Life up and down method for determining WFD average behavior of aircraft structures

The widespread fatigue damage (WFD) problem is a serious threat to the integrity and safety of aircraft structures. Evaluation of the WFD average behavior is the base for establishing a limit of validity of the engineering data that supports the structural maintenance program. A life up and down method for evaluating the WFD average behavior of aircraft structures was proposed based on the theory of strength up and down method in fatigue applied statistics. The residual strength test was conducted following the fatigue test with a specified fatigue life level (FLL) to determine whether the residual strength meet the requirement. Taking the mean value of the adjacent two FLL with contrasted results as the FLL could lead to a residual strength equal to the required one. By repeating the tests and statistical analysis of the results, the WFD average behavior could be found out. Test on multiple site damage structure with 5-details was conducted with the proposed method, and a WFD average behavior, with a requested residual strength and a specified loading condition, was provided. The proposed method based on the fatigue reliability can be used both in multiple site damage structures and multiple element damage structures, independent of the specific forms of the structures and the load conditions.