(Received 15 September 1995; accepted 24 November 1995)
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The physical laws governing the forces encountered when two materials in contact move, or tend to move, relative to another, have been developed over the last three hundred years. The assumptions used in these theories considered materials to be elastic in nature. A number of problems arise in applying these theories to footwear outsole materials. For example, the materials can be viscoelastic, which can alter their response to applied loads. Friction in elastomers has been shown to be made up of two different terms: adhesion and deformation. Waves of detachment have been observed in sliding elastomers that are the result of viscoelastic instability at the contact surface. Although this has been established in various laboratory tests, it has not been shown that typical conditions encountered during gait (forces, walking/sliding speed, temperatures, pressures) fit the adhesion/deformation model of elastomers. Available methods to measure/simulate friction during gait are controversial. Viscoelastic materials properties can vary with rate of loading, including friction and mechanical. Tensile tests at various strain rates were performed on typical commercially available shoe outsole materials-leather, and Neolite®. The results did not show any significant trend with respect to strain rate and measured mechanical properties.
Pennsylvania State University, Ogontz Campus, Abington, PA
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