Computational simulation of the relaxation of local stresses due to foreign object damage under cyclic loading

Abstract

In this study, the phenomenon of residual stress relaxation from foreign object damage (FOD) is numerically simulated using a hybrid explicit-implicit finite element method. The effects of both low-cycle and high-cycle fatigue loadings on stress relaxation were studied. FOD is first simulated by firing a 3mm cube impacting onto a plate of Ti-6Al-4V at 200m/s. The FOD impact produces two distinct stress concentrations: one is compressive directly beneath the impact site, the other is tensile around the outer edge of the impact. The plate was then assumed to be subjected to a cyclic fatigue loading. The stress relaxation was investigated under a range of stress ratios and maximum applied stresses. Two different material models were considered for the simulations, namely an elastic-perfectly plastic model and a non-linear kinematic hardening model. Keywords: finite element; impact; foreign object damage (FOD); residual stress; stress relaxation. normal stress component in x-direction, i.e. in the direction of applied load σ ys yield stress 2 σ ys (  ) strain rate sensitive yield stress σ y stress component in y-direction σ z stress component in z-direction 1. Introduction Dynamic impact and subsequent fatigue under cyclic loading conditions demonstrate a very demanding failure mode, which occurs in a number of mechanical applications. Impact leads to either beneficial or detrimental effects on the fatigue performance. For example, the cold working process of shot-peening is used to create a beneficial compressive residual stress field over the surface of the component. This compressive residual stress inhibits surface crack growth and thereby leads to an increase in fatigue life. Spherical shots made from hardened materials are typically used for the shot peening, fired at velocities in the range of 40-100 m/s. [1]. The detrimental effects of impacts are observed in such a phenomenon as Foreign Object Damage (FOD), which occurs primarily during the takeoff and landing periods of aircraft flights, where millimetre sized debris in the form of hard particles such as sand and grit are ingested into aero engines [2][3][4]. These particles impact the fan and early stage compressor blades, typically made from the titanium alloy Ti-6Al-4V, at speeds in the range of 200 to 350 m/s. Impacts on the leading edge of the blades cause the most severe damage, which is in the form of notch like craters and gives rise to stress concentrations, residual stresses, microcracks around the impact site and distortion of the materials microstructure [3]. The combination of the fatigue loading and the residual stress field induced by the impact can make the impact region a preferential site for both fatigue crack initiation and growth. This reduces the fatigue resistance of the blades and in some cases causes them to fail prematurely. It has been reported that the cost of FOD to both the civil and military aircraft industries is greater than $4 billion annually [4]. NotationTo gain insight into the interaction between FOD and fati...