A strain energy based damage model for fatigue crack initiation and growth

Dr. P.J. Huffman from John Deere in the United States proposed a strain energy based fatigue damage model which uses the strain energy from applied loads and the strain energy of dislocations to calculate stress-life, strain-life, and fatigue crack growth rates. Stress ratio effects intrinsic to the model are also discussed, and he parameterized in terms of the Walker equivalent stress and a fatigue crack growth driving force. Then the method is validated using a variety of different metals with strain-life data and fatigue crack growth rate data available on the SAE Fatigue Design & Evaluation subcommittee database.

The work presented here intends to establish a generalized relationship between the strain energy density imparted by external loads, dislocation strain energy density, and fatigue life in the form of stress-life, strain-life, and fatigue crack growth, which is a unique. The work is unique in the following way: He differs by using the complimentary plastic strain energy density, and includes both elastic and plastic components, but does not sum the elastic and plastic strain energy density terms. This is the only calibration necessary for the strain–life equations.

The model shows a softening of the ‘‘knee” at high and low stress ratios. The ratios at which this softening becomes noticeable dependent on the value of n0. It is important to include residual stress effects from the plastic strains ahead of the crack tip. And also that the residual stresses for positive stress ratios can be accounted for by the estimation, which is another class of good result in this work.

The results show that the strain-life behavior calculated using the present fatigue damage model fits measured data for a variety of different metals including various steels, aluminums, titaniums, nickel alloys, and copper, as well as nodular cast iron. The results are promising as it can be seen that in addition to temperature and processing effects, stress ratio effects are intrinsic to the model.

Dr. Huffman also found that a similar stress ratio effect is shown for the fatigue crack growth rate the model is seen to agree with measurement at higher fatigue crack growth driving forces. However, at low fatigue crack growth driving forces, where influences such as crack closure are more apparent, the model diverges from measurement. The author also identified that particularly at very low strains, the model presented in this work can be used to predict stress-life and strain-life, as well as stress ratio effects from relatively few high strain amplitude measurements for well behaved materials. With additional information or assumptions, this work can also be used to predict fatigue crack growth rates in the Paris’ law regime, along with the corresponding stress ratio effect.