For any company and its products to be successful in the market-place, most products must have a worthwhile lifetime and reliability in service to match the expectations of the purchaser. In order to achieve this the designer, among others, must have some feel for the ways in which the useful life in service of a material can be estimated, and must reflect these factors in his design. The purpose of this chapter is to discuss briefly some aspects of prediction of service life and the design needed to ensure that this life and a safe product results. Consideration of the design decisions for correct materials choice and the mechanisms of failure of materials in service is focused on metals and plastics as examples of the whole field of materials.

Predicting service life

Materials respond to their environments in a time-dependent manner, which is usually forecast from accelerated laboratory experiments; results of experiments are compared with what has been seen under real service conditions to prove and extend their validity. Provided that accelerating factors for laboratory tests are correctly chosen, and that the tests themselves are correctly representative of real life service, it is reasonable to expect that results can be used to calculate what must be done to achieve a predetermined life in service. Laboratory tests which attempt to simulate, in an accelerated way, real life exposures must of course be truly representative in a mechanistic way.