TABLE OF CONTENTS In the last two chapters, we discussed the quantification of ESS by thermal cycling and by random vibration, separately. However, the combined thermal cycling and random vibration scenarios are often used in ESS to ensure more effective screening. These two stresses have been found to be the two most effective stresses among all kinds of screening stresses. They are two different stresses, but they are not independent during combined ESS. The vibration resisting stress capability (fatigue strength) of hardware has been shown to be successively and incrementally degraded by thermal action [1; 2].
Thermal stresses during thermal cycling are produced by restrictions on the natural expansions and contractions of a material. These restrictions can be an external constraint that prevents free expansion and compression of the entire item, such as the interaction of two or more material interfaces in a structure which have different coefficients of thermal expansion. A more common occurrence is that of varying the temperature throughout the volume of the item. This will cause a nonuniform distribution of stresses. Thermal cycling as a life accelerating process, has been quantified in Chapter 7 using the modified Arrhenius model.
From the fracture mechanics point of view, thermal cycling reliability analysis can be based on an application of the Coffin-Manson model [3; 4; 5; 6; 7] which treats thermal expansions and compressions, and their associated stress-strain relations under temperature cycling [8; 9], as a low-cycle fatigue problem. Therefore, coupled thermal-vibration screening can be treated as...
