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

DSTO-TR-1477beads, fragments of which may become embedded in the surface, excessive dwell times due tomanual application of the peening process resulting in deep folds and laps, and incorrect peeningof corners – folding over sharp corners by peening towards the edge, then burring this materialaround the edge preventing the surface beneath the burr from being peened.To reduce the risk of bead fracture while maintaining the controllability of glass beads, ceramicbeads were introduced. These are denser and stronger than glass while being lighter than steelshot. Tests comparing ceramic bead peened specimens to glass bead peened specimens found thaton average they gave a similar life – the difference was that the glass bead peened specimens hada larger scatter in fatigue life as the result of the damage caused by fractured beads (Sharp et. al.,1994). Given that glass bead peening if correctly used, generally gives reasonable lifeimprovements, and that several of the critical areas of the F/A-18 aircraft structure have beenglass bead peened, this method was adopted to prepare the surfaces of the coupons tested andreported on here.3.1 Introduction3. Fatigue testingA series of fatigue tests designed to represent typical RAAF service loading were carried out. Thespecimens were of a type used previously to compare spectra developed during the lead-up to thetesting of full-scale F/A-18 structure both in Canada and Australia (DSTO) under the IFOSTP,Simpson et. al., 2002. These previous tests also included tests carried out with etched surfaces,surfaces similar to those of F/A-18 fracture critical 7050 components, using the same spectrum asused here, Barter, 2003. Prior to the etched surface specimen testing, testing was either carried outusing different spectra or unrepresentative surface conditions. In the case of the peened specimens(Sharp and Clark, 2001) testing was mostly aimed at disclosing some of the critical factors inproducing a reliably peened surface rather than establishing the LIF for varying stress levelsunder a representative spectrum.3.2 Specimens geometryThe specimens were of the simple ‘dog bone’ type with a continuous 127mm radius on either sideresulting in a 15mm wide test section. The thickness of the specimens was 6.35mm giving a crosssectionalarea of 95.25mm 2 at the specimen centre. A schematic of the specimens is shown inFigure 5. The K t of 1.037 for the radius was calculated by FE analysis. This specimen was originallydesigned to mimic a critical low K t detail of an F/A-18 Y488 centre-section bulkhead, for which alot of data has already been generated using this specimen or similar specimens of a closelyrelated configuration. Although the Y488 bulkhead detail in question has a 6” (152.4mm) radius,the K t was not significantly different (Molent, Ogden and Pell, 2000) and the 127mm radiusallowed a shorter specimen that used less material. Experience with fatigue cracking in thesespecimens indicated that critical cracks could occur up to about 15mm on either side of thespecimen centre. Using, for simplicity a constant width of 10mm over this region an estimate ofthe approximate surface area likely to generate a critical crack in each specimen was about850mm 2 . Therefore for each stress level with five specimens tested about 4270mm 2 of peenedsurface was tested. Over this area the specimen varies in width from 15mm at the centre to about16mm at 15mm from the centre. This will result in about a 5.6% drop in the net section appliedstress at 10mm from the centre. Countering this, the rounding of the corners prior to peeningresulted in a measured drop in cross-sectional area of between 1 and 3.5%. More on the criticalregions for peened surfaces will be presented later.5