NDT

Nondestructive testing


Nondestructive testing (NDT), also called nondestructive evaluation (NDE) and nondestructive inspection (NDI), is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components and structures. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo blip on the screen). This article is aimed mainly at industrial NDT, but many of the methods described here can be used to test the human body. In fact methods from the medical field, where there tends to be more development funding available, have often been adapted for industrial use, as was the case with Phased array ultrasonics and Computed radiography.
While destructive testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. Destructive testing is also inappropriate in many circumstances, such as forensic investigation. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively; destructive testing is performed on a sampling of test objects that is drawn randomly for the purpose of characterizing the testing reliability of the nondestructive test.

The need for NDT
It is very difficult to weld or mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential. During the process of molding a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.
During their service lives, many industrial components need regular nondestructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:
aircraft skins need regular checking to detect cracks;
underground pipelines are subject to corrosion and stress corrosion cracking;
pipes in industrial plants may be subject to erosion and corrosion from the products they carry;
concrete structures may be weakened if the inner reinforcing steel is corroded;
pressure vessels may develop cracks in welds;
the wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.
Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique. In the cowboy days, it was quite common for a gun to kill the shooter rather than the person they were aiming at. From the 1992 Clint Eastwood western Unforgiven, here's a quote that reflects historical reality:
Little Bill Daggett: "... Bob's as good as dead because ... Corky ... takes careful aim and BAM!, the cylinder explodes in that Walker Colt he was carrying; a failing common to that model. It would have been better if Corky had two guns..., 'cause Bob walks over and shoots him."
[edit] Methods and techniques
NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT.

Methods and Techniques

Liquid penetrant testing (PT or LPI)
Radiographic testing (RT) (see also Industrial radiography and Radiography)
Digital Radiography (real-time)
Computed radiography
SCAR (Small Confined Area Radiography)
Neutron radiographic testing (NR)
Computed tomography (CT)
Ultrasonic inspection (UT)
Phased array ultrasonics
Time of flight diffraction ultrasonics (TOFD)
Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)
Internal Rotary Inspection System (IRIS) ultrasonics for tubes
Visual and optical testing (VT)
Ellipsometry
Pipeline video inspection
Electromagnetic testing (ET)
Eddy-Current Testing (ECT)
Remote field testing (RFT)
Magnetic-particle inspection (MT or MPI)
Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope
Barkhausen testing
Acoustic emission testing (AE)
Infrared and thermal testing (IR)
Thermographic inspection
Laser testing
Profilometry
Holography
Shearography
Leak testing (LT)
Tracer-gas method testing
Bubble testing
Absolute pressure leak testing (pressure change)
Halogen diode leak testing
Mass spectrometer leak testing

the Article above is taken from www.wikipedia.org