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

More documents

Recommendations

Info

DSTO-RR-0208 Figure 15b: Scanning electron microscope pictures <strong>of</strong> a replica from a RAAF reworked surface, Aircraft A21-110 (left) and Aircraft A21-15 (right) (Sharp et al. 1997). 3.2.2 Surface roughness/residual stress measurement Since a smoother surface appeared to provide substantial improvements in fatigue life, it was desirable to explore the possibility that a correlation could be found between surface roughness condition and fatigue life. AMRL purchased a laser surface pr<strong>of</strong>iler, which allowed very accurate non-contact measurements <strong>of</strong> the surface roughness. <strong>The</strong> laser surface pr<strong>of</strong>iler is accurate to 0.1µm over a range <strong>of</strong> 500µm. While this level <strong>of</strong> accuracy is not required in this case, many “stylus” type <strong>of</strong> surface roughness measuring systems grind through the surface due to the complex surface roughness. <strong>The</strong> AMRL instrument is now computerised to allow a 3-D surface pr<strong>of</strong>ile to be built up. For the RAAF aircraft <strong>of</strong>f-cuts, the wide difference in surface quality is very evident in the measured surface roughness, Table 3, where R max is the max roughness depth, R z is the mean roughness depth and R a is the arithmetical mean deviation (See Appendix 1). Table 3: Some Typical F/A-18 FS488 Bulkhead Off-cut Surface Roughness Measurements (Sharp et al 1997). Tail No. R max (µm) R z (µm) R a (µm) A21-13 63.96 30.95 3.66 A21-18 73.12 41.36 5.03 A21-109 103.1 35.43 3.72 A21-112 124.6 55.53 6.23 Table 4 compares coupon surface roughness with the coupon fatigue life at 410MPa (F/A-18 peak applied stress under a USN ST-16 load spectrum, which gives a k t s=432MPa). 22
DSTO-RR-0208 Table 4: Comparison <strong>of</strong> fatigue life and surface roughness for a number <strong>of</strong> peening processes. <strong>The</strong> specimens were tested using the ST-16 load spectrum with a peak stress <strong>of</strong> 432MPa. Surface Process R max (µm) R z (µm) X-ray Residual Stress (MPa) Fatigue Life (Programs) AMRL Glass Bead 19.7 14.5 -230±30 150 Peen Commercial Glass 28.2 21.5 -230±30 65 Bead Peen AMRL Steel Shot 50.3 40.5 -230±30 55 Peen AMRL Polish #800 3.4 2.9 -20±30 63 <strong>The</strong> results appear to show a direct correlation between surface roughness and fatigue life <strong>of</strong> peened aluminium alloys; as roughness increases, life decreases,Figure16. It was therefore considered that it might be possible to quantify the effects <strong>of</strong> F/A-18 fleet peening on fatigue life by measuring surface roughness from replicas obtained from peened areas. However, further results (discussed below) obtained using a more aggressive peening method indicated that the apparent relationship broke down, the problem being that excessive or aggressive peening leads to defects being buried sub-surface and to very deep laps, which do not influence the surface roughness measurement, while greatly reducing fatigue life. <strong>The</strong> conclusion at this stage is that some correlation exists between roughness and life, but where the history <strong>of</strong> the part may have included some aggressive peening, this relationship is unreliable. Roughness vs Fatigue Life Roughness (micrometre) 100 80 60 40 20 0 Roughness Rmax Roughness Rz 0 50 100 150 200 Fatigue Life (Blocks) RAAF Aircraft Figure 16: <strong>The</strong> relationship between measured surface roughness and fatigue life for three different peening conditions. <strong>The</strong> surface roughness on RAAF aircraft samples was R max =64-125µm and R z =30-60µm as marked, indicating a possible shorter fatigue life than the laboratory conditions indicated. 23
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 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

Create successful ePaper yourself

Delete template?

Save as template?