Mechanical relaxation of localized residual stresses associated with foreign object damage

Abstract

Foreign-object damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue (HCF) life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations. However, recent experimental studies have shown the unexpected propensity for impact-induced fatigue crack formation at locations of compressive residual stress in the vicinity of the impact site. To address this issue, in situ and ex situ spatially-resolved X-ray diffraction and numerical modeling are utilized to show that the initial residual stress state can be strongly relaxed during the fatigue loading process. The magnitude and rate of relaxation is strongly dependent on the applied loads. For a Ti–6Al–4V turbine blade alloy, little relaxation was observed for an applied maximum stress of 325 MPa (0.35σ_y, where σ_y is the yield stress), and cracks tended to form in subsurface zones of tensile residual stress away from the damage sites. In contrast, at an applied maximum stress of 500 MPa (0.54σ_y), equal to the smooth-bar 10⁷-cycle endurance strength, cracks tended to form at the damage sites in zones of high stress concentration that had initially been in strong compression, but had relaxed during the fatigue loading.

Introduction

The ingestion of airborne objects or debris into aircraft turbine engines can lead to severe reductions in the expected high-cycle fatigue (HCF) life of impact-damaged components [1], [2]. In a simplified simulation of such damage using high-velocity impacts of hard spherical objects onto a Ti–6Al–4V blade alloy, studies [3], [4] have shown that the endurance strength can be reduced by as much as 50% and the number of cycles to failure at a given stress amplitude decreased by several orders of magnitude. The reduction in fatigue life due to such foreign-object damage (FOD) was reasoned to be associated with four main factors: (i) impact-induced residual stresses, (ii) microcracks formed upon impact, (iii) the stress-concentrating effects of the impact site, and (iv) distortion of the microstructure.

In an effort to specifically quantify the contribution of residual stresses, spatially-resolved synchrotron X-ray diffraction experiments were carried out to measure local residual stress values in the vicinity of the impact sites [5]. While the presence of such local stresses was thought to influence the premature initiation of fatigue cracks and their early growth during subsequent fatigue cycling, these preliminary studies [4], [5], [6], [7] indicated the potential relaxation of the impact-induced residual stresses during fatigue loading, implying that in this relaxed state, the residual stresses might not be a significant driving force (or mitigating force) in crack formation and propagation.

It is the purpose of the current paper to evaluate the redistribution and/or relaxation of impact-induced residual stresses during mechanical fatigue loading using in situ and ex situ synchrotron X-ray diffraction, to compare the observed behavior with numerical modeling of the damage and subsequent relaxation processes, and to use this information to develop a better understanding of the driving force for the initiation and subsequent propagation of cracks at sites of impact damage during HCF loading.