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John Field was a brilliant experimental physicist who made major contributions to the physics and chemistry of solids. His research interests spanned a very wide range of topics, most of them involving energetic phenomena. These areas included the strength properties of solids, fracture growth, impact and erosion phenomena, shock physics, reactivity of solids, explosive initiation, lasers, acoustics and medical physics. Within the Physics and Chemistry of Solids Group in the Cavendish Laboratory, he developed the best-equipped high-speed camera facility in any university in Europe, including seven that achieved frame rates in excess of 10 6 frames per second. In addition to the cameras, extensive use was made of ultrasonics, optical and electron microscopy, mass spectroscopy and thermal techniques. He played an important national role in advising the Ministry of Defence on a wide range of topics in energetic phenomena and materials science, which led to practical engineering solutions. He was an outstanding supervisor of doctoral students, who remember him fondly.

This review critically examines the various ways in which the mechanical properties of wood have been understood. Despite the immense global importance of wood in construction, most understanding of its elastic and inelastic properties is based on models developed for other materials. Such models neglect wood’s cellular and fibrous nature. This review thus questions how well models that were originally developed for homogeneous and effectively continuous materials can describe wood’s mechanical properties. For example, the elastic moduli of wood have been found by many authors to depend on the size of the test specimen. Such observations are incompatible with classical elasticity theory. There is also much uncertainty about how well elastic moduli can be defined for wood. An analysis of different models for size effects of various inelastic properties of wood shows that these models only approximate the observed behaviour, and do not predict or explain the scatter in the results. A more complete understanding of wood’s mechanical properties must take account of it being in some sense intermediate between a material and a structure.

A previous review of micro- and nano-indentation hardness tests and their analyses gave emphasis to obtaining measurements of continuous nano-indentation load, P, versus, depth, h, recordings that monitor the full elastic–plastic deformation behavior of a localized crystal volume [1]. Attention is given to determining the complete, indentation-based, elastic–plastic deformation properties of the local volume, including the initial crystal elastic deformation behavior and, especially to evaluation of post pop-in plastic strain hardening. Stress–strain calculations are presented for an initial Hertzian elastic loading and follow-on crystal micro- and nano-scale plastic deformation responses [2]. Applied load, P, dependencies on contact diameters, di, of silicon crystals are compiled on the basis of elastic, plastic and cracking predictions, giving indication at the lowest P values of an indentation size effect (ISE) for the crystal hardness. Elastic–plastic stress–strain curves are presented for sodium chloride and tungsten crystals. The hardness and strain hardening calculations also demonstrate an influence of the ISE. The exceptional plastic strain hardening behaviors scale dimensionally with corresponding dislocation interactions and sessile reactions within the very localized plastic indentation zones. There is usefulness in determining elastic modulus values from the initial loading record. Micro- and nano-scale dislocation interactions/reactions account for the high stress and strain hardening levels as well as the occurrence of an ISE.

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.

We review some of the factors that influence the hardness of polycrystalline materials with grain sizes less than 1 µm. The fundamental physical mechanisms that govern the hardness of nanocrystalline materials are discussed. The recently proposed dislocation curvature model for grain size-dependent strengthening and the 60-year-old Hall–Petch relationship are compared. For grains less than 30 nm in size, there is evidence for a transition from dislocation-based plasticity to grain boundary sliding, rotation, or diffusion as the main mechanism responsible for hardness. The evidence surrounding the inverse Hall–Petch phenomenon is found to be inconclusive due to processing artefacts, grain growth effects, and errors associated with the conversion of hardness to yield strength in nanocrystalline materials.

