Shape optimal design of an engine connecting rod for minimum weight is presented, subject to upper and lower bound constraints on principal stresses. A two-dimensional plane stress model with multiple external loads is used to determine optimum boundary shape and thickness distribution. The variational equations of elasticity, the material derivative idea of continuum mechanics, and an adjoint variable technique are employed to calculate shape design sensitivities of stress. A 20-percent weight reduction is achieved in the neck region of the connecting rod, with reasonable computation cost. The method, combined with a steepest descent optimization method, is shown to be stable and applicable to broad classes of elastic structural shape optimization problems.

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