Definition and importance of NDT
Non-destructive testing is the
use of physical methods which will test materials, components and assemblies
for flaws in their structure without damaging their future usefulness. NDT is concerned with revealing
flaws in the structure of a product. It, however, cannot predict where flaws
will develop due to the design itself. All NDT methods have the following
common characteristics:
(a) The application of a testing medium to the product to be tested.
(b) The changes in the testing medium due to the defects in the
structure of the product.
(c) A means by which it detects these changes.
(d) Interpretation of these changes to obtain information about the
flaws in the structure of the product.
Importance of NDT
NDT plays an important role in
the quality control of a product. It is used during all the stages of
manufacturing of a product. It is used to monitor the quality of the:
(a) Raw materials which are used in the construction of the product.
(b) Fabrication processes which are used to manufacture the product.
(c) Finished product before it is put into service.
Use of NDT during all stages of
manufacturing results in the following benefits:
(a) It increases the safety and
reliability of the product during operation.
(b) It decreases the cost of the
product by reducing scrap and conserving materials, labour and energy.
(c) It enhances the reputation of
the manufacturer as producer of quality goods.
All of the above factors boost
the sales of the product which bring more economical benefits to the
manufacturer. NDT is also used widely for routine or periodic determination of
quality of the plants and structures during service. This not only increases
the safety of operation but also eliminates any forced shut down of the plants.
Types of NDT methods
The methods of NDT range from the
simple to the complicated. Visual inspection is the simplest of all. Surface
imperfections invisible to the eye may be revealed by penetrant or magnetic
methods. If really serious surface defects are found, there is often little
point in proceeding to more complicated examinations of the interior by ultrasonics
or radiography. NDT methods may be divided into groups for the purposes of
these notes: conventional and nonconventional.
To the first group may belong the
methods which are commonly used and include visual or optical inspection, dye
penetrant testing, magnetic particle testing, eddy current testing, radiographic
testing and ultrasonic testing. The second group of NDT methods are those used only
for specialized applications and consequently are limited in use. Some of these
methods which are being mentioned here merely as a curiosity for the reader
include neutron radiography, acoustic emission, thermal and infrared testing,
strain sensing, microwave techniques, leak testing, holography etc. It must
also be remembered that no one of these methods can give us solutions to all
the possible problems, i.e. they are not optional alternatives but rather complementary
to each other. The basic principles, typical applications, advantages and limitations
of the methods of group one will now be briefly described.
source : International Atomic Energy Agency; TRAINING COURSE SERIES No. 11
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Elcometer
Which principle is used for testing.....I mean echo or variation density of medium
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