An accelerated life test approach for aerospace structural components

In this article, analysis and testing processes of the prototype of a helicopter’s mission system sensor (MSS)-cowling assembly is described. MSS is an add-on unit which is mounted on the helicopter body. In order to ensure that the attached unit functions for a specified life without structural failure, a careful analysis and testing program had been implemented. The approach described here which involves accelerated life testing (ALT) establishes an appropriate methodology for such “integration” projects. The expected mode of failure for the MSS-cowling assembly was metal fatigue under random vibrations. Operational flight tests were performed for obtaining the loading data (accelerations) at the analyzed location. This data was utilized both in the numerical simulation (vibration fatigue analysis through finite element modeling) and in the actual testing of the assembly. In order to qualify the design life in the laboratory, ALT profiles were created by synthesizing the collected data. Fatigue test of the assembly was executed by using these profiles. At the same time strain gage data was collected at the critical locations. Representative alternating stress values were obtained by processing this strain data. By changing the test durations and accordingly the mission profile amplitudes a simple equation is proposed which relates ALT durations with the equivalent alternating stresses. The laboratory testing of the assembly was continued until failure. Results of numerical simulations and testing are discussed.