The Effect of Pitting on Fatigue Lives of Peak-Aged and Overaged 7075 Aluminum Alloys
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INTRODUCTION
CORROSION, especially pitting, can adversely affect aircraft structural integrity, since fatigue cracks can nucleate from corrosion pits and grow at an accelerated rate in corrosive environments.[1] In the absence of corrosion-induced flaws, fatigue cracks initiate from extrinsic defects (such as surface roughness, machining marks, and edges) or from constituent particles prevalent in the aluminum alloys.[2,3] However, in the presence of pits, a large fraction of crack initiation is found to occur at the pits. Pitting, on the other hand, is nominally associated with some fraction of the constituent particles present in the aluminum alloy.[4–8] The fatigue life,[9,10] fatigue crack growth rate (FCGR),[11,12] and the effects of microstructural factors and that of aging on fatigue[2,11,13] in uncorroded samples have been documented for 7075 and similar alloys of the 7XXX series. The important observations from these and other studies are that the constituent particles, grain boundary precipitates, and the precipitate-free zone are significant microstructural factors influencing fatigue life and FCGR of 7075 and similar alloys. The 7XXX series is strengthened by aging. Thus, the effect of aging on performance becomes an important issue. One of the temper variants (T73) of the 7XXX alloys was specifically developed with the aim of reducing stress corrosion cracking. In these alloys, the peak-aged temper (T6) has significantly less stress corrosion resistance than the overaged temper (T73). However, studies on a 7010 alloy[11] have revealed that the benefits of higher stress corrosion resistance are not S. DEY, CSIR-Senior Research Fellow, S.K. DAS, Scientist, and I. CHATTORAJ, Scientist and Head, Business Development & Monitoring Division, are with the National Metallurgical Laboratory (CSIR), Jamshedpur 831007, India. Contact e-mail: ichatt_62@ yahoo.com A. BASUMALLICK, Professor and Head, is with the Department of Metallurgy and Materials Engineering, Bengal Engineering & Science University, Shibpur, Howrah 711103, India. Manuscript submitted August 18, 2009. Article published online August 21, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
translated to better corrosion fatigue resistance in the overaged alloy. It is well known that corrosion pitting has a strong effect on the fatigue life of structural aluminum alloys.[5,14–19] The presence of localized corrosion pits modifies the local stress and may ultimately shorten fatigue life.[20] Pidaparti et al.[21] revealed that the pitinduced stresses were responsible for the crack initiation in 2024 T3 aluminum alloy. Many researchers have discovered a connection between corrosion pitting and fatigue cracks.[22–24] Predicting remaining fatigue life of corroded components or samples has been the subject of various studies.[25–31] Some researchers[26–31] had adopted a fracture mechanics approach to life, adapted to the case of pits serving as the initial flaws. Ghidini et al.[22] suggested a probabilistic model to predict fatigue life of 2024-T3 aluminum alloy using f
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