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The effect of fiber tow waves on the static strength and in-service fatigue life of the Hercules Graphite Torsion Spring used in the Boeing 767 airline is presented. A concise process overview is given. Process variables affecting tow wave formation are identified. The design of quality control specimens is presented. Pictures of tow waves preceding translaminar fractures are shown. The functional relationship of nested tow wave amplitude and length to the maximum strain is briefly reviewed. Tow wave strength and in-service fracture cycle data are fit to regression curves. The potential fatigue life of the springs is predicted. Finally, no effect of fatigue on spring rate and permanent set was found.

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