Non-Destructive Testing techniques attempt to examine the state (of repair) of an object, without causing any deterioration of the object under test.
A Practical Example
Imagine a painful, possibly broken arm. An NDT approach would be to X-Ray the arm. A destructive test would be to (possibly) amputate the arm, peel off the skin, and... well you get the idea.
A non-destructive test has the distinct advantage of leaving the test subject intact. As well as the obvious benefits this entails, it also allows for testing of multiple possible problem causes. Finally, a non-destructive test lets us check for problems when there is no other evidence that a problem exists -- think of ultrasounds for pregnant women.
So Why Use Destructive Testing At All?
NDT techniques require interpretation as they are using indirect observation techniques. A destructive test has the advantage of direct observation, so its results are usually conclusive.
There are times when a conclusive answer is the primary objective. To take a topical example, when suspicious packages are left on underground trains, authorities regularly conduct controlled explosions to conclusively determine the existence of explosives.
Non-Destructive Testing in Industry
The term Non-Destructive Testing (also NDT) is most commonly used in inspection industry, civil and mechanical engineering, as well as materials science, and the construction industry. The companies who provide equipment of services to conduct NDTs are referred to as NDT or inspection companies, and the people who do the tests are NDT technicians.
Traditional industrial NDT techniques generally revolve around the detection of cracks or flaws within metallic objects. NDT is particularly useful in industrial applications as it lets us check for defects where they might exist. Also, potential problems can be diagnosed without shutting down processes or replacing significant (costly) components.
Common industry non-destructive tests include:
Flooded Member Detection
Magnetic Resonance Imaging
ACFM and ACPD
Ultrasonic Testing (Ultrasounds)
Close visual inspection
While the terminology may be uncommon outside of the industries listed, NDT techniques are frequently used in many other industries.
Other Uses for NDT
Archaeology. Archaeologists and palaeontologists must perform a number of tests to determine the authenticity, origin, and composition of their finds. NDT techniques allow these tests to be conducted without damaging valuable finds.
Medicine. The medical community makes extensive use of NDT techniques. In fact many of the more industrial NDT techniques have grown from medicine, for example X-Rays and MRIs. The obvious reason for this is that people would generally prefer to know if they have a brain tumor without a surgeon removing and dissecting their grey matter.
Schooling. Personally, I believe High school and University exams to be great examples of destructive testing, at least when it comes to your self esteem and feelings of self worth. But more generally, they aim to aim to discover things like learning potential without cracking open your skull and weighing your brain.