Effects of Load Sequence and Block Loading on the Fatigue Response of Fiber-Reinforced Composites

Abstract

The vast majority of fatigue loading experiments are constant-amplitude tests, although this type of fatigue loading is hardly present in real in-service fatigue loading conditions. However, due to the expensive and time-consuming nature of variable-amplitude experiments, their effect is often assessed by performing block loading experiments with various low-high and high-low sequences. In this article, the effects of load sequence and block loading on the fatigue damage development in fiber-reinforced polymer composites is investigated. First it is shown that the opinions in the open literature on the damaging effect of low-high and high-low load sequences are divided. Next the effect of block loading on the bending fatigue behavior of composites is experimentally tested and numerically simulated with a newly developed fatigue damage model. Finally, numerical simulations show that the transitions from low to high stress levels are the most damaging, and that the number of transitions and their relative importance in particular determine which block loading sequence (low-high or high-low) is the most devastating.