Failure mode transition speeds in an impact loaded prenotched plate with four thermoviscoplastic relations

Four different thermoviscoplastic relations, namely, the Litonski-Batra. Jo hnson-Cook, Bodner-Partom and the power law are used to model the thermovis coplastic response of a material. Each one of these relations accounts for strain hardening. strain-rate hardening and thermal softening of the materi al. The material parameters in these relations are found by solving an init ial-boundary-value problem corresponding to simple shearing deformations so that the computed effective stress vs, the effective plastic strain curves match closely with the experimental data of Marchand and Duffy who tested thin-walled HY-100 steel tubes in torsion. These four viscoplastic relation s are used to analyze dynamic thermomechanical deformations of a prenotched plate impacted on the notched side by a cylindrical projectile made of the same material as the plate. The impact loading on the contact surface is s imulated by prescribing the time history of the normal component of velocit y and null tangential tractions. A plane strain state of deformation is ass umed to prevail in the plate and its deformations are studied for different values of the impact spc rd. The in-house developed finite element code em ploys constant strain triangular elements, one point integration rule, and a lumped mass matrix. The Lagrangian description of motion is used to descr ibe deformations of the plate. The coupled nonlinear partial differential e quations are first reduced to coupled nonlinear ordinary differential equat ions (ODEs) by using the Galerkin approximation. The ODEs are integrated by using the stiff solver, LSODE, which adaptively adjusts the time: step siz e and computes the solution within the prescribed accuracy. Results compute d with the four constitutive relations are found to be qualitatively simila r to each other and the general trends agree with the experimental observat ions in the sense that at low speed of impact, a brittle failure ensues at a point on the upper surface of the notch tip. However. at high impact spee ds, a ductile failure in the form of a shear band initiates first from a po int on the lower surface of the notch tip. The predicted speed at which the failure mode transitions from brittle to ductile is different for the four viscoplastic relations. The effects of the notch tip radius and the yield strength of the material in a quasistatic simple tension/compression test o n the failure-mode transition speed have been ascertained.