Environmental resistance applied-stress effects

A plastic material that is resistant to a given chemical in an unstressed state may crack when exposed under stress to the same chemical. This stress may arise not only from externally applied forces but also from internal stresses due to molding operations. Most plastics suffer from this phenomenon of environmental stress cracking, and it is the cause of many failures in service. Several test methods have been developed to quantify this effect. 

Thermal effects are the most cormnon of the environmental effects, since all components will have a temperature coefficient of expansion and matty will have a temperature coefficient of resistance. Thns, besides possible resistance changes with temperatnre, there will be dimension changes, with the resulting possibility of stress forces being applied to mounted components. Since semiconductor devices are pressine sensitive, ambient temperature changes may have a marked effect on the performance of equipment and circuits incorporating any snch... 

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