High Strain Rate Compression Tests on Aluminium at High Temperatures

Abstract

The studies in metallic and non-metallic materials have made considerable progress in recent years. Still they have some problems concerning their properties which have not been made clear. In particular, it has become increasingly important to clarify the behaviour of the materials when they deform at high strain rates.The high strain rate compression testing apparatus which is manufactured for trial, and the experimental results which are obtained by using the apparatus to clarify the effect of the temperature and the strain rate on the strength of aluminium, are described in this paper.The principle of the experimental method is as follows. The mechanical parts consist of a projectile apparatus, a striking bar, an input bar, an output bar and a throw-off bar. The specimen is mounted between the input bar and the output bar. The impact load is provided with the striking bar which is projected from the projectile apparatus. A pulse of compression caused by impact, travels into the input bar, compresses the specimen, and is transmitted to the output bar and finally to the throw-off bar. The throw-off bar absorbes the surplus energy.The striking bar, the input bar and the output bar are all steel bars of 17mm diameter and 500mm length.The stress and strain to be produced in the specimen can be obtained from the analysis of the stress-time patterns in the input and the output bars. The patterns are drawn on the screen of the two-elements synchroscope by the strain gauges cemented at the midpoints of the input bar and the output bar.The specimens are machined to 17mm diameter and 17mm length from the 99.997% purity polycrystalline drawn aluminium bar.The velocities of the striking bar are 2.5m/sec, 3.3m/sec and 4.2m/sec for this experiment. The testing temperatures are room temperature, 220°C, 290°C and 360°C. The temperature of the specimen is measured by two Chromel-Alumel thermocouples fastened to the specimen's surface.In each impact test, a specimen is impacted repeatedly with the same velocity of the striking bar.The following results are obtained within the range of strain rate of 50∼200/sec.(1) The compressive flow stress of aluminium becomes higher with the increase of the strain rate, and becomes lower with the elevation of the temperature.(2) The static flow stress is more sensitive to temperature compared with the dynamic flow stress.(3) The ratio of the compressive flow stress to cause a certain amount of strain at elevated temperature to the flow stress at room temperature is adopted as a parameter to represent the temperature sensitivity on the compressive strength. The ratio for the static load is far smaller than the ratio for the dynamic load.(4) Comparing the dynamic flow stress with the static one at the same temperature and at the fixed strains, it is concluded that the ratio, σdynamic/σstatic, has the higher value with the rise of temperature. But the value has the tendency to approach 1 rapidly with the increase of strain.