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Also serious is the permanent distortion that results from yielding during service, from creep, and from buckling (or compression instability).
Yielding during service. If a part yields or distorts permanently during service after one or more load applications, the stress on the part has obviously exceeded the yield strength (actually, the elastic limit). If the part is a spring, we say that the spring has "taken a set," indicating that the spring is permanently distorted and can no longer perform its intended function. Identification of this type of failure is quite simple and obvious.
Less simple and obvious, however, may be the means for correcting the problem so that the same type of failure will not occur on other similar springs. In performing this function, the analyst will be tried to the limit of his or her ability - for tracking down the specific cause of yielding really becomes a challenge. It is vital to learn, for example, if this yielding is an isolated problem or is occurring on similar parts. If it is isolated, it is necessary to learn the details of what occurred to make this specific part yield. Obviously, it is necessary to measure the distorted spring and compare it with a new spring of the same original dimensions. Photographic comparisons are often useful with the new and yielded parts in the same photograph with a scale, if necessary, to show the distortion.
Detailed study of the part from a metallurgical standpoint is essential, with attention to such questions as the following: Is the microstructure the same as was originally specified, or has it been altered by exposure to an elevated temperature? Has the hardness been reduced by tempering during service? If so, what temperature would have caused that reduction in hardness level? Was the temperature abnormally high for the application? If so, why?
If investigation indicates that this is only one instance of a general and widespread problem, it may be necessary to redesign the assembly to reduce the stresses and temperatures, or to obtain a material with a higher yield strength at the operating temperature.
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