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

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DSTO-RR-0208 Table 7: <strong>The</strong> average log fatigue life results for each peening/rework simulation. Simulation Process Av. Fatigue life after last rework (SFH) Addition <strong>of</strong> fatigue life between reworks (SFH) Total Av. Fatigue Life (SFH) C1 9287 0 9287 C2 16550 0 16550 F1 16026 3000 19026 F2 17624 3000+3000 23624 F3 20327 6000 26327 F4 17865 3000+3000 23865 <strong>The</strong> results are also shown in Figure 25. For comparison a polished specimen (C1) fatigue life is 9287SFH. Column 2 in Table 7 shows that all peening gives an improvement in subsequent fatigue life at the test stress k t s= 410MPa. All other tests were completed on new coupons, which is why C2 (OEM simulated original peening) average fatigue life is slightly lower than the results presented in Table 5 for the same condition. 30000 Polish Peen Fatigue 3000hrs-rework-fatigue Fatigue 3000hrs-rework-fatigue 3000hrs-rework-fatigue Fatigue 6000hrs-rework-fatigue Fatigue 3000hrs-rework-fatigue 3000hrs-ceramic rework-fatigue Number <strong>of</strong> Cycles 25000 20000 15000 10000 5000 0 Cycles to failure from last rework Total Cycles (rework+failure) Figure 25: Left –Average number <strong>of</strong> cycles to failure after the last rework , on the Right-Total number <strong>of</strong> cycles to failure. As can be seen there is little variation in the cycles to failure after the last rework, indicating that sufficient material has been removed before final peening to restore complete life. A significant result in terms <strong>of</strong> aircraft lifing is the reduction in fatigue life scatter when using ceramic beads compared to glass beads. This reduction can be attributed directly to the quality <strong>of</strong> the beads and the reduction in the number <strong>of</strong> broken beads embedding in the specimen surface. It is also interesting that ceramic beads provide a similar fatigue life to glass beads. This result was observed by MCAIR but not by earlier AMRL testing that 34
DSTO-RR-0208 showed a significant improvement in fatigue life using ceramic beads compared to glass beads. <strong>The</strong> reason for the different results is not clear, but may be due the different testing conditions or process details. Figure 25 indicates that an operator can rework a local area a number <strong>of</strong> times and each time the original peened fatigue life is restored to that area. On an aircraft <strong>of</strong> course, the reduction in thickness (increased stress) with each polish would have to be taken into account. 3.3.3 Summary <strong>of</strong> AMRL Rework <strong>Peening</strong> Specification AMRL recommendation was that any future rework peening be done to PS14023 with the following changes; 1) Remove original peening surface damage (or other damage) before peening, 2) Polish surface prior to peening, 3) ALMEN intensity 6-8A, 4) 200% coverage in main area, 5) Nozzle perpendicular to the surface, 6) Total loss glass bead system or recycled (filtered) ceramic beads, 7) Start peening in corners and work to flat region to reduce laps and folds, 8) Ensure peening runout region ie 100% coverage. <strong>The</strong> improvement over the specification set out in section 3.2.3 is the verification that the removal <strong>of</strong> “damage layer” is critical to the process and the life improvement factor derived from any peening process. This complete process was published and distributed in late 1996 (Sharp and Byrnes, 1996) and has been used on both fleet aircraft and structural test articles (Section 3.5). CF adopted this process for any peening repairs to the FT55 test article. To enhance control <strong>of</strong> the amount <strong>of</strong> material to be removed AMRL developed a spring punch process for controlled surface removal (Barter and Houston, 1997). <strong>The</strong> process essentially places a large dent into the surface <strong>of</strong> a controlled depth. Once the dents have been polished away a controlled depth <strong>of</strong> material has been removed. 3.4 <strong>Peening</strong> Life Improvement Factor (LIF) <strong>The</strong> basis <strong>of</strong> any repair is the LIF that can be derived from that repair. For the F/A-18 while many components are peened no LIF is taken account <strong>of</strong> in the lifing. <strong>The</strong> main reason for this has been the unreliability with the peening process. Figure 26 shows how the coupon fatigue life varies with the peening parameters ALMEN intensity and peen coverage. <strong>The</strong> variability in life is an indication <strong>of</strong> why robotic or automated peening is far better than human operator peening. Figure 26 also shows that using 6-8A ALMEN intensity at 200% coverage provides maximum LIF for 7050-T7451 aluminium alloy coupons. 35
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 20 and 21: DSTO-RR-0208 Figure 8: 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 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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