Analytical Simulation and Verification of Air Gun Impact Testing

Report No. ARL-TR-3559
Authors: Adam Bouland and Mostafiz R. Chowdhury (ARL)
Date/Pages: August 2005; 55 pages
Abstract: This report presents an analytical method to simulate the acceleration pulses and forces experienced by test objects during air gun impact testing. The purpose of an air gun impact test is to determine the survivability of sensitive artillery components during launch. It does so by simulating the acceleration pulse and shock waves experienced by such objects during firing. In an air gun impact test, these forces are generated by the impact of a "bird"/test object with an energy-absorbing mitigator complex, which consists of an aluminum honeycomb mitigator and a large momentum exchange mass (MEM). The front of this mitigator is shaped as a wedge or cone, which plastically deforms to provide for a gradual deceleration. Customers often request the peak acceleration they wish to achieve in a given test; therefore, a simulation is necessary to design a test around a client?s needs. This report develops a discrete element simulation to predict the acceleration pulse experienced by an object during an air gun test. The model accounts for the frequency contents of different regions of the test object, which had been simplified to a single frequency or omitted altogether from previous models (1). The model also accounts for the elastic and plastic deformation of the aluminum honeycomb mitigator. The strain rate dependency of the plastic crush force is derived with experimental data, confirmed with reference materials, and integrated into the model. A plastic wave front is tracked through the geometry of the mitigator, and the inertial mass transfer of the crushed portion of the mitigator from the MEM complex to the "bird" is accounted for as well. After the crush phase is complete, the mitigator is allowed to elastically unload. This model is governed by a damped spring mass system in which the mitigator is represented as a dynamic force. The stiffness of the components is derived geometrically. The simulation is programmed into Visual Basic1 via a finite difference timestepping scheme. Results of the model are verified by comparison with actual test results as well as previous simulations. The new model accurately predicts the peak acceleration pulse, the duration of the acceleration pulse, and the frequency content of the test and shows significant improvement over previous models. 1Visual Basic is a registered trademark of Microsoft.
Distribution: Approved for public release
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Last Update / Reviewed: August 1, 2005