Non-destructive testing (NDT) is an analysis technique used in scientific fields to determine the state or function of a system by comparing a known input with a measured output, without the use of invasive approaches like disassembly or
failure testing. Because NDT does not require the disabling or sacrifice of the system of interest, it is a highly-valuable technique that saves both money and time in product evaluation, troubleshooting, and research. Common NDT methods include acoustic testing,
liquid penetrant testing, and
radiographic testing. NDT can be used with any isolated input / output system, and is a commonly-used tool in
forensic engineering,
mechanical engineering,
electrical engineering,
civil engineering,
systems engineering, and
medicine.
NDT involves comparing a known input to a measured output and comparing to a known model - does not require the sacrifice of the physical system, as would be the case with disassembly, dissection, or failure testing. Such methods are known as Non-Destructive Testing techniques. Because the physical system does not need to be sacrificed or damaged for NDT tests, such techniques are valued for saving time and money. On the other hand, NDT methods do not always reveal hidden defects, and skill is usually needed in interpreting the results. Such NDT methods are important in litigation because the material evidence is preserved intact.
NDT methods usually rely on use of electromagnetic radiation to examine samples. Initially, this includes most kinds of microscopy to examine external surfaces in detail. The examination is often reasonably obvious especially when different light sources are used. Thus glancing light on a fracture surface will reveal details not immediately obvious to sight. The inner parts of a product can be examined using other kinds of radiation which can penetrate the material, such as X-rays or ultrasound. Contrast between a defect and the bulk is always an important consideration, and may be enhanced by using liquids for example to penetrate fatigue cracks, provided that the liquid has absolutely no effect on the sample being examined.
In manufacturing, welds are commonly used to join two or more metal surfaces. Because these connections may encounter loads and fatigue during product lifetime, there is a chance that they may fail if not created to proper specification. During the process of casting a metal object, for example, the metal may shrink as it cools, which may introduce voids or cracks inside the structure. Some typical weld defects that need to be found and repaired in order to ensure the safe operation of a product are lack of fusion of the weld to the metal, porous bubbles inside the weld, and variations in weld density, all of which could cause a structure to break or a pipeline to rupture.
