(a) Phase One (Cylindrical projectiles fired onto a rigid anvil)
In the early stages of the research, ballistic test specimens were made of HE15, HE30TF and DTD5044 aluminium alloys. The specimen size was chosen to be 20mm in length and 9.52mm in diameter, in order to suit the firing device. Accuracy in length of the test specimen was important for correct impact velocity readings. Diametral accuracy was essential for close fit of the projectiles in the barrel; this prevented air from passing through the gap between the projectile and the barrel inside surface.
HE30TF and HE15 aluminium alloy specimens were machined to size from as received round bars, size 12.7mm and 15.87mm in diameter respectively. The DTD5044 specimens were machined from a rectangular cross-sectional bar 76mm x 50mm obtained from British Aerospace PLC, Aircraft Group, Weybridge Division.
Using such projectile dimensions it was possible to
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obtain strain rates ranging from 2.5 x 10 to 1.25 x 10 per second. The cylindrical ballistic test specimens were finely ground to obtain a smooth flight through the barrel, and the flat ends surfaces were polished result ing in a mirror finish to achieve uniform deformation across the diameter and to minimise friction.
(b) Phase Two (Tool steel projectiles fired onto small billets)
In this phase of the investigation the specimen size was kept to 5mm in length and 5.2mm in diameter in order to
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obtain strain rates of the order of 10 per second. Other dimensions were introduced later on for the three aluminium alloys in order to investigate radial inertia and size effects during deformation. The aspect ratio (length/diameter) was kept less than unity to avoid buckling and undesirable barrelling during deformation. En-8 test specimens, 5mm in length and 5.2mm in diameter were machined to size from as received 25.4mm diameter round bars. All test specimens in this phase, including the FMP-338 tool steel projectiles, were finely ground and the ends polished resulting in surfaces as flat as possible in order to achieve homogenous deformation. Using such test specimens it was possible to achieve
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strain rates ranging from 4 x 10 to 2.2 x 10 per second.
2.3.5 Preparation of Tool Steel Projectiles for Phase Two Tests The FMP-338 tool steel projectile dimensions were 19mm in length and 9.52mm in diameter and were initially machined to size from as received 76.2mm diameter round bars. The projectiles were then heat treated at a temperature of 970°C, oil quenched and tempered at a temperature of 150°C to increase the strength. The chemical composition and hardness of the projectile material are shown in tables 2.1 and 2.2 respectively. Mass of FMP-338 tool
The FMP-338 projectiles were machined to size to fit accurately in the loading throat of the test rig and all finely ground. Their flat surface were polished to a mirror finish. Other dimensions for the tool steel projectiles were considered to achieve higher impact velocities and consequently higher strain rates. However, it was not possible to use impact velocities higher than about llOm/s for the aluminium alloys and En-8 steel respectively, due to severe compression, fracture and noticeable barrelling. Thus all these tests were conducted with only one sized projectile.
2.4 Calibration of Test Equipment
2.4.1 Calibration of Pressure Transducer
The pressure transducer was calibrated using a dual range dead weight pressure gauge tester (Budenberg Gauge Tester). This tester uses hydraulic oil and ratio
weights. Equal amounts of oil are pumped to the output end of the equipment where the pressure transducer is
connected and to the other end where a piston carries the weight. Pumping was continued until the weights were rotating freely on the piston. The voltage reading corresponding to the pressure applied was noted.
Table 2.3 shows the pressures with their corresponding voltages for the pressure transducer.
2.4.2 Air Pressure - Impact Velocity Calibration
The ballistic rig was used to obtain air pres sure-impact velocity calibration charts for FMP-338, HE15, HE30TF
and DTD5044 projectiles as shown in figure 2.4. This chart facilitates easy readings of the air pressure corresponding to a required impact velocity for initial guide only. The calibration chart was constructed from experiments carried out using the standard test procedure detailed in section 2.5. After selecting a certain air pressure (using the Roband Voltmeter) the projectile was fired and the velocity noted.
2.5 Experimental Procedure 2.5.1 Introduction
A number of preliminary tests were carried out involving full use of the experimental rig in order to obtain
(i) Calibration of test equipment
(ii) Impact velocity and air pressure relationship (iii) Laser tests involving velocity readings correspon
ding to each aluminium alloy and tool steel projectile.
Several quasi-static compression tests were carried out using different lubricants such as oil, petroleum jelly and tallow-graphite (8/1 weight) to decide which of these lubricants was most suitable. These quasi-static com pression tests were carried out at different compression
speeds in the quasi-static range in order to gather information regarding stress-strain and strain rate characteristics.
These experiments have been conducted to establish and maintain an appropriate test procedure in order to obtain