J A Leighs

An overview on the effect of manufacturing on the shock response of polymers

Scatter and non-linearity of the Hugoniot in the Us-up plane has been seen in a number of polymers including poly(methyl methacrylate) (PMMA), the polymer considered here. In this study the plate impact technique has been used to investigate the shock response of PMMA between particle velocities of 0.13 and 0.77 mm μs−1. From this data no scatter was seen between our data and the experimental data of Barker and Hollenbach, and Carter and Marsh. Also a linear Hugoniot in the Us-up plane was found, with the equation Us = 2.99 + 0.92up. The non-linearity observed by Barker and Hollenbach was not present in this data, probably due to the non-linearity occur at particle velocities of below 0.13 mm μs-1, within their experimental data. Gruneisen gamma has also been briefly considered using a shock reverberation experiment but more work is needed before a value can be ascertained.

A sealed capsule system for biological and liquid shock-recovery experiments.

This paper presents an experimental method designed to one-dimensionally shock load and subsequently recover liquid samples. Resultant loading profiles have been interrogated via hydrocode simulation as the nature of the target did not allow for direct application of the diagnostics typically employed in shock physics (e.g., manganin stress gauges or Heterodyne velocimeter (Het-V)). The target setup has been experimentally tested using aluminium flyer plates accelerated by a 50-mm bore single-stage gas-gun reaching projectile impact velocities of up to ~500 ms(-1) (corresponding to peak pressures of up to ca. 4 GPa being experienced by fluid samples). Recovered capsules survived well showing only minor signs of damage. Modelled gauge traces have been validated through the use of a (slightly modified) experiment in which a Het-V facing the rear of the inner capsule was employed. In these tests, good correlation between simulated and experimental traces was observed.

On the effects of lateral gauge misalignment in shocked targets

Plate-impact experiments have been used to interrogate the influence of gauge alignment on the shock response of wire-element lateral manganin stress gauges in PMMA and aluminium targets. Embedded gauges were progressively rotated relative to the target impact face. Peak stress and lateral gauge rise-times were found be proportional (negatively and positively, respectively) to the resolved angle of the embedded gauge element. However, lateral stress gradients behind the shock were found to be relatively insensitive to gauge alignment. In addition, investigation of the effects of release arrival showed no connection to either peak stress or behaviour behind the shock.

The effect of hydrostatic vs. shock pressure treatment of plant seeds

The hydrostatic pressure and shock response of plant seeds has been investigated antecedently, primarily driven by interest in reducing bacterial contamination of crops and the theory of panspermia, respectively. However, comparisons have not previously been made between these two methods ofapplying pressure to plant seeds. Here such a comparison has been undertaken based on the premise that any correlations in collected data may provide a route to inform understanding of damage mechanisms in the seeds under test. In this work two varieties of plant seeds were subjected to hydrostatic pressure via a non-end-loaded piston cylinder setup and shock compression via employment of a 50 mm bore, single stage gas gun using the flyer plate technique. Results from germination tests of recovered seed samples have been compared and contrasted, and initial conclusions made regarding causes of trends in the resultant data-set. Data collected has shown that cress seeds are extremely resilient to static loading, whereas the difference in the two forms of loading is negligible for lettuce seeds. Germination time has been seen to extend dramatically following static loading of cress seeds to greater than 0.4 GPa. In addition, the cut-off pressure previously seen to cause 0% germination in dynamic experiments performed on cress seeds has now also been seen in lettuce seeds.

Ramp wave generation using graded areal density ceramic flyers and the plate impact technique

A requirement exists to generate realistic insults in energetic targets, for example ramp loadings leading to shock waves. This paper examines the development of a ceramic flyer ramp wave generation technique. Ceramic stereolithography was used to produce fully-dense, graded areal density alumina ceramic flyers. These flyers consisted of multiple square pyramids arranged on a solid base. The gas gun plate impact and electromagnetic particle velocity gauge techniques were used to observe the ramp waves generated when the flyers impacted a Kel-F 81 polymer target. Ramp waves of varying properties were successfully generated in the targets, and good agreement was obtained with 3D hydrocode modelling.

The bactericidal effect of shock waves

There are a variety of theories relating to the origins of life on our home planet, some of which discuss the possibility that life may have been spread via inter-planetary bodies. There have been a number of investigations into the ability of life to withstand the likely conditions generated by asteroid impact (both contained in the impactor and buried beneath the planet surface). Previously published data regarding the ability of bacteria to survive such applied shockwaves has produced conflicting conclusions. The work presented here used an established and published technique in combination with a single stage gas gun, to shock and subsequently recover Escherichia coli populations suspended in a phosphate buffered saline solution. Peak pressure across the sample region was calculated via numerical modelling. Survival data against peak sample pressure for recovered samples is presented alongside control tests. SEM micrographs of shocked samples are presented alongside control sets to highlight key differences between cells in each case.

The effect of fibre orientation on a TWCP composite

Multiple authors have shown that orientation can greatly affect the shock profiles seen in composites. Carbon fibre composites are employed in multiple sectors, with their use in the aerospace industry becoming more prevalent. An angle of 20° between the outer surface and the weave direction has been shown to provide a good compromise between strength and ablation, making orientation an important property. Using a single stage gas gun with manganin pressure gauges the shock response of both a 90° and 45° layup TWCP composite was investigated up to a particle velocity of approximately 1 mm μs−1, in both the Us-up and pressure-volume planes. Comparisons in terms of shock propagation were also made with a previously investigated TWCP orientation of 0° . This allowed a detailed interrogation of the effects of weave orientation in this important TWCP composite to be made. It was found that the shock response was not greatly altered by orientation of the carbon fibre weave above a certain particle velocity. This was due to the 90° behaving elastically until a particle velocity of 0.6 mm μs−1. Above this value the experimental data had very little deviation regardless of the angle.

The use of lateral gauges in the assessment of shear strength in a carbon fibre composite

Laterally orientated manganin stress gauges have been used to obtain strength measurements in multiple materials, most commonly polymers and metals. Composites such as carbon fibre provide an interesting challenge for lateral gauges, as any long range order within the composite will be broken up by the inclusion of the gauge. This study has investigated the shear strength of multiple orientations of a carbon fibre composite (TWCP) which has then been compared with the matrix material. The Hugoniot elastic limit of the 90° fibre weave TWCP composite was 2.27±0.25 GPa, compared to 1.53±0.20 GPa found for the fibre weave orientated at 0° with respect to the shock front. The lateral stress in both orientations however, was found to be the same, at a given particle velocity. This implies that either the matrix material dominates the lateral stress behaviour of this composite, or laterally orientated gauges are too intrusive and break up any long range order of the fibre weave. Further work utilising other strength assessment techniques will be employed to fully validate these experimental results.

On the shock response of pisum sativum and lepidium sativum

The high strain-rate response of biological and organic structures is of interest to numerous fields ranging from the food industry to astrobiology. Consequently, knowledge of the damage mechanisms within, and the viability of shocked organic material are of significant importance. In this study, a single-stage gasgun has been employed to subject samples of Pisum sativum (common pea) and Lepidium sativum (curled cress) to planar shock loading. Impact pressures of up to ~11.5 GPa and ~0.5 GPa for pea and cress seed samples respectively have been reached. The development of the experimental approach is discussed and presented alongside results from modelled gauge traces showing the sample loading history. Viability of the shock-loaded pea and cress seeds was investigated via attempts at germination, which were unsuccessful with pea seeds but successful in all tests performed on cress seeds. This work suggests that organic structures could survive shockwaves that may be encountered during asteroid collisions.