Experimental and postprocessing procedures for the response of sheet metals to high strain rate

Metals subjected to high strain rates in Hopkinson bar testing remarkably increase their temperature, so that thermal and dynamic effects are always interleaved. Experimental procedures in such field are not clearly standardized and the most common methods for tensile testing and postprocessing cannot highlight some crucial aspects of the dynamic response of metals. When sheet metal specimens are tested instead of bulk specimens, the accurate derivation of the flow curves are further complicated due to intrinsic strain nonuniformities induced by the specimen geometry and to possible material anisotropy. The local strain peaks calculated by digital image correlation, together with the specimen elongation / shrinking evaluated on the deforming specimen by optical methods, deliver remarkably different estimates of the stress-strain curves and of the strain rate histories for the same given test. Such differences are discussed in this paper together with the assessment of the anisotropic response of the material at static, intermediate and high strain rates.