Chaotic behavior of an elastoplastic bar in tensile test over a wide range of strain rate: a numerical investigation

Numerical tensile tests of an elastoplastic cylindrical bar at various high strain rates are performed by the use of a dynamic explicit FEM code (DYNA3D, a public domain version). The effects of mass (inertia) of the body, strain rate and strainrate sensitivity exponent (m-value) on the deformation pattern and the load curve are investigated. As for the material, strain-dependent nth power work hardening property is given by σ=Kεn(1+ε̇)m where ε̇ is strain rate. The range of the prescribed average strain rate is 50–1000/s where the tensile tests with constant average strain rate and with constant tensile velocity are performed. Materials with higher density exert a greater influence on deformation mode at a high strain rate. Even if the tensile speed is less than that of plastic wave propagation, the deformation becomes non-uniform remarkably due to mass effect. It is unexpected that double necking occurs at certain computational conditions. The strain at maximum load point predicted by the numerical simulation does not coincide with the analytically predicted one. Maximum rate of decrease in cross sectional area within the straight portion of the bar is compared with Hartʹs instability criterion based on the imperfection in the cross section. In high-rate tension over ε̇=400/s, deformation behavior is chaotic in the sense that it varies very sensitively with a trivial change in material properties and/or in the prescribed strain rate.