The use of the Hopkinson bar to validate constitutive relations at high rates of strain

Abstract

Attempts to validate constitutive relations at high rates of strain have often used finite element codes, incorporating the proposed constitutive relation, to predict the deformed shape of a given component, e.g. an explosively formed projectile or a Taylor impact test specimen, resulting from the high-rate loading. A more sensitive check, however, of the validity of the given constitutive relation, is its ability to predict the loads required to produce this deformed shape. Use of the Hopkinson-bar test, where the load at which the specimen deforms may be measured, allows such a check to be made. In the present paper tensile Hopkinson-bar tests are performed on Remco iron specimens and high-speed photography is used to monitor the changing specimen geometry, allowing a further check of the validity of the proposed constitutive relation to be made in terms of its ability to predict the observed reduction in diameter at the neck. In addition a thermal imaging camera monitors the specimen surface temperature following the high-speed deformation and allows a comparison to be made between the temperature profile measured experimentally and that predicted in the numerical analysis using two forms of constitutive relation.