Fatigue Life Estimation of Pre-corroded 7075 Aluminium Alloy

Abstract

Heterogeneous microstructures of aluminium alloys such as 7075, which are extensively used in aircraft structures, render them highly susceptible to corrosion. Corrosion can adversely affect aircraft structural integrity since fatigue cracks can nucleate from corrosion pits and grow at an accelerated rate in corrosive environment. During its service life, an aircraft structure typically experiences corrosion between flights and fatigue loading during flight, thus pointing to the need for quantifying the effects of ‘prior corrosion’ on fatigue behaviour. This phenomenon, termed corrosion-fatigue, has an especially detrimental effect on the longevity of structures and leads to the premature failure of a component in service.
In this work the fatigue life of as-received (pristine) specimens were determined at various stress levels. Specimens were also corroded in two conditions viz. 3.5% NaCl for 50 h and applying variable potential at a constant scan rate in same 3.5% NaCl solution. The fatigue life of pre-corroded specimens were also determined and correlated with that of the as-received specimens. Scanning Electron Microscopy was conducted for observation and analysis of fracture surface and corrosion pits. The average pit size, depth and density were also calculated. It was confirmed that corrosion pits and constituent particles act as preferential sites for crack nucleation. Fine non-critical cracks on the corroded surface were also observed.