Application of the Hopkinson Bar to Advanced Materials in Tension, 18-R9781

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Principal Investigators
Sidney Chocron
Arthur Nicholls
Kathryn A. Dannemann
Dan Nicolella
Charles E. Anderson

Inclusive Dates:  01/01/08 – 10/28/08

Background - SwRI has a long and successful history in measuring the response of materials at high strain rates. Recently our split-Hopkinson bar (SHPB) was named a historical engineering landmark by ASME.

New advanced materials like composites, foams, nanomaterials and biomaterials are of increasing importance in the marketplace. The new Boeing 787, for example, is fully made of carbon reinforced plastic. However, SwRI's facilities are not suitable for characterizing advanced composites and other non-metallic materials. There are currently a few tensile SHPB systems in the world; but these systems have rarely been used in armor composites or soft materials because of the inherent difficulty in the gripping system and the interpretation of the results.

Approach - A new SHPB system is being designed and fabricated. It will advance the state of the art in tensile SHPB testing by incorporating the following innovations: 1) pulse shaping to ensure equilibrium of the sample, 2) a new gripping system, and 3) an optical strain measurement system that will allow measuring large strains. Numerical simulation will assist in the design and the testing phase of the system. Additionally, the system is being designed to test strong structural composites, pliable but strong fabric-like composites, and weaker, pliable biological materials (e.g. ligaments, tendons, etc.). Simulations are important in the interpretation of the waves recorded in the bar when exotic materials are tested.

Accomplishments - Extensive 3-D numerical simulation of the full SHPB system has been accomplished to ensure that the samples reach equilibrium before failing. The figure shows a detail of specimen's section of the bar. Both grips and the specimen (a cow ligament) are shown. The colors represent strains in the specimen, maximum strain is around 50 percent.

All the parts needed for the Hopkinson bar have already been designed and machined and are waiting for assembly. Samples are being prepared to calibrate and test the new device.

Figure 1: A tensile wave is traversing the specimen (a cow ligament) and stretching it up to 50 percent. Simulations were performed using LS-DYNA.

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