Stress cycle random

The overall response of the crystal to such a stress cycle is shown in Fig. 8.16. When the stress a0 is applied suddenly, the crystal instantaneously undergoes an ideally elastic strain following Eq. 8.62. As the stress is maintained, the crystal undergoes further time-dependent strain due to the re-population of the interstitials. When the stress is released, the ideally elastic strain is recovered instantaneously and the remaining anelastic strain will be recovered in a time-dependent fashion as the interstitials regain their random distribution. 

Figure 8.17 Variation of stress with time that accounts for fatigue failures, (a) Reversed stress cycle, in which the stress alternates from a maximum tensile stress (+) to a maximum compressive stress (-) of equal magnitude, (b) Repeated stress cycle, in which maximum and minimum stresses are asymmetrical relative to the zero-stress level mean stress cr, range of stress cr and stress amplitude cr are indicated, (c) Random stress cycle.

Since fatigue cracks often start at a random surface imperfection, considerable scatter occurs in fatigue data, increasing with the increasing lifetime wherever crack initiation occupies most of the fatigue life of a specimen. When a line of the best fit is drawn from the available data points on an S-N curve, this represents the mean life expected at any given stress level or the stress that would cause, say, 50% of the product failures in a given number of cycles. 

Life test A test in which random samples of a product are checked to see how long they can continue to perform their functions satisfactorily. A form of stress testing is used, including temperature, current, voltage, and/or vibration effects, cycled at many times the rate that would apply in normal usage. 

Environmental validation programs are tests performed to verify adequate workmanship in the construction of a test item. It is often necessary to impose stresses beyond those predicted for the mission in order to uncover defects. Thus, random vibration tests are conducted specifically to detect bad solder joints, loose or missing fasteners, improperly mounted parts, and so on. Cycling between temperature extremes during thermal-vacuum testing and the presence of EMI during EMC testing can also reveal the lack of proper construction and adequate workmanship. 

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