A general approach for fatigue life prediction

The crack nucleation and growth regions are traditionally modeled with completely different methods with no quantitative relationships between them. In addition, a number of fitting parameters are needed in order to consider different effects. The objective of this work is to develop a robust approach for the prediction of fatigue life from crack initiation to final fracture. The approach bridges the gap between crack initiation and crack growth. Based upon the conception that fatigue damage is directly related to the stresses and strains inside the material, it is assumed that both crack nucleation and crack growth are governed by the same fatigue damage mechanisms and that a single fatigue damage criterion can model both stages. The basic rule is that any material point will fail and form a fresh crack on a material plane if the total accumulated fatigue damage on this material plane reaches a limit. The corresponding material plane is called 'critical material plane'. Crack growth is treated as a process of continuous crack nucleation without using the stress intensity factor or J-integral concept. The approach consists of two steps: stress analysis and fatigue damage prediction. With the implementation of a robust and accurate cyclic plasticity model, elastic-plastic stress analysis is conducted for the component to obtain the detailed stress-strain responses. Using the stress and strain outputted from the stress analysis, a general fatigue criterion is developed to predict fatigue crack nucleation and growth. Due to the usage of the concept of critical material plane, the crack growth direction can be determined in this approach as well as the crack growth rate. Notched specimens made of 1070 steel were tested from crack initiation till fracture under constant amplitude loading (Mode I), constant amplitude loading with artificially induced surface contact (Mode I) and loading with direction change (Mixed Mode I/II). The approach was applied to predict the total fatigue life of 1070 steel. The predicted fatigue lives were in excellent agreement with the experimental observations. The predicted crack growth direction for the cases with loading direction change (Mixed Mode I/II) correlates well with the results from the experiments. All the material constants used in the approach were obtained from testing smooth specimens for crack initiation. Therefore, the crack growth behavior can be predicted without any crack growth experiments.