Ballistic Properties of Projectile Material

Othman, Ahmad ; Barrett, Søren ; Christiansen, Rasmus Viking Lømand Ravgård

3. term

Design of Mechanical Systems, Master

2016

2016-12-20

99 pages

This project treats terminal ballistics and the determination of parameters and effects influencing projectile failure when impacting an armour plate. By increasing this knowledge, it is perhaps possible to increase the efficiency of the armour solutions available for clients of Composhield, the proposer of this project, in theatres of operation around the world. An analytical, a numerical and an experimental approach is taken in an attempt of determining the governing effects and three different materials are used, namely a steel, an aluminium and a brass. Furthermore, only a cylindrical projectile with a blunt face and a length of 15 mm and a diameter of Ø10 mm is used in the models and experiments. Such a projectile is known as a fragment simulating projectile (FSP) and represents projectiles or fragments often experienced in connection with improvised explosive devices (IED). In the analytical approach method of determining the retarding pressure of the armour plate on the projectile without use of empirical constants is derived along with an analytical approach of determining the dynamic yield strength of the projectile material as long as it is used for impacts below the plastic wave velocity in the material. Based on this work, a model capable of determining the amount of deformation, both in the longitudinal and radial direction including the stress-distribution and the penetration depth into the target, is set up. This model yields a very good correlation with experimental and numerical findings. Less successfully, a model for impact of projectiles on ceramics is adopted and modified for projectile to steel impact is adopted. This model is capable of treating impacts above the plastic wave velocity and thereby erosion, i.e. mass loss, of the projectile. The modifications made for this model is however not sufficient, and a rather poor correlation with experiments and simulation is found. In the numerical approach, use of hydrocode in ANSYS Autodyne and Explicit Dynamics is made. Different methods and material models are presented. Assumption and validity of axisymmetry is verified for the case of cylindrical projectiles. A convergence study is conducted to validate and determine optimal mesh size. Simulations mimicking the experimental set up is conducted and post impact length of projectiles are obtained for steel and aluminium. Numeric element erosion and failure are omitted from final simulations. The conducted simulations shows good correlation with experimental results. In the experimental work, as wide a velocity range as possible on the available test equipment has been tested in an attempt to verify the models above in as wide a range as possible. Furthermore, observations in the projectiles after impact yield some additional information models and simulations do not show. A test campaign of the three materials with two shots at six different velocities for a total of 28 impacts is conducted in a velocity range of {220 - 530} m/s. Both ductile fractures, i.e. plastic deformation in a shape known as mushrooming, and brittle fractures are observed. The models are not capable of modelling the brittle failures, and these are therefore omitted in the comparison of the results from the different approaches.

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