ORAL/POSTER REFERENCE: ICF 100580OR DYNAMIC COMPRESSIVE BEHAVIOR OF CLOSED-CELL ALUMINUM FOAMS* J. Lankford, Jr., A. E. Nicholls, and K. A. Dannemann Mechanical and Materials Engineering Division, Southwest Research Institute, San Antonio, TX 78228, USA ABSTRACT Compression experiments were conducted on a closed-cell aluminum foam at strain rates ranging from 10-5s-1 to 2000 s-1. The materials demonstrated a strain-rate effect over the range of strain rates investigated; in particular, a marked strength increase at high strain rates is attributed to the stabilizing influence of the gas (i.e., air) pressure within the closed-cell structure. This was confirmed by testing samples with holes drilled through their cell walls to permit intercellular gas flow. An ultrahigh speed imaging system was used to capture the damage development sequence at high strain rates. Evaluation of sectioned microstructures following interrupted testing at a series of strains provided further insight on the deformation mechanism. The strain-rate sensitivity of the foam material appears to be related to sequential (time dependent) rupture of the cell walls that control the exit of cell-wall stabilizing gas from the structure. This conclusion was supported by the results of identical experiments involving a similar foam with very small pre-existing cell wall flaws. INTRODUCTION The contributions of intrinsic strength and relevant material properties to quasistatic deformation resistance of foam metals has been modeled by various authors [1-3]. Further theoretical analysis [4,5] suggests that cell wall imperfections (waviness; variation in wall thickness; non-uniform cell shape) probably are significant factors in local (initial) foam deformation. Experiments have shown [6] that such deformation proceeds by the formation of macroscopic deformation bands that initiate well below the steady-state flow stress. Recently, several investigators have reported [7-10] strain-rate strengthening of Alporas closed-cell Al foam. Several mechanisms have been proposed to explain such behavior, including microinertial cell wall effects [11] on the kinetics of gas flow through broken cell walls [9]. The work reported here was aimed at defining the specific factors that control the strength-strain rate dependence of closed-cell metal foams. __________________ * The authors appreciate the support of the Office of Naval Research under ONR Contract N00014-98-C-0126. They also greatly appreciate the advice and encouragement of their ONR technical monitor, Dr. Steven Fishman.
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