In the case of Fraunhofer foam, it appears that at high loading rates, all of the cells must be able to release pressure as the load increases. This controlled release forestalls to a much higher overall stress level the formation of the first unstable deformation band, whose presence then drops the stress to that (flow stress) needed to continue its propagation/expansion. The instability point may be related to the pressure-driven growth of the natural flaws, an intrinsically rate-dependent process. This behavior contrasts with that of the artificially interconnected (drilled) Alporas foam. Evidently 300 µm holes are sufficient to permit virtually unimpeded (rate-insensitive) egress of internal gas, while the more than order-of-magnitude smaller natural flaws in the Fraunhofer foam require rate-dependent growth before gas loss is sufficient to destabilize the foam structure. It should be noted that the two foams are fabricated via entirely different processes, i.e., Alporas through foaming an alloy melt by adding a foaming agent (TiH2), and Fraunhofer through the powdermetallurgical mixing of alloy powder with a metal hydride foaming agent, followed by compaction heating of the precursor material. Hence, it is reasonable that the relative porosities of their respective cell walls might differ, and that Alporas foam could be more truly closed-cell than the Fraunhofer material. REFERENCES 1. Gibson, L. J. and Ashby, M. F. (1997). Cellular Solids: Structure and Properties, 2nd edition, Pergamon Press, Oxford. 2. Sugimara, Y., Meyer, J., He, M.Y., Bart-Smith, H., Grenestedt, J. and Evans, A. G. (1997). Acta Mater. 45, 5245. 3. Banhart, J. and Baumeister, J. (1998). J. of Mat. Sci. 33, 1. 4. Simone, A. E. and Gibson, L. J. (1998). Acta Mater. 46, 11, 3929. 5. Grenestedt, J. L. (1998). In Porous and Cellular Materials for Structural Applications, pp. 3-13, Schwartz, D. S., Shih, D. S., Evans, A. G., Wadley, H. N. G. (Eds.). Materials Research Society Symposium Proceedings, Vol. 521. 6. Bart-Smith, H., Bastawros, A. F., Mumm, D. R., Evans, A. G., Sypeck, D. J., Wadley, H. N. G. (1998). Acta Mater. 46, 3583. 7. Mukai, T., Kanahashi, H., Miyoshi, T., Mabuchi, M., Nieh, T. G. and Higashi, K. (1999). Scripta Met. 40, 921. 8. Paul, A. and Ramamurty, U. (2000). Mat. Sci. & Engr. A281, 1. 9. Dannemann, K. A. and Lankford, J., Jr. (2000). Mat. Sci. and Engr. A293, 157. 10. Dannemann, K. A., Lankford, J., Jr. and Nicholls, A. E. (2000, in press). “The Effect of High Strain Rate Compression on Closed-Cell Aluminum Foams.” In Proc. of Mtls. Conf. on Fundamental Issues and Applications of Shock Wave and High-Strain-Rate Phenomena, Explomet™ 2000, Albuquerque, NM. 11. Park, C. and Nutt, S. R. (in press). Mat. Sci. & Engr. 12. Davidson, D. L., Chan, K. S. and Page, R. A. (1989). AMD-Vol. 102, Micromechanics: Experimental Techniques, Sharpe, W. N., Jr. (Ed.) Book No. H00539.
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