Material models are widely used in finite element codes for analysis of material deformations particularly at high strain rates and elevated temperatures. The problems such as necking and bulging limit the conventional test techniques to measure the stress–strain curves only up to small strains. This is while in some deformation processes, the strain can be greater than 1. In this study, steel shots of 6 mm in diameter are impacted on specimens at high impact velocities and at elevated temperatures using shot impact test. Strains up to 1.6 and strain rates up to about 4 × 106 s−1 are achieved in this study. The geometry of the crater created by the shot impact on the specimen is used for determination of the constants of Johnson–Cook material model. A combined experimental, numerical and optimization approach is used for determination of the constants. The experimental and numerical crater geometries coincide when the constants of material are chosen correctly. The selection of the constants is performed using an optimization technique such as genetic algorithm. The computed constants are verified by quasi-static tests. With this new technique, stress–strain curves are no longer needed to be obtained by experiment at high strain rates and elevated temperatures.

Abstract. Dynamic behavior of materials is investigated using different devices. Each of the devices has some restrictions. For instance, the stress-strain curve of the materials can be captured at high strain rates only with Hopkinson bar. However, by using a new approach some of the other techniques could be used to obtain the constants of material models such as Johnson-Cook model too. In this work, the restrictions of some devices such as drop hammer, Taylor test, Flying wedge, Shot impact test, dynamic tensile extrusion and Hopkinson bars which are used to characterize the material properties at high strain rates are described. The level of strain and strain rate and their restrictions are very important in examining the efficiency of each of the devices. For instance, necking or bulging in tensile and compressive Hopkinson bars, fragmentation in dynamic tensile extrusion and petaling in Taylor test are restricting issues in the level of strain rate attainable in the devices.

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The relation between hardness, strain and temperature has been widely investigated over the past decades. However, less attention has been paid to the effect of strain rate on hardness induced in high rate deformations. In this investigation, the relation between strain, temperature and strain rate is studied and a new model is proposed. The investigation is performed by experiment and numerical simulation. The simulations are used to predict the distribution of the strain and strain rate within the specimen. The high rate experiments are conducted using Taylor test, compressive and tensile Hopkinson bar. Quasi-static tests are also carried out using Instron testing machine. The results show a quadratic relation between the hardness and strain rate. The results also indicate that the relation between hardness, strain and temperature is the same for the compressive and tensile loading but the relation between hardness and strain rate is different for the compression and tension. The hardness increases with the increase of the strain and strain rate but decreases with the temperature. The reason is believed to be due to dislocation pile up which is denser in compression than that in tension.

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