3D Image Correlation Studies of Geometry and Material Property Effects During Split Hopkinson Bar Experiments

Full field measurement of strain and strain rate in split Hopkinson bar experiments using the Aramis three-dimensional image correlation system has previously been thoroughly validated. Local strains and 3D displacements are measured at thousands of points, with data acquisition rates as high as 300,000 frames per second. The new capability of direct full-field local strain measurements at high strain rates provides orders of magnitude more information than the traditional method of inferring the average specimen strain based on elastic wave behavior. For example, now the dynamic necking behavior of a ductile specimen can be directly observed, and valid strains can be obtained regardless of whether stress equilibrium and strain homogeneity exist. Shear bands and complex local strain fields can be quantified. Aramis strains for OFHC copper and 2024 aluminum are compared to a specimen gauge and to elastic wave results. In compression and shear, the results agree, but it is found that in tension, there can be errors in the elastic wave results due to significant deformation outside the gauge region. The influence of specimen geometry and material properties on the accuracy of the elastic wave strain time history and stress/strain curves is explored. The ability to profile a shear localization throughout the complete load time history for a torsion test is also presented.