Papers by Keyword: High Strain Rate

Abstract: The mechanical properties and microstructure evolution of a one step quenching and partitioning steel containing bainite/martensite/retained austenite mixed microstructure was studied though static and dynamic tensile tests (strain rates ranging from10^-3 s^-1 to 5×10² s^-1 ).Scanning electron microscopy (SEM) and electron-back-scattered diffraction (EBSD) were applied to describe the microstructure evolution near the fracture. XRD characterization shows the volume fraction of retained austenite decrease exponentially with the strain rates increased.EBSD phase maps reveal that the first type of retained austenite is less sensitive to strain rate than the second type.

Abstract: In traditional sheet metal forming processes, a thin sheet metal is deformed at relatively low speeds. However, more and more the speed of production processes is increased, very often to take advantage of the beneficial implications that high strain rates can have on the formability of certain metals. In this work, the effect of strain rate on the anisotropic behavior of Ti6Al4V is investigated performing tensile tests in different orientations, namely 0^o, 45^o and 90^o, with respect to the rolling direction (RD), and in-plane shear experiments along RD. Three nominal strain rates, i.e., , 0.5 and 1000, are considered. A novel high-speed bulge (HSB) test developed at Ghent University, designed to deform sheet metals in nearly equibiaxial loading conditions at high strain rates, is used for the dynamic bulge test. The experimental results, presented in this paper, clearly show the need of including the effect of strain rate, and eventually temperature, into the formulation of the yield function in order to correctly model and predict deformation processes performed at high speed.

Abstract: Many mechanical material properties show a dependence on the strain rate, e.g. yield stress or elongation at fracture. The quantitative description of the material behavior under dynamic loading is of major importance for the evaluation of crash safety. This is carried out using numerical methods and requires characteristic values for the materials used. For the standardized determination of dynamic characteristic values in sheet metal materials, tensile tests performed according to the guideline from [1]. A particular challenge in dynamic tensile tests is the force measurement during the test. For this purpose, strain gauges are attached on each specimen, wired to the measuring equipment and calibrated. This is a common way to determine a force signal that is as low in vibration and as free of bending moments as possible. The preparation effort for the used strain gauges are enormous. For these reasons, an optical method to determine the force by strain measurement using DIC is presented. The experiments are carried out on a high speed tensile testing system. In combioantion with a 3D DIC high speed system for optical strain measurement. The elastic deformation of the specimen in the dynamometric section is measured using strain gauges and the optical method. The measured signals are then compared to validate the presented method. The investigations are conducted using the dual phase steel material HCT590X and the aluminum material EN AW-6014 T4. Strain rates of up to 240 s-1 are investigated.

Abstract: Al₂O₃/SiC_p composite ceramics with 2 vol%, 5 vol% and 10 vol% SiC nanoparticles additions were hot pressed at 1650 °C. The polished ceramic discs were prepared and indented at high strain rate using a compressed gas gun with tiny tungsten carbide bullets. The microstructure and mechanical properties were obtained to explain the dynamic deformation behavior of Al₂O₃/SiC_p ceramics. Cr³⁺ fluorescence mapping was used to examine the residual stress and plastic deformation induced on the surface of each target. The residual compressive stress area around the crater was evenly distributed, while the greatest plastic deformation was found at the hitting point of the bullet tip. It can be calculated that the high temperature of 1400K may be produced at the instant of the bullet impact and result in large plastic deformation region and low residual stress of the Al₂O₃/SiC_p ceramic.

Abstract: The mechanical responses of Ti-5Al-2.5Sn alloy at low temperatures were investigated under quasi-static and dynamic tensile loads using MTS system and SHTB system, respectively. Tensile stress-strain curves were obtained over the temperature range of 153 to 298K and the rate range of 0.001 to 1050 s^-1. Experimental results indicate that the tensile behavior of Ti-5Al-2.5Sn alloy is dependent on strain rate and temperature. Yield stress and flow stress increase with increasing strain rate and decrease with increasing temperature. Results also indicate that strain hardening rate of Ti-5Al-2.5Sn alloy is lower at high strain rate, while strain hardening rate varies little with testing temperature. The Khan-Huang-Liang constitutive model was chosen to characterize the tensile responses of Ti-5Al-2.5Sn alloy at low temperatures and different strain rates. The model results coincide well with the experimental results within the tested temperature and rate ranges.