Up until the Industrial Revolution, the dynamic mechanical properties of materials were only of importance in warfare, particularly after the powder-driven gun was invented. With the invention of the steam engine, the explosion of steam boilers (which is similar to the explosion of cannon) became a concern. When railways began to be built, the lack of knowledge of the dynamic properties of the iron alloys used in rails and railway bridges was understood to be a problem, but no way of measuring them was devised until the end of the nineteenth century. Ingenious mechanical (and later electromechanical) methods of recording signals onto rotating drums or moving smoked glass plates began to be developed from the middle of the nineteenth century onwards. Optical/photographic methods of recording information from dynamic experiments date from the 1890s. The rod-on-anvil technique (later named after Taylor) was developed in France at the beginning of the twentieth century but not mathematically analysed until the 1940s. The Hopkinson pressure bar was invented just before the start of the First World War and found to be useful in improving British artillery shells. It was then forgotten about until the Second World War when a two-bar version was developed for measuring the dynamic properties of soft materials such as explosives and polyethylene. As the story of high rate mechanical testing from about 1950 onwards is quite well known to the high rate testing community, this date is taken as the end point of this article.

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.

The field of rock mechanics is concerned with the response of rocks to the forces acting on them, characterizing this behaviour in specific environments and under varying loading conditions. This review discusses the suitability of current mechanical testing methods for inhomogeneous rock bodies, with a specific focus on the use of artificial samples in place of real rocks in these tests. The use of artificial materials such as cement, resins and sand-based mixtures is reviewed, as is the manufacture of three-dimensionally printed samples. The benefits and drawbacks of using such specimens in mechanical tests are discussed, and the validity of their simulation of real rock discussed. There is evidence that 3D printed samples have the advantage of overcoming the problems of specimen reproducibility and also make possible the ability to test the response of specific defects to loading. There is thus great potential for the use of 3D printing in this field. However, the review concludes that further post-processing and careful thought about the materials used must be carried out to ensure that printed samples are of adequate strength and brittleness to accurately simulate real rocks.

Space penetrators are a potential method of inserting instrumentation onto ice-covered bodies in the solar system. Part of a study to see whether this is feasible involves numerically simulating impact of the penetrator into ice at impact velocities of a few 100 m/s. In order to do this accurately, it is necessary to have a constitutive model for water ice that is valid at the strain rates and temperatures relevant to impact in the Outer Solar System. This paper reports certain issues and difficulties that arose during a study to obtain this data.

The dynamic strength of distilled water ice, porous distilled water ice and sintered ice powder have been measured in uniaxial compression at a range of temperatures from 173 to 263 K and at strain rates in the range 103− s−1 to 10+3 s−1 using a split Hopkinson pressure bar (SHPB) and an Instron mechanical testing machine. The peak compressive strength of ice as a function of density was found to be described by a linear relationship. The average peak compressive strength for high density ice was 49.5 ± 2.0 MPa. Low density (porous) ice was found to have a lower strength of 36.5 ± 1.9 MPa. Porous low density ice specimens also had a second peak in their stress-strain curves after failure which we attributed to the effect of internal flaws. The peak compressive strength of ice as a function of density was found to be described by a linear relationship. No effect of strain rate was discernable over the strain rate range investigated. Sintered ice powder with a density greater than 735 kg m−3 had an average peak compressive strength of 27.1 ± 1.7 MPa. Sintered ice powder with a density lower than 735 kg m−3 had a strength of 20.4 ± 1.1 MPa.

There is expanded interest in the long-standing subject of the hardness properties of materials. A major part of such interest is due to the advent of nanoindentation hardness testing systems which have made available orders of magnitude increases in load and displacement measuring capabilities achieved in a continuously recorded test procedure. The new results have been smoothly merged with other advances in conventional hardness testing and with parallel developments in improved model descriptions of both elastic contact mechanics and dislocation mechanisms operative in the understanding of crystal plasticity and fracturing behaviors. No crystal is either too soft or too hard to prevent the determination of its elastic, plastic and cracking properties under a suitable probing indenter. A sampling of the wealth of measurements and reported analyses associated with the topic on a wide variety of materials are presented in the current Special Issue.

