Materials and methods

Materials

Moulds of dimension 9” x 4” x 4” (228.6 mm x 101.6 mm x 101.6 mm) (+/- 0.25 mm) were manufactured by the Cranfield University workshops using Aluminium. Aluminium was selected as the material of choice primarily due to is proven ability to act successfully as a mould for this type of application [56, 57], its workability, and ability to provide an economical solution to this studies requirement. To ensure transparency of the material was maximise, consideration of surface sidewall damage during decanting was given, and minimised by ensuing as much of the mould was manufactured from a single sheet of Aluminium before being welded to create a watertight seal. Additionally, the application of Liquid Wax release agent was utilised to reduce the potential of adhesion and therefore damage during decanting.

The Perma-Gel material was supplied whole and stored before use, before being shot once per block, before being exposed to its melting/re-casting cycle. Post melt, all samples within this study were individually poured into the moulds before being left to solidify at room temperature (18 °C) to set. Once each mould contained a solidified material sample that had undergone its designated melt and re-cast cycle, a surgical scalpel was used to ensure any cohesion between the material and mould walls were reduced before being carefully extracted (Fig. 1).

Fig. 1: Perma-Gel material extraction post cure

Perma-Gel

10 Blocks of Perma-Gel were purchased from Ballistic Dummy Lab [44] at a cost of $69.99 per block¹.

As this study is primarily focussed on the mechanical performance of Perma-Gel when subjected to thermal cycling, a Heraeus D-6450 Hanau Oven (Fig. 2) was pre-heated to 117 °C which was ascertained via a calibration test prior to this study and consistent with other studies and technical notes associated with the same material [24, 37]. A series of procedures were followed to ensure the material was melted under controlled conditions, before being allowed to cure at a consistent temperature. Table 2 articulates the conditions and observations.

Fig. 2: Left: Heraeus oven. Right: Material condition post artificial ageing arranged by number of melt cycles (Yellowing shown bottom right sample)

Number of Samples | Oven Temperature (°C) | Cure Temperature (°C) | Melt/Recast Cycles | Observations
3 | 117 for 3 h | 18 for 5 h | 5 | N/A
3 | 117 for 3 h | 18 for 5 h | 10 | N/A
3 | 117 for 3 h | 18 for 5 h | 15 | Yellowing present on 1 x material sample after thermal cycling.

Dynamic testing projectile

AISI 420 grade stainless steel ball bearings of 4.5 mm diameter were integrated to a pre-stressed plastic sabots to ensure no energy would be lost during firing from air passing around the projectile in the gun barrel [14, 15]. The sabots had been pre-stressed to ensure projectile separation prior to material impact and minimise any risk of impeding the visual inspection of the projectile entry site. Each ball bearing, with a mass of 0.4 g, underwent a visual inspection before being inserted into the gas gun breech.

Footnotes

1. Cost correct as of March 2024. ↩