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

Fatigue Crack Growth in 7050T7451 AluminiumAlloy Thick Section Plate with a Glass Bead PeenedSurface Simulating Some Regions of the F/A-18StructureExecutive SummaryAVD research into material factors likely to affect F/A-18 fatigue life has highlightedthe critical role played by surface condition in determining the service fatigue life ofaircraft structure. This report presents the results of a fatigue coupon test program.The primary purpose was to obtain results from coupons with a glass bead peenedsurface condition typical of some regions of the critical structure of the F/A-18 aircraft.Another surface condition that could possibly initiate critical cracks was compared tothese results.These specimens were loaded with a representative wing root-bending spectrumderived from the F/A-18 FT488/2 centre barrel bulkhead fatigue test. Several peakstress levels were used. The coupons were representative of the material and geometrytypical of a structural detail that has been found to be fatigue-critical in the 7050T7451high strength aluminium alloy wing carry through bulkheads. The spectrum used hadadditional marker loads added to aid quantitative fractography. These marker loadsare briefly discussed.Following the tests, quantitative fractography was used to produce crack growthcurves for each of the fatigue specimens. This allowed a detailed interpretation of thecrack growth to be made, including a measure of the severity of the flaws from whichthe fatigue cracks initiated. In addition to an examination of the effect of the flaws, itwas found that the peening produced a retarding effect on the early part of the crackgrowth. The extent of this effect was examined by comparison to previously examinedetched specimens. These comparisons are reported.