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Inelastic wave propagation in metal rods Santosham, Thomas V.


Experimental results are presented on the propagation of strain waves in long rods of mild steel (type-1020) aluminum (AA2024S) and copper (soft electrolytic). Strain pulses of order 500 μ in/in amplitude were generated by mechanical impact. The average strain rate during loading was of order one per second. The length and diameter of the specimens and the dominant frequencies in the strain pulses were such that one-dimensional conditions prevailed. Budd Metalfilm resistance type strain gauges were used for recording the strain pulses. Electromagnetic disturbances were effectively eliminated by proper grounding and shielding. Elastic wave propagation in mild steel, aluminum, and copper was studied. For the steel specimen, there was no apparent attenuation or dispersion of elastic waves. However, significant attenuation and dispersion were observed in aluminum and copper specimens, a 30 percent reduction in amplitude occurring in aluminum over a distance of 8 feet. Comparison of the Fourier transforms of the strain pulses in copper and aluminum at different positions along each specimen revealed that amplitude decreased exponentially with distance and that phase angle varied linearly with distance. Furthermore, the observed attenuation and phase velocity were frequency dependent. These results conform to the behaviour of strain pulses propagating in linear visco-elastic materials. Complex compliances for aluminum AA2024S and soft electrolytic copper were derived over the frequency range 400-6000 c.p.s. from the variation of attenuation and phase angle measured in these tests. Approximate three-parameter models suitable for estimating internal damping in these two materials were also determined. Plastic wave propagation in statically prestressed rods of aluminum and copper was investigated. In copper, it was observed that strain increments propagate at constant velocity along the rod and that the velocity of propagation decreases with increasing strain. Strain-rate independent theory is thus applicable to the description of plastic wave propagation in copper, but the dynamic stress-strain curve for the material lies well above the quasi-static one. Furthermore, experimentally observed loading-unloading boundaries in copper resemble the shape predicted by Skobeev and calculations based on these boundaries are compatible with the strain-rate independent theory. It was found that annealed aluminum (AA2024S-0) does not possess a smooth quasi-static stress-strain relation and exhibits unstable behaviour under dynamic loading.

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