Optimization Based Approach to Automate Fatigue Durability Test Load Development

Loads for structural fatigue durability tests are often developed by considering only the load data and ignoring the structure and its failure modes. Typically, in these procedures, the load data is directly used with a fatigue damage calculation method to compute the most damaging loading direction and magnitude. These processes ignore the underlying structure and hence may not arrive at loading modes that are sensitive to the failure modes of the structure. The structure, with its failure modes, wield considerable influence on the test load selection and ought to be considered in the test development. FEA tools can be employed for this purpose. However, due to the iterative nature of the test development process and the repeated FEA analysis it entails, the development task can become tedious. Here, an optimization based approach to automate the test load development process is proposed. This methodology leverages optimization algorithms to arrive at the test load cycles with proper load phasing even when a large number of load channels are involved. This method permits linear or nonlinear FEA procedures with component or system level test setups. This method also allows for maximizing the fatigue damage at the primary ‘key life’ failure location. A range of loading constraints — from constraints based on durability loading histories to constraints due to testing rig limitations — may be applied. In this discussion, a unique approach to setup lab test development problems that are conducive to optimization algorithms is delineated. As a part of this process, a novel approach to set loading constraints by utilizing multidimensional scatter plots of the existing loading histories will also be shown. The effectiveness of well known optimization methods in searching and arriving at the test load cycles will be also highlighted.© 2007 ASME