P. Church

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.

THE DYNAMIC COMPACTION OF SAND AND RELATED POROUS SYSTEMS

Porous and granular materials are widely found in a number of environments. One of the most important groups both geographically and in the construction industry are the sands. A review of the response of sand (42% porous) over a wide range of strain rates is presented. Factors such as water content and density variation are addressed. In addition a very low‐density silica dust (95% porous) is also discussed in relation to its contrasting behaviour.

A method for determining the main mechanical properties of soft soils at high strain rates (103–105 s−1) and load amplitudes up to several gigapascals

A new method has been developed for determining the main laws of the deformation of soft soils under conditions of dynamic loading with amplitudes of up to several gigapascals. The method is based on the results of high-strain-rate tests under uniaxial deformation conditions, which were obtained using a modified Kolsky method and the plane shock wave technique. The possibilities of the proposed method are illustrated by determining the dynamic properties of sand.

Using the split Hopkinson pressure bar to validate material models.

This paper gives a discussion of the use of the split-Hopkinson bar with particular reference to the requirements of materials modelling at QinetiQ. This is to deploy validated material models for numerical simulations that are physically based and have as little characterization overhead as possible. In order to have confidence that the models have a wide range of applicability, this means, at most, characterizing the models at low rate and then validating them at high rate. The split Hopkinson pressure bar (SHPB) is ideal for this purpose. It is also a very useful tool for analysing material behaviour under non-shock wave loading. This means understanding the output of the test and developing techniques for reliable comparison of simulations with SHPB data. For materials other than metals comparison with an output stress v strain curve is not sufficient as the assumptions built into the classical analysis are generally violated. The method described in this paper compares the simulations with as much validation data as can be derived from deployed instrumentation including the raw strain gauge data on the input and output bars, which avoids any assumptions about stress equilibrium. One has to take into account Pochhammer–Chree oscillations and their effect on the specimen and recognize that this is itself also a valuable validation test of the material model.

High Strain Rate Characterisation of a Polymer Bonded Sugar

The mechanical properties of a polymer bonded sugar consisting of 78% sugar crystals, of modal particle size 310 μm, dispersed in an HTPB binder have been characterized in a split Hopkinson pressure bar system at a strain rate of 103 s−1 and temperatures from −100 to +20 °C. These high rate experiments were supplemented by further experiments in an Instron at 10−3 s−1. The material behavior is compared to that of other polymer bonded explosives and simulants. In order to further understand the structural deformation mechanisms specimens of both pristine material and that after Instron testing were examined using X‐ray microtomography.

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.

Dependence of Measured Lateral Stress on Thickness of Protective “Padding” around Gauge

Earlier work found that lateral stress gauges were unusable when the longitudinal stress in the sample exceeded around 13 GPa in samples of steel. This pressure corresponds to a phase transition found in iron and its alloys. It was hoped that protective padding would allow measurement of stresses higher than this. It was therefore necessary to investigate the effect of the padding on the stress measured. A series of experiments were carried out using mild steel with different thicknesses of mylar or polycarbonate padding. This research has been supported by simulation studies using the Eulerian hydrocode GRIM, which has indicated some issues with the representation of the lateral gauge in the hydrocode. The simulations have confirmed the experimental trends observed with additional padding, but they have also indicated the potential need for additional analysis on the raw lateral gauge data.

Experiments and simulation of split Hopkinson Bar tests on sand

Static triaxial cell data and Split Hopkinson Bar data has been generated for well controlled dry and wet sand under confined and unconfined conditions. This has demonstrated that the dry sand is rate independent in its behaviour, whereas the wet sand exhibits a strain rate dependency in its behaviour. Simulations have been performed with the Lagrangian hydrocode DYNA using a Porter-Gould equation of state (EOS) and Johnson-Holmquist type constitutive model. Comparison with the raw strain gauge data is qualitatively reasonable, although some of the details of the trace are not reproduced. Sensitivity studies have also been performed, which has demonstrated some deficiencies in the constitutive model, relating to wave-speed and definition of moduli in a granular material. This has given some insights into how the constitutive model should be improved and which future experimental tests will be required.

Demonstration of survivable space penetrator

This work was performed in support of MoonLITE which is a proposed UK space mission to the moon. The basic premise is to deploy 4 instrumented penetrators, one each on the near-side, farside and at the poles of the moon, with an impact velocity of approximately 300m/s. The primary science aims are to set up a passive seismometer network, investigate the presence of water and volatiles and determine thermal gradients in the lunar soil (i.e. regolith). A key requirement is that the penetrator shell survives the impact together with the instrument payload and supporting subsystems. The material chosen for the penetrator shell was 7075 aluminium alloy, which is a good compromise between high compressive strength and low mass. The baseline penetrator design was evaluated and refined using the DYNA3D hydrocode to determine the survivability of the penetrator in sand at an impact velocity of 300m/s and an attack angle of 8°. The simulations predicted that the penetrator design would survive this severe impact co...

Characterising the Response of Energetic materials and Polymer-Bonded Explosives (PBXs) to High-Rate Loading.

Polymer-bonded explosives (PBXs) are being increasingly used as energetic fillings and components in many systems. They are perceived as more chemically and mechanically stable than traditional fillings such as RDX/TNT. They are castable into predetermined shapes, machinable and can be used as structural components. However, along with all these undeniable advantages, as a class, these materials are now undergoing extensive characterisation to ensure they comply with both the legal and technical requirements in energetic systems. It is well-known that polymers display non-linear behaviour and are much more complex than, for example, simple metal systems at any rate of strain. The understanding of PBX systems involves areas as diverse as polymer chemistry, chemical compatibility, mechanical properties, impact tests, and thermal stability. In this paper, aspects of energetic material response are outlined which are relevant to the understanding of PBX sensitivity.