Application of the S-N Curve Mean Stress Correction Model in Terms of Fatigue Life Estimation for Random Torsional Loading for Selected Aluminum Alloys.

Michał Böhm,Mariusz Kupina,Sławomir Pochwała,Krzysztof Kluger

doi:10.3390/ma13132985

Abstract

The paper presents the experimental fatigue test results for cyclic constant amplitude loading conditions for the case of the torsion of the PA4 (AW-6082-T6), PA6 (AW-2017A-T4) and PA7 (AW-2024-T3) aluminum alloy for a drilled diabolo type test specimen. The tests have been performed for the stress asymmetry ratios R = −1, R = −0.7, R = −0.5 and R = −0.3. The experimental results have been used in the process of a fatigue life estimation performed for a random generated narrowband stress signal with a zero and a non-zero global mean stress value. The calculations have been performed within the time domain with the use of the rainflow cycle counting method and the Palmgren−Miner damage hypothesis. The mean stress compensation has been performed with the S-N curve mean stress model proposed by Niesłony and Böhm. The model has been modified in terms of torsional loading conditions. In order to obtain an appropriate R = 0 ratio S-N curve fatigue strength amplitude, the Smith−Watson−Topper model was used and compared with literature fatigue strength amplitudes. The presented solution extends the use of the correction model in terms of the torsional loading condition in order to obtain new S-N curves for other R values on the basis of the R = −1 results. The work includes the computational results for new fatigue curves with and without the mean stress effect correction. The results of the computations show that the mean stress effect plays a major role in the fatigue life assessment of the tested aluminum alloys and that the method can be used to assess the fatigue life under random conditions.

Highlights

The central motivation of this paper is the fatigue life assessment of the PA4, PA6 and PA7 aluminum alloys under random loading conditions under torsion stress with a global non-zero mean stress value
Given the shear stress distribution performed with the finite element method, we were expecting expecting the place of the initiation of the fatigue crack to be on the surface of the specimen in the place of the initiation of the fatigue crack to be on the surface of the specimen in the lower parts, as it lower parts, as it was mounted to the experimental stand
The own tests as well as modify their calculation procedures by taking into account the mean stress effect in calculation procedure presented in this paper focuses on the application of two popular models, situations where weSmith−Watson−Topper are missing the experimental curves forthe

Summary

Introduction

The central motivation of this paper is the fatigue life assessment of the PA4, PA6 and PA7 aluminum alloys under random loading conditions under torsion stress with a global non-zero mean stress value. Aluminum alloys have many applications these days due to their weight, and they can be used in the automotive, maritime, as well as in the aviation industry These applications are very often connected with the bearing of torsional forces, as the materials are being subjected to variable loading conditions. These conditions cause the premature failure of the material, which can be explained as the effect of material fatigue.

Methods

Discussion

Conclusion