Half-cycle theory fatigue life prediction of an aircraft monolithic beam and webs under diagonal tension

A fatigue test of a simply supported monolithic aluminum (7050-T7451 plate) beam with webs under diagonal tension was performed to investigate the fatigue life characteristics using fracture mechanics and the half-cycle theory. The intent of this thesis is to explore the use of new simplified formulas based on the half-cycle theory to predict fatigue life and then compare it with the experimental results. The monolithic beam fatigue test was done under constant load cycles to simulate an aircraft bulkhead in a constant pressure environment and its fatigue life was calculated using the half-cycle first and second order approximation formulas. The first and second order approximations significantly over estimated the remaining number of flights by a range of 25 to 828 percent. The fatigue life using linear elastic fracture mechanics was also calculated for the same monolithic beam using the crack growth Walker equation and the remaining number of flights estimation resulted in non-reliable over estimation. Overall, the half-cycle theory second order approximation proved to be a more reliable fatigue life prediction, especially when the initial crack growth was derived for each stress interval (only a 25 to 139 over estimation).