An Approach for Fatigue Life Prediction

Fatigue damage is generally described as the nucleation and growth of cracks to final failure. These two stages of fatigue damage are often 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 current work is aimed to develop a robust approach for the prediction of fatigue life from crack initiation to final fracture. The approach bridges the gap between the crack nucleation and growth regions. 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 a single fatigue damage criterion can model both stages. A basic rule is that any material point fails to form a fresh crack if the total accumulated fatigue damage reaches a limit. 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. Elastic-plastic stress analysis is conducted for the component to obtain the detailed stress-strain responses. By using a general fatigue criterion, fatigue crack nucleation and growth are predicted. Notched specimens made of 1070 steel were experimentally tested from crack initiation till fracture. The approach was applied to predict the fatigue life of 1070 steel and the predicted fatigue lives were in excellent agreement with the experimental observations.