Progress in testing sheet material at high strain rates

Abstract
In this paper the requirements for the evaluation of yield behaviour at high strain rates up to 1000 s ^-1 will be discussed. The main topic is the performance of tests at high strain rates preventing oscillations caused by impact and the use of special solutions for force measurement of sheet material. The importance of obtaining stress-strain curves at high strain rates is increasing due to the need of finite element simulation of higher accuracy, especially in the field of crashworthiness. Finite element codes are now able to use records of stress-strain curves for different strain rates instead of simple models of strain rate dependence of the yield behaviour. The use of light weight metal sheet and newly developed steel sheet material needs the determination of the yield behaviour at high strain rates. Experience in testing and measuring was applied and improved in many tests with round tensile specimens made of steel at strain rates up to 1000 s ^-1. hall parts connecte with the specimen have to have light weight to reduce the influence of inertia effects. This can be achieved with optical and strain gauge measurements. The time resolution and frequency bandwidth has to be sufficiently in order to obtain reliable data. Because of the low limit frequency, standard force measurement techniques can only be used at strain rates up to 100 s ^-1. The strain gauge measuring technique for measuring the actual load, approved in many round tensile specimen tests, could not be directly applied to sheet specimens. Based on the experience with round tensile tests a modified procedure has been developed. In order to reduce the experimental effort additional sheets with strain gauges can be connected to the specimen for force measurement. The signal from this force measurement device shows a significant reduction of oscillation. The quality of this method will be demonstrated in test results of different materials. With this method an accurate, reliable and economical test procedure has been established.