Daniel R. Drodge

The mechanical response of a PBX and binder: combining results across the strain-rate and frequency domains.

The mechanical response of a polymer bonded explosive (PBX) has been measured using a Split Hopkinson Pressure Bar at a strain-rate of 2000 s−1, across a range of temperatures from 173 to 333 K, with the aim of observing its behaviour in the glassy regime. The yield stresses increased monotonically with decreasing temperature and no plateau was found. The failure mechanism was found to transition from shear-banding with crystal debonding fracture to brittle failure with some evidence of crystal fracture. Similar experiments were performed on samples of its nitrocellulose-based binder material, at a strain-rate of 3000 s−1 across a temperature range 173–273 K. The failure stresses of the binder approach that of the composite at temperatures near −70 °C. The elastic moduli were estimated from post-equilibrium regions of the stress–strain curves, and compared with those obtained for the composite using 5 MHz ultrasonic sound-speed measurement, and powder dynamic mechanical analysis measurements and quasi-static behaviour reported in a previous paper. The moduli were plotted on a common frequency axis: a temperature shift was applied to collapse the curves, which agreed with the Cox–Merz rule.

HOPKINSON BAR STUDIES OF A PBX SIMULANT

Experiments were carried out, at a strain rate of  1600 s−1, to characterise an HTPB/sugar propellant simulant within the temperature range of 50 °C to −100 °C. Optical techniques, namely high‐speed photography with digital speckle analysis for flow visualisation, and line‐laser occlusion to measure diametric expansion, were deployed. Agreement was found with the results of a previous study on a similar material: failure occurs by dewetting at temperatures above the glass transition temperature and brittle fracture occurs below. The peak strength of the material was recorded to be 80 MPa, measured at a temperature of −80 °C.

Stress gage system for measuring very soft materials under high rates of deformation

Soft materials have seen continued growth in industrial importance, but are difficult to test at relevant, particularly at high, rates of deformation and relevant temperatures. This is mainly due to the low stresses supported by these materials, which mean that very sensitive force measurements are required. In this paper, a split-Hopkinson pressure bar method for testing very soft materials and elastomers at high rates of deformation is presented and applied. Experiments are conducted in compression on hydroxyl terminated polybutadiene, a very soft rubber, at strain rates of about 2000?s?1. Titanium alloy bars are used, and in addition to the usual strain gauges on the bars, forces at both ends of the specimen are measured using a piezoelectric material, lead zirconium titanate (PZT), which is much more sensitive than the quartz crystal gauges typically used in previous literature. The piezoelectric constant of PZT ranges between 290?630???10?12?C N?1, making it 100?times more sensitive than quartz crystal (2.3???10?12?C N?1). Results obtained from the experiments show that the gauges are able to measure the forces on both ends of the specimen with excellent signal to noise ratios.

COMPARISON OF PORTER‐GOULD CONSTITUTIVE MODEL WITH COMPRESSION TEST DATA FOR HTPB/SUGAR

We have been developing the physically based QinetiQ Porter‐Gould (P‐G) model for the mechanical response of PBXs over a number of years and applying it to the solution of real scenarios involving impact and blast. The main difficulty with these models is predicting the intermediate strain rate regime where the relaxation time for the polymer is of the same order as the duration of the loading (e.g. as in a Hopkinson bar test). The other main issue is the ability of the model to predict the stress/strain data as a function of temperature up to and through the glass transition temperature. The paper presents predictions from the QinetiQ P‐G model compared to quasi‐static compression and Hopkinson bar compression test data and discusses the results in terms of requirements for future developments of the model.

THE EFFECTS OF PARTICLE SIZE AND SEPARATION ON PBX DEFORMATION

A Hall‐Petch‐like relationship between yield stress and particle size has previously been documented for some monomodal PBX compositions loaded in uniaxial compression. However, due to the fixed fill‐fraction of these materials, the particle separation was proportional to particle size. Thus either or both parameters could be responsible for the relationship. A set of inert monomodal composites have been produced to resolve this ambiguity and supply modellers with validation data for their PBX codes. So far, uniaxial compression response has been measured at a strain‐rate of 103 s−1.

Mechanical response of damaged explosive compositions

Polymer‐bonded explosive (PBX) materials often exhibit strain‐softening as a consequence of increasing microstructural damage. Good mechanical models thus require an account of loading path dependence. For validation purposes, a series of experiments have been carried out on a PBX system, introducing damage through uniaxial compression to fixed strains, with accompanying X‐ray microtomographic imaging to provide insight into the structural changes that occur. The resulting datasets should provide a thorough test of the various PBX models abounding in the literature.

Hierarchical Bionanomaterials Under the Hammer: High-Rate Response of Silks

Silks are of significant interest to scientists and the public due to their high specific strength and unsurpassed toughness. The study of their properties and formulation of physically-based models is ongoing in the biomaterials community. Interesting models and simulation data are appearing in the literature but there is a paucity of experimental data at high strain-rate or high frequency. To remedy this, high strain-rate characterisation has been undertaken alongside conventional low-rate tests, under a range of conditions. The methods reported here represent large-strain, high-rate, i.e. transverse impact; and small-strain, high-rate, i.e. vibration. Both have relevance to the use of silk in nature by organisms (protection, predation and communication) and the application/imitation of silk by materials scientists. Here we report the methodology and results to date in our investigations on silkworm and orb weaver silks.

TWO‐STEP LOADING IN A SPLIT HOPKINSON PRESSURE BAR

In conventional Split Hopkinson Pressure Bar (SHPB) experiments the striker bar is a single rod and the sample is loaded at one strain rate. In this study, we present results from a system that uses a striker bar formed from two rods of different materials. A two level loading pulse is generated, subjecting the sample to two different strain‐rates: either a low strain‐rate followed by a higher strain‐rate (“step up”) or vice versa (“step‐down”). The sample does not experience repeat loading or significant unloading between the two regimes. The “step‐down” loading sequence, of equal durations at 2300 s−1 and 400 s−1, has been applied to iron and copper samples. History independence is observed for iron, as expected from its BCC structure.