Wear Out Failure Patterns and Their Interpretation

It is shown theoretically that fatigue of a component will result in a failure pattern which consists of an initial period of intrinsic reliability, or near zero failures, followed by a rapid increase in failure rate when loss of fatigue strength becomes operative, to be followed in turn by a period during which the failure rate decreases with time or maybe remains constant. By contrast other wear-out modes involving a continuous loss of strength give rise to a steadily increasing failure rate after the period of intrinsic reliability has expired. Practical examples of each type are quoted to substantiate the theoretical deductions. The interpretation of wear out characteristics by Weibull distributions is discussed.