Model for predicting the load ratio for the shortest fatigue life

When designing composite structures it can be valuable to know which load ratio results in the shortest fatigue life. A model is developed which predicts this. It assumes the fatigue life of specimens to be limited by delamination growth. The delamination growth follows the Paris law. A recently developed delamination-growth model is used to find the load ratio which gives the highest delamination growth rate and shortest fatigue life by calculating the range of change in energy release rate for different load ratios. The model predicts that a load ratio R=−1 gives the shortest fatigue life which is supported by experimental results in the literature.