Cumulative fatigue damage modeling—crack nucleation and early growth

Abstract

Prior research into linear and nonlinear fatigue damage accumulation in metals and alloys is briefly reviewed, and general trends are identified. Nonlinear damage rules offer substantially more accurate, and hence less unconservative, fatigue life predictions than the classical linear damage rule (LDR). Increased accuracy does not require excessive cost of analysis or additional databases. The nonlinear damage curve approach and its double linear damage rule (DLDR) equivalent are models that, at most, require only twice the effort of the classic LDR. These models require no increase in complexity of use, nor do they require additional material property or mission loading information to achieve the improved accuracy. Greatest improvements (factors of 5–10 or more in mission lifetime) accrue when missions include considerable high-cycle fatigue with some severe low-cycle fatigue loadings. Criteria are proposed for judging those severe mission loading circumstances when the classical LDR becomes unacceptably nonconservative.