Testing of Composite Materials Used in Orthotics and Prosthetics
DOUGLAS A. TAYLOR, C.P., JEREMY C. GILBERT, Ph.D., AND EUGENE P. LAUTENSCHLAGER, Ph.D.
A composite material is a combination of two or more materials which retain their identities as they act in concert. These materials are usually composed of a reinforcement such as a fiber and a matrix such as a resin. Nylon, fiberglass and carbon fibers and polyester, acrylic and epoxy resins are the commonly used composite materials in prosthetics and orthotics. Several factors can greatly effect the strength and performance of composite materials. The adhesion at the interface between the resin and fiber, and the mechanical properties of the resin and fiber, greatly effect composite performance. The fiber length, orientation and ratio of fiber to resin, and processing techniques used to fabricate the composite also effect composite properties.
The first phase of my testing involved the tensile testing of composites (ASTM D 3039). Commonly used fibers with varying orientations (fiberglass, nyglass, and carbon fiber) were tested in combination with epoxy and acrylic resins. Standard resin transfer molding techniques used in orthotics and prosthetics were used to fabricate test samples. The Table-1 below displays the results of tensile testing.
The results show that the plain weave (bidirectional) carbon fiber with a 0 degree/90 degree orientation to the axis of pull performed the best. The type of resin effected tensile strength but not as much as the type and orientation of fiber. The samples with large fiber bundles (6K) and the samples with 3M Super 77 adhesives an application binder reduced tensile strength. The samples that were resin dependent such as the braided fiberglass and the off axis carbon fibers (45/45) performed very poorly. The Nyglass samples which were almost all resin performed the worst. The carbon fiber should be used as the primary structural material. The Nyglass would be undesirable as a structural material. Tension testing is not the only measure of a composite materials properties. A compressive test and a interlaminar toughness test are the next steps to define the role of the resin in adhesion between fiber and resin.
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