S.M. Walley

A study of the rapid deformation behaviour of a range of polymers

Polymers are increasingly being used in applications where they are rapidly deformed. However, compared with metals, relatively few studies of their mechanical properties at high rates of strain have been published. This paper describes an investigation of the rapid deformation behaviour in compression of a number of widely used polymeric materials. The necessity of properly characterizing polymers is discussed, as the variation of commercial grades bearing the same name is considerable, and furthermore these materials are much more susceptible to change during storage than say metals. The importance of thermal properties to rapid, and hence adiabatic, deformation is pointed out, and tables of such properties are presented. Extensive use was made of high-speed photography (interframe time 7 (is) to study qualitatively the behaviour of solid discs of polymers at strain rates of 2.5 x 103 s -1. The framing speed was sufficiently fast to capture fracture initiation and subsequent failure of all the polymers studied, including polycarbonate (PC), which fails in an almost explosive manner. The darkening of heat-sensitive film in contact with deforming discs was also investigated. Quantitatively, this technique was used to check the applicability of Avitzur’s analysis (Avitzur (Israel J. Technol. 2, 295-304 (1964)) of a deforming annulus to polymers. Agreement was found to be good and hence friction could be measured during deformation at high rates of strain for the first time. Studies were also carried out to determine the best lubricant for rapid compressive testing. Petroleum jelly was found to reduce the friction closest to zero. An optically identical system was set up in an Instron mechanical testing machine both to perform friction studies and to explore deviation from incompressible behaviour. Agreement with Avitzur’s analysis was found to be poorer, and no lubricant was found to reduce friction below about 3-4 %. PC, with a very high value of frictional stress, showed evidence of a change in volume. Allowances were made for the elastic indentation of the anvils. Higher strain rates were achieved by using an instrumented drop-weight machine and a direct impact Kolsky bar, both developed in this laboratory. Care was taken to eliminate sources of error, including friction and calibration errors. The strain rate sensitivity of the polymers ranged from 5—15 MPa per decade of strain rate. However, most showed some softening as the strain rate was raised from 103 to 104 s-1, the exceptions being polybutylene teraphthalate (PBT) and polyvinylidene difluoride (PVDF).

Mechanical properties of SnPb and lead-free solders at high rates of strain

The mechanical properties of 63% Sn–37% Pb and lead-free solders have been measured at high strain rates (500–3000 s−1) using a split Hopkinson pressure bar. The solders were produced by quenching in water from the melt, to give the phase structure associated with rapid cooling. Measurements were made at −40 °C, room temperature and +60 °C. The Sn–Pb solder was strongly strain rate and temperature dependent, whereas the lead-free solders showed only a weak dependence on these parameters. All of the materials behaved elasto-plastically until a plateau stress of circa 200 MPa. An unexpected, and possibly important, feature of the lead-free solders was the division of the specimens into two groups with different strengths at low temperatures.

Particle size effect on strength, failure, and shock behavior in polytetrafluoroethylene-Al-W granular composite materials

The variation of metallic particle size and sample porosity significantly alters the dynamic mechanical properties of high density granular composite materials processed using a cold isostatically pressed mixture of polytetrafluoroethylene (PTFE), aluminum (Al), and tungsten (W) powders. Quasistatic and dynamic experiments are performed with identical constituent mass fractions with variations in the size of the W particles and pressing conditions. The relatively weak polymer matrix allows the strength and fracture modes of this material to be governed by the granular type behavior of agglomerated metal particles. A higher ultimate compressive strength was observed in relatively high porosity samples with small W particles compared to those with coarse W particles in all experiments. Mesoscale granular force chains of the metallic particles explain this unusual phenomenon as observed in hydrocode simulations of a drop-weight test. Macrocracks forming below the critical failure strain for the matrix and un...

Crystal sensitivities of energetic materials

Abstract Organic and inorganic explosives were first developed and put into service in the 19th century, before there was much understanding of how the energy release mechanisms differed from those of the long established gunpowder. Theoretical advances in the understanding of shock waves combined with improvements in photographic and electronic techniques led to the hypothesis that a detonation is a shock wave maintained by the rapid release of chemical energy. Studies of accidental ignitions/initiations showed that explosive events can occur even when the energy input is much less than that required to heat the bulk explosive to the deflagration temperature. Hence, the highly fruitful idea of the localised hot spot was conceived. Apart from electrical stimuli, the main hot spot mechanisms are currently accepted as being adiabatic asymmetric collapse of gas spaces (producing gas heating, jetting, viscoplastic work) and the rubbing together of surfaces as in friction or adiabatic shear. Initiation mechanisms are also connected with the anisotropy of plasticity and fracture in explosive crystals. Decomposition of molecules can take place as they are forced past each other in a deforming crystal. There is, however, still much to discover about reaction pathways. Novel optical and electron microscopy techniques have given a great deal of new and precise information about displacements and failure mechanisms when explosive crystals are bonded together using polymers. The deflagration–detonation transition (DDT) has been extensively studied in model one- and two-dimensional systems.

Behaviour of ammonium perchlorate-based propellants and a polymer-bonded explosive under impact loading

The response of a range of ammonium perchlorate–based propellants and a polymer–bonded explosive to drop–weight impact loading has been studied using high–speed photography. This technique allows the generation of ‘hot spots’ and the subsequent growth of reaction to be recorded. In separate experiments, the mechanical properties of these materials were measured over the range of strain rates 0.01−8×103 s–1 using an Instron 1122 and a split Hopkinson pressure bar. In addition, the effect of temperature on their high–strain–rate properties was examined over the range −60 to +60 °C. Scanning electron microscopy studies were performed, on both as–received specimens and material which had been recovered from interrupted drop–weight experiments, to investigate the connection between microstructure and ignition sources. A close link was established between the mechanical properties at the appropriate strain rates and the ignition response to impact.

