What is NDT? Introduction to NDT
NDT is an acronym that stands for Nondestructive Testing.
NDT is a field of engineering encompassing the testing and inspection of materials and equipment to evaluate condition, find flaws and defects, and extend the useful life of the infrastructure all around us.
Sometimes broadly termed 'industrial inspection', NDT is the interdisciplinary engineering field dedicated to ensuring the structural components, systems, and infrastructure used in construction and industry stay in a reliable and safe state. Besides ensuring the cost-effective operation of mills, factories, airplanes, and municipal systems, NDT also ensures the safety of all persons in and around these systems.
The key difference between NDT and other forms of materials evaluation is that NDT allows in-situ evaluation or inspection of the part without modifying or permanently damaging the part. Many of the technologies developed for medical imaging are used in NDT, in addition to many other high-energy forms of inspection, unique to NDT, that would be harmful if used on living tissue.
The term NDE (Nondestructive Evaluation) is sometimes used to describe NDT methods that result in quantitative measurements of material characterization, for instance the size of an indication, a measure of remaining life of a part, or another quantitative characteristic of a material or part.
Most people have already had experience with NDT technologies from the medical industry. X-ray, Ultrasound and flexible scopes are all used in both medicine and industry, with some of the devices being almost exactly the same. The major categories, or disciplines, of NDT are described here.
Visual or Optical Testing - VT
Visual testing usually utilizes miniature, robotic, or flexible cameras to examine areas inaccessible under normal circumstances. Examples include using a wheeled crawler which carries a pan-and-tilt camera up to thousands of yards down a small diameter pipe, or a waterproof pan-tilt-zoom camera to inspect radioactive spent fuel rods in a spent fuel pool of a nuclear reactor, or perhaps a flexible, articulating borescope to inspect the turbine blades of a jet engine. The main categories of VT include microscopy, fiberscopes, borescopes, crawlers, pan-tilt-zoom (PTZ) cameras, and flying drones. Commonly viewed indications, or imperfections, include corrosion, pitting, cracking, obstructions, dislocations, and leaks.
Radiography - RT
RT uses penetrating gamma and X-ray radiation that creates an image of the inside of a surface, for example the wall of a steel pipe. The X-rays are generated by an electronic generator or an isotope source of radioactive material, usually created from spent nuclear fuel. X-ray film or an electronic collector is placed behind the part, and when exposed to the X-rays traveling through the part, creates a visible image similar to a medical X-ray.
The level of radiation required to shoot through metal parts is quite high, making RT a potentially dangerous discipline, and is highly regulated to ensure safety. Common indications found are voids, cracks, internal corrosion, stress corrosion cracking, and defective welds. CT, or computed tomography, is becoming ever more popular, allowing inspectors to use radiographic testing to create 3D images of the internal structure of a solid part.
Magnetic Particle Testing - MT
MT is completed by inducing a magnetic field in a ferromagnetic material and covering the excited part with iron particles. The particles may be dry or suspended in a fluid. Defects in the surface like voids and cracks cause disruptions in the magnetic field lines of the part, and force the iron particles to concentrate at the indications due to magnetic flux leakage. MT makes it easy to see cracks and other defects in the surface of a part that would normally be too small to see by eye alone. Common indications found with MT include cracks, pits, voids, and sometimes even indications below the surface.
Ultrasonic Testing - UT
Ultrasonic testing is the most common form of NDT used in industry. Similar to an ultrasound used in medicine, UT uses high frequency sound waves that travel through material to create a signal or image showing the internal structure of the material. The simplest form of UT is thickness measurement where the thickness of material can be measured to evaluate the remaining life of a pipe or tank. More complex methods, including pulse echo, time-of-flight diffraction, and shear wave, measure the reflected sound waves returning to the probe and, using complex calculations, create 3D images of the internal structure of a part.
UT is useful for both metallic and composite parts, making it a very important disciplines to find cracks, voids, delamination, and other indications in modern construction for aerospace, wind power, and automotive inspection.
Penetrant Testing - PT
PT is one of the oldest methods of finding indications in parts which are not visible by eye. This method utilizes a fluorescent or vividly colored dye that glows brightly when exposed to UV light. To complete this method, a part is coated with the fluorescent dye and then wiped clean. A developer, a liquid which activates the color or fluorescence, is then applied to the surface. When brightly lit or lit with a UV lamp, excess dye will remain in cracks and voids on the surface, making many types of indications easy to see by eye.
Electromagnetic Testing - ET
Eddy current testing is the most common form of electro-magnetic testing. Eddy current testing takes advantage of the fact that rapidly changing the magnetic field around a metallic part creates electrical currents in the part. These currents are disrupted by indications like cracks and voids, and measuring these electrical currents allows the device to create a measurement of defects within the part. Eddy currents are also changed by the electrical conductivity and magnetic permeability of the material, allow Eddy Current testing to identify or classify materials.
Leak Testing - LT
Leak testing is used extensively in industry, construction, and municipal infrastructure to ensure the safety of gas lines, boilers, oil pipelines, and many other high-pressure and hazardous material storage applications. Techniques include many simple and complex methods that generally provide a go/no-go evaluation of an installed system. Tanks and vessels are leak and pressure tested by filling with water and pressurizing to operating pressures, with the water preventing dangerous and catastrophic failure should the test fail. Soap bubble testing is commonly used on installed gas lines to see small gas leaks on gas lines. Testing for hydrogen gas lines is much more complex due to the small size of a hydrogen molecule, so electronic sniffing and sampling devices are often used for this application. Other methods include penetrant testing, audio and listening devices, and electronic monitoring.
Acoustic Emission Testing - AE or AET
AET is used in a fashion similar to ultrasonic testing where sound waves give us a picture of inside of a part. However, it differs in that the defects in part itself create distinctive sounds when the part is placed under load. Somewhat similar to how a crack in a wine glass changes the sound when it is tapped from a clear ring to a less sonorous sound, defects in a stressed or loaded part can emit a sound of their own. Special electronic devices can listen for these sounds and help make judgements about the internal structure or presence of defects in a part or structure.