NDT is popular because of its harmless and unfaltering association with the final product to check its quality. As we acknowledge, all materials are imperfect, but this is only of concern if the imperfections adversely affect intermediate processing or use of the finished product. In order to detect imperfections, some form of testing is necessary that will not have a detrimental effect on the materials/components.NDT encompasses all the test methods that, when applied to a component, do not impair its subsequent utilization (Colangelo, p 44) If the testing does not destroy or damage the material in any way it is known as Non - Destructive Testing (NDT). NDT is crucial in characterizing final products into "zero defect" and "potentially flawed" parts, this characterization is fast and easy. Hence, the quality control is easy itself. For instance, let us take an example of a steel plant which makes "railroads". There is a specific dimensional requirement of every piece and also there is some maximum tolerance level for cracks and porosity holes in those pieces. Using NDT (laser and X-Ray techniques, we can determine the dimensions and flaws inside the rails in a very short period of time). A number of techniques are used in NDT; each is generally dependent on a different energy system. Techniques range from ordinary macroscopic examination with white light to the complex procedure of neutron radiography, each method having an area in which it yields optimum performance (Colangelo, p44) though it can often be used successfully on marginal situations when the need arises.
In present case we will discuss TWO vastly used NDT methods:
1. Magnetic Particle Inspection (MPI)
2. Ultrasonic Testing
Magnetic Particle Inspection offers a means for the detection of surface and slightly subsurface discontinuities in ferromagnetic materials (Colangelo, p 48). MPI is not applicable to non-ferromagnetic materials thus many structural metals like austenitic stainless steel, aluminum magnesium, copper and titanium are excluded from this inspection. Only ferromagnetic materials are inspected through this method. Magnetic particle inspection (MPI) is a widely used nondestructive inspection method for aerospace applications essentially limited to experiment-based approaches (Betz 1997). The analysis of MPI properties that affect sensitivity and reliability contributes not only reductions in inspection design cost and time but also improvement of analysis of experimental data. Choosing a particle medium, consider the application. For convenience, select dry particles when inspecting large components such as forgings. Wet-particle inspection, often requiring a tank complete with stirring and pumping machinery, works well for production-line Magnetic Testing inspection. This approach is especially useful when the operator must examine large numbers of small, similarly sized components. When portability is important, particularly for field