A nonlinear explicit dynamic GBT formulation for modeling impact response of thin-walled steel members

A nonlinear explicit dynamic finite element formulation based on the generalized beam theory(GBT)is proposed and developed to simulate the dynamic responses of prismatic thin-walled steel members under transverse impulsive loads.Considering the rate strengthening and thermal softening effects on member impact behavior,a modified Cowper-Symonds model for constructional steels is utilized.The element displacement field is built upon the superposition of GBT cross-section deformation modes,so arbitrary deformations such as cross-section distortions,local buckling and warping shear can all be involved by the proposed model.The amplitude function of each cross-section deformation mode is approximated by the cubic non-uniform B-spline basis functions.The Kirchhoff s thin-plate assumption is utilized in the construction of the bending related displacements.The Green-Lagrange strain tensor and the second Piola-Kirchhoff(PK2)stress tensor are employed to measure deformations and stresses at any material point,where stresses are assumed to be in plane-stress state.In order to verify the effectiveness of the proposed GBT model,three numerical cases involving impulsive loading of the thin-walled parts are given.The GBT results are compared with those of the Ls-Dyna shell finite element.It is shown that the proposed model and the shell finite element analysis has equivalent accuracy in displacement and stress.Moreover,the proposed model is much more computationally efficient and structurally clearer than the shell finite elements.

Compressive Mechanical Properties of Normalized 10^#Low Carbon Steel

Compressive mechanical properties of 10^# lowcarbon steel with normalizing heat treatment are studied. A Gleeble system is adopted to analyze the quasi-static properties and thermal softening effects of heat treated 10^# steel,while a Hopkinson bar apparatus is used to investigate its dynamic characteristics under different strain rates. The results showthat yield stress of heat treated 10^# steel is more than that of untreated one at room temperature. When the specimens are tested at different temperatures,yield stresses decrease with increasing temperature except 573 K. Moreover,the influence of strain rate on yield stress are verified,which shows that the yield stress increases sharply from 500 s^-1 to 1 890 s^-1,while it changes a little from 1 890 s^-1 to 4 850 s^-1. The results indicate that yield stress is mainly influenced by hardening effect at lowstrain rate and controlled by both thermal softening effect and strain rate hardening effect at high strain rate.

SHEAR BAND FORMATION IN THERMO-VISCOPLASTIC SOLIDS UNDER PLANE STRAIN COMPRESSION

The plane strain compression of a rectangular block is numericallyinvestigated for the study of dynamic shear band development inthermo-elasto-viscoplastic materials from an internal in-homogeneity. As expected, it plays an important role in triggeringthe onset of shear localization as well as thermal softening. And thecompetition between the strain, strain-rate hardening and thermalsoftening exists throughout the process. It is found that shear banddevelops at a 45-degree angle to the compression axis. In the lightof given patterns of deformation and temperature, shear bandevolution accelerated by thermal softening is retarded by theinertial effects.

Experimental and numerical study on the dynamic shear banding mechanism of HfNbZrTi high entropy alloy

High-entropy alloys(HEAs) have attracted considerable attention in recent years because of their unique mechanical properties.In this work, the mechanism of dynamic shear banding(also called adiabatic shear bands, ASBs) in a BCC HEA HfNbZrTi was investigated combining dynamic experiments and numerical simulations. The temperature evolution during dynamic shear banding, which has been believed to play a dominant role during ASB formation in the literature, was measured using high-speed infrared thermal detectors synchronized with a split Hopkinson pressure bar system. The dynamic mechanical behavior of the BCC HEA was described using the Johnson-Cook model accompanied by damage accumulation. The process of ASB formation,considering potential contributions from thermal softening and damage softening, was numerically investigated by controlling the activation of each softening mechanism separately. Based on the results of experimental observation and numerical analysis,dynamic shear banding in this BCC HEA is proposed to be dominated by damage softening, and thermal softening only plays a secondary role, which differs from the thermal-softening-dominated ASB formation in typical FCC HEAs such as the Cantor alloy.

Shear Band Formation in AISI 4340 Steel Under Dynamic Impact Loads:Modeling and Experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.