Non Destructive Testing

Introduction to Non-Destructive Testing (NDT)

History of NDTThe art and science of Non-destructive Testing (NDT) are very old. Probably one of the most famous and well known examples is that of Archemedes and Hiero’s Crown. In performing a test to determine if the king had been defrauded by the silversmiths, Archemedes discovered the principle that now bears his name. The art of NDT is used in many fields of endeavour without even being considered in the realm of NDT. To give an example, the fruit vendor who can tell if a watermelon is ripe by ‘thumping’ or if a cantaloupe is ripe by shaking and listening for the ‘rattle’ of the seeds is using NDT.

Since, 1920, the art of NDT has developed from a laboratory curiosity to an indispensable tool of production (1). However, the real revolution in NDT took place during World War II. The progress in materials engineering in identifying new and improved materials subsequent to a number of catastrophic failures in World War II like the brittle fracture of Liberty ships, necessitated the requirement to test and improve material properties.

This requirement resulted in a wider application of the then existing NDT methods and techniques and also paved the way for development of new methods and techniques. Though in the beginning, NDT was used primarily for process control and secondarily for quality control, subsequently, the use of NDT was recognised by management as a means of meeting consumer demands for better products, reduced cost and increased production. NDT tests were used world-wide to detect variations in structure, minute changes in surface finish, the presence of cracks or other physical discontinuities, to measure thickness of materials and coatings and to determine other characteristics of industrial products.

NDT became a vital ingredient of modern engineering practice to achieve the required standards of quality in manufacturing and fetched reputations and profits to many industries. This traditional role changed steadily and NDT was relegated to the role of an inspection tool, popularly known as Non-destructive Inspection (NDI) and Non-destructive Evaluation (NDE), catering to the safety needs of components in aircraft, nuclear rectors, offshore installations, petrochemical plants, gas turbines, bridges etc.

It is not uncommon to find the use of ultrasonics to detect submarines, schools of fish, and as navigational aids; eddy currents for baggage control at air ports as well as metal detectors and infrared techniques for detecting heat losses from buildings, hot spots in electrical equipments, defects/stresses in metals. NDT has become a vital ingredient of modern engineering practice contributing significantly to overall safety, reliability and confidence at economic cost


(2).Role, Benefits and Components of NDT
NDT is a branch of the materials sciences that is concerned with all aspects of the uniformity, quality and serviceability of materials and structures. Essentially, NDT refers to all the test methods which permit testing or inspection of material without impairing its future usefulness. The science of NDT incorporates all the technology for detection and measurement of significant properties

(3). In other words, from an industrial viewpoint, the purpose of NDT is to determine whether a material or a component will satisfactorily perform its intended function. By use of NDT methods and techniques, it is possible to decrease the factor of ignorance about material without decreasing the factor of safety in the finished product

(4). In general, the purpose of NDT will fall into one of the following categories:
1. Determination of material properties
2. Detection, characterisation, location and sizing of discontinuities/defects
3. Determining quality of manufacture or fabrication of a component/structure
4. Checking for deterioration after a period of service for a component/structure The benefits derived from NDT to the industry are many


(5). The contribution which NDT tests can make to the industry can de divided into four categories:
1. increased productivity
2. increased serviceability
3. safety
4. identification of materials.

By detecting faulty material and thus preventing loss of material, manpower and shop time non-destructive tests will increase productivity, and hence the economic gains. NDT techniques can be used to as an aid in new process and manufacturing techniques. Preventive maintenance tells if parts are still satisfactory for use, it pays off in dependable predictable production, fewer repairs, less accidents and lower over-all operating costs. Increased serviceability of equipment and materials will result through the application of NDT methods and techniques by finding and locating defects which may cause malfunctioning or breakdown of equipment

(6). In the field of safety proper use of NDT will aid in the prevention of accidents, with their possible loss of life, property, and vital equipment. The identification of materials differing in metallurgical, physical or chemical properties can often be done by using NDT methods. A variety of NDT techniques have been developed to detect and characterise the above types of defects. All the NDT techniques are based on physical principles. Nearly every form of energy has been utilised in NDT. Likewise nearly every property of the materials to be inspected has been made the basis for some method or technique of NDT. Nearly all methods of NDT involve subjecting the material being examined to some form of external energy source and analysing the detected response signals.

The essential parts of any NDT test are

1) application of a testing or inspection medium
2) modification of the testing or inspection medium by defects or variations in the structure or properties of the material
3) detection of this change by suitable detector
4) conversion of this change into a suitable detector
5) interpretation of the information obtained.

For example, in the case of X-ray film radiography,
1) the X-rays are the testing or inspecting medium,
2) any defects in the material being radiographer will modify the intensity of the radiation reaching the film on the opposite side of the specimen,
3) certain silver bromide emulsions are sensitive to X-rays and can be used as a detector,
4) the emulsions are capable of recording variations in X-ray intensity and by the proper developing procedures can be made to give a permanent record, and
5) interpretation is then a process of explaining variations in density of the radiograph. In most of the instances, NDT results are indirect measurements.

Hence, it is essential that the interpretation be made by an experienced or skilled person. The person interpreting the results sometimes determines the success or failure of a test method or technique.