An inevitable consequence of testing materials at high strain rates is rapid acceleration of at least part of a specimen. As well as inducing waves that propagate in the specimen, at least during the initial stages of loading, this rapid acceleration gives rise to inertial stresses that affect the force measurements at the interfaces between the specimen and the input and output Hopkinson bars. In this chapter, we will be mainly concerned with the effects of macro-inertia on the observed specimen response. Inertial effects due to any internal structure of, for example, a foam will not be considered. Another cause of departure from simple compression (upsetting) is friction between the faces of the specimen and the ends of the bars. Friction produces three effects: (i) it changes the state of stress from uniaxial to triaxial; (ii) the specimen bulges (barrelling); (iii) deformation localizes producing (in cross-section) an X-shaped shear band. Various lubricants and lubrication techniques have been developed over the years, but none have been found to reduce friction to zero for metals. However for polymers, hydrocarbon greases have been found to eliminate friction up to true strains of around 0.4. The ring and aspect ratio tests are discussed as two methods of quantifying friction in compression. Lubricants suitable for high and low temperatures are also discussed.

Although Bertram Hopkinson had a research interest in the dynamic strengths of materials, the bar technique he published in 1914 was developed to roughly determine the shapes of stress pulses produced by bullet impact and explosions. His pressure bar proved useful to the British explosives and armour industry during the Great War allowing them to improve their munitions. It also allowed the phenomenon of spall of metal plates produced by ballistic impact or by explosions to be understood. Another greater war had to occur before Hopkinson’s pressure bar was applied by Taylor, Davies and Volterra in the UK to the measurement of the dynamic compressive properties of explosives and polymers. Unlike after the First World War, why this research was thought important to the British armed forces during the Second World War was not explained in the declassified reports published once the war was over. Soon after the end of the Second World War, Kolsky made a number of improvements to the double Hopkinson bar system including using detonators to substantially increase the stresses that could be applied to specimens. This made possible for the first time the accurate determination of the dynamic compressive stress-strain curves of metals. Detonators are, however, impractical for all but defence labs to obtain and use. So an alternative method of loading was needed, and this was developed by Krafft and co-workers at the Naval Research Laboratory in Washington DC in the early 1950s. Their design used a gun to launch a projectile rod against one end of the double bar system and this American method eventually became standard in all laboratories that use the split Hopkinson pressure bar technique. However, it was not until the end of the 1970s that split Hopkinson bars began to be built in laboratories in an ever increasing number of countries around the world.

Most of the time, improvements to armour, as in other areas of science and technology, are gradual. From time to time, however, substantial changes take place. What I have attempted to do in this chapter is (i) to identify these “step changes”, (ii) to give an impression of what was going on politically and militarily at the time that influenced technology and design, and (iii) to introduce the people involved.

Conventional impact sensitiveness machines (e.g. Rotter and BAM) do not provide detailed information about hot-spot ignition mechanisms in energetic materials. To gain further insight, a glass anvil technique was developed at Cambridge in the early 1950s, which allowed high-speed photographic studies to be performed during impact on energetic liquids, explosive powders and crystals. Herein we review such investigations which continue up to the present day.

The ASB phenomenon was first described by Russian researchers in 1928 and 1935. Translations of both papers are included.

