The Influence of Residual Stress in Wrought Wire Clasps on Bending Properties

概要

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The purpose of this study was to assess the influence of residual stress produced after the bending manipulation of a straight wire on flexibility and permanent deformation of wrought-wire clasps. A three-dimensional finite element model of a straight wire (12 mm in length and 1.0 mm in diameter) was created. Elastic and plastic properties of a commercial Co-Cr straight wire measured by means of tensile test were input into the program. We simulated the bending of the clasp shoulder and clasp arm to an angle of 90° or 120° and subsequent unloading processes with spring-back phenomenon. The maximum residual tensile stress appeared at the inside of the bending angle after unloading of the bending force. Thereafter, the clasp models were deflected 0.25, 0.50 mm, and 0.75 mm at the tip to simulate the removal and insertion of a denture. The maximum tensile stress at the inside corner of the bending angle under the deflection was considerably higher than the residual stress before deflection. The stress increase by the deflection was larger at the shoulder than the arm. The load required for deflection was larger in the models with the arm of 120° angle than models with the arm of 90° angle. The results suggest that the permanent deformation of wrought-wire clasps is likely to initiate at the clasp shoulder, while the clasp flexibility is dependent on the bending angle of the clasp arm.
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