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Investigation of the SwRI Split Hopkinson Pressure Bar System for Testing Polymers and Other Novel Low Strength Materials, 18-9317

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Principal Investigator
Kathryn A. Dannemann

Inclusive Dates: 05/13/02 - 09/13/02

Background - Split Hopkinson pressure bar (SHPB) testing is commonly used to characterize the dynamic behavior of high-strength materials in the strain rate range from 100 s-1 to 5000 s-1. High strain rate characterization of polymers, rubber materials, foams, and other novel low-strength materials are of current interest owing to recent development of these materials for energy-absorption applications. There is limited information and experience in the high strain rate community for SHPB testing of these new materials using a low modulus, metallic bar material (e.g., Al or Mg).

When conventional SHPB test systems with metal bars are used to investigate the high strain rate properties of low-strength materials, low signal-to-noise ratios and short loading times result, leading to questions about the accuracy and interpretation of the inferred results. Stress state equilibrium, assumed in the test analysis, is a concern for these low sound speed dispersive materials owing to large differences in mechanical impedance and wave propagation velocity relative to the metallic bar materials. The major technical barrier for conducting SHPB tests on polymeric materials using the current SwRI system with metal bars is establishing the validity of the testing methodology. This project was undertaken to establish the validity of the testing methodology for low-strength materials using existing equipment, and the validity of the interpretation of the test data.

Approach - The objective of this project was to determine if reliable high strain rate compression data can be obtained for polymers and other low strength, dispersive materials using the SwRI traditional split Hopkinson pressure bar (SHPB) systems, with aluminum alloy bars. SHPB experiments were performed on a polycarbonate material (i.e., Lexan). The experimental findings were compared to numerical simulations. Agreement between numerical modeling and experiments imply the efficacy of the testing technique. Similarities in 1-wave (incident) and 2-wave (incident + reflected) stress outputs were also explored.

Accomplishments - Limited SHPB tests were performed on a polycarbonate (i.e. Lexan) material. The test results show good agreement with published test results for Lexan material using a polymer bar SHPB system. The experimental results were further compared with numerical simulations of the SHPB test. Successful numerical simulations of the SHPB system were conducted using LS-DYNA. The effects of varying the sample L/D ratio on stress equilibrium were also investigated for the low-strength materials. Generally, samples with smaller L/D ratios are preferred. From this comparison, it was determined that the SwRI conventional SHPB system (with aluminum alloy bars) can be used to obtain reliable high strain rate compression data for polymer materials. The numerical model derived for the SwRI SHPB test setup is applicable to all current and future SHPB testing on novel, as well as traditional materials. It provides a means of establishing test conditions without conducting numerous setup and calibration test runs.

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