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

DSTO-TR-1477Sections through one of the specimens surfaces showing examples of cracks growing from a lapand a sharp glass bead cut are presented in Figure 16.Figure 16 Sections through the surface of one of the specimens, showing examples of fatigue cracks growingfrom a lap and a sharp glass bead cut3.7 Quantitative fractography3.7.1 IntroductionQuantitative fractography (QF) was carried out on the largest crack in each specimen. Themethod of calculating the crack depth during measurement of crack growth is set out in Barter,2003. These analyses were made possible by the existence of a fairly distinct repeating patternaided by the marker loads as discussed earlier. The repeating markings were found to be veryconsistent with the type and spacing expected for fatigue growth per spectrum under the testconditions. Usually the repeating pattern from the repeat of the spectrum was fairly easily foundparticularly around the originating flaws, although its distinctiveness did vary from area to areaon the fracture surface. By tracking the most distinct areas and some of the less distinct areas ofthe repeating patterns and combining the data from these regions, a relatively complete picture ofthe crack growth could be assembled assuming that each repeat of the pattern was equivalent to asingle load spectrum. This is supported by previous examinations of fatigue fracture surfaces in7050 aluminium alloy, which have undergone similar fatigue testing, Barter, 1990a, Barter, 1991,Bishop and Clark, 1991, Barter, 1998, Barter and Price, 2000.For the peened specimens, the crack shape could be very distorted from the semi-elliptical shapenormally found in the absence of residual stresses, as noted in the previous section. In all thesecases the QF was carried out on the region as close as possible to the centre and deepest part of thecrack to avoid artefacts in the crack growth curves.3.7.2 Quantitative fractography measurementsFigure 17A&B to Figure 20A&B present the crack growth curves produced from examining eachof the main cracks on the peened specimen fracture surfaces. The crack growth curves areconsidered to be reasonable representations of the rate of cracking of the largest crack in each ofthese specimens. These curves do not contain the flaw depth, which was also measured duringthis examination. Consequently the data adjacent to the origin is distorted with an accelerationrate higher than the true picture. This will be discussed in Section 4. To improve the clarity of therate of growth while the crack is small the data have also been plotted on log crack depth versus15