVENTIONAL TESTING
1.1.1 MERITS OF CONVENTIONAL TESTING
(DESTRUCTIVE TESTING):
 Provides direct and reliable measurements
 Quantitative measurements
 Result interpretation is easy
 It can be performed without very high skilled professional
 Correlation between test measurements and material properties is
direct
 Allows a roughly identify the mechanical properties of the adhesive
joint (fracture strength, elongation, modulus of elasticity
 The mechanical properties of the adhesive or adhesive bonding can
be defined according to the different types of stresses (tension,
compression, shear, peel, dynamic forces of impact)
 There are many standards to be followed on destructive testing
 The costs of equipment for destructive testing are cheaper compare
with the equipment used in nondestructive testing.
 Ability to compare adhesives with this type of testing
 Verification of surface preparation, curing conditions, working
conditions and adhesives system products (primers, activators,
adhesives)
 Predict and identify the approximate nature of the failure or
breakdown that may occur during the lifetime of the bonded joint in
use, when the specimen is previously submitted to an accelerated
ageing
 Tests on a relatively cheaper cost

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 Tests usually simulate one of more service conditions.
Consequently, they tend to measure serviceability directly and
reliably
1.1.2 LIMITATIONS OF CONVENTIONAL TESTING
(DESTRUCTIVE TESTING) :
 Tests are performed only to a sample and the sample may not be a
representative of the group
 Tests parts are destroyed during the testing
 Specimens cannot be reused once have been tested again
 Usually cannot be used the same specimen for multiple destructive
testing
 May be restricted for costly or few in number parts
 Difficult to predict cumulative effect of service usage
 Hard to apply to parts in service if done testing terminates their
useful life
 Extensive machining or preparation of test specimen is often
required
 Capital equipment and labor cost re often high
 Test ofte require more time
 It’s not possible to identify internal defectology (bubbles,
delaminating, pores, wrong thickness) of the real bonded joint,
preventing repairs before being put in use or during their lifetime
 Test equipment is usually not portable.
1.1.3 MERITS OF NON-DESTRUCTIVE TESTING:
 Tests are done directly on objects

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 100% Testing (or representative samples) on actual components
can be performed
 Different NDT methods can be applied on the same components
and hence many or all properties of the interest can be examined
 Non – Destructive Test can be repeated on the same specimen
 It can be performed on components which are in-service
 Little or no specimen preparation is required
 The test equipment is often portable
 Labor costs are usually low
 Most NDT methods are quick
1.1.4 LIMITATIONS OF NON-DESTRUCTIVE TESTING:
 Measurements are indirect
 Reliability to be verified
 Measurements are often qualitative or comparative
 Result interpretation is often difficult
 Skilled personal are required for testing and result interpretation
 Different observers may interpret the test results differently
 Some test equipment requires a large capital investment
Some Uses of NDE Methods:
 Flaw Detection and Evaluation
 Leak Detection
 Location Determination
 Dimensional Measurements
 Structure and Microstructure Characterization
 Estimation of Mechanical and Physical Properties
 Stress (Strain) and Dynamic Response Measurements

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 Material Sorting and Chemical Composition Determination
 Fluorescent penetrant indication
NDE Methods are used when,
 there are NDE application at almost any stage in the production or
life cycle of a component
 to assist in product development
 to screen or sort incoming materials
 to monitor, improve or control manufacturing processes
 to verify proper processing such as heat treating
 to verify proper assembly
 to inspect for in-service damage
Most Common NDT Methods
 Visual Inspection
 Liquid Penetrant Test
 Magnetic powder Test
 Ultrasonic Test
 Thermography Test Eddy Current Test
 Radiography Test (X-ray, Gamma Ray etc.)

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1.2 VISUAL INSPECTION
 Visual Inspection is the fastest, simplest and by far the most
commonly used non-destructive testing method.
 As the name suggests, visual inspection relies primarily on good
eyesight and can be carried out with the naked eye (known as
unaided visual inspection) or using some optical aids (aided
visual inspection) such as mirrors, magnifying glasses and
microscopes.
 Definition : Visual inspection is commonly defined as “ the
examination of a material, component or product for conditions of
non-conformance using light and eyes, alone or in conjunction with
various aids”.
 Visual inspection is also involves shaking, listening, feeling and
sometimes even smelling the component being inspected.
Other NDE Methods Rely On Visual Testing
 Visual testing is inherently part of all other NDT test method. Visual
inspection is commonly employed to compliment / support other
NDT method.
 Other NDT method required visual interventions to interpret images
obtained while carrying out the examination. At some point, all NDT
methods fall back on visual testing.
 For example, liquid penetrating method uses dyes that rely on the
inspector’s ability to visually identify surface indications.
 Radiographic technique requires that the technician use visual
judgement to determine the soundness of the object being tested.

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1.2.1 BASIC PRINCIPLE OF VISUAL INSPECTION

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1.2.2 TYPES OF VISUAL TESTING

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1.3 THERMAL INSPECTION

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1.4 LIQUID PENETRATE INSPECTION
1.4.1 PRINCIPLE OF LIQUID PENETRANT INSPETION

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1.4.2 LIQUID PENETRANT TEST PROCESS

CONCEPTS OF NDT

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CONCEPTS OF NDT