Reliability and failure patterns arising from fatigue

Abstract

This paper examines the cause of 'knees' or 'freaks' in the life-failure patterns resulting from some fatigue processes. A theory is developed which takes into account the extra fatigue damage associated with the non-elastic behaviour of the material. It appears to account for the knee and agrees well qualitatively with test results, though quantitative comparisons are not possible because the stochastic data required for the calculation are not available. It is shown that 'S'-shaped curves sometimes experienced at higher loading roughness are also due to the same mechanism. The theory suggests that the undesirable features of the failure patterns, whose origin has not hitherto been fully investigated, can be eliminated by a design which is intrinsically reliable with respect to the elastic limit (or with respect to the yield strength if a small loss of reliability is acceptable). Because the knees or freaks appear to be a consequence of the fatigue process and arise with conventionally distributed mechanical properties it is suggested that this design action would be more effective than eliminating the items by quality control.