Nondestructive Testing (NDT) is the name given to the processes designed to verify the integrity of a structure in such a way that the performance of that component is not impaired by the test. It has applications in many diverse fields from the industrial such as medical, construction and automotive to the entertaining such as amusement park rides. In all cases, the role of NDT is now recognized as vital to ensure the reliability of the product. This vital role is never more important than it is in the aerospace industry.
In its infancy, NDT was quite a crude technology. Going back hundreds of years, metalworkers such as blacksmiths and bell makers would tap their work to listen to the ringing noise this produced. This would indicate the strength of the material. As recently as the early twentieth century, this methodology was still in use: people were employed as “wheel tappers.” When locomotives were stationary, these individuals would test the wheels by tapping them to find cracks, which were typically caused by fatigue. This test is still performed - albeit by instrumentation and it is now referred to as the acoustic impact technique. In testing of honeycomb structures in aircraft a backup used to be a “coin tap” to verify a dis-bond.