Fracture Mechanics Analysis of Aero-Engine Compressor Disc

Abstract

Fracture is a natural reaction of solids to relieve stress and shed excess energy. The design philosophy envisions sufficient strength and structural integrity of the aircraft to sustain major damage and to avoid catastrophic failure. However there are inherent limitations in the methodology, resulting in significant under utilization of component lives and an inability to account for non-representative factors. Ductile materials used in aircraft engine are likely to experience fatigue and stable crack growth before the occurrence of fast fracture and final failure. Fatigue crack propagation can be characterized by a crack growth-rate model that predicts the number of loading cycles required to propagate a fatigue crack to a critical size. Stress Intensity Factors under fatigue loading are below the critical value for quasi-static or unstable crack propagation. Under these circumstances, Linear Elastic Fracture Mechanics helps to characterize the crack growth-rate model. Stable crack growth and final failure generally occur at the very last loading cycle of the life of aircraft. Crack propagation at this stage involves elastic-plastic stable tearing followed by fast-fracture. Since crack growth is no longer under small-scale yielding conditions, Elastic-Plastic Fracture Mechanics is needed to characterize the fracture behavior and to predict the residual strength. The most likely places for crack initiating and development are bolt holes in a compressor disk. Such cracks may grow in time leading to a loss of strength and reduction of the life time of the disc. The objective of this work is to determine Stress Intensity Factor for a crack emanating from a bolt hole in a disk and approaching shaft hole. The objective is achieved by developing a 2D finite element model of a disk with bolt holes subjected to a centrifugal loading. It was observed that stress concentration at the holes has a strong influence on the value of Stress Intensity Factor. Also, fatigue life prediction was carried out using AFGROW software. Different fatigue crack growth laws were compared. This provides necessary information for subsequent studies, especially for fatigue loads, where stress intensity factor is necessary for the crack growth rate determination and prediction of residual strength.