Identification of fatigue damage mechanism in PA38-T6 aluminum alloy under multiaxial loadings – initial research

Abstract

This paper presents results of the initial research which originated from previous work, which was aimed to study the effect of asynchronous loadings in elastic-plastic strain range. Among the others, a study of fatigue cracks on the fatigued specimens’ surfaces, was performed.The aim of the present work is to identify the mechanisms of initiation and propagation of small fatigue cracks in selected materials under multiaxial loadings, including asynchronous cases. To study the evolution of small cracks and main crack formation, the cellulose acetate thin foil replication technique was utilized. Fatigue tests were performed on thin-walled tubular specimens, which were mirror-polished after CNC machining, to avoid the influence of unwanted factors.This paper presents the first part of fatigue tests, performed on specimens manufactured from PA38-T6 (AW 6060-T6) aluminum alloy. Fully reversed axial, torsional and 90º out-of-phase loadings were applied in elastic-plastic strain regime, with strain control. For these loading cases a shear damage mechanism was identified, based on the observation of small cracks. The small cracks initiated ate grew on maximum shear strain planes. The main crack formed at the very end of fatigue life by coalescence of small cracks of high density, regardless the applied loading case and level. An interesting difference in cracking behavior was observed in case of out-of-phase loading, on the low loading level. Small cracks propagated, and the main crack formed by linking of a few propagating cracks.The research is in progress and will be extended to the lower levels of strain as well as other cases of loadings, including asynchronous loadings. It is planned to conduct the research using other materials, for example non-alloy steel and austenitic stainless steel.