Innovative Unified Damage Mechanisms-Based Model to Predict Remaining Useful Life for Rotorcraft Structures

Recent studies by the PI and the Co-PI of this STTR effort have shown that aluminum alloys and stainless steel in Low Cycle Fatigue (LCF) fail upon reaching a critical entropy; claimed to be an intrinsic material property. The critical entropy has been used as damage metric for prediction of remaining useful life (RUL). Also, similar entropic concept has previously been applied to LCF of composites by the Co-PI of this STTR effort.This encouraging finding motivated us to further advance the analytical, computational, and experimental aspects of entropic concept for High Cycle Fatigue (HCF), Combined Cyclic Fatigue (CCF) and variable amplitude loading. In this STTR effort, we have therefore focused on applying fundamentals of physics and mechanical principles to predict fatigue damage and RUL of metals under a variety of loading conditions. Emphasis is placed on modeling the physical mechanisms underlying fatigue damage initiation and accumulation via irreversible thermodynamics by a detailed study of damage accumulation at the micro and macroscale, then use this basic modeling information for prediction of RUL via measured changes in field variables such as temperature and acoustic emission signals.The systematic approach taken to characterize the entropic state of damage emerges from smart use of sensor technologies, simulations and modeling in a concise form which will result in a robust fatigue damage prognosis tool. Sensor data provide information required to evaluate critical entropy. We have made significant progress by achieving the objectives of Phase I and are therefore confident to continue our progress during Phase II.