B3 S N data for metal foams

Test results in the form of S-N curves are shown in Figure 8.5 for a number of aluminum alloy foams. Tests have been performed at constant stress range, and

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° Compression (0.1) ■ Compression (0.5) x Tension (R=0.1) o Shear (R=0.1)

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° Compression (0.1) ■ Compression (0.5) x Tension (R=0.1) o Shear (R=0.1)

(Shear)

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(Shear)

111 lid

104 105 Cycles

106 107 1

Higher Density Materials, p/ps ~ 0.3

Higher Density Materials, p/ps ~ 0.3

Materials, p/ps ~ 0.15

102 103 104 105 106 107 108 Cycles (b)

Figure 8.5 (a) S-N curves for compression-compression and tension-tension fatigue of Alporas foam (relative density 0.11). (b) S-N curves for compression-compression fatigue of Alulight foam (R = 0.1). (c) S-N curves for compression-compression and tension-tension fatigue of Alcanfoam (R = 0.1 and 0.5). (d) S-N curves for compression-compression and tension-tension fatigue of Duocel Al-6101-T6 foam of relative density 0.08

Endurance limit

101 102 103 104 105 106 107 108 Cycles

Endurance limit

101 102 103 104 105 106 107 108 Cycles

a max a pl apl apl max

102 103 104 105 106 107 Cycles (d)

Figure 8.5 (continued)

a max the number of cycles to failure relates to specimen fracture in tension-tension fatigue, and to the number of cycles, N/, to initiate progressive shortening in compression-compression fatigue. Results are shown in Figure 8.5(a) for an Alporas foam, in Figure 8.5(b) for an Alulight foam, in Figure 8.5(c) for an Alcan foam and in Figure 8.5(d) for a Duocel foam. The following broad conclusions can be drawn from these results:

1. The number of cycles to failure increases with diminishing stress level. An endurance limit can usefully be defined at 1 x 107 cycles, as is the practice for solid metals.

2. The fatigue life correlates with the maximum stress of the fatigue cycle, CTmax, rather than the stress range Act for all the foams considered:

compression-compression results for R = 0.5 are in good agreement with the corresponding results for R = 0.1, when amax is used as the loading parameter.

3. There is a drop in fatigue strength for tension-tension loading compared with compression-compression fatigue. The fatigue strength is summarized in Figure 8.6 for the various aluminum foams by plotting the value of <7max at a fatigue life of 107 cycles versus relative density, over a wide range of mean stresses. The values of amax have been normalized by the plateau value of the yield strength, apl, in uniaxial compression. The fatigue strength of fully dense aluminum alloys has also been added: for tension-tension loading, with R = 0.1, the value of amax at the endurance limit is about 0.6 times the yield strength.

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Relative density, p/ps

Figure 8.6 Ratio of amax at the endurance limit to the monotonie yield strength apl for foams, compared with that for tension-tension fatigue of fully dense aluminum alloys at R = 0.1

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Relative density, p/ps

Figure 8.6 Ratio of amax at the endurance limit to the monotonie yield strength apl for foams, compared with that for tension-tension fatigue of fully dense aluminum alloys at R = 0.1

We conclude from Figure 8.6 that the fatigue strength of aluminum foams is similar to that of fully dense aluminum alloys, when the fatigue strength has been normalized by the uniaxial compressive strength. There is no consistent trend in fatigue strength with relative density of the foam.

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