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

DSTO-TR-1477The largest cracks were found growing from all positions (sides, edges and corners up to about10mm either side of the centre of the specimen) from the specimen surface, although thepredominant position was at or near the external radii in the specimen edge (5”radius) as shownin Figure 13. Here an example of a main crack with two main regions of initiation, both of whichare just within the radius of the corner which was a favoured site for initiation for many of themain cracks. Although the corners were favoured as sites for initiation, these corners had beenrounded with a 1 to 2mm radius to prevent them from being overly significant as initiation sitesand to mimic the type of peening rework carried out on F/A-18 structure.Figure 13 Typical region of crack initiation near a corner. This was the most common position for crackinitiation in these peened specimens. Note that the initiation sites are not in the middle of thecurved section but closer to the transition region.The number of cracks that could be observed intersecting the fracture appeared to increased withthe stress level and specimen life (at a particular stress level).The crack growth around the origins of the cracks, particularly the cracks which occurred in thespecimens loaded at the lower stress levels revealed considerable distortion of the normal crackfront shape. Some examples are shown in Figure 14. Typically the cracks initially grew with alonger surface length than depth compared to cracks in the etched specimens (Barter, 2003), thenrapidly deepened resulting in a bulging out of the centre of the crack front. This is evident inFigure 14B. (The banding in this Figure shows the repeat in the block loading). The shape was tosome extent distorted in this manner due to the deepest point of the crack moving into thematerial that is less affected by the residual stresses produced by the peening, while the remainderof the crack fount is still affected by the residual stress. Odd shaped crack fronts can also occurduring cracking from cold worked fastener holes, Clark, 1991.13