The response of structures and materials subject to ballistic impacts or blast loads remains a field of intense research. In a blast or impact load a sharp pressure wave travelling at supersonic speed impinges on the structure surface where deformation will develop at very high strain rates and stress waves may form and travel through the continuum solid. Both the dynamic loading and the temperature increase will significantly affect the mechanical and failure response of the material. This rate dependent behaviour has been very intensely investigated for metals and composites. In broad terms, strains due to explosive blast will increase at rates from 10^2 to 10^4 /s, leading to regimes of elastic and plastic wave propagation, plane stress and adiabatic deformations. To understand the response of engineering materials to blast loads many constitutive models have been proposed that can be used in numerical analysis and a compilation and discussion may certainly be of help for those involved in the field. Few models consider high strain rate effects, however constitutive approaches that were developed and tested at low strain rate regimes will also be addressed due to their relation with other models. Specific reference will be made to strain rate regimes close to 10^4 /s, where shock waves may be present in the material. A choice was made on constitutive models suitable for polycrystalline face centred cubic (FCC) metals since their behaviour under high strain rate regimes is not fully well understood and poses difficult problems due to their path loading dependency. Reference is also made to results obtained with aluminium alloys since they are widely used in virtually all fields of industry (e.g. aeronautical, automotive, marine and civil) and are increasingly used for armour and protective structures and systems. Even with such narrowing of the review many models are available, each one emphasising different aspects of high strain rate material response but none being completely satisfactory. A basic review of the main theoretical aspects that constitute the basis for most of the constitutive models described is also presented and the main features of each model are thoroughly discussed.

The dynamic friction behaviour of polycarbonate (PC) was investigated at ambient (26°C) and low temperature (−60°C) using a split Hopkinson pressure bar (SHPB) and specimens of varying thicknesses. At ambient temperature, polytetrafluoroethylene (PTFE) and molybdenum disulphide (MoS2) were found to be good lubricants, but not as good as petroleum jelly, which reduced friction to zero (within experimental error). In the low-temperature regime, PTFE was not a good lubricant. MoS2 was not as good a lubricant as ice at −60°C. However, again petroleum jelly provided perfect lubrication within experimental error. This was confirmed by a ring upsetting test in a drop-weight apparatus using high-speed photography. The PC used had a glass transition temperature Tg of 422 K as determined by differential scanning calorimetry (DSC). The intrinsic (zero friction) yield stress of PC in compression at a strain rate of 2500±200 s −1 was found to be 110 MPa at a strain of around 0.1 at 26°C and 175 MPa at a strain of approximately 0.13 at −60°C.

Two RDX/HTPB polymer bonded explosives (PBXs), with different explosive particle size, were studied in a Hopkinson bar system at three different temperatures. Three temperatures were chosen, two above, and one below, the glass transition temperature of the binder material. The PBX consisted of cyclotrimethylene trinitramine (RDX) crystals in a hydroxyl-terminated-polybutadiene (HTPB) binder. Overall the larger particle sized material was weaker, and exhibited a more distinct yield point than the finer sized material. Both materials showed temperature sensitivity, the effect being greater in the material with the smaller particles.

Compared to the many thousands of studies that have been performed on the energy release mechanisms of high energy materials, relatively few studies have been performed (a few hundred) into their mechanical properties. Since it is increasingly desired to model the high rate deformation of such materials, it is of great importance to gather data on their response so that predictive constitutive models can be constructed. This paper reviews the state of the art concerning what is known about the mechanical response of high energy materials. Examples of such materials are polymer bonded explosives (used in munitions), propellants (used to propel rockets), and pyrotechnics (used to initiate munitions and also in flares). © 2010 TMS.

Time-resolved and integrated diagnostics including high-speed photography, mass and optical spectroscopy, and optical-radiometry were used to study the impact response of high explosives in substantially more detail than possible with conventional sensitiveness tests. Specifically, we compare pentaerythritol tetranitrate (PETN) and cyclotetramethylene tetranitramine (HMX), with 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105), the latter of which is currently receiving a great deal of interest as a promising insensitive high explosive. We conclude that under modest impact conditions, LLM-105 can undergo a low-level reaction whose characteristics explain apparently contradictory sensitiveness results obtained using standard tests. Our results demonstrate how more sophisticated diagnostics with large dynamic ranges, can quantify the hazard response of energetic materials in detail, enabling complex behaviour to be distinguished.

