A model for fatigue crack growth, part II: modelling*

A fatigue crack growth prediction model for metals is presented. It is based on experimental evidence obtained on the aluminium alloy Al 7475‐T7351, which is presented in Part I of this paper. This part of the paper generalises the results from Part I and develops a new fatigue crack growth prediction model for variable amplitude loading. The central parameter of this model is the crack propagation stress intensity factor,KPR . The general description of the relatively simple loading sequences in Part I of this paper in terms ofKPRleads to a scheme that is denoted as the ‘Fatigue Crack Growth Map.’ This map defines all possible crack growth conditions and is the heart of the crack growth prediction method. It is shown that any arbitrary loading spectrum consists only of three different types of load cycles, Type I, II and III. This paper elaborates on the specific conditions that lead to a specific type of cycle and explains the determination of the correspondingKPRvalue. The model determinesKPRcycle by cycle throughout a loading spectrum.KPRdetermines the driving force, ΔKeff , for the following cycle. The model requires the knowledge of the yield strength and the intrinsic threshold value, ΔKT , for the material, as well as five different functions that also depend on the material. Phenomenologically, the model accounts for residual compressive stresses ahead of the crack front, theintrinsicresponse of the material and crack closure, which is anextrinsiceffect. A few variable amplitude loading sequences are presented to compare predictions ofKPRwith experiments. The results are in encouraging agreement.