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

Most of current available methods for prediction of fatigue failure -such as cumulative damage models, cyclic plastic energy hypothesis, crack propagation rate models, and empirically-derived relationships -are based on empirical relations and their application requires many unknown parameters that must be experimentally determined or calibrated for specific locations/critical areas. Also, the aforementioned methods concentrate on specific types of loading and single fatigue modes, i.e., bending, or torsion, or tension-compression. In practice, however, fatigue involves simultaneous interaction of multimode processes. Further, their use and execution requires consideration of large factors of safety, often leading to gross over designs that waste resources and cost more fleet maintenance. In this STTR effort, we therefore focus on applying fundamentals of physics and mechanical principles to predict fatigue damage and remaining useful life (RUL) of metals under a variety of loading and environmental conditions. The proposed RUL approach is mechanism-based with little or no reliance on phenomenological parameters. First we develop physics basis for fatigue damage initiation and accumulation via irreversible thermodynamic framework by a detailed study of damage accumulation at the micro-meso-macro scales with and without environmental elements, then use this basic modeling information for prediction of RUL via measured changes in the field.