Damage in Polymer Bonded Energetic Composites: Effect of Loading Rate

Particulate composites are widely used in the materials world. An understanding of their damage behaviour under a variety of loading conditions is necessary to inform models of their response to external stimuli. In the present experimental study, fine and coarse grained RDX-HTPB composites have been used to investigate the effect of loading rate on the degree of damage produced in polymer bonded explosives subjected to varying degrees of uniaxial compression. High strain rate loading (4×10⁺³ s⁻¹) was achieved using a direct impact Hopkinson pressure bar and low strain rate loading (1×10⁻² s⁻¹) using an Instron mechanical testing machine. The causal metrics are the degree to which the samples were strained and the mechanical energy expended in straining them. The damage metric is the residual low rate compressive modulus of the samples. The quantitative, physically based, results discussed in terms of the Porter-Gould activated debonding damage model clearly demonstrate that for both fine and coarse fills there is a marked reduction in residual moduli as a function of imposed strain, and substantially less specific energy is required to cause the same level of damage at the lower strain-rate. In the case of the coarse grained composite there is some evidence for a change in damage mechanism at the higher strain-rate. We obtain a value for the measured work of adhesion and a measure of the effective modulus local to the damage site, as damage is actually occurring. The observed underlying behaviour should be broadly applicable to particulate composites, whenever stiff filler particles are held in a viscoelastic matrix.The authors wish to acknowledge financial support in the form of an Industrial CASE PhD Studentship for RLB funded by the UK Engineering and Physical Sciences Research Council (EPSRC) and by QinetiQ [EP/I501290/1]; UK MOD via a WSTC contract; DMW and APJ acknowledge the financial support of AWE.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s40870-016-0050-x The data underlying this article can be found at the following persistent URL: https://www.repository.cam.ac.uk/handle/1810/25319

Behaviour of moist and saturated sand during shock and release

Relatively little is known about the changes that occur in the shock compaction and release of granular matter with varying levels of moisture. Here, we report a series of plate impact experiments giving shock Hugoniot and release data for a well characterized sand at dry, 10% moist, and saturated water contents. The results reveal that at low moisture content the shock impedance is slightly reduced, while the release remains predominantly inelastic. Close to saturation, much more substantial changes occur: the shock impedance stiffens substantially, the Hugoniot appears to split into two branches, and the release becomes almost completely elastic. We discuss mechanisms underpinning these changes in behavior.This work was supported through the Force Protection Engineering research programme led by QinetiQ Plc. on behalf of DSTL.This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493468

Characterisation of the impact response of energetic materials: Observation of a low-level reaction in 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

Time resolved and integrated diagnostics including high speed photography, mass and optical spectroscopy, and optical-radiometry used to study impact response of high explosives in far more detail than possible with conventional sensitiveness tests.The authors wish to acknowledge the funding and provision of samples for this research by AWE plc.This is the author accepted manuscript. The final version is available from the Royal Society of Chemistry via http://dx.doi.org/10.1039/C6RA03096

Unlocking new contrast in a scanning helium microscope.

Delicate structures (such as biological samples, organic films for polymer electronics and adsorbate layers) suffer degradation under the energetic probes of traditional microscopies. Furthermore, the charged nature of these probes presents difficulties when imaging with electric or magnetic fields, or for insulating materials where the addition of a conductive coating is not desirable. Scanning helium microscopy is able to image such structures completely non-destructively by taking advantage of a neutral helium beam as a chemically, electrically and magnetically inert probe of the sample surface. Here we present scanning helium micrographs demonstrating image contrast arising from a range of mechanisms including, for the first time, chemical contrast observed from a series of metal-semiconductor interfaces. The ability of scanning helium microscopy to distinguish between materials without the risk of damage makes it ideal for investigating a wide range of systems.This research was supported under the Australian Research Councils Discovery Projects (Project No. DP08831308) funding scheme. Postgraduate research scholarships (M.B., A.F.) from the University of Newcastle gratefully acknowledged. We thank the Newcastle and Cavendish workshops, Donald MacLaren and Kane O’Donnell for technical support, insightful discussions and assistance. This work was performed in part at both the Materials and ACT nodes of the Australian National Fabrication Facility, which is a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for Australia’s researchers.This is the final version of the article. It was first available from NPG via http://dx.doi.org/10.1038/ncomms1018

How Atomic Steps Modify Diffusion and Inter-adsorbate Forces: Empirical Evidence from Hopping Dynamics in Na/Cu(115).

