Fracture surface formation of notched 2017A-T4 aluminium alloy under bending fatigue

Wojciech Macek

doi:10.1007/s10704-021-00579-y

Abstract

The effect of cyclic loading on facture surface topology in notched components made by aluminium alloys is not completely clear. Fractography and fracture mechanics can help to understand this interdependency. This paper aims to study the distribution of the fracture surface roughness of notched 2017A-T4 aluminium alloy after bending fatigue using an optical focus-variation surface measurement technique by applying the fracture zone concept. The effects of stress level at the notch root and the load ratio on fatigue crack growth and fracture surfaces are analysed. Profile and areal surface parameters of four fracture surface regions were investigated at incremental crack lengths of the specimens. Studies have confirmed that the surface areas associated with the main stages of fatigue phenomenon (i.e. crack initiation, crack propagation, and final rupture) have significant differences in roughness which can be explained by the different loading scenarios. Overall, fatigue fracture surfaces have smallest roughness values at the crack initiation stage and a gradual increase during the fatigue crack growth stage.

Highlights

There are numerous methods to study the different mechanisms of fatigue phenomena, which are reflected in the increasing number of fatigue testing approaches for the most critical cases (McDowell 1989; Berto and Zappalorto 2011; Martins et al 2020; Moreira et al 2020; Mendes et al 2020)
The failure element on the fracture surface depends on the material type and the loading conditions under which it is subjected during its lifetime (Carpinteri et al 2002; Faszynka et al 2016; Branco et al 2018; Correia et al 2018; He et al 2021)
This paper aims to investigate the effects of bending stress level and stress ratio on fatigue crack growth rates and correlate the effects with the associated values of the fracture surface parameters measured on different locations of the fracture zones, i.e. different areas and profiles of fracture surfaces collected in the experiments

Summary

Introduction

There are numerous methods to study the different mechanisms of fatigue phenomena, which are reflected in the increasing number of fatigue testing approaches for the most critical cases (McDowell 1989; Berto and Zappalorto 2011; Martins et al 2020; Moreira et al 2020; Mendes et al 2020). One of the methods of fracture-surface topography analysis is FRASTA, developed by SRI International (Kobayashi and Shockey 2010) and further improved by (Cao et al.2014; Martelo et al 2019) Another approach, called total fracture area method, was used by Macek to analyse the fracture topographies for various engineering materials and loading cases, including single bending, single torsion and their combinations (see, for example, the reference (Macek et al 2020a, 2021)). This and other cited works present robust attempts to apply various fatigue fracture surface parameters (such as Rx profile parameters, areal Sx, volume Vx, as well as the fractal dimension Df) to explain the effect of various fatigue variables, namely loading type and specimen geometry. Crack lengths and the number of cycles to failure were compared to the metrological parameters of fractured surfaces, showing a direct interdependence

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