This paper discusses the technique of Digital Speckle Radiography (DSR) and its application to the measurement of the internal flow fields in penetration of sand by long-rod projectiles at velocities up to 200 m/s. Three different rod nose shapes were studied: flat-ended, ogive-2, and hemispherical. Impacts performed on gelatine and concrete gave significantly different displacement fields to sand. Sand, therefore, cannot either be modelled as a fluid or as a conventional solid. Simulations performed using a code written by two of the authors (Bobaru and Promratana) showed that the velocity distribution has a very different appearance to the force distribution. This suggests that processes such as reorganisation, sliding and void filling take place, allowing the grains to move in directions other than the applied force. The resulting velocity distribution bears a strong resemblance to the experimentally measured displacement fields.

The normal impact of chrome steel spheres upon mild steel and copper plates of various thicknesses was investigated. The bulging of their rear surfaces was monitored during the first 30 μs after impact using stereo digital speckle photography. The experimental data for copper was compared with two path-dependent models, one optimised for annealed and the other for pre-stressed (work-hardened) copper. It was not possible to differentiate between the two copper models using single-point displacement data. However, full-field data revealed clear differences in the shapes of the bulges, with greater global deformation and closer agreement with experiment for the model that included pre-stress. For both mild steel and copper, thin plates resulted in localised bulging whilst thicker plates produced global dishing. The bulge peak also grew earlier and more quickly in thinner specimens. The greater ductility of copper resulted in narrower, faster-growing bulges compared with mild steel of comparable thickness.

This paper reports an initial study into the benefits of determining two-dimensional flow fields for low velocity impact on a small-scale model of explosive reactive armour (ERA) using digital speckle radiography (DSR). The model system consisted of a polymer-bonded sugar (PBS) (otherwise known as a sugar mock) confined between two mild steel plates. The DSR technique relies upon creating a layer within the specimen that is seeded with lead particles. So although radiography itself is mechanically non-invasive, the lead layer needed may change the mechanical properties of the material. DSR revealed where regions of intense shear occurred in normal impact. These regions are likely to be where a polymer-bonded explosive (PBX) would initiate.

The ballistic impact properties of a borosilicate (‘pyrex’) glass was studied using mild steel rods accelerated using a light gas gun. High-speed photography at sub-microsecond framing rates was used along with schlieren optics to investigate the propagation of elastic shock waves and fracture fronts. Flash X-radiography was used to visualise the deformation of rods as they penetrated the comminuted glass normally. The rod was seen initially to dwell on the surface for at least 3 μs creating a Hertzian cone-crack. Later on, between 40 and 60 μs, self-sharpening of the projectile was observed as the ‘wings’ of the heavily deformed front end sheared off. After this event, the front of the rod speeded up. X-rays also showed that the pattern of fissures within the comminuted glass was observed to be very similar shot-to-shot. X-radiography was also used to examine the mechanisms occurring during oblique impact of rods at 45°. In oblique impact, bending of the rod rather than plastic deformation (‘mushrooming’) takes on the role of distributing the load over an area larger than that of the original rod diameter. High-speed photography of the rear surface of a glass block on which a fine grid had been placed confirmed that the comminuted glass moved as larger interlocked blocks. The experiments were modelled using the QinetiQ Eulerian hydrocode GRIM making use of the Goldthorpe fracture model. The model was found to predict well the transition from dwell to penetration.

An integrated investigation of rod-on-rod (symmetric Taylor) impact of annealed copper was conducted using the single-stage gas-gun facility at the Cavendish Laboratory as a validation study of the Armstrong–Zerilli constitutive model, as modified by Goldthorpe. Two main techniques were used for obtaining data from the experiments: high-speed photography (up to 20 million frames s−1framing rate) and a velocity interferometer system for any reflector (VISAR). The symmetric configuration was used to minimize friction effects and eliminate target indentation seen in classic Taylor tests, where a rod is fired against a massive target block. However, the need for coaxial alignment of the two rods made the experiments considerably more challenging to perform than the classic case. The propagation of plasticity along the rods was monitored using high-speed photography and VISAR. It was found to propagate with a logarithmically decelerating velocity. The rod profiles and VISAR traces can be understood in terms of material properties such as strain hardening. No asymmetry between the responses of the two rods involved (moving and stationary) was observed within the resolution of the techniques employed. A modified Armstrong–Zerilli material model for copper predicted intermediate profiles well, but slightly overestimated the material strength.

