Fibre-reinforced composites (FRPs) are strong, light and corrosion resistant. From marine to aerospace and space applications, potential benefits from wider use of high-strength FRPs include increased fuel efficiency and service life. However, they are also highly complex, with strongly anisotropic properties which depend not just on their constituents but also their meso- and micro-structural properties like fibre architecture and fibre-matrix bonding. Developing a more robust understanding of how they behave is therefore critical if we hope to use FRPs more. High-strain-rate loading conditions are of particular importance for composites, as their high specific strength makes them ideal for many applications involving rapid accelerations, which are often at risk of high-speed impact threats. Here, FRPs with similar fibre and matrix components, but differing fibre architectures, have been studied at loading rates of the order of 0.01/s and 1000/s under both ‘pristine’ condition and after ageing by full saturation in a demineralised water bath at 40 °C. Experiments were designed to ensure equilibrium is reached early in the deformation process to discern information about stiffness and toughness, and the use of energy flux is considered in terms of power and total work done to characterise both experimental conditions and material response.

The mechanical response of the body centred cubic metal vanadium has been studied across a wide range of strain-rates through uniaxial stress compression. Samples of polycrystalline material have been tested up to strain-rates of 10 5 s −1 using miniature split and direct impact Hopkinson bars. The strain-rate sensitivity of the yield strength is shown to be significant with an apparent increase around 10 2 –10 3 s −1 and another increase ~ 10 5 s −1 . This later increase is discussed as being consistent with uniaxial strain experiments and shown to compare favourably with a Preston-Tonks-Wallace model modified to fit data out to strain-rates of ~ 10 7 s −1 . The benefits of the miniatured direct impact Hopkinson bar experiment are discussed along with the importance of velocimetry diagnostics and the requirement to understand and correct for friction at these high rates. Multiple repeat experiments and comparison of data taken with different loading platforms and different sample sizes are shown to produce a consistent and reliable data set which is complementary to existing sparse data on vanadium.

We have used a split Hopkinson pressure bar arrangement to investigate impact-induced reaction of the secondary explosives HMX, RDX and PETN in granular form. Sentencing of the experiments was performed by detecting reaction light emission, the spectral analysis of which can also provide information about the temperature of reaction. We measure the fraction of the mechanical energy that passes through the specimens that is absorbed in the run up to reaction, which we refer to as the efficiency factor, and for these experiments is of order 5–10%. We postulate that the efficiency factor is a function of the microstructure. The measured amounts of energy that were absorbed are comparable to those amounts required to bulk heat the samples to their melt points. A critical absorbed energy for reaction implies a minimum duration of loading for a given mechanical power and efficiency factor, and this idea is supported by the observation that the more intense the loading, the shorter the time to reaction. Additionally, we postulate a critical minimum mechanical power below which heat is redistributed faster than it can be accumulated. A minimum mechanical power threshold in turn dictates a minimum pressure threshold; but the idea of a stand-alone critical pressure is not experimentally supported.

A direct consequence of time-temperature superposition is that the observed glass transition temperature of polymers increases with strain rate. It also increases with heating rate, as has been observed in our experiments and by other authors. Our research has established a clear relationship between heating rates in thermal experiments and strain rates in mechanical experiments, by considering thermal expansion and mechanical strain to be equivalent quantities. The time-temperature superposition principle is most commonly used in the context of mechanical measurements, where the observation timescale is related to the strain rate, or oscillation frequency of the experiment. Thermal experiments access significantly larger timescales than mechanical experiments; therefore, they can extend the timescale range over which time-temperature superposition methods are validated. We use input from thermal expansion and calorimetric measurements to validate models used to predict the glass transition temperature observed via mechanical measurements, across an extended range of timescales.

Professor John Edwin Field passed away on October 21st, 2020 at the age of 84. Professor Field was widely regarded as a leader in high-strain rate physics and explosives. During his career in the Physics and Chemistry of Solids (PCS) Group of the Cavendish Laboratory at Cambridge University, John made major contributions into our understanding of friction and erosion, brittle fracture, explosives, impact and high strain-rate effects in solids, impact in liquids, and shock physics. The contributions made by the PCS group are recognized globally and the impact of John’s work is a lasting addition to our knowledge of the dynamic effects in materials. John graduated 84 Ph.D. students and collaborated broadly in the field. Many who knew him attribute their success to the excellent grounding in research and teaching they received from John Field.

Nematic liquid crystal elastomers (LCE) exhibit unique mechanical properties, placing them in a category distinct from other viscoelastic systems. One of their most celebrated properties is the 'soft elasticity', leading to a wide plateau of low, nearly-constant stress upon stretching, a characteristically slow stress relaxation, enhanced surface adhesion, and other remarkable effects. The dynamic soft response of LCE to shear deformations leads to the extremely large loss behaviour with the loss factor tanδ approaching unity over a wide temperature and frequency ranges, with clear implications for damping applications. Here we investigate this effect of anomalous damping, optimising the impact and vibration geometries to reach the greatest benefits in vibration isolation and impact damping by accessing internal shear deformation modes. We compare impact energy dissipation in shaped samples and projectiles, with elastic wave transmission and resonance, finding a good correlation between the results of such diverse tests. By comparing with ordinary elastomers used for industrial damping, we demonstrate that the nematic LCE is an exceptional damping material and propose directions that should be explored for further improvements in practical damping applications.

