Lateral confinement effect in the SHPB tests of polycarbonate specimens

Abstract

Split Hopkinson pressure bar (SHPB) technique is widely used to characterize dynamic behavior of mate-rials at high strain rates ranged from 103 to 104 s-1. In the SHPB tests, the measurement accuracy is prone to error due to lateral confinement of a tested specimen and this is critical especially for polycarbonate (PC) which is severely pressure dependent materials. The confinement is mainly produced due to two factors: inertial stress; and the interfacial friction between the specimen and pressure bars. Those factors tend to increase the yield strength of a tested specimen. Moreover, both of the inertial stress and interfacial friction is known to be rate dependent resulting that the inertial stress and friction will increase as the strain rate increases. Those rate de-pendent phenomena make the confinement increases as well. Such effect makes the measured yield strength in-crease as the strain rate increases. It is, indeed, true that the yield strength increases when the loading rate in-creases due to the material behavior itself. However, it was often misinterpreted that such an increase is purely due to the rate-dependent behavior of the tested material. This study newly proposes a methodological based on SHPB tests with its associated numerical analysis and the linear Drucker-Prager yield criterion to quantify the lateral confinement effect by utilizing an inverse method with finite element analysis (FEA). With the proposed quantification procedure, the purely strain rate dependent behavior of a tested material could be obtained. The key idea is to simulate the SHPB tests with strain rate insensitive model with finite element method to show the existence of the lateral confinement effect as a primary analysis. To obtain the amount of lateral confinement effect, an inverse method was used to obtain the input stress-strain curve data iteratively through FEA such that the output of FEA will be close to the experi-mental data obtained from SHPB tests. When s...