However, test objects can be in various forms and conditions. Alternate embodiments of the NASA system that may meet some field-testing requirements include the use of thermal test chamber. The test chamber would induce stresses by heating and cooling the test structure. Residual stress can be determined by calibrating the eddy current signal to a predetermined thermal response curve. Residual stress establishes the initial stress state of an element.
Total stress is the sum of residual stress and stress resulting from any externally applied loads.
The yield stress of a material dictates the maximum stress a structure can sustain before plastic deformation begins and ultimately leads to failure. Thus, it is important to know the residual stress so that the total stress does not exceed the yield stress. Other methods for measuring stress such as strain gauges, linear variable displacement transducers, and ultrasonic systems are not able to measure residual stress. Numerous researchers have demonstrated that eddy current measuring techniques can be used for stress field mapping of a material. This is the basis of the NASA technique.