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

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

Inclusive Dates:  01/01/09 – 12/31/09

Background - SwRI has a long and successful history in measuring the response of materials at high strain rates. In 2006 its split-Hopkinson bar (SHPB) laboratory was named a historical engineering landmark by the American Society of Mechanical Engineers (ASME). New advanced materials such as 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 non-metallic materials, for example biological materials such as 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 in interpreting results.

Approach - The objective of the project is to design and fabricate a direct tension Hopkinson bar system.

Accomplishments - A new SHPB system was designed and fabricated, see Figure 1. 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 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 Dyneema, a material used in body armors, 4) baboon bone, see Figure 2a, and 5) baboon ligaments, see Figure 2b.

An interesting result 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 than the bone from young animals. This result was presented at the October 2010 American Society of Bone and Mineral Research Conference. The observation that the strength of bones in old animals might depend on the rate of load is potentially very important and could help elucidate the underlying mechanisms of osteoporosis. Consequently it will be the subject of future research projects.

The new system has already been used for multiple commercial and government projects to test aluminum, steel, 3-D glass fiber specimens and other exotic materials and geometries.

New tensile split Hopkinson bar. The bar was completely designed and fabricated at SwRI.

Figure 1. New tensile split Hopkinson bar. The bar was completely designed and fabricated at SwRI.


 

Figure 2.Dynamic Leg Extension. Left: initial position.

Figure 2.Dynamic Leg Extension. Right Final Extended Position. Note: only one half of the model was used in this simulation.

Figure 2. Example of testing on biological materials: a) baboon bone; b) baboon ligament.


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