Influence of Bending Operation on Mechanical Properties of Wrought-Wire Clasps

Objectives: The purpose of this study was to assess the influence of bending operation on mechanical properties of the wrought-wire clasps. Methods: Three-dimensional finite element model of a straight wire (1.0 mm in diameter) was created. The elastic and plastic properties of a commercial Co-Cr wire measured by means of a three point bending