Clive R. Siviour

Particle size effects on the mechanical properties of a polymer bonded explosive

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.

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.

High strain rate properties of a polymer-bonded sugar: their dependence on applied and internal constraints

This paper describes research performed on a polymer-bonded sugar (PBS) consisting of 66% caster sugar in a hydroxyl-terminated polybutadiene (HTPB) binder The mechanical response of the PBS and pure HTPB to applied loading at a strain rate of approximately 2000 s−1 at temperatures from −80 to +22°C is presented. The materials were also characterized using dynamic mechanical analysis, X-ray tomography and quasi-static loading. These measurements are required for the development of intermediate strain rate constitutive models of polymer-bonded explosives, for which PBSs are a commonly used mechanical simulant. The current constitutive modelling suffers from a lack of experimental data on well-characterized composites and binders, especially at intermediate strain rates. This is particularly important for understanding the effects of mixing two materials. Applications of such modelling include explosive safety and fundamental understanding of the various deformation mechanisms. In this paper, the dependences of strength and deformation mechanism on temperature, and, in particular, the glass transition temperature of the binder, are shown. Physical damage plays an important role; X-ray tomography scans support debonding as the primary form of damage during room-temperature deformation. These results are in agreement with previous investigations and are discussed in this context.

High-strain rate Brazilian testing of an explosive simulant using speckle metrology

The application of high-speed photography and image correlation has allowed dynamic Brazilian tests to be carried out using the split Hopkinson pressure bar (SHPB) on PBS9501, an explosive simulant. The essential features of low-rate Brazilian tests are found to occur in the high-rate experiments, with the samples reaching equilibrium quickly and remaining in equilibrium throughout the experiment. The advantage of the approach described here is the ability to make tensile stress/strain measurements in the high-strain rate regime using a compression Hopkinson bar. This allows smaller sample sizes, making testing of expensive or dangerous materials easier, while expanding the capabilities of the compression bar.

Beyond Hopkinson's bar

In order to perform experimental identification of high strain rate material models, engineers have only a very limited toolbox based on test procedures developed decades ago. The best example is the so-called split Hopkinson pressure bar based on the bar concept introduced 100 years ago by Bertram Hopkinson to measure blast pulses. The recent advent of full-field deformation measurements using imaging techniques has allowed novel approaches to be developed and exciting new testing procedures to be imagined for the first time. One can use this full-field information in conjunction with efficient numerical inverse identification tools such as the virtual fields method (VFM) to identify material parameters at high rates. The underpinning novelty is to exploit the inertial effects developed in high strain rate loading. This paper presents results from a new inertial impact test to obtain stress–strain curves at high strain rates (here, up to 3000 s−1). A quasi-isotropic composite specimen is equipped with a grid and images are recorded with the new HPV-X camera from Shimadzu at 5 Mfps and the SIMX16 camera from Specialised Imaging at 1 Mfps. Deformation, strain and acceleration fields are then input into the VFM to identify the stiffness parameters with unprecedented quality.

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.

Temperature-time response of a polymer bonded explosive in compression (EDC37)

The compressive strength of the energetic composition EDC37 has been measured at a temperature of 293 ± 2 K over a range of strain rates from 10−8 to 103 s−1, and at a strain rate of 10−3 s−1 over a range of temperatures from 208 to 333 K. The results show that failure stress is a monotonic function of applied strain rate or temperature, which is dominated by the relaxation properties of the polymeric binder; this is confirmed by dynamic mechanical thermal analysis performed on both EDC37 and its binder. Similarities between the compressive strain rate/temperature data sets can be understood by temperature–time superposition; data collected at a strain rate of 10−3 s−1 over a temperature range 208 to 333 K were mapped onto a plot of strain rate dependent strength at 293 K, using an empirically determined sensitivity of −13.1 ± 0.3 K per decade of strain rate. Sample size was noted to have a modest effect on the stress–strain behaviour; small length to diameter ratios gave results consistent with an increased degree of confinement. Samples taken to large strains exhibited strain localization in the form of shear bands.

3D deformation and strain analysis in compacted sugar using x-ray microtomography and digital volume correlation

Understanding the displacement of granular beds under compaction is important for a range of industrial, geological and civil engineering applications. Such materials exhibit inhomogeneous internal displacements including strain localization, which mean that a method for the in situ evaluation of internal 3D displacement fields at high spatial resolutions would be a major development. This paper presents results from the compaction of a cylindrical bed of sugar, with diameter 7.0 mm and height 8.2 mm, using x-ray microtomography to evaluate the internal structure and digital volume correlation to calculate 3D displacement information from these data. In contrast to previous studies, which generally track a small number of marker particles, the research here uses the natural structure of the sugar to provide a random pattern for 3D image correlation, allowing full-field information to be captured. The results show good agreement when compared with a well-established 2D image correlation technique; moreover, they indicate structural features associated with deformation of granular materials that would not necessarily be observed in a 2D slice.

Cross-section reconstruction during uniaxial loading

The inelastic response of materials to applied uniaxial loading is typically measured using tensile or compressive specimens of an initially circular cross-section. Under deformation, this cross-section may become elliptical due to anisotropic material behaviour. An optical technique for measuring the elliptical deformation of anisotropic, homogeneous cylindrical specimens undergoing uniaxial deformation is presented. It enables the quantification of anisotropic deformation in situ and provides data for material characterization. Three or more silhouette views of a specimen are obtained using multiple cameras or mirrored views. The positions of the edges are computed using a sub-pixel edge detection method, and 3D tangent rays from the camera through these positions are calculated. These bounding tangents are used as the basis for an elliptical fit by least squares at cross-sections along the length of the specimen. Stochastic error estimates are performed by simulation of the experiment. Error estimates, for the experimental set-up used, are also calculated by reconstructing elliptical prisms of precisely measured dimensions. Example reconstructions from specimens of rolled titanium deformed plastically in tension at quasi-static (7 × 10-4 s-1) and high strain rates (3 × 103 s-1) are presented.

The Strain Rate Dependent Material Behavior of S-GFRP Extracted from GLARE

S-Glass fiber reinforced epoxy (S-GFRP) extracted from GLARE has been experimentally characterized at three distinct strain-rates (5 × 10−4 s−1, 10 s−1, and c.a. 2000 s−1) and in four loading directions (0°, 90°, and 45° to the fiber direction and in the through-thickness direction). A novel specimen clamping mechanism was developed and full-field optical strain measurement was applied. With the aid of these techniques, a significant increase in failure-strength and apparent elastic modulus in all loading directions, particularly in fiber direction, was observed with an increased strain-rate; strain to failure increased in the fiber direction and decreased in all other loading directions.