94 Creep data for metallic foams

The limited creep data for metallic foams are consistent with equation (9.3). Figure 9.3 shows log(steady-state creep rate) plotted against log(stress) and against 1/T for a Duocel 6101-T6 aluminum foam, allowing the power, n, and activation energy, Q, to be determined. The measured creep exponent n = 4.5

0.0017

• Compression, Q = 157 kJ/mole a Tension, Q= 174 kJ/mole -

Regression fits

Regression fits

0.0018 0.0019

0.0020

Figure 9.3 Secondary creep strain rate plotted against (a) stress (T = 275 °C) and (b) 1/Temperature (a = 0.42 MPa) for an open-cell aluminum foam (Duocel Al 6101 T6foam, p*/ps = 0.09; Andrews et al., 1999)

is close to the value n = 4.0 for solid 6101-T6 aluminum. The measured activation energy, 166kJ/mole, corresponds well with that for the solid metal (Q = 173 kJ/mole). The dependence of the strain rate on foam density is found from a plot of log(steady-state strain rate) against log(relative density), shown in Figure 9.4: the measured power of —6.4 compares well with the expected value of —6.5 from equation (9.3). The steady-state strain rate of the foam is the same in tension and compression. The reference stress o0 = 31.6MPa.

The results of creep tests on a closed-cell Alporas aluminum foam indicate that it, too, is well described by equation (9.3) at low stresses and temperatures (o < 0.42 MPa and T < 250°C) (Andrews and Gibson, 1999). At higher stresses and temperatures, the behavior becomes more complicated.

10-5

10-6

10-8

10-9

0.01 0.10 1.00 Relative Density (p/ps)

Figure 9.4 Secondary creep strain rate plotted against relative density at constant stress and temperature for an open-cell foam (T = 275 °C, o = 0.42 MPa, Duocel aluminum 6101-T6 foam; Andrews et al., 1999)

Figure 9.5 shows the time to failure tR of a metal foam loaded in tension and compression plotted against the secondary creep strain rate on double-log axes. The value of tR in compression, based on an instantaneous strain rate of five times the steady-state value, is slightly longer than that in tension. The slopes of the lines give values of the parameter, m, in equation (9.2) of 0.96 and 0.83, for tension and compression, respectively, close to the range found by Monkman and Grant (Hertzberg, 1989). The values of the parameter log(C) in equation (9.2) are —2.21 and —1.16, meaning that C is smaller than that found by Monkman and Grant, indicating that the aluminum foams have lower creep ductilities than solid aluminum alloys.

10-6

10-8

0.01 0.10 1.00 Relative Density (p/ps)

Figure 9.4 Secondary creep strain rate plotted against relative density at constant stress and temperature for an open-cell foam (T = 275 °C, o = 0.42 MPa, Duocel aluminum 6101-T6 foam; Andrews et al., 1999)

Creep strain rate (1/s)

Figure 9.5 Time to failure in tension and compression plotted against secondary creep strain rate for an open-cell aluminum foam for a range of stresses and temperatures (Duocel aluminum 6101-T6 foam, p*/ps = 0.09; Andrews et al., 1999)

Creep strain rate (1/s)

Figure 9.5 Time to failure in tension and compression plotted against secondary creep strain rate for an open-cell aluminum foam for a range of stresses and temperatures (Duocel aluminum 6101-T6 foam, p*/ps = 0.09; Andrews et al., 1999)

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