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

DSTO-TR-1477A (KSIF152)B (KSIF107)Figure 14 Two views of the regions about initiations of the fatigue cracking in the peened specimens. ‘B’shows the approximate position of one of the marker bands – red dashed line, indicating the oddshaped crack front while the crack was small. The arrows point out some of the block repeats.Crack initiation was in all cases found to be associated with flaws in the surface, as was the casewith the etched specimens. Most usually the origins consisted of closely spaced multipleinitiations along the edges of laps produced during peening. These were usually the mostprevalent type of flaw in the glass bead peened surface and were found, to a grater or lesserdegree, to depend on the quality of the peening, as has been noted previously by Sharp et al, 2001.An example of such an origin site is shown in Figure 15A. Those laps that are most likely toinitiate cracking are usually at the base of peening dents. The second view in Figure 15B shows adeep cut produced by a fractured glass bead. This also produced multiple origins around its edge.Although multiple origins initially occurred the crack quickly formed a single crack front, whichprogressed more rapidly into the material than it did at its flanks.14A (KSIF193)B (KSIF142)Figure 15 Two examples of the types of flaws that initiate fatigue cracks in peened 7050T7451. ‘A’ showsthe closely spaced multiple initiations associated with a broad lap produced during peening(approximately outlined). ‘B’ shows a deep cut and associated damage produced by a fracturedglass bead impact (approximately outlined). This also produced multiple origins around its edge.