0 10 20 30 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 Strain Rate (s-1) σ (MPa) Peak Stress Plateau Stress Figure 5: Flow stress (ε = 5%) and initial peak stress versus strain rate for Fraunhofer foam. Lines are drawn to indicate trends. Analysis of high-speed photos of SHPB tests [12] showed that, as for quasistatic tests [6], isolated deformation/damage bands nucleate early in Alporas foam, at stress levels well below the plateau flow stress. Local strains in these bands approached 50%, and lead to flexing, kinking, and buckling of cell walls. Within the local buckled zones, explosive rupture (“blowout”) of the cells was observed for samples tested at high (ε& > 400 s-1) strain rates, versus more stable tearing at lower strain rates. Microstructural investigations demonstrated that Fraunhofer foam also fails via discrete deformation bands, which likewise form under both quasistatic and dynamic loading conditions. However, local failure at high strain rate generally seems to be a consequence of the growth (tearing) of pre-existing cell wall flaws, rather than through cellular “blowout.” Microscopic examination of untested specimens appeared to reveal tiny (∼ 10 µm) holes, whose existence was verified by flotation experiments. Identical samples of Alporas and Fraunhofer foam were immersed in water; the former floated for days, while the latter sank irreversibly in less than 30 minutes. DISCUSSION The implication of the Alporas results is that buildup of gas pressure within cells eventually causes some of the cell walls to rupture. Specimen collapse occurs when there are no longer sufficient cell walls intact to support the compressive load. This view is supported by the absence of strain rate strengthening (other than what is probably a mild intrinsic material contribution) in the drilled versus the closed-cell specimens tested in the SHPB. Clearly, it is not the rate of gas flow that is paramount since the average strength of drilled samples essentially coincides with that of closed-cell ones. Thus, the fact that the cell walls must rupture sequentially in order to permit the internal gas to exit the structure must account for the strain-rate sensitivity of the closed-cell structure. Such a surface-to-interior rupture sequence obviously is not required for the collapse of the drilled foam metal.
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