The high strain rate dependence of the flow stress of metals and alloys is described from a dislocation mechanics viewpoint over a range beginning from conventional tension/compression testing through split Hopkinson pressure bar (SHPB) measurements to Charpy pendulum and Taylor solid cylinder impact tests and shock loading or isentropic compression experiment (ICE) results. Single crystal and polycrystal measurements are referenced in relation to influences of the crystal lattice structures and nanopolycrystal material behaviours. For body centred cubic (bcc) metals, the strain rate sensitivity (SRS) is in the yield stress dependence as compared with the face centred cubic (fcc) case of being in the strain hardening property. An important consequence is that an opposite ductility influence occurs for the tensile maximum load point strain that decreases with strain rate for the bcc case and increases with strain rate for the fcc case. Different hexagonal close packed (hcp) metals are shown to foll...

Dynamic compression experiments were performed on a pressed PTFE/Al/W mixture to understand the composite behavior at high-strain and high-strain rate. The high-strain-rate tests were carried out in a drop-weight apparatus at impact velocities of 3.5 and 5 m/s, providing strain rate so f approximately 4 × 10 2 s −1 . Aluminum jackets of varying thickness were used to ensure that specimens underwent confined deformation but did not separate into fragments. Failure was preceded by extensive plastic deformation concentrated primarily in the PTFE component. W particle–PTFE interface separation provided initiation and propagation of cracks. In extensively deformed specimens (strains of up to −0.875), PTFE nanofibers formed along cracks as a result of shear localization and significant softening caused by plastic deformation. The Zerilli–Armstrong constitutive equation for polymeric solids was used to simulate the response of the composite. Its use is justified by the fact that the majority of plastic strain is concentrated in the PTFE polymer. © 2007 Elsevier B.V. All rights reserved.

Advances in experimental, high-speed techniques can be used to explore the processes occurring within energetic materials. This review describes techniques used to study a wide range of processes: hot-spot formation, ignition thresholds, deflagration, sensitivity and finally the detonation process. As this is a wide field the focus will be on small-scale experiments and quantitative studies. It is important that such studies are linked to predictive models, which inform the experimental design process. The stimuli range includes, thermal ignition, drop-weight, Hopkinson Bar and Plate Impact studies. Studies made with inert simulants are also included as these are important in differentiating between reactive response and purely mechanical behaviour. Graphical abstract

This article discusses the fundamental physics of granular systems. It begins with an overview of the science of granular matter, followed by a description of the ‘micro’-structure on the granular level. It then considers stress transmission in mechanically equilibrated granular assemblies, focusing on conditions for marginal rigidity, isostaticity theory, and limitations of linear stress theories. It also examines the use of statistical mechanics to analyse and classify granular materials, taking into account the micro-canonical volume ensemble, structural degrees of freedom, the canonical volume ensemble and the quasi-particles of the volume ensemble, the stress ensemble, and the relationship between the volume and stress ensembles. The article concludes with an assessment of recent advances in the ongoing attempt to construct a statistical mechanical theory of granular systems.

We have used digital speckle radiography, the X-ray version of digital image correlation (DIC), to quantify the internal flow fields in dry sand ballistically impacted by rods of different nose shapes. We found that a standard DIC algorithm gave nonsensical results for the flow field close to the metal penetrator. We assessed various contrast normalization algorithms for their suitability in overcoming this problem. We found that a window averaging approach worked best. High-resolution displacement data were generated for specimens large enough to be representative of bulk material.

