The Effect of Peening on the Fatigue Life of 7050 Aluminium Alloy

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DSTO-RR-0208 Figure 8: <strong>The</strong> polished FS488 bulkhead before peening (left) showing the polished surface and the finished product (right) after peening during ECP45. <strong>The</strong> reason for this change <strong>of</strong> process was AMRL concern that peening over a damaged surface would probably do nothing to improve fatigue life and might in fact degrade the component fatigue life. <strong>The</strong> recommendation from AMRL was; “Given that when shot peening is ineffective it is because <strong>of</strong> a poor surface finish from the peening, and that the original machined pr<strong>of</strong>ile (143 pr<strong>of</strong>ile) will contribute to this effect, it is recommended that the critical flange regions be polished then shot peened. This recommendation does not guarantee any life improvement, but is made on the basis that it is more likely to give an improvement than the alternative.” Sharp et al. (1996), later examined in detail the OEM peened surface finish quality from FS488 bulkheads <strong>of</strong>f-cuts provided during ECP45 (machining 143 pr<strong>of</strong>ile to 6 inch pr<strong>of</strong>ile). <strong>The</strong> results indicated that the surface finish was very poor, with lots <strong>of</strong> embedded glass fragments, and some areas showing no indication <strong>of</strong> peening. <strong>Peening</strong> is now a standard localised rework treatment on the F/A-18 and is used on numerous structural components. Since the FS488 bulkhead rework process was developed, the biggest change has been the progression to ceramic bead peening. Ceramic beads are denser than glass and do not fracture as readily, though it is noteworthy that the peening conditions ie. Almen intensity have not changed. Northrop has conducted research into this area (Lambase (1990), Gallardo (1991) and Brenashan et al. (1990 and 1991)). <strong>The</strong> results indicate an improvement in fatigue life using ceramic beads compared with glass bead 12
DSTO-RR-0208 peening, due to the reduction in surface defects which are caused by glass bead fragments embedding into the surface. Northrop also determined from experiments that a peening intensity <strong>of</strong> 8A provided optimum life improvement for 7050 material. Ceramic bead peening is now used on all RAAF aircraft where localised peening is performed. To ensure complete coverage, a layout ink is used on the area to be peened; this ink allows improved assessment <strong>of</strong> the area coverage. <strong>The</strong> area is peened twice to ensure saturation and coverage. <strong>The</strong> introduction <strong>of</strong> these extra variables (a change in peening media and changed intensity) on the fleet aircraft also complicates the matter <strong>of</strong> determining a life improvement factor (ie fatigue life as a multiple <strong>of</strong> the un-peened fatigue life). Using a specimen that replicated the FS488 bulkhead lug area and a test surface fatigue stress defined by k t s=69ksi 1 in the critical area the results listed in Table 1were obtained by MDA. <strong>The</strong> specimens were peened using a shot bonded flapper wheel to an Almen intensity <strong>of</strong> between 0.006-0.010A. Table 1: Results supplied by McDonnell Douglas Aircraft Company for a peened specimen simulating the FS488 wing attachment lug region. Peened Life (hours) Average Life (hours) No 16960 15030 15995 Yes, initially 21460 21460 Yes, initially and every 6300 hrs 36630 36630 From these results MCAIR arrived at a single shot peening average life improvement factor (LIF) <strong>of</strong> 1.34 and a multiple shot peening average LIF <strong>of</strong> 2.29, although very few coupons were tested. This was then used to support fatigue life estimation for the F/A-18 aircraft. In another series <strong>of</strong> experiments, examining a number <strong>of</strong> variables (pre-cycle [ie. fatigue cracking before treatment] and bead type) Smith (1990) made the following observations in relation to pre-cycling and glass and ceramic bead peening; • For specimens pre-cycled to 2800 Simulated Flying Hours (SFH), shot peening only is required to yield more than 12000SFH <strong>of</strong> crack initiation life (life for a crack to reach 0.01inch), • For specimens pre-cycled to 4200SFH both shot peening and a “bonded doubler” are required to yield more than 12000SFH <strong>of</strong> crack initiation life, • Regardless <strong>of</strong> stress level the shot peening LIF is 3.0 (average) for these experiments. <strong>The</strong> load spectrum used was the USN ST-16. What was particularly interesting about this result is the large improvement in average LIF from 1.34 for flapper wheel to a value <strong>of</strong> 3.0 for glass and ceramic bead peening. It was also interesting that while numerous glass beads fragments were found embedded in the surface, in only two cases (out <strong>of</strong> 6) did the fatigue 1 At all times during the report the specimen k t s stress will be quoted. Appendix 1 has the conversion factors for determining applied stress. 13
Page 1 and 2: The Effect
Page 3: The Effect
Page 7: Contents 1. INTRODUCTION ..........
Page 10 and 11: DSTO-RR-0208 extension method has n
Page 12 and 13: DSTO-RR-0208 Figure 2: A graph illu
Page 14 and 15: DSTO-RR-0208 the total life is long
Page 16 and 17: DSTO-RR-0208 Figure 4: This figure
Page 18 and 19: DSTO-RR-0208 Figure 6: The<
Page 22 and 23: DSTO-RR-0208 crack initiate from an
Page 24 and 25: DSTO-RR-0208 Figure 10: SEM photogr
Page 26 and 27: DSTO-RR-0208 550 USN Spectrum (ST-1
Page 28 and 29: DSTO-RR-0208 Table 2: Four point be
Page 30 and 31: DSTO-RR-0208 Figure 15b: Scanning e
Page 32 and 33: DSTO-RR-0208 Research by Fair et al
Page 34 and 35: DSTO-RR-0208 3.2.3 Summary
Page 36 and 37: DSTO-RR-0208 Peak Spectrum Stress (
Page 38 and 39: DSTO-RR-0208 Figure 21b: A typical
Page 40 and 41: DSTO-RR-0208 10 Crack Depth (mm) 1
Page 42 and 43: DSTO-RR-0208 Table 7: The</
Page 44 and 45: DSTO-RR-0208 200 180 160 8A ALMEN 6
Page 46 and 47: DSTO-RR-0208 Figure 28: A schematic
Page 48 and 49: DSTO-RR-0208 Table 9: Residual Stre
Page 50 and 51: DSTO-RR-0208 Like all the structura
Page 52 and 53: DSTO-RR-0208 Y470 X19 pocket Peened
Page 54 and 55: DSTO-RR-0208 4. Discussion
Page 56 and 57: DSTO-RR-0208 5. Conclusion 1. <stro
Page 58 and 59: DSTO-RR-0208 Clayton J.Q. and Clark
Page 60 and 61: DSTO-RR-0208 Sharp P.K. and Clark G
Page 62 and 63: DSTO-RR-0208 Figure 23: The
Page 64 and 65: DSTO-RR-0208 A.4. Surface Roughness
Page 66 and 67: UK Defence Research Information Cen
Page 68: Page classification: UNCLASSIFIED D

The Effect of Peening on the Fatigue Life of 7050 Aluminium Alloy

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