We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico- to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces.This work was supported by the German-Israeli Foundation for Scientific Research and Development, the Israeli Science Foundation (Grant No. 2011185), the German Science Foundation (DFG) through contract MO 960/18-1, the Cluster of Excellence RESOLV (EXC 1069), and the European Research Council under the European Union’s seventh framework program (FP/2007-2013)/ERC Grant 307267.This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acs.jpclett.5b0193

A method for constrained optimisation of the design of a scanning helium microscope.

We describe a method for obtaining the optimal design of a normal incidence Scanning Helium Microscope (SHeM). Scanning helium microscopy is a recently developed technique that uses low energy neutral helium atoms as a probe to image the surface of a sample without causing damage. After estimating the variation of source brightness with nozzle size and pressure, we perform a constrained optimisation to determine the optimal geometry of the instrument (i.e. the geometry that maximises intensity) for a given target resolution. For an instrument using a pinhole to form the helium microprobe, the source and atom optics are separable and Lagrange multipliers are used to obtain an analytic expression for the optimal parameters. For an instrument using a zone plate as the focal element, the whole optical system must be considered and a numerical approach has been applied. Unlike previous numerical methods for optimisation, our approach provides insight into the effect and significance of each instrumental parameter, enabling an intuitive understanding of effect of the SHeM geometry. We show that for an instrument with a working distance of 1 mm, a zone plate with a minimum feature size of 25 nm becomes the advantageous focussing element if the desired beam standard deviation is below about 300 nm.The work was supported by EPSRC grant EP/R008272/1. M.B. acknowledges an EPSRC studentship and a Leathersellers Graduate scholarship

An evaluation of the kinematic approximation in helium atom scattering using wavepacket calculations

We use 2-D wavepacket calculations to examine the scattering of helium atoms from dynamic assemblies of surface adsorbates, and in particular to explore the validity of the widely used kinematic scattering approximation. The wavepacket calculations give exact results for quasi-elastic scattering that are closely analogous to time-of-flight (TOF) experiments and they are analysed as such. A scattering potential is chosen to represent 8 meV helium atoms scattering from sodium atoms adsorbed on a Cu(001) surface and the adsorbates in the model move according to an independent Langevin equation. The energy broadening in the quasi-elastic scattering is obtained as a function of parallel momentum transfer and compared with the corresponding results using the kinematic scattering approximation. Under most circumstances the kinematic approximation and the more accurate wavepacket method are in good agreement; however, there are cases where the two methods give different results. We relate these differences to pathological features in the scattering form-factor.EPSRC Studentship, Royal Society University Research Fellowshi

The significance of grain morphology and moisture content on the response of silica sand to ballistic penetration

The dynamic response of sand is of interest for a wide range of applications, from civil engineering to asteroid impact, in addition to defense and industrial processes. Granular dynamics are controlled by a complex network of intergrain force chains; yet, our understanding of how grain morphology, moisture, rate, and loading geometry affect the response to rapid compaction remains limited. Here, we show how just 1% moisture can significantly reduce penetration resistance in silica sand, while smoother-grained material—with a similar bulk density, grain size, and mineralogy—exhibits markedly improved stopping power. Cylindrical targets are impacted by spherical steel projectiles, with Digital Speckle Radiography employed to determine both the penetration depth and the sand bed displacement at a series of incremental time steps after impact. The results provide substantial insight into how slight adjustments to grain-grain contact points can affect the bulk dynamic response of brittle granular materials.</jats:p