1.1. Basic principles of non-destructive testing (NDT)
1.1.1. 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 predictwhere 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 thestructure of the
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 putinto 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
(c) It enhances the reputation of the manufacturer as producer of quality goods.
All of the above factors boost thesales 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.
1.1.2. Types of NDT methods
The methods of NDT range from the simple to the complicated. Visualinspection 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 andnonconventional.
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 beingmentioned 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 basicprinciples, typical applications, advantages and
limitations of the methods of group one will now be briefly described.
1.1.3. Visual testing (VT)
Often overlooked in any listing of NDT methods, visual inspection is one of the most
common and most powerful means of non-destructive testing. Visual testing requires adequate
illumination of the test surface and proper eye-sight of the tester. To bemost effective visual
inspection does however, merit special attention because it requires training (knowledge of
product and process, anticipated service conditions, acceptance criteria, record keeping, for
example) and it has its own range of equipment and instrumentation. It is also a fact that all
defects found by other NDT methods ultimately must be substantiated by visual inspection....