Fatigue size effect due to defects in an AA7050 alloy

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.