Two-Parametric Nature of Fatigue and the Intrinsic Mechanisms

Fatigue damage of structural materials is an age-old problem causing the premature failure of components in service. Fatigue requires two load parameters for quantification. For SN-fatigue, Goodman and others have used stress amplitude and mean stress. However, this two-parametric nature is ignored for fatigue crack growth. Instead, the extrinsic crack-closure concept was introduced by Elber in 1970 and has been used extensively since then. We consider the load-ratio effects are intrinsic to fatigue arising from its two-parametric nature. For S-N fatigue, stress range Delta sigma and maximum stress, sigma(max), and for crack growth, stress intensity factors, Delta K, and K-max provide the required parameters. While the cyclic damage is governed by the amplitude, the monotonic modes enter via the maximum loads. The monotonic modes include environmental effects, corrosion fatigue, sustained load crack growth, hydrogen embrittlement, creep effects, transformation-induced plasticity, etc. Thus, the two parameters reflect the microstructural mechanisms contributing to the fatigue damage. We present a systematic analysis of the load-ratio effects of materials including metals and alloys, ceramics, and composites to show the universal application of the two-parametric nature of fatigue reflecting the material response and governing mechanisms, without the need to invoke any crack-closure arguments.