Influence of the microstructure of aluminium alloys on their residual impact properties after a fatigue loading program

Consecutive loadings of fatigue and impact have been carried out on aluminium alloys. The aim of this study is to quantify the influence of the microstructure on the residual impact behavior after a prior fatigue loading. Two alloys with different chemical compositions and hardening modes have been investigated: 2017A-T3 used in the aircraft industry and 5454-O used in automotive applications. The fatigue pre-loadings were carried out under fully reversed tensile-compression with several pairs (stress level, number of cycles) in the high cycle fatigue zone (10(5)-10(6) cycles). The residual impact behavior was determined under tensile loading, in the range of medium strain rates (about 300 s(-1)). To assess the prior fatigue damage and to follow its evolution during the impact loading, observation of the specimens (surface and fracture surfaces) were made. From this study, two conclusions have been highlighted: (1) there is no direct correlation between a given prior loading and residual behavior, whatever the material; (2) the material aspect is fundamental. At the mechanical (macroscopic) scale, the Al-Mg alloy (5454-O) remains insensitive to the prior fatigue loading whereas the Al-Cu alloy (2017A-T3) undergoes a large modification in its residual performance. At a lower scale, the pre-damage signature appears for the insensitive as well as for the sensitive material. The prior damage and its contribution to the process fracture appear to be strongly linked with the material's microstructure. (C) 2008 Elsevier B.V. All rights reserved.