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The effect of shear stress on the dynamic failure of ceramic materials is not sufficiently investigated in the published literature. With the use of a bespoke split Hopkinson pressure bar, this paper presents an effort to investigate the dynamic shear compressive response of Macor, a model ceramic material with zero porosity and light weight characteristics. A cone specimen and cylindrical specimens with varying inclined angles are used to introduce the shear stress to the Macor ceramic. The dynamic failure initiation and crack propagation are monitored by the high speed photography and Digital Image Correlation techniques. It is found that the equivalent stress of Macor at the initiation of failure decreases nonlinearly with the increase of shear stress. The high speed images show that the crack originates from the minimum cross-section of the cone specimen and the obtuse angle corner of the inclined cylindrical specimens. The cracks propagate parallel to the inclined plane instead of the axial loading direction. The fractographic analysis shows the compacted zone in the shear fracture surfaces of the cone specimen and the inclined cylindrical specimens. This indicates a significant role of shear loading in the dynamic failure process of Macor.

This paper studies the uniaxial and multiaxial dynamic compressive behaviour and failure of PA66-GF30 (30 wt % glass fibre reinforced polyamide 66), a typical light weight and high strength composite increasingly used in automobiles including the electric cars in applications from ambient temperature to elevated temperatures up to 90 °C. Likewise, the constitutive relation of PA66-GF30 is characterized from quasi-static to high strain rates. Constant high strain rate loading is achieved by pulse shaping technique on a bespoke split Hopkinson bar. The stress-strain relations are pressure, strain rate and temperature dependent. Effects of strain rate and temperature are found to be decoupled on the pressure sensitivity of PA66-GF30. Beyond maximum stress, micro crack has already formed in dynamically deformed PA66-GF30, which corresponds to macro strain localization monitored by high speed photography and digital image correlation techniques. The PA66-GF30 with confinement shows adiabatic shear failure, with fibres coated by severe shear matrix facets and evenly distributed filaments. This is different from the unconfined PA66-GF30 which shows fibres pull out in the fractured matrix. A modified Drucker-Prager model is proposed to describe the pressure dependent compressive strength of PA66-GF30 over various strain rates and temperatures.

Polymethylmethacrylate (PMMA) has been widely used as transparency in aerospace and automobile engineering to enhance structural reliability under impact loading such as bird strike events. This study investigates the confined compressive behaviour of PMMA at quasi-static 0.01 s−1, medium rate 1-100 s−1 and intermediate strain rates 100-1000 s−1 from room temperature to elevated temperatures. Constant intermediate strain rate loading is achieved by using pulse shaping technique on a bespoke in-house developed split Hopkinson compression bar equipped with a high-speed camera and an environmental chamber. The material presents significant strain rate, temperature and pressure sensitivities. The failure mode at intermediate strain rate changes from brittle fragmentation without confinement to adiabatic shear banding with medium lateral confinement (65.3 MPa), which can be seen from the post-peak slope in the constitutive behavior, melting flow PMMA filaments observed in high speed photography and microstructural analysis. A series of intermediate strain rate experiments at elevated temperatures indicate that the higher temperature results in ductile failure for corresponding pressures. The temperature and strain rate dependent Drucker-Prager (DP) model is found to describe the response of PMMA. The brittle-ductile transition and pressure dependent model provide a better understanding of PMMA at intermediate strain rates, which is appropriate for engineering applications.

Ductile polymers have been increasingly applied in engineering applications to enhance the structural reliability under impact loading. Due to the limitation of experimental setup to achieve large tensile deformation and the difficulty to achieve dynamic force equilibrium, the localization and post-necking stages up to fracture of ductile polymers at high strain rates have less been investigated. In the present work, the dynamic strain localization of ductile polymeric rods under large tensile deformation up to fracture is studied on the bespoke Hopkinson tension bar synchronized with a high-speed camera. Transparent polycarbonate (PC) is used as a model material in the present study. Likewise, the constitutive response and fracture behaviour of polycarbonate are also characterized with the assistance of Digital Image Correction (DIC) from low to high strain rates under various temperature conditions. The results quantitatively show that the dynamic local strain rate initially increases dramatically to 200 % of the nominal strain rate due to strain localization. This is followed by a rapid drop with necking propagation, and finally tends to stay at strain rate of approximately 20 % of the nominal strain rate until fracture. The elevated temperatures would result in higher local strain rates. Two constitutive models with and without the consideration of constant strain rate condition are constructed for PC and incorporated in finite element simulations. The trend of dynamic local strain rate history with respect to nominal strain rate is successfully reproduced in simulations. The constitutive models particularly the simple dynamic amplification model, are able to reflect the phenomenological key features of the experimentally observed macroscopic and local responses of polycarbonate, and would find their potential applications in impact resistant transparency design.

A dynamically adjustable terahertz (THz) broadband absorber based on a vanadium dioxide (VO2) hybrid metamaterial is theoretically designed. During the design, we first consider working frequency, bandwidth, excitation method and fabrication process. And then, a commercialized full wave electromagnetic (EM) calculation software CST microwave 2019 based on finite integration technology is used to simulate and optimize the performance of the device. Due to the device is excited by light, it consists of a top layer of VO2 oblique-split-ring-resonator (OSRR) array, a polyimide (PI) spacer and a bottom metal film. The simulation results show that, when the VO2 changes from insulating state to metal state under light stimulus, the absorption increases from 3% to over 90%. Meanwhile, when the VO2 is in metallic phase, the bandwidth of absorption greater than 90% reaches to 1.0 THz and the relative bandwidth is about 83%. The potential applications of the device are active EM stealth, THz wave detection and thermal imaging. This work paves a new way for simply constructing a low-cost dynamically adjustable THz broadband absorber.