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

DSTO-TR-1477To build the required level of understanding, DSTO has over the years undertaken a number ofresearch programs investigating the effects of steel shot, glass and ceramic bead peening on 7050aluminium alloy - an alloy which is used extensively in RAAF F/A-18 aircraft. The overall aim ofthis previous work was to establish a LIF for the peening undertaken on various parts of the F/A-18. This previous work also attempted to provide a means of measuring peening quality byassessing the surface condition after peening, and thereby the reliability of the fatigue lifeimprovement, Sharp & Clark, 2001. While the flaws that initiated cracking in the peening wereunderstood to be an important factor in the effectiveness of the peening, no attempt was made toquantify the way the fatigue cracks grew from them.2.2 Peening and its relevance to the F/A-18Peening attempts to deform material surrounding the impact point of a projectile impacting thetarget surface, against the resistance of the material surrounding the impact. In so doing, acomplex sub-surface residual stress distribution is generated in which, generally, the material justbelow and to the sides of the impact are in elastic compression as shown in Figure 1, produced bythe enlarged surface layer. To balance this compression zone an elastic tension zone surroundsthe compression zone. The transition to this tension zone can be rapid. For the case were thesurface has been covered by impacts the zones may be considered to be in layers receding fromthe surface so that the surface is generally in compression followed by a layer of tension which, onmoving deeper decays to zero as shown in Figure 2. These deformation induced residual stressesare triaxial with a balanced biaxial state and as such will affect the apparent loading of anydiscontinuity, such as a fatigue crack, that is growing through the layers. The compressiveresidual stresses keep the crack closed when a far field tensile stress is applied to a component, toa level related to the magnitude of the compressive residual stresses, reducing the amount of cracktip opening. This reduction in crack opening, and therefore crack tip stress intensity range (∆K)retards the cracks growth. The retardation caused by these compressive stresses still operateswhen the crack tip has grown into the tensile stress field at deeper depths, where the crack tip isheld open by the tensile residual stresses since the crack behind the tip (wake) is still clampedclosed by the compressive residual stresses. Therefore the effect of the residual stresses producedby peening is always to retard the crack growth by raising the far field stress required to open thecrack from its far field unloaded state.ImpactingprojectileFigure 1Surface of metalRegion of material held incompression by surroundingmaterialThe process of deforming the surface as the result of the impact of a projectile produces residualstrain which is more commonly referred to as residual compressive stress. From “Shot PeeningApplications”, 2001.2