Background on Nondestructive Testing (NDT)
Nondestructive testing has been practiced for many decades. One of the earliest applications was the detection of surface cracks in railcar wheels and axles. The parts were dipped in oil, then cleaned and dusted with a powder. When a crack was present, the oil would seep from the defect and wet the powder providing visual indicating that the component was flawed. This eventually led to oils that were specifically formulated for performing these and other inspections and this inspection technique is now called penetrant testing.
X-rays were discovered in 1895 by Wilhelm Conrad Roentgen(1845-1923) who was a Professor at Wuerzburg University in Germany. Soon after his discovery, Roentgen produced the first industrial radiograph when he imaged a set of weights in a box to show his colleagues. Other electronic inspection techniques such as ultrasonic and eddy current testing started with the initial rapid developments in instrumentation spurred by technological advances, and subsequent defense and space efforts following World War II. In the early days, the primary purpose was the detection of defects. Critical parts were produced with a "safe life" design, and were intended to be defect free during their useful life. The detection of a defects was automatically a cause for removal of the component from service.
In the early 1970's, two events occurred which caused a major change in the way inspections were viewed. The continued improvement of inspection technology, in particular the ability to detect smaller and smaller flaws, led to more and more parts being rejected (even though the probability of part failure had not changed). At this time the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size would fail under a particular load if a particular material property or fracture toughness were known. Other laws were developed to predict the rate of growth of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for a new design philosophy called "damage tolerant designs." Components having known defects could continue to be used as long as it could be established that those defects would not grow to a critical size that would result in catastrophic failure.
A new challenge was thus presented to the nondestructive testing community. Mere detection of flaws was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics calculations to predict the remaining life of a component. These needs, which were particularly strong in the defense and nuclear power industries, led to the creation of a number of research programs around the world and the emergence of nondestructive evaluation (NDE) as a new discipline.