Application of the Hopkinson Bar to Advanced Materials in Tension, 18-R9781Printer Friendly Version
Inclusive Dates: 01/01/09 10/28/09
Background - SwRI has a long and successful history in measuring the response of materials at high strain rates. In 2006, SwRI's 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. The new Boeing 787, for example, is fully made of carbon-reinforced plastic. However, SwRI's facilities were not suitable for characterizing advanced composites and other nonmetallic materials, for example, biological materials like baboon ligaments or bones. 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.
Accomplishments - A new SHPB system was designed and fabricated as shown in the illustration. The system design is sufficiently flexible to allow testing materials with a wide range of compliance and strength. For example, strong structural composites, pliable but strong fabric-like composites, and weaker, pliable biological materials (e.g., ligaments, tendons) can be tested. Special grips were also fabricated for both weak and strong specimens. Numerical simulations assisted in the design and the testing phase of the system. Five different materials were tested during this project: 1) glass-fiber reinforced plastic used in vehicle armor, 2) a rubber also used in vehicle armors, 3) unidirectional samples of Dyneema, used in body armors, and as shown in the second illustration, 4) baboon bone and 5) baboon ligaments. A very interesting result (still under analysis) is that the baboon bones taken from young and old animals have a similar strength under quasistatic loads, while at high strain rates (~100 s-1), the bone from old animals is weaker that the one from young animals. The new system has already been used for commercial projects to test aluminum and steel samples in tension. This allowed researchers to validate the system because the same material was tested with the old (indirect tension) Hopkinson bar and the new direct tension system, resulting in the same constitutive properties.