Analysis of Failure Mode and Stress

The probability of a device failing at any instant is very sensitive to the stress applied to it. Stresses, which can be classified as environmental or self generated, include:The overall sum of these stresses is often pictured as constantly varying, having peaks and troughs, and superimposed on a distribution of strength levels for a group of devices. A failure is assumed to be the result of stress exceeding strength. The average strength of the group of devices will increase during the early failures period due to the elimination, from the population, of the weaker items. During wear out strength declines as a result of physical and chemical processes. An overall change of the average stress will cause more of the peaks to exceed the strength values and more failures will result. Figure 7.1 illustrates this concept showing a range of strength throughout the bathtub together with a superimposed strain.