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

DSTO-TR-14774.2.1 Position of the cracking and crack originsCrack initiations in the peened specimens were all found to be associated with flaws in thesurface, as was the case with etched specimens for the same specimen type and material. Usuallythe origins of the peened specimens consisted of closely spaced multiple initiations along theedges of laps produced during the peening as noted previously. These are the most prevalent typeof flaw in these and other glass bead peened aluminium alloy surfaces, populating the surface, toa greater or lesser degree, depending on the quality of the peening, as was shown by Clayton &Clark, 1988, Sharp et al, 1994 and Sharp & Clark, 2001. Those laps that are most likely to initiatecracking are usually at the base of peening dents. Deep cuts and embedded fractured glass beadswere also noted as crack starters. These flaws also produced multiple origins around their edges.Although multiple local origins initially occur around a flaw, cracks quickly combined to form asingle crack, which progress more rapidly into the material at its centre than its flanks.Although the predominant position of the largest cracks was at or near corner radii, large crackswere found in all positions; sides, edges, close to the corners and away from the corners, generallywithin 10mm of the specimen centre. The corners, or at least close to the corners of the specimenswould appear to be the dominant regions of cracking due to a number of interacting effects: thecorners have less constraint over the flat surfaces as noted before, the probability of folding at thecorners is higher since the peening is bound to be angled to the surface at some time during thepeening process (for example see Figure 22) and over peening is more likely at the corners sincethey receive peening when the flat surfaces are peened and when the radii are peened as aseparate peening operation.Figure 22 An example of a poorly peened corner. Note the laps (arrows) that indicate surface flow awayfrom the corner in both directions (Not from this specimen series).The dominance of a single crack while many other very small cracks were present indicated thatalthough there may be many crack initiations in the peened specimens only a few cracksdominate. This is most likely the result of the very slow crack growth through the residual stressaffected layer; once a crack has passed though, it grows relatively rapidly to failure, dominatingthe damage to the specimen. This results in the specimen failing before most, if any of the othercracks have exited the region of residual stress influence. This led to some very jagged fracturesurfaces (Figure 10B) due to the fast fracture running along the grain orientation rather thanlinking together significant cracks during failure. Fractures similar to this have also been found in7050 thick section plate specimens with a similar grain orientation with machined surfaces,Aktepe, 2000. In these cases the jagged nature of the final fracture was attributed to a lack ofsecondary cracking due to a low population of surface flaws, which led to the final fracturerunning along the grain orientation rather than across the specimen test section.19