The stress-strain behaviours of polycarbonate (PC) and polyvinylidene difluoride (PVDF) have been measured over a range of strain rates at room temperature and a range of temperatures at high strain rate. Both materials show an approximately bilinear dependence of yield stress on strain rate over the rates examined. The experiments at different temperatures allow the high strain rate glass and β transitions to be identified in PC, and the melting point and glass transition to be identified in PVDF. These can be confirmed by comparison to Dynamic Mechanical Analysis (DMA) measurements on the materials. Applying a time-temperature superposition to the data shows that these transitions are the cause of the bilinearity in the strain rate dependence of the materials. The behaviour of nominal (or engineering) stress in PC is also examined.

Measurements are presented of the compressive stress–strain behaviour of polycarbonate (PC) and polyvinylidene difluoride (PVDF) at strain rates from 10 −4 to 10 4 s −1 at room temperature, and temperatures from −50 to +150 °C at 10 3 s −1 . These results, obtained using a split Hopkinson pressure bar and Instron testing machine, are supported by dynamic mechanical analysis (DMA) measurements on the materials. Previous researchers have observed that the yield stress of these materials is bilinearly dependent on the logarithm of strain rate. The data presented here show that the bilinearity is due to the movement of low order transitions in the materials, so that they occur at temperatures above room temperature at the higher strain rates. In particular, these transitions are the β transition in PC, and the glass transition in PVDF. In addition, Appendix A presents measurements of a high strain rate Poisson's ratio of polycarbonate and its evolution with strain.

A variety of techniques used to obtain the mechanical properties of materials at high rates of strain (⩾10 s−1) are summarised. These include dropweight machines, split Hopkinson pressure bars, Taylor impact and shock loading by plate impact. High-speed photography, particularly when used in association with optical techniques, is a key area and recent advances and applications to studies of ballistic impact are discussed. More comprehensive bibliographies and a fuller discussion of the history may be found in earlier reviews published by us in 1994, 1998 and 2001 (J Phys IV France 4 (C8) (1994) 3; Review of experimental techniques for high rate deformation studies, Proceedings of the Acoustics and Vibration Asia ’98, Acoustics and Vibration Asia 98 Conference, Singapore, 1998; Review of experimental techniques for high rate deformation and shock studies, New Experimental Methods in Material Dynamics and Impact, Institute of Fundamental Technological Research, Warsaw, Poland, 2001).

By giving careful thought to the design of the force transducer, a dropweight machine has been constructed capable of giving mechanical data of comparable accuracy to a split Hopkinson pressure bar. This has proved particularly useful in deforming large cylinders of hard metals to large plastic strains at high strain rates (ca. 500 to 1000 s -1 ). In conjunction with high-speed photography and finite element analysis, this device is proving to be a valuable stringent validation test for advanced constitutive models.

In conventional Hopkinson bar measurements the sample is required to be in stress equilibrium and homogeneous. By using an optical technique to measure the strains in the sample these problems can be overcome. This has already been done in Tensile Hopkinson bars [1], but not in compression. In this paper, a Polybutaleneteraphthalate (PBT) sample is tested using a compression Hopkinson bar combined with speckle metrology. Speckle measurements allow high accuracy displacement fields, and hence strains, on the material's surface to be measured. The speckles are produced either by laser illumination, or by using the natural relief and features of the surface. By capturing the speckled images using high-speed photography, the progression of strains on the sample surface can be followed throughout the experiment. The combination of Hopkinson bar data with the optical technique will also provide modellers with both macro and meso-scale data from the same experiment.

In this study, the rod-on-rod impact geometry was used at impact speeds of ca. 200 m s -1 . This is widely believed to be equivalent to the impact of a single rod on an infinitely rigid target and hence an ideal form of the Taylor test as no real target is infinitely rigid. However, our modelling studies showed that the two rods do not behave symmetrically. Two metals were studied: AQ85 iron and XM copper. These were machined into cylinders 15mm long and 5.35mm in diameter. High-speed photographic sequences of the impacts were taken with an interframe time of 5μs. These pictures were then digitised to obtain profiles as a function of time. The bcc material (iron) exhibited the classic 'mushroom' shaped profile which was relatively easy to model as a function of time using existing constitutive models. However, the fcc material (copper) exhibited an additional bulge a few millimetres from the impact face. This behaviour proved impossible to simulate using path-independent models. It proved, however, possible to simulate the behaviour of fcc copper in the 'symmetric' Taylor impact configuration using a newly-developed path-dependent constitutive model. The paper contains the results of this comparison between experiment, theory and modelling.

