Nondestructive Testing Types

Nondestructive Testing and its Types:

Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used.

 In contrast to NDT, other tests are destructive in nature and are therefore done on a limited number of samples ("lot sampling"), rather than on the materials, components or assemblies actually being put into service.

 These destructive tests are often used to determine the physical properties of materials such as impact resistance, ductility, yield, and ultimate tensile strength, fracture toughness and fatigue strength, but discontinuities and differences in material characteristics are more effectively found by NDT.

 Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public. Here a deeper look at some of the most common types of NDT/NDE:

Radiographic Testing 

(RT) is a nondestructive examination (NDE) technique that involves the use of either x-rays or gamma rays to view the internal structure of a component. In the petrochemical industry, RT is often used to inspect machinery, such as pressure vessels and valves, to detect for flaws. RT is also used to inspect weld repairs.

Compared to other NDE techniques, radiography has several advantages. It is highly reproducible, can be used on a variety of materials, and the data gathered can be stored for later analysis. Radiography is an effective tool that requires very little surface preparation. Moreover, many radiographic systems are portable, which allows for use in the field and at elevated positions.

Ultra Sonic Testing

It is a non-destructive testing (NDT) method in which beams of high frequency sound waves that are introduced into the material being tested are used to detect surface and sub-surface flaws. The sound waves travel through the materials with some attenuation of energy and are reflected at interfaces. The reflected beam is detected and analyzed to define the presence and location of flaws. Cracks, laminations, shrinkage, cavities, bursts, flakes, pores, bonding faults and other discontinuities can be easily detected. Inclusions and other in homogeneities in the metal being inspected can also detected by causing partial reflection or scattering of the ultrasonic waves, or by producing some other detectable effect on the ultrasonic waves. Ultra-high frequency sound is introduced into the part being inspected and if the sound hits a material with different acoustic impedance (density and acoustic velocity), some of the sound will reflect back to the sending unit and can be presented on a visual display.

 By knowing the speed of the sound through the part (the acoustic velocity) and the time required for the sound to return to the sending unit, the distance to the reflector (the indication with the different acoustic impedance) can be determined. The most common sound frequencies used in UT are between 1.0 and 10.0 MHz, which are too high to be heard and do not travel through air. The lower frequencies have greater penetrating power but less sensitivity (the ability to "see" small indications), while the higher frequencies don't penetrate as deeply but can detect smaller indications (the ability to "see" small indications), while the higher frequencies don't penetrate as deeply but can detect smaller indications.

Liquid Penetrant Inspection

The basic principle of Liquid Penetrant Inspection (LPI) is that when a liquid with low viscosity – called a penetrant, is applied to the surface of a part, it will penetrate into fissures and open to the surface.

Once the excess penetrant is removed, the penetrant trapped in those voids will creep back out through capillary action, creating an indication.

Penetrant testing can be performed on magnetic and non-magnetic materials, but it doesn’t work well on porous materials. Penetrants may be “visible”, meaning they can be seen in ambient light, or fluorescent, requiring the use of a “black” light.

After applying the penetrant, it sits on the surface for a specified period of time, coined as the “penetrant dwell time”, then the part is carefully cleaned to remove excess penetrant from the surface. A light coating of developer is then applied to the surface and given time to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication.

Following the developer dwell time, the part is inspected visually, with the aid of a black light when using fluorescent penetrants. Most developers are fine-grained, white talcum-like powders that provide a color contrast to the penetrant being used.