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

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DSTO-TR-14774. Discussion4.1 IntroductionA number of the observations made during this examination will be discussed in the followingSections, and the results are compared with the results of similar testing on specimens with etchedsurfaces (Barter, 2003).4.2 An examination of testing resultsThe results of the testing are presented in Table 5 and Figure 9. The peening resulted in lifeimprovement over the four peak stress levels tested, when compared to the same specimenswhich had been etched (simulating the etching that occurs to F/A-18 structure prior to theapplication of a corrosion protective aluminium alloy coating by an ion vapour depositionmethod, generally known as an IVD coating). This life improvement is variable and is greatest atthe lower stresses and least at the highest stress while the scatter increases with the lower stresslevels. If the range of three log standard deviations above and below the log mean of the results iscompared to the values for the etched specimens at the same stress levels then the LIF (peenedvalues normalised by dividing by the etched log mean values for each of the stress levels) can becalculated. These results are shown graphically in Figure 21. Exponential curves have been fittedto the data sets, although power curves also fit the data very well. Examining these curvesindicates that the LIFs, measured by the three methods pass through one (no improvement) atabout 475MPa, which is very close to the stated yield stress of this material (461MPa). The factthat the curves indicate a loss of peening life extension at about the yield stress might beintuitively expected, although some workers in this field have suggested that ‘shake down’ atstresses below yield would reduce the effectiveness of the cold working at high sub-yield cyclicstress levels. For this spectrum ‘shake down’ does not appear to be significant. From the data it isclear that the LIF depends on the loading spectrum stress (and probably the spectrum form) andshould not be considered to be a constant for the purposes of calculating the effect of a peeningoperation – the life of the detail that is proposed to be peened must be established given theexpected loading spectrum and the local stressing.1000y = 1.29e+06 * e^(-0.025x) R 2 = 0.983y = 7.16e+07 * e^(-0.0334x) R 2 = 0.962y = 2.31e+04 * e^(-0.0165x) R 2 = 0.971y = 574 * e^(-0.00866x) R 2 = 0.999100y = 2.24e+03 * e^(-0.0163x) R= 0.981y = 1.25e+05 * e^(-0.0247x) R= 0.964y = 40.2 * e^(-0.00789x) R= 0.94210010101Peening log meanPeening + 3 logSDPeening -3 LogSDEtched Log meanPeening log mean/etched meanPeening + 3 logSD/etched meanPeening -3 LogSD/etched mean1350 400 450 500Stress Level MPa0.1350 375 400 425 450 475 500Peak stress level MPa.Figure 21 The log mean curves for the etched and peened tests are shown along with the + and – 3 log SDsfor the peened specimens. Dividing the log mean life and the + and – 3 log SDs of the peenedspecimens by the log mean etched specimen lives gives an indication of the ‘life improvementfactor’ for glass bead peening. Exponential curves have been fitted to the data sets and indicate aloss of peening effect at about the yield stress of this material.18
DSTO-TR-14774.2.1 Position of the cracking and crack originsCrack initiations in the peened specimens were all found to be associated with flaws in thesurface, as was the case with etched specimens for the same specimen type and material. Usuallythe origins of the peened specimens consisted of closely spaced multiple initiations along theedges of laps produced during the peening as noted previously. These are the most prevalent typeof flaw in these and other glass bead peened aluminium alloy surfaces, populating the surface, toa greater or lesser degree, depending on the quality of the peening, as was shown by Clayton &Clark, 1988, Sharp et al, 1994 and Sharp & Clark, 2001. Those laps that are most likely to initiatecracking are usually at the base of peening dents. Deep cuts and embedded fractured glass beadswere also noted as crack starters. These flaws also produced multiple origins around their edges.Although multiple local origins initially occur around a flaw, cracks quickly combined to form asingle crack, which progress more rapidly into the material at its centre than its flanks.Although the predominant position of the largest cracks was at or near corner radii, large crackswere found in all positions; sides, edges, close to the corners and away from the corners, generallywithin 10mm of the specimen centre. The corners, or at least close to the corners of the specimenswould appear to be the dominant regions of cracking due to a number of interacting effects: thecorners have less constraint over the flat surfaces as noted before, the probability of folding at thecorners is higher since the peening is bound to be angled to the surface at some time during thepeening process (for example see Figure 22) and over peening is more likely at the corners sincethey receive peening when the flat surfaces are peened and when the radii are peened as aseparate peening operation.Figure 22 An example of a poorly peened corner. Note the laps (arrows) that indicate surface flow awayfrom the corner in both directions (Not from this specimen series).The dominance of a single crack while many other very small cracks were present indicated thatalthough there may be many crack initiations in the peened specimens only a few cracksdominate. This is most likely the result of the very slow crack growth through the residual stressaffected layer; once a crack has passed though, it grows relatively rapidly to failure, dominatingthe damage to the specimen. This results in the specimen failing before most, if any of the othercracks have exited the region of residual stress influence. This led to some very jagged fracturesurfaces (Figure 10B) due to the fast fracture running along the grain orientation rather thanlinking together significant cracks during failure. Fractures similar to this have also been found in7050 thick section plate specimens with a similar grain orientation with machined surfaces,Aktepe, 2000. In these cases the jagged nature of the final fracture was attributed to a lack ofsecondary cracking due to a low population of surface flaws, which led to the final fracturerunning along the grain orientation rather than across the specimen test section.19
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Page 8 and 9: DSTO-TR-1477To build the required l
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Page 12 and 13: DSTO-TR-1477127mm Radius (5’)230m
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Page 18 and 19: DSTO-TR-1477Figure 11 The fracture
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Page 22 and 23: DSTO-TR-1477life axes. The data poi
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Page 28 and 29: DSTO-TR-1477poorly defined areas on
Page 30 and 31: DSTO-TR-1477Simulated flight hours0
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Page 35 and 36: DSTO-TR-1477390MPa(higherslope)420M
Page 37 and 38: DSTO-TR-1477Table 9Specimen No.A co
Page 39 and 40: DSTO-TR-1477considerable concern to
Page 41 and 42: DSTO-TR-1477Barter, S. A., Fatigue
Page 43 and 44: DSTO-TR-1477126.5 41102 0.1638 0.25
Page 45 and 46: DSTO-TR-1477248.5 80743 0.1195 0.12
Page 47 and 48: DSTO-TR-147730.5 9910.1 0.0713 0.10
Page 49 and 50: DSTO-TR-1477104.5 33954 0.0733 0.08
Page 51 and 52: DSTO-TR-147743.5 14134 0.042 0.0468
Page 53 and 54: DSTO-TR-147716.5 5523.6 0.0406 0.05
Page 55 and 56: DSTO-TR-147718.5 6011 0.0264 0.0452
Page 57 and 58: DSTO-TR-147723.5 7635.6 0.0481 0.08
Page 59 and 60: DSTO-TR-1477Table A-13. Results of
Page 61 and 62: DSTO-TR-1477Table A-16. Results of
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