Fatigue size effect due to defects in an AA7050 alloy

Foued Abroug,Franck Morel,Guénaël Germain,Etienne Pessard,G Hénaff

doi:10.1051/matecconf/201816514015

Abstract

One objective of this project is to propose a fatigue design approach that is able to account for a large range of machining surface defects and different component sizes and geometries. Due to the huge size difference between a typical fatigue specimen and large aircraft components it was first necessary to confirm if a size effect can indeed be observed. This was done by introducing different numbers of artificial surface defects on smooth specimens. The material investigated is a 7050 Aluminium alloy (Al Zn6CuMgZr). Plane bending specimens both without and with artificial hemispherical surface defects were tested. The number of defects was varied from 1 to 44 defects per specimen and the defect size ranged from 60 μm to 800 μm in diameter. The test results allow the characterization of both the defect effect and scale effect on the fatigue response of the material. A probabilistic approach based on the weakest link concept together with a proper fatigue crack initiation criterion is used to account for the stress distribution and the size of the highly stressed volume. Predictions using FE simulations show a good agreement with experimental results and illustrate the importance of taking the scale effect into account in HCF.

Highlights

The effect of component size on fatigue strength is a well know problem in fatigue and has been investigated by many researchers since many years
The Kitagawa-Takahashi diagram can be justified as an illustration of the scale effect
The Kitagawa diagram obtained shows a low sensitivity of the AA7050 alloy to the defect size compared to usual fatigue criterion approach (LEFM and Murakami)

Summary

Introduction

The effect of component size on fatigue strength is a well know problem in fatigue and has been investigated by many researchers since many years. Pogoretskii and Karpenko in 1965 [1] observed for example a high fatigue strength decreasing with the specimen length in rotating pure bending on a 37Cr4 steel (figure 1). The Kitagawa-Takahashi diagram can be justified as an illustration of the scale effect. The drop in the fatigue strength that is observed with increasing volume is very often explained by an increase in the probability of finding a critical defect or a weak zone as the volume of highly stressed material increases. The impact of defect size on the fatigue strength is well known and the Kitagawa-Takahashi diagram [2] is a useful tool to design material containing defects (see figure 2). The observed specific size effect will be modeled by using Finite Element simulations together with the weakest link concept

Material

Fatigue results

Probabilistic fatigue criterion

Finite element simulation

Predictions

Conclusions