An Experimental Investigation of High-Shear-Strain-Rate Behavior of Metals

Citation

Deshpande, Nitin Ashok (1999) An Experimental Investigation of High-Shear-Strain-Rate Behavior of Metals. Engineer's thesis, California Institute of Technology. doi:10.7907/naah-mx91. https://resolver.caltech.edu/CaltechETD:etd-02062008-080229

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The present study investigated the mechanical behavior of metals under high-shear-strain-rates and large shear strains. Numerical and experimental investigation of different specimen geometries for shear testing of materials was carried out. Results of numerical simulations showed that shear stress-strain curves calculated from boundary measurement of load and displacement did not match with the constitutive law for the material. The yield stress for the shear stress-strain curve from the boundary measurement was considerably lower than that of the constitutive law whereas hardening exponent was almost the same for two curves. Considerable bending was observed in the shear zone. The results from the boundary measurements were close to the constitutive law for H specimen, an analogue of axisymmetric top hat specimen with top replaced by tool steel punch. A planar version of axisymmetric top hat specimen geometry was studied using finite element analysis. The plane specimen was chosen since it is suitable for temperature measurements in the shear zone. In order to reduce the bending, three types of constraints were considered in the experiments. Quasi-static and high strain rate experiments were carried out on different geometries in the strain rate range, 10[...] to 10[...] s[...]. Relatively rate insensitive material, 2024-T3 aluminum was used to establish the relationship between numerical and experimental results. Experimental results for the axisymmetric top hat specimen were found to be in good agreement with the finite element results, but there was discrepancy between the stress-strain curve from the boundary measurements and the constitutive law. Shear stress-strain curve from the quasi-static test for the plane specimen with external constraint reproduced the results of numerical simulation. A planar specimen with built-in constraint was fabricated using wire EDM. Both quasi-static and high strain rate results matched with the numerical simulation of the same specimen geometry. High shear strains of the order of 1.5 were reached in the experiments on the Kolsky pressure bar. Some amount of thermal softening was observed in high strain rate experiments. It was concluded that both numerical simulations and experiments are required in order to obtain accurate constitutive behavior of the material using top hat specimen geometries. A relationship can be established between the numerical tests and the experiments by conducting the experiments at strain rates where the constitutive behavior of the metal is well known. This relationship then can be used to predict the constitutive law at higher strain rates from the experimental data obtained at high strain rates.

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