James Wilgeroth

On the dynamic behavior of three readily available soft tissue simulants

Plate-impact experiments have been employed to investigate the dynamic response of three readily available tissue simulants for ballistic purposes: gelatin, ballistic soap (both subdermal tissue simulants), and lard (adipose layers). All three materials exhibited linear Hugoniot equations-of-state in the US-uP plane. While gelatin behaved hydrodynamically under shock, soap and lard appeared to strengthen under increased loading. Interestingly, the simulants under test appeared to strengthen in a material-independent manner on shock arrival (tentatively attributed to a rearrangement of the amorphous molecular chains under loading). However, material-specific behavior was apparent behind the shock. This behavior appeared to correlate with microstructural complexity, suggesting a steric hindrance effect.

On the interpretation of lateral manganin gauge stress measurements in polymers

Encapsulated wire-element stress gauges enable changes in lateral stress during shock loading to be directly monitored. However, there is substantial debate with regards to interpretation of observed changes in stress behind the shock front; a phenomenon attributed both to changes in material strength and shock-dispersion within the gauge-encapsulation. Here, a pair of novel techniques which both modify or remove the embedding medium where such stress gauges are placed within target materials have been used to try and inform this debate. The behavior of three polymeric materials of differing complexity was considered, namely polystyrene, the commercially important resin transfer moulding (RTM) 6 resin and a commercially available fat (lard). Comparison to the response of embedded gauges has suggested a possible slight decrease in the absolute magnitude of stress. However, changing the encapsulation has no detectable effect on the gradient behind the shock in such polymeric systems.

The shock response of a rendered porcine fat

Characterization of the shock response of biological materials is required in order to develop an understanding of how such materials behave under high strain-rate loading. In this work, a predominately linear Us-up Hugoniot relationship for a rendered porcine fat has been established using the plate-impact technique. This has been shown to take the form Us=1.58+2.47up (ρ0=0.945 g/cc) and comparison has been made between the dynamic behavior of the adipose material and both 20 wt % ballistic gelatin and water. The adipose material has been shown to behave in likeness with simple polymers such as polyethylene and to strengthen under shock loading, unlike ballistic gelatin, which has been shown to behave hydrodynamically. An experimental design incorporating direct insertion of lateral stress gauges within the rendered fat has given insight into both the behavior of lateral gauges and the lateral stress response of the material under dynamic loading.

Controlling blast wave generation in a shock tube for biological applications

The shock tube is a versatile apparatus used in a wide range of scientific research fields. In this case, we are developing a system to use with biological specimens. The process of diaphragm rupture is closely linked to the shock wave generated. Experiments were performed on an air-driven shock tube with Mylar® and aluminium diaphragms of various thicknesses, to control the output. The evolution of shock pressure was measured and the diaphragm rupture process investigated. Single-diaphragm and double-diaphragm configurations were employed, as were open or closed tube configurations. The arrangement was designed to enable high-speed photography and pressure measurements. Overall, results are highly reproducible, and show that the double-diaphragm system enables a more controllable diaphragm burst pressure. The diaphragm burst pressure was linearly related to its thickness within the range studied. The observed relationship between the diaphragm burst pressure and the generated shock pressure presents a noticeable difference compared to the theoretical ideal gas description. Furthermore, the duration of the primary shock decreased proportionally with the length of the high-pressure charging volume. Computational modelling of the diaphragm breakage process was carried out using the ANSYS software package.

Aspect ratio compression effects on metals and polymers

Previous research has studied the effects of altering the shape and size of samples over a range of compression rates. Several discrepancies exist in the published data, probably due to the wide range of techniques used. The research here was carried out to bridge this gap. Cylindrical polycarbonate and aluminium (6082) samples, with varying aspect ratios, were subject to a range of strain rates between 10-4 − 102 s−1, with further data gained for polycarbonate at higher strain rates between 102 − 103 s−1, and the stress-strain relationships obtained, compared and discussed.

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.

Temperature effects on the mechanical behaviour of PZT 95/5

This research is to develop a better understanding of the piezoelectric ceramic lead zirconate titanate (PZT) 95/5 with varying temperatures, porosities and strain rates. Here, unpoled PZT samples of two different porosities were subjected to a range of compression rates, using quasi-static loading equipment, drop-weight towers and Split Hopkinson Pressure Bars (SHPBs). Varying temperatures were achieved using purpose-made environmental chambers. The resulting stress-strain relationships are compared. The samples were square tiles, 7.5 × 7.5 mm and 3 mm thickness. The density of the standard PZT used here was 7.75 g cm−3 (henceforth described as PZT), whilst the density of the higher porosity PZT was 7.38 g cm−3 (henceforth described as PPZT). This research is part of a wider study.

The dynamic response and shock-recovery of porcine skeletal muscle tissue

A soft-capture system allowing for one-dimensional shock loading and release of soft tissues via the plate-impact technique has been developed. In addition, we present the numerical simulation of a shock-recovery experiment involving porcine skeletal muscle and further investigate the effects of the transient wave on the structure of the tissue via transmission electron microscope (TEM). This paper forms part of an ongoing research programme on the dynamic behaviour of skeletal muscle tissue.

Comparison of epoxy-based encapsulating materials over temperature and strain rate

The highly insulating, adhesive and bonding properties of thermosetting epoxies, their ability to be injection moulded in an uncured state, as well as their presence in a wide number of composites, has resulted in their widespread use in both electrical and aerospace applications. There is thus a need to understand the compressive response of epoxies over the range of temperatures likely to be experienced within their working environment.The effects of varying strain rates and temperatures on an epoxy resin (Scotchcast 8) and an epoxy-based syntactic foam (Stycast 1090) were investigated. The samples were studied from −20 °C to +80 °C over a range of strain rates (10−4 - 10+3 s−1). Stress-strain data was obtained, with further analysis from high-speed images. Dynamic Mechanical Analysis (DMA) was also performed on the two materials.Data obtained from these experiments demonstrated key differences in the behaviour of the two materials, forming a basis for comparison with numerical simulations.