System ringing in impact test triggered by upper-and-lower yield points of materials

Abstract

In the mechanical tests of materials under impact loading, the measured load signal usually exhibits severe oscillation known as system ringing. A lightweight load sensor was designed and has successfully overcome this problem for many metal materials, but was found to fail when dealing with steel with apparent upper-and-lower yield points. In order to investigate this problem, two representative steels, DP780 without yield peak and B340LA with significant yield peak, are chosen for comparison. Using both experimental and numerical analysis of the test system consisting of specimen, gripper and load sensor, it is found that upper-and-lower yield points can trigger the system ringing under impact loading. Furthermore, a series of preloading-reloading tests are conducted, the results of which can validate the experimental conclusion. Additional numerical simulations of the preloading-reloading tests are carried out to verify the test results, and the simulation results indicate that the tangent modulus change of stress-strain curve in the transition from elastic stage to plastic stage can generate an excitation to the test system. This excitation can trigger system resonance and result in severe oscillation in load signal. Based on the test and simulation results, it is suggested that the specimen should be loaded statically to a strain beyond its yielding range before dynamic test in order to obtain an accurate hardening curve.