The Erosion and Deformation of Polyethylene by Solid-Particle Impact

In recent years, polyethylene (PE) has found increasing use in applications involving impact and erosion. This paper describes a detailed study of the properties of PE subjected to solid particle impact. Flat discs of the material were eroded by sieved sand (300-600 pm) accelerated by using an air blast rig in which the important variables of velocity, angle and mass flux rate are accurately controllable and measurable. Scanning electron microscopy of lightly eroded specimens enabled four basic crater types to be identified: smooth, ploughed, cut, and dented. The proportions of each were established over a range of angles. Long time erosion experiments were conducted in which the flux rate for each angle was adjusted to keep the number of impacts per unit time constant. The dimensionless erosion parameter, e (mass lost per unit mass of erodent that has struck) was computed by using the rate of mass loss when steady-state erosion had been established. Most erosion was found to occur at an angle of 20—30°, the mass loss becoming zero at around 80°. An analysis by D. R. Andrews is presented, showing that the flux rates used in these experiments are well below those needed to cause wear by thermal mechanisms, and this was confirmed by changing the flux rate: mass loss increased in proportion. Macroscopic particles were used to model sand grain impacts, spheres for rounded particles and square plates for sharp ones. A range of techniques was used in this study including high-speed photography (framing speed of 5 x 104 s-1), scanning electron microscopy, and moire methods (both in-plane and out-of-plane). A deformation map was constructed for steel sphere im pacts giving the type of crater to be expected at a given angle and speed. It was observed that sand grains required much lower speeds at a given angle to produce a given crater type. High-speed photography enabled mass-loss mechanisms for single-particle impact to be identified. These were the drawing-out of filaments and the machining-out of chips. Quantitative data on kinetic energy losses were obtained, and these, combined with moire methods that gave the sizes of deformed zones, enabled an estimate of the temperature rise per impact to be made (25 K).

Impact Sensitivity of Propellants

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.

ELASTIC, PLASTIC, CRACKING ASPECTS OF THE HARDNESS OF MATERIALS

The hardness properties of materials are tracked from early history until the present time. Emphasis is placed on the hardness test being a useful probe for determining the local elastic, plastic and cracking properties of single crystal, polycrystalline, polyphase or amorphous materials. Beginning from connection made between individual hardness pressure measurements and the conventional stress–strain properties of polycrystalline materials, the newer consideration is described of directly specifying a hardness-type stress–strain relationship based on a continuous loading curve, particularly, as obtained with a spherical indenter. Such effort has received impetus from order-of-magnitude improvements in load and displacement measuring capabilities that are demonstrated for nanoindentation testing. Details of metrology assessments involved in various types of hardness tests are reviewed. A compilation of measurements is presented for the separate aspects of Hertzian elastic, dislocation-mechanics-based plasticity and indentation-fracture-mechanics-based cracking behaviors of materials, including elastic and plastic deformation rate effects. A number of test applications are reviewed, most notably involving the hardness of thin film materials and coatings.

Major Steps in the Discovery of Adiabatic Shear Bands

The standard story of the discovery of adiabatic shear bands is that it began with the American researchers Zener and Hollomon’s famous 1944 paper where the phenomenon was first reported and named. However, a recent discovery by one of us (SMW) in the Cambridge University Library has shown that the phenomenon was discovered and described by a Russian researcher, V.P. Kravz-Tarnavskii, in 1928. A follow-up paper was published by two of his colleagues in 1935. Translations of the 1928 and 1935 papers may be found at http://arxiv.org/abs/1410.1353.

The Shock Initiation and High Strain Rate Mechanical Characterization of Ultrafine Energetic Powders and Compositions

ABSTRACT This paper reviews the techniques that have been developed at the Cavendish Laboratory for the study of the mechanical and ignition properties of energetic materials. HIGH-SPEED PHOTOGRAPHY A number of techniques have been developed and applied in our laboratory for the investigation of the properties of energetic materials. One method we have used in a wide range of such studies has been high-speed photography e.g. refs [1-11]. The advantage is that it is possible to see directly what is going on in, for example, hot spot initiation of energetic materials. In recent years, it has increasingly been desired to model the impact response of structures containing energetic materials using numerical methods. If meaningful numerical results are going to be obtained for, say, munitions or rockets, it is of vital importance that constitutive relations be constructed which describe the mechanical response of unreacted energetic materials over the temperature and strain rate ranges of interest. With this in mind, we have developed a range of techniques for obtaining the mechanical properties of energetic materials over a wide range of strain rates and temperatures. Examples of publications where such data have been published include refs [4, 7, 12-20]. One problem with conventional mechanical testing methods is that they only allow the measurement of the global response of a specimen. Energetic materials usually consist of a viscoelastic binder heavily loaded with explosive crystals. In order to develop realistic and physically-based constitutive models for such unusual composites, it is vital to determine how these materials deform on the mesoscale. To this end, we have developed a range of optical and microscopy techniques. These have been used for both quasistatic [14, 15, 18, 21-23] and dynamic studies [19, 20, 24-26].