Perforation resistance of lightweight protective structures: An experimental and analytical study
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In international peacekeeping missions or other situations where the threat level is high, both military stabilization forces and civilian organizations are in the need of a sufficient protection system and shelter, to protect both personnel and sensitive equipment against ballistic impacts. It is during the deployment and establishment of new camps, the threat levels from insurgents are at the highest, and it is therefore important that the system is light-weighted and easy to mount. Aluminium alloys are of particular interest in the design of these types of protection systems, and there have earlier been some studies on this where different alloys have been considered. A further study on this has been carried out in the present thesis where the emphasis has been the experimental and some analytical work. In the present thesis, both normal and oblique impacts on two different aluminium targets have been studied experimentally and analytically. A set of 20 mm thick monolithic AA6082-T651 aluminium target plates have been struck at 0°, 15° and 30°obliquity, and 65 mm thick extruded aluminium panels filled with two different granular materials (0-2 mm and 2-8 mm) have been struck at normal 0° obliquity. Three different projectiles with the same calibre were tested: 7.62 x 63 mm -APM2 (with a hard steelcore), -NM60 NATO Ball (with a soft lead core) and -NM231 NATO (with a soft steel core).During testing initial and residual bullet velocity was measured by the use of both a laserbase doptical device and high-speed cameras. Pictures were also taken of the penetration process. Ballistic limit velocity was found for all the different target situations, using the Lambert and Jonas model. By the use of cylindrical cavity-expansion theory as ananalytical prediction, results from both normal and oblique impacts by the APM2 bulletcore was found and validated against the experimental data for the AA6082-T651 alloy. Amaterial test program with quasi-static compression and tension tests of the target materials, were also conducted to provide data for the analytical predictions and for further use in numerical simulations. Constitutive relations like Power Law, Voce and modified Johnson-Cook were used to find the different parameters. Results from the ballistic perforation tests were validated against each other and earlier conducted experiments on different target materials, showing validations like the effect of yield strength and areal density compared to ballistic limit velocity. This showed aclear trend in the importance of high yield strength for aluminium as a ballistic protection system. Further investigations showed also that aluminium plates with high yieldstrength gave very good results in ballistic limit velocity versus areal density for APM2 bullets, compared to some high-strength structural steel plates. It was found that the extruded aluminium panels filled with granular material gave a poor ballistic protection against small-arms bullets, and a too low ballistic limit velocity with regard to areal density compared to other protection materials as mentioned above. The analytical predictions from the cylindrical cavity-expansion model for the hard steel core of the APM2 bullet were in good agreement with the experimental data of normal impacts by APM2 bullets on 20 mm thick AA6082-T651 target plates. It also showed a proportional similarity to the oblique impacts obtained experimentally.