Abstract: Material characteristics at high strain rates are used for various applications involving high speed loading, one of which is plasticity-based impact energy absorbers. In this paper, experimental result of Split-Hopkinson Pressure Bar (SPHB) test of three common structural steels is reported and discussed. The three materials consist of JIS G3101 SS400, API5L Grade B, and S355JR. The material characteristics are presented in Johnson-Cook constitutive equations, and the corresponding parameters have been found through curve fitting. The results are comparable with those of literatures for other carbon steels, hence the acquired data may enrich the database of constitutive equations for various materials, especially carbon steels. The data can then be used for crashworthiness simulation in the future with the corresponding materials.

Abstract: This paper presents an experimental study on the high strain rate compressive behavior of micro-fibre reinforced ultrahigh performance cementitious composite, which is intended to be used as a matrix for slurry infiltrated fibre concrete (SIFCON). Cementitious composite specimens with 5 different types of microfibres, namely aramid, carbon, wollastonite, polypropylene and glass in amounts of 1.5-2.0% by volume were prepared and investigated. Split Hopkinson pressure bar (SHPB) equipment was used to determine the cementitious composite behavior at strain rates up to 1600 s^-1. Quasistatic tests were performed, as well and ratios of these properties at high strain rates to their counterparts at static loading were compared. The dynamic increase factors were calculated. Strain rate sensitivity was observed - compressive strength was found to be increased with strain rate for all tested specimens. Peak stress values, critical compressive strain and post peak behaviour varies for specimens with different micro-fibre reinforcement, which allows to find the optimal reinforcement for high strain rate impacted structures.

Abstract: An experimental characterization of the austempered ductile iron ISO 17804/JS/1050-6/S was performed carrying out tensile tests under different strain rates, temperatures and stress triaxiality levels. Then, composing a yield function surface, a hardening relation, and a damage criterion, a constitutive model was developed to describe the salient features of the observed macroscopic response. In particular, the Mohr-Coulomb yield function was selected to account for the pressure effect observed on the yield surface. A new hardening relation was proposed in order to account for both strain rate and temperature effects. The Bonora’s damage model, developed in the framework of the continuum damage mechanics, was adopted to capture the failure condition under different stress triaxiality levels. The damage model was appropriately modified to account for the effect of strain rate and temperature on the failure strain.

Abstract: We have studied spallation in single crystal of metals under shock at very high strain rate. Our work has been devoted to understanding, and predicting the dynamic ductile damage processes of nucleation, growth and coalescence of voids in these extreme conditions of impact. Recovered sample only indicates final state of damage. Molecular Dynamics calculations are predicting the phenomenon over time. However we need experimental results to validate and improve simulations and models. X-ray tomography analyses are appropriate to extract pore volume distributions. Our study on ductile materials allowed us to conclude that experimental analyses exhibit two power laws attributed to growth and coalescence regimes. Moreover power law is scale invariance so it is possible to compare experiment (macroscopic) to calculation (microscopic). We show that there are good correlations between experimental and Molecular Dynamics pore volume distribution. Thanks X-ray microtomographies findings we progress in understanding the phenomenon of dynamic damage.

Abstract: In this paper was studied dynamic behavior of the armor steels Armox 500T and Secure 500 by testing specimens in quasi-static tensile test with strain rate 1∙10^-3 s^-1 and high-speed tensile test within range of intermediate strain rates from 100 s^-1 to 400s^-1 at the room temperature. Hardness test and quasi-static tensile test confirmed material properties specified by the manufacturer. Stress-strain diagrams showed very low strain-rate hardening effect at investigated strain rates. Total elongation at fracture was larger in case of Armox 500T for the whole strain rate range. Deformation energy density was calculated from the stress-strain curve and temperature rise due to adiabatic heating was estimated. Because of higher total elongation, Armox 500T was able to withstand higher deformation energy.