Quantification of the fatigue severity of porosity in aluminium alloy 7050-T7451 thick plate

Abstract

Porosity is a well-known cause of fatigue cracking in aluminium alloy components when they are subjected to cyclic loading. This is well understood for many situations where castings are used in critical structure. However, fatigue cracking from porosity can also occur in thick wrought plates of high strength aluminium alloys. Such plates are used in aircraft components and can have significant amounts of shrinkage porosity present due to the lack of sufficient rolling prior to machining. An example of such behaviour has been observed in early production F/A-18 A-D Hornet components, where fatigue cracks nucleated from porosity during several structural durability tests. This demonstrated the potential for porosity to cause fatigue cracks in these airframe components during service. The present study quantifies the fatigue severity of the porosity present in the Aluminium Alloy (AA) 7050-T7451 thick plate that is used for some F/A-18 A-D Hornet critical structural components. To this end, over one hundred polished AA7050-T7451 coupons were fatigue tested to failure and the crack growth from the porosity that precipitated the fatigue failure of each coupon was measured using quantitative fractography. The data measured for each porosity discontinuity was then used to determine the fatigue crack depth that would have produced equivalent crack growth if it had existed at the start of the fatigue life. This crack depth, denoted the equivalent pre-crack size of the porosity, is considered to represent the fatigue severity for each porosity discontinuity examined here and the collated data were used to estimate the distribution of porosity fatigue severities in AA7050-T7451 thick plate. The authors propose that using such a metric to describe the initial condition of a structure facilitates the use of fatigue crack growth prediction models to make deterministic and probabilistic fatigue life predictions for realistic service loading spectra. Moreover, it allows the fatigue severity of porosity to be compared to those of the other discontinuity types that cause fatigue cracks in similarly manufactured AA7050-T7451 components.