Fatigue Crack Growth in 7050T7451 Aluminium Alloy Thick Section ...

DSTO-TR-1477considerable concern to aircraft structural integrity. This is borne out by the case where peeningdid not appear to be effective in the KSIF178 specimen, as well as previous experience with theFT55 and the FT488/2 full-scale and component tests (Barter, 1996, Athiniotis et. al, 2003). This,while pessimistic about the practical advantage of glass bead peening for aircraft repair, does notconsider ceramic bead peening which has been shown (Sharp & Clark, 2001) to produce surfaceswith a reduced scatter in their fatigue lives, and thus may produce a considerably smaller 1 in 1000EPS than for glass bead peening.5. ConclusionA series of fatigue coupon tests were carried out to investigate the effect of glass bead peening onF/A-18 aircraft 7050T7451 components. This type of surface treatment has been used as a repair toextend the life of many critical regions on several of the fracture critical bulkheads of the aircraft.These tests produced the following results:1. The peening had in all cases (including KSIF 178) resulted in an extension of the specimenlife when compared to the etched specimens from Barter, 2003.2. The peened specimens gave a scatter on life dependent on the peak stress level. The lowerthe stress the higher the scatter.3. The Life Improvement Factor for the peened specimens compared to the etched specimensreported in Barter, 2003, varied exponentially with the peak stress level reducing to noapparent life improvement when the spectrum peak stress level was about the yield stressof the material.4. The crack growth curves for all specimens had growth rates that were well represented bytwo phases of exponential growth, with a early stage of retarded growth as the crack grewthrough the residual stress layer and a faster growth acceleration phase almost as fast asthe growth of cracking without the effect of peening. A small beneficial effect on thegrowth of the faster phase was noted, although it would need to be further investigated toconfirm this as a consistent effect.5. All lead cracks had initiated in the first applied block of loading. Many other secondarycracks existed in the specimens at failure although most of these were very small.6. Several different measures of the effectiveness of the crack initiating defects in the peeningwere evaluated. The Log normal distributions of the measures appeared to be similarwhen used to predict a large flaw of low probability.In general the quantitative fractography gave valuable information about the nature of the crackgrowth. This allowed a better understanding of the life results and the way that peening improvedthe fatigue life of these specimens.The estimated pre-crack size (EPS) values calculated in this series of tests would appear to showpromise as the starting point for a fracture mechanics based prediction of life in peened 7050T7451alloy parts.6. AcknowledgementsThe author gratefully acknowledges the work of Chris Niessen and Brian Jones of DSTO-PSL forspecimen preparation and performing the fatigue tests.33