David Townsend

Testing low-impedance materials such as polymeric foams under dynamic compression presents challenges for traditional Hopkinson-type methods, which often fail to maintain stress equilibrium or transmit sufficient force. This study introduces a modified Direct Impact Hopkinson Pressure Bar (mDIHPB) system with an intermediate loading stage, addressing some of these limitations. Closed-cell Rohacell IG-71 foam was tested at strain rates of 100-1000/s, demonstrating improved resolution and reliability in material characterisation, supported by a numerical model. A rate-dependent foam model was developed and calibrated using low-rate experimental data, and subsequently implemented in the Abaqus/Explicit solver to validate the experimental method and provide insight into stress wave interactions and deformation mechanisms. This numerical framework complements the experimental setup, offering predictive capability and a foundation for future dynamic testing of low-impedance materials. Compared with conventional SHPB and DIHPB techniques, the mDIHPB enables longer pulse durations, enhanced force sensitivity, and effective observation of foam deformation through densification, providing a compact and versatile solution for low-impedance material testing at medium-to-high strain rates.

This work studies the dynamic strain localization and constitutive relationship of a Ti3Al2.5V alloy in jet engine containment system and a transparent polycarbonate conceived for aircraft canopy application by Digital Image Correlation (DIC) technique from quasi-static condition to high strain rates at different temperatures. The responses of two materials show significant strain rate and temperature sensitivities. Observations of Ti3Al2.5V alloy show that the dynamic local strain rate can reach values up to 1000 % of the nominal strain rate in the necking zone. However, dynamic local strain rate of polycarbonate soars up during strain softening then decreases rapidly with necking propagation, and eventually becomes 20 % of the nominal strain rate until fracture. Appropriate viscoplastic constitutive models are determined for both materials, which are incorporated in finite element simulations to reveal the trend of dynamic local strain rate evolution in dynamic tensile tests. The present work shows two different kinds of strain localization in typical lightweight materials, which should be addressed carefully from Split Hopkinson Tension Bar (SHTB) tests.