A Model Unifying Both Fatigue Nucleation And Crack Growth Based On Continuous Accumulated Damage

Fatigue process is often described as the nucleation and growth of cracks to final failure. Under the frame of the fatigue analysis, continuum mechanics methods and fracture mechanics concepts are adopted for the crack initiation life and the propagating life, respectively. However, the nucleation and growth of cracks are not two independent stages, but closely related to the continuous fatigue damage process. Therefore, the establishment of an analysis method on predicting the total life of notched components unifying the fatigue nucleation and growth of cracks has both values of theory and engineering application.On the basis of the propagating crack scheme and the continuous accumulated process of the fatigue damage of the crack tip, the nonlinear mathematical relation between the accumulative history of fatigue damage and the crack growth rate was established. A new model unifying the fatigue nucleation and crack growth based on the continuous accumulated damage was proposed. This model can be applied to the constant and variable amplitude loading. The proposed unified model bridges the gap between fatigue nucleation and growth of cracks during the fatigue damage process. The model can describe well the effects of the loading history and the interaction mechanism between the residual stress and the crack contact on the crack growth behavior.Two fatigue life prediction methods was proposed based on the Armstrong-Frederick type cyclic plasticity theory well describing the non-Masing behavior and the multiaxial fatigue damage theory. One was the fatigue life prediction method based on the finite element method. The other was the multiaxial local stress-strain approximate method predicting the notched component with the non-Masing behavior. The first method considers the effect of the local plastic deformation on the stress state at the notch root. The second method is fit for the notched components with small plastic deformation. The capability of predicting fatigue life for the two methods was checked against the fatigue experimental data of the notched components made of 16MnR steel. It is found that two methods can predict well the fatigue lives of notched components under the axial-torsion loading.The fatigue initiation life, the crack growth rate and the fatigue total life of notched components under the constant amplitude loading, the high-low block loading and the over-loading were predicted based on the proposed model. The unified model was verified by the crack growth experimental results of the notched components made of 16MnR steel. It shows that not only both crack nucleation and crack growth can be modeled simultaneously, but also the initial acceleration and the delayed retardation of crack growth under the overload or the high-low sequence loading can be prediced quantitatively. The histories of the stress and the accumulated fatigue damage of the crack tip were studied based on the 2D finite element method. It shows that the notch influencing zone and the contact of the cracked surfaces are the most important factors resulting in the short crack behavior. The overall crack growth process under the variable amplitude loading is controlled by the interaction mechamsm among the crack surface contact and the residual stress field and the enhanced cyclic plasticity due to the crack advance.