The paper begins with a review of the literature on high pressure and shear properties of materials. This is an area in which further interest was recently stimulated by published Russian work on s...

Pour verifier le modele de comportement d'un metal, il est important de determiner comment il faut corriger les courbes contrainte-deformation obtenues en compression afin de tenir compte des effets de frottement. Des erreurs graves viennent d'etre decouvertes dans les modeles analytiques standards utilises pour determiner les dites corrections dans les barres de Hopkinson. Dans le present article nous proposons une nouvelle methode pour determiner le coefficient de frottement dans la compression axiale dynamique d'un anneau La methode utilise la modelisation numerique et la photographie ultrarapide. Elle est validee en utilisant un acier a blindage.

It has long been known that explosives can be ignited by raising their temperature. What was a puzzle around the time of the Second World War was the observation that during dynamic loading, explosives could also be ignited even when the mechanical work dissipated within them was much less than that required to raise the temperature of the bulk to that required for ignition. It was such observations that led to the concept of the ‘hot spot’, i.e. that only very small volumes of energetic material need to be raised to a critical temperature for rapid decomposition to spread throughout the bulk. Since then, several mechanisms whereby energy can be localized have been identified of which adiabatic shear banding is one. This chapter will review what is known about strain localization in energetic granular materials (e.g. polymer-bonded explosives) as well as inert granular materials (such as sand, soil and sedimentary rocks).

This paper reports an experimental study of the rapid deformation and ignition behaviour of a number of cast double-base propellants both at room temperature and at temperatures below the glass transition of those compositions that were elastomer modified. A range of techniques were used to obtain stress/strain data on the materials. A drop-weight machine with transparent anvils was used in conjunction with a high-speed camera to observe the deformation behaviour during impact. The aim was to gain understanding of the key parameters leading to the impact initiation of propellants. The only propellants that ignited violently in solid disc form at room temperature were the conventional compositions, the most sensitive being a composite modified propellant containing aluminium and ammonium perchlorate. All compositions were sensitized if they contained gas spaces. In addition, all the elastomer modified compositions were sensitized by cooling them below their glass transition ( ca .210K). The mechanisms leading to ‘hot spot’ formation and ignition of the propellants are discussed.

This paper describes the response of explosives to stress and impact and in particular the mechanisms of ‘ hot-spot production. Samples in the form of single crystals, powder layers, pressed pellets, gels, polymer bonded explosives (PBXS) and propellants have been studied. Techniques used include a drop-weight facility with transparent anvils which allows photography at microsecond framing intervals, an instrumented drop-weight machine, a miniaturized Hopkinson bar system for high strain rate property measurement, laser speckle for studying the deformation and fracture of PBXS, an automated system for analysing speckle patterns and heat sensitive film for recording the positions and temperatures of hot spots. Polishing and staining methods have been developed to observe the microstructure of PBXS and failure during quasi-static loading. Ignition, when it occurred, took place at local hot-spot sites. Evidence is discussed for a variety of ignition mechanisms including adiabatic shear of the explosive, adiabatic heating of trapped gases during cavity collapse, viscous flow, friction, fracture and shear of added particles and triboluminescent discharge.

Symmetric Taylor impacts of copper and RHA rods were performed using a light gas gun. High-speed photography was used in back illumination to obtain silhouette sequences of the deforming rods. Predictions were then made of the rod profiles using an Armstrong-Zerilli path- dependent model within a Lagrangian finite element code. These predictions were then compared with the experiments and excellent agreement found for both materials.