Fatigue crack growth of aluminium alloy 7075-T651 under non-proportional mixed mode I and II loads

Xiaobo Yu,Gwenaelle Proust,Ling Li

doi:10.3221/igf-esis.38.20

Abstract

This study aims to investigate fatigue growth behaviour in AA7075-T651 under non-proportional mixed mode I and II loads. Fatigue tests were performed under cyclic tension and torsion using a thin-walled tubular specimen with a key-hole style crack starter. After the generation of a single-side mode I pre-crack, varied forms of mixed mode loads were applied, which in most cases led to a short distance coplanar growth followed by a long and stable crack path deviation. It was found that under most of the non-proportional mixed mode load cases, the direction of the deviated crack path could not be reasonably predicted using the commonly accepted maximum tangential stress criterion. Meanwhile, in some cases, the crack path directions could be approximately predicted using the maximum shear stress criterion. It was also confirmed for the first time that a long, stable and noncoplanar shear mode fatigue crack growth could be produced in AA7075- T651 under non-proportional mixed mode I and II loads.

Highlights

Fatigue of aircraft structures is traditionally managed based on the assumption of uniaxial loads
It is not clear whether this finding applies to other materials, such as aluminium alloy (AA) 7075-T651, which is typically used for aircraft structures, as no similar tests have been reported for the latter in the open literature
Tab. 1 compares the measured crack directions with those predicted by the maximum tangential stress (MTS) [9] and maximum shear stress (MSS) [10] criteria, which are expressed in Eqs. 1 and 2, respectively: MTS criterion:

Summary

Introduction

Fatigue of aircraft structures is traditionally managed based on the assumption of uniaxial loads. Limited results on A106-93 mild steel [3] reveal that a long and stable shear mode FCG – which is significantly different from the commonly understood open mode FCG – could be produced by non-proportional loads. It means that a stage II fatigue crack can propagate in other than open mode growth, and in such a case, the crack path is absolutely unpredictable by the commonly accepted MTS criterion. For the sake of comparison, the specimen design and majority of the mixed-mode load cases used in the present study are the same as reported in [3]

Objectives

Results

Conclusion