Flow Stress Measurement and Dynamic Response Analysis of Hot Compression Test Machine at High Strain Rates

Flow stress is the most important information for hot strip rolling as it affects the rolling force and the thickness of the rolled product. A high-speed compression test up to a strain rate of 300 s(-1), which is the compression speed of 3 600 mm.s(-1) for a 12-mm-high cylindrical specimen, is necessary as a strain rate of 100-300 s(-1) is the normal rate in the production of hot-strip-rolled steel sheets. An experiment is conducted using a servo-hydraulic compression test machine, which enables a high compression speed and a high temperature, but the oscillation is observed in stress-strain curve at high strain rate over 50 s(-1). To determine the natural frequency of the compression test machine, the Savitzky-Golay filtering method is used for regression and the fast Fourier transformation (FFT) is adopted. To explain the mechanism of this phenomenon, a spring-mass-damper model is used and the results are compared with the FFT analysis result. After eliminating oscillation on the time versus load curve, a flow curve is obtained by inverse analysis, which compensates for the nonuniform strain rate, the inhomogeneous distribution of deformation, and the temperature increase during deformation.