Defining NDT

Also known as Non Destructive Evaluation (NDE), NDT refers to the method of examining materials and components in order to identify and quantify defects and degradations in their material properties before they result in failure. The aim of NDT is to ensure the safe utilization of engineering structures, as well as to ensure product quality and performance upon production. To put it simply, NDT encompasses techniques to evaluate defects in objects without having to physically break them up to test them. This is achieved through a number of non-invasive measurement techniques that draw their origin from areas as diverse as medicine, geophysical prospecting, sonar and radar.

NDT inspection techniques are essential tools toward ensuring quality assurance in a host of applications. These techniques are required right from product or material development stage, through manufacture and inspection at final application level. Applications for NDT range from inspection of aircrafts, automobiles, railway, foundry applications, to defense, nuclear power plants, oil pipelines and rigs. NDT is also essential in inspection of welds and detection of cracks, flaws and defects at surface and subsurface levels. Inspection of such flaws and defects is paramount as avoidance or inadequate NDT inspection could potentially end in catastrophic consequences. These could range from wastage of material, time and other resources to even endangering safety of human lives and the environment.

The use of NDT is thus taking on greater significance in today's market scenario due to the following reasons:
• Increasing concern on impact of products and services on the safety of human lives and the environment;
• Stringent regulatory edicts and standards governing quality of materials, products and services;
• Competitive market conditions forcing end-user organizations to identify means to optimize costs; and,
Growing focus on quality in emerging Asia Pacific and East European markets to cater to needs of domestic and international demand.

Over the years, the emphasis of NDT has moved from being a qualitative method aimed only for defect identification, towards a more quantitative tool. Most of the leading NDT techniques typically are labor intensive in nature and thus depend considerably upon the skill levels of technicians performing inspection. The move toward quantitative NDT has meant increasing statistical trending for identifying residual life of assets as well as reduction in defects on the production line. This growing importance toward the quantitative analysis has necessitated the effective capture, storage, analysis and reporting of NDT test results, thus impacting the evolution of the various NDT product types. Before understanding more about the product and technology trends in this market, let us understand the various NDT techniques that have governed the development of these equipment.

NDT Techniques

There are a number of NDT techniques that have been developed over the years. As mentioned earlier, most of these techniques trace their origin to non-invasive measurement methodologies. As is the case in other industries, each of these techniques is constantly evolving to match the needs of the developing end-user markets. Each technique has its set of benefits, as well as limitations, and the most suitable technique to be used depends upon the physical property of the material to be tested. Chart 1.1 provides a view of the leading NDT techniques used in the NDT market.



Ultrasonic Inspection

The Ultrasonic inspection process involves transmission of sound waves of short wavelength at high frequencies in order to identify flaws and/or measure the thickness of materials. An ultrasonic instrument works with the principle of sending a pulsed beam of high-ultrasound from a handheld transducer, which is placed upon the surface of the object being tested (also known as specimen). The sound waves (or echo) from the pulse that returns to the transducer is displayed upon the screen of the Ultrasonic equipment, presenting the amplitude of the pulse and the duration taken for return to the transducer. By measuring this sound that bounces back through the thickness of the specimen, a trained operator can identify defects as well as calculate the flaw-size, distance and reflectivity.

Surface Inspection

As the name suggests, surface inspection refers to the method of inspecting the surface or near-surface of materials or the object, using inspection techniques towards identifying flaws, cracks and other defects. Surface inspection can be basically classified as:
• Liquid penetrant Inspection (LPI) - Also known as dye penetrant inspection, this method aids in revealing surface breaking flaws by bleeding out of a colored or fluorescent dye from the flaw. LPI works on the principal of capillary action, and involves stages such as cleaning the surface to be inspected, application of penetrant, clearing out excess penetrant and application of developer to display defects through regular white light, or ultraviolet black light for fluorescent penetrants.
• Magnetic particle inspection (MPI) - This is used in inspection of ferromagnetic materials such as steel and iron. This is based on the principle that magnetic lines of force (flux) would get distorted by the presence of a flaw and thus reveal its presence. MPI involves application of fine iron particles to the area under examination, and measuring the variations once a magnetic field is applied.

Eddy Current Testing

Eddy current test is an electromagnetic technique primarily performed on conductive materials. It can be used for identifying cracks, as well as rapidly sorting small components for flaws, size and/or material variations. This technique works with the principal of bringing an energized coil near the surface of a metal component to generate eddy currents into the specimen. The currents develop magnetic field that typically opposes that of the original magnetic field, and the presence of flaws or variations affects the impedance in the coil. Measuring this change and displaying it aids in identifying the nature of the flaw or material condition.

Visual Inspection

At its most basic level Visual inspection can be performed by the naked eye of the operator. Visual inspection refers to the examination of surfaces using direct viewing or low magnification techniques. A number of products such as light sources and video display units such as borescopes and videoscopes are used toward inspecting an object or surface visually. These equipment are further affixed to a processing unit where the images can be processed using a software and interpreted accordingly. The visual inspection process is particularly of use in inspection of surfaces with complex geometries by using flexible borescopes or videoscopes.

Radiography

Radiography involves the process where radioactive rays are directed at the object to be inspected, to pass through it and the resulting image is captured on a film. This film is in-turn processed and image displayed as a sequence of grey shades between black and white. Radiography encompasses sources such as X-rays, Gamma rays as well as newer methods such as real time radiography, computed radiography (CR) and computed tomography (CT). Considering the radioactive nature of this technique, special protective care has to be taken by the technician taking the radiography measurements to avoid exposure and the resultant harmful side effects.