A Study on Material Tests in High Strain Rates
Date Issued
2009
Date
2009
Author(s)
Lee, Chih-Chun
Abstract
The finite element method has been widely applied to simulate the crashworthiness tests in the automotive industry. However, in the high strain-rate deformation, the yield strength and ultimate tensile strength of a material may be changed. In order to obtain accurate results, the stress-strain relations of the material in high strain rates are required for the simulations of the crashworthiness tests. There are various high strain-rate tests available to obtain the stress-strain relations, such as the split Hopkinson bar system, direct impact method, and servo hydraulic system. Each test method is applicable in certain strain-rate range. In the present study, the servo hydraulic system MTS819 was adopted to implement the high rate tests in a strain-rate range below 500s-1. The testing equipment including the machine frame, load cell, and data acquisition system was fine tuned first to make it suitable for the tests. As strain rate increasing, the amplitude of the stress vibration acquired from load cell increases. Hence, the efforts to determine the cause of the stress vibration and the remedy approaches were made. The actual strain rate measured in specimen during the test was considered. It was found that there is an acceleration zone in the beginning of the test. So it is important to determine when the strain rate comes to a constant strain rate. The stress-strain curves acquired from the experiments conducted in the present study were fitted by the Cowper-Symonds equation and then input to the finite element software for simulations. Through the finite element simulations, the actual strain rates in specimen and stress distributions in the grip during experiment were investigated. The finite element simulations were also performed to examine the strain rate effect on the impact of a hydro-formed engine cradle to a rigid wall. The experimental approach and the finite element simulations results obtained in the present study could be valuable references for the future researches in field of deformation on the high strain rates.
Subjects
high strain rate test
servo hydraulic system
stress-strain curve
Cowper-Symonds equation
strain rate effect
Type
thesis