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Fracture of bone is investigated by measuring the energy required to propagate a crack and by observing, microscopically, the mechanism of fracture. At low strain rates bone displays very high resistance to crack propagation comparable even to some metallic materials. The type of fracture is similar to fiber-reinforced composite materials.

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SummaryProperties of injection-molded high-density polyethylene reinforced with carbon fibers were investgiated. With 20% by weight carbon fibers tensile strength of polyethylene was almost doubled. Further increase of volume fraction of fibers did not produce as dramatic results. With 40% by weight carbon fibers, elastic modulus increased one order of magnitude: from 0.225 X 106 psi to 2.48 x 106 psi. This value is close to the value of the elastic modulus of bone, hence the material may substitute for bone where the same stiffness is required. The total strain a t fracture was reduced two orders of magnitude, but still stayed comparable to total strain in bone failures (1-2'%).No appreciable changes were noted in shear strength, impact strength and wear resistance. The work to fracture increased from 0.9 to 2.9 kg cm/cmz with 40% of carbon fibers.The most significant improvement of polyethylene with addition of carbon fibers was demonstrated in measurements of viscoelastic properties of the material.In summary, it can be stated that injection-molded reinforced polyethylene has much superior properties to the nonreinforced P.E. which can be further improved if better adhesion between P.E. matrix and reinforcing fibers can be secured.Creep characteristics were dramatically improved.

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