Introduction
Both Destructive and Non-destructive tests are essential
for ensuring quality.
While destructive tests, such as mechanical and micro
structural tests, are used for evaluation and ensuring of the desired properties
of the materials or components,
Non-destructive test (NDT) techniques, as the name implies,
are the methods used to determine the performance capabilities of materials,
components or products, without destroying them and allowing them to be used
for their intended end use, if they are found suitable.
NDT supplements the destructive tests and is used
for detecting unacceptable defects in materials / components, which may lead to
failures.
NDT will thus help in ensuring product quality, maintaining
of appropriate quality standards, right from raw material step, through
fabrication, processing to pre - service and in – service inspection stages.
Thus practice of NDT, can lead to achieving a better
quality of life, by ensuring quality in processes, products and services,
reducing wastages and consequently increasing the profitability.
It is an accepted fact that, the practice of NDT has
to keep pace with needs and advances in fundamental science, engineering and
technology.
Thus NDT ensures synergy between quality assurance
and cost effectiveness.
State of the art NDT, is one of the important means
of ensuring production of high quality products, for fabrication works, in critical
areas, of sophisticated, strategic and other core industries, like Atomic
energy, Space, Defense, Power, Steel, Chemical, Transport etc. where stringent
quality requirements are to be met.
This will also help in preventing, even the remotest
chances of catastrophic failures in any industrial plant.
NDT also assists in the condition monitoring of the
critical components, which can also be used for assessment of remnant life of
plants and its components.
Quality
Assurance
Quality Assurance work activities are mainly
technical in nature and normally covered by procedures for carrying out the
work as also procedures for verification of the quality of work (viz. various
NDE procedures).
QA Manuals, guide lines etc are programmatic and work
oriented.
QA in design is also very much essential. This
involves adequate review of the design basis report, work environment etc.
Quality
Surveillance
This refers to the day to day quality survey of
works being executed and covers
Base material Identification
Welding procedure Qualification
Welder Performance Qualification
Qualification of NDT procedures and Operators
Stage wise Inspection of Work activities
Random Evaluation
Final Testing
It also covers control on Materials and performance records,
Certification and Documentation, keeping in view the safety, reliability
function of the work performed, for the safe and satisfactory performance of
the plant was also the plant performance.
Other
Aspects
A (QA) Programme
An organization
Documentation control
Procurement control
Inspection Test control
Non conformance control
Corrective actions
System Preservation
Records
Audit
Method
of NDT Techniques
There are various methods to gauge the quality of
the raw materials, to test at any stage of production and also to check, the
components at the final stage.
The following NDT techniques are used in testing of bulk
metals, for their flaws, in homogeneities, surface cracks, blowholes,
laminations etc. Commonly used Techniques are,
VISUAL AND OPTICAL,
PENETRANT FLAW DETECTION,
MAGNETIC PARTRICLE INSPECTION,
Other NDT Techniques Include
RADIOGRAPHIC INSPECTION,
ULTROSONIC INSPECTION,
EDDY CURRENT INSPECTION
ACOUSTIC EMISSION INSPECTION,
LEAK TESTING,
Though these examinations are mutually exclusive, they
are complimentary.
Essential
elements common for all NDT methods are,
A Probing Medium,
A test specimen appropriate for the medium being
used
A detector to enable measurement of distribution or
alterations in the media
A technique for recording or displaying information
for suitable evaluation Interpretation of the results
Visual
and Optical Test
In this method examination procedures range from a
very simple to a very complex one
Some methods involve comparison of workmanship, by
comparison of samples with production parts. Others use sophisticated imaging
technology to permit viewing of internal parts in large components without the
need for disassembly.
Human eye plays an important role in this technique.
Acceptance is based on dimensional checks and
examination, of surface roughness, weld spatter, unfilled craters, and arc
strikes, under cuts, overlap, and surface cracks.
Penetrant
Flow Detection
This is a non-destructive method for finding discontinuities
that are open to surface, of solids and essentially non-porous materials.
It is an extension of the method of visual inspection,
as majority of the discontinuities found by penetrant flaw detection can be
seen visually, if viewing conditions are perfect.
The basic principle is that a liquid penetrant,
which wets a surface, migrates in to the cracks through minute surface openings,
as fine as 4 microns width, by capillary action.
Penetrant testing makes them much easier to detect.
Indication of flaws can be found regardless of the
size, configuration of the work being inspected and also regardless of the flaw
orientation.
Magnetic
Particle Testing
Inducing a magnetic field in a ferromagnetic material
and spraying the surface with iron particles either dry or suspended in liquid,
accomplish this test.
Surface and near surface imperfections distort the
magnetic field and the iron particles concentrate near imperfections.
Ultrasonic
Inspection
This inspection technique uses transmission of high
frequency sound waves in to a material to detect imperfections / changes in
material properties.
The most commonly used ultrasonic testing technique
is the PULSE ECHO, where in sound waves are directed in to the test object and reflections
(echoes), from the imperfections are returned to a receiver.
Eddy
Current Inspection
It is also called as the electromagnetic test, in
this technique electric currents are generated in a conductive material, by an
induced alternating magnetic field.
Interruptions in the flow of electric currents (eddy
currents), caused by imperfections or changes in material’s conductivity and or
properties will cause changes in the field.
These changes, when detected indicate in homogeneity
in the test piece.
Radiography
This involves use of penetrating Gamma rays or
X-radiation to examine parts and products for imperfections.
An X-ray machine or radioactive isotope is used as
source of radiation.
Radiation is directed through a part and on to a
film or any other media.
The resultant image on the film indicates the soundness
of the component.
Imperfections if any are shown up in the film
density changes.
Acoustic
Emission Inspection
When a solid
material is stressed, growing imperfections, if any within the material, emit short
bursting acoustic energy called emissions.
As in ultrasonic testing acoustic emissions can be
detected by special receivers
Emission sources can be evaluated through the study of
their intensity, rate and other characteristics.
The growing defects can be located by using triangulation
technique, analogous to earth quake epicenter location.
Leak
Testing
Several techniques are used to detect and locate leaks
in pressure retaining components such as pressure vessels and pipelines.
Leaks can be detected by using electronic listening devices,
Pressure gauge measurements, liquid and gas penetrating techniques, a simple
soap bubble test or more sensitive Halogen diode / Helium leak testing
System
Testing
Is required to be done to demonstrate the integrity of
the system, so that failures do not occur during the service period of the
system
This is done by subjecting the material of construction
to stress levels nearer to yield point and holding for about ten minutes.
Bring down the pressure of system to design pressure
and check the weld & mechanical joints for leak- tightness.
Any leaks found are to be repaired and system subjected
to retest.
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