The visible wavelength spectrum of HMX was studied during the different reactions rates associated with burning, deflagration and detonation. For burning, the material was ignited by a butane flame in air at atmospheric pressure leading to millisecond burn times. A modified BAM impact test was used for deflagration, resulting in a 20 mu s impact-initiated partially confined reaction. Detonation was achieved in a column of HMX pressed to a density of 84 +/- 2 % TMD; PDV measurements allowed the CJ-pressure to be calculated at 24.0 +/- 0.5 GPa, and the reaction front velocity was measured at 7.8 +/- 0.3 kms(-1). When burning spectral emission was found to originate mainly from alkali metal impurities, with the 589 nm sodium peak dominating the spectrum. With the higher reaction temperatures and pressures of deflagration, the redshift and broadening of the Na spectral peak were measured, along with the continuous competing greybody emission. From greybody portions of the spectra, temperatures of 4000 K in deflagration and 7000 K in detonation were calculated. The temperature increase is likely caused by the higher pressure shock of the detonation front compressing air filled interstitial pores in the material, leading to multiple localization mechanisms that drive a greater temperature than that achievable by chemical reaction alone.

An experimental approach for studying the interfacial strength of polymer bonded composite constituents is discussed. Macroscopic spherical caps formed of filler material are pressed against half-planes of matrix material and pulled off. The experimentally measured parameters are force, contact-area and displacement. Measuring both the bonding and debonding curves provides enables the study of hysteresis in the system. Measurements performed on Sylgard 184 polydimethylsiloxane shows increasing hysteresis as the loading rate is raised, and the critical tensile load required to separate the system is observed to increase by an order of magnitude compared to that expected for a purely elastic material.

A split Hopkinson pressure bar that is specifically designed for testing low impedance materials in uniaxial tension is discussed. The input and output bars are each 2.1 m long and formed of titanium alloy tubes and are of low mechanical impedance (Grade 9 alloy, 16 mm outside diameter, 1mm wall thickness). Tensile pulses are generated by the action of an impedance matched striker-tube which is coaxial to, and rides over the top of, the input-tube. The striker tube is propelled along the input-tube away from the specimen by a gas-gun with a wrap-around breech to strike a stop mounted at the end of the input-tube. The subsequent compressive pulse is inverted to become tensile by reflection from a free surface, and propagates back up the input-tube to interact with the specimen, causing it to be placed into a state of uniaxial tension with associated displacement rates of between 0.5 and 10 meters per second. In addition to force-displacement information obtained from strain gauges mounted on the bars, sample deformation is monitored using high-speed video and sample strains are extracted using digital image cross-correlation. Recent results from experiments on the highly filled particulate composite PBS 9501are presented.

Though a vast amount of literature can be found on modelling particulate reinforced composites and suspensions, the treatment of such materials at very high volume fractions Vf (>90%), typical of high performance energetic materials, remains a challenge. The latter is due to the very wide particle size distribution needed to reach such a high value of Vf. In order to meet this challenge, multiscale models that can treat the presence of particles at various scales are needed. This study presents a novel hierarchical multiscale method for predicting the effective properties of elasto-viscoplastic polymeric composites at high Vf. Firstly, simulated microstructures with randomly packed spherical inclusions in a polymeric matrix were generated. Homogenised properties predicted using the finite element (FE) method were then iteratively passed in a hierarchical multi-scale manner as modified matrix properties until the desired filler Vf was achieved. The validated hierarchical model was then applied to a real composite with microstructures reconstructed from image scan data, incorporating cohesive elements to predict debonding of the filler particles and subsequent catastrophic failure. The predicted behaviour was compared to data from uniaxial tensile tests. Our method is applicable to the prediction of mechanical behaviour of any highly filled composite with a non-linear matrix, arbitrary particle filler shape and a large particle size distribution, surpassing limitations of traditional analytical models and other published computational models.

The sodium D-line is often present in optical spectra of combustion due to its high prevalence and emissivity. Collision theory predicts the spectral peak to have a red-shift dependent on pressure, P , and temperature, T . Here we show that the conditions reached during deflagration of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) permit the red-shift of the sodium D-line to be calibrated to 1.5 GPa. Deflagration at these pressures is achieved using a split Hopkinson pressure bar apparatus, with temperatures of circa 2900 K from the greybody continuum away from spectral features. Lower deflagration pressures, of 0.5 to 0.9 GPa, are achieved in a fallhammer test, with temperatures of circa 4000 K. The red-shift exhibits the predicted PT −0.7 dependence with a constant of proportionality of (950 ± 30) GPa -1 · K 0.7 · nm. Using the serendipitous presence of sodium, this optical technique allows for fast measurements of both pressure and temperature from the same light source in one measurement.

