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

DSTO-TR-1477Figure 2Schematic of residual stress distribution below a peened surface. With traditional bead peeningtechniques this compressive layer generally extends 0.2-0.4mm below the surface (from Sharp &Clark, 2001).In reality peening does not produce a consistent surface condition since the resultant residualstresses are the product of many individual impacts by the peening media. These impacts are notdistributed evenly over a surface, so the result is a variation in the final residual stress field. Anexample of a peened 7050T7451 surface as imaged by backscattered electron channellingdiffraction pattern mapping methods in the scanning electron microscope (SEM) is shown inFigure 3. Here the variation in colour shows the variation in the intensity of the residual strains.Surface20 micronsFigure 3Example of strain variation in a peened section of 7050T7451 aluminium alloy as shown bybackscattered electron channelling diffraction pattern mapping. The strain appears to be highestdirectly below the centre of a peening dent (arrow). The lines indicate the sub-grain boundaries.(Note that the image does not show the full extent of the peening, just the variation in strain)Along with the above local variations, the residual stress field is reliant on the constraint that ismaintained by the material surrounding the deformed material. This constraint disappears at afree surface, and can be reduced at external radii (re-entrant radii usually have excellentconstraint). This leads to the most favourable region for crack propagation being at externalcorners of peened components. Since the surface is unconstrained the compressive residual stressusually drops very close to the surface and, because the surface is not smooth, may containregions of residual tension. This allows surface flaws to initiate fatigue cracks more favourably3