Shock-tube detonators, used in the mining industry to initiate charges of blasting explosives, often contain pyrotechnic delay-lines for controlling the sequence of blasts. Traditionally, delay-lines consist of pyrotechnic compounds of toxic metals such as lead which are pressed into the metallic tubing. Since lead is well-known to be a hazard both to people and the environment, lead-free alternatives were investigated to create a delay-line composition. Further, to create delay-lines cheaply and reproducibly, a composition was developed that could be printed onto a substrate. Screen printing was investigated for this purpose. The pyrotechnic ink that we developed consists of two inert inks (a fuel and an oxidiser) so that it can be stored and transported safely and conveniently without being classified as an energetic. After being transported to its destination, the two inks can be mixed to form an energetic (wet) ink, printed, and used as a delay-line when dry. A silicon-bismuth trioxide composition bonded together using nitrocellulose was found to be a suitable ink for screen printing. The burning rate was measured as 52 +/- 7 mm s(-1) for a thickness of 100 mu m. This thickness reduces the raw delay-line material by over 90 % compared to traditional 3D delay-lines. The target burning rate was 24 mm/s, but the measured rate is sufficiently close to this that changes in geometry could meet this requirement. The critical thickness for a self-sustainable burn was measured as 40 +/- 5 mu m. The thermal output and viscosity of the ink were determined as functions of the particle size. The viscosity of the ink was determined as 7.04 +/- 0.05 Pa s, which is suitable for screen printing. Milling was used to reduce the particle size below 1 mu m. Ignition of the printed delay-line using shock tubing was achieved through overprinting with a shock-sensitive ink. The burning rate of a delay-line is one of its most defining properties when used in mining applications. Thus the effect on the delay-line's burning rate of the pyrotechnic composition printed thickness, and the ink solvent was investigated. We found that the burning rate could be modified by changing the silicon content in the pyrotechnic, and to some extent, by adding aluminum. Adding a third, slowly evaporating solvent, to the ink vehicle was found to reduce the critical thickness to less than 40 mu m, decrease the burning rate to 46 +/- 7 mm s(-1), and enhance the ink's flow properties. We found that the width of printed lines did not affect the burning rate for widths between 1-5 mm. This is an interesting result and could be used to determine the most cost-effective shape to produce printed delay-lines. Ignition of the printed delay-line using shock tubing was achieved through overprinting with a shock-sensitive ink.

The mechanical behaviour of a rubbery nitrocellulose/nitroglycerine double-base propellant was probed at a range of strain rates. The propellant was relatively soft, and inhomogeneous on the millimetre scale, presenting a few experimental difficulties. These difficulties were overcome in a shock experiment using a plate reverberation technique. The resulting information on the shock Hugoniot and release isentrope of the propellant was compared against a hydrocode using a group interaction based material model.

Stress relaxation experiments are used to probe the time dependent properties of materials such as polymers: a constant strain is applied to the sample and the stress is observed to decay as a function of time. Such measurements are typically performed quasi-statically with corresponding timescales of minutes to hours. Here we describe a dynamic stress relaxation experiment using a specially configured split Hopkinson pressure bar with a corresponding timescale of microseconds. In a conventional bar system the signals that immediately follow the accumulation of sample strain are typically overwritten by reflected waves within the input and output bars. The bar configuration described here extends the measurement window to include not only the accumulation of strain as per a conventional SHPB experiment but also a subsequent period of near-constant strain during which the stress in polymer samples can be observed to relax. The technique is demonstrated by application to specimens of a double base propellant.

The aim of the research reported here was to investigate the strain rate and temperature sensitivity of Rowanex 1100 Type 1A, a polymer-bonded explosive (PBX). The stress supported by this PBX at high rates of deformation (1750 ± 225 s−1) was found to be about an order of magnitude greater than that supported at low rates (0.015 s−1). Temperature was also found to have a large effect, with the strength of the material decreasing exponentially with temperature over the range studied (–60 to +60 °C). The exponents for the decay of the PBX’s strength with temperature at both low and high strain rates were the same within experimental error. So a temperature/strain rate shift factor could be determined and was found to be 31.2 ± 2.4 K/decade of strain rate.

There is a significant challenge in simulating the behaviour of PBXs under high strain rate impact loading. A Porter-Gould physically based constitutive model has been developed for the DPX2 explosive. A series of quasi-static compression and tensile tests over a range of temperatures were performed together with DMA tests to calibrate the model. In particular tests were performed for different L/D ratios to understand the complex localisation and damage behaviour of the material. High rate tests on the compression Split Hopkinson Pressure Bar (SHPB) for a range of temperatures were then used for validation of the model under idealised stress states. Some model development is still required, particularly at lower temperatures near the glass transition temperature. In addition a series of classical Taylor Tests were used to validate the model under impact loading conditions at room temperature. The DYNA3D simulations gave very good results compared to the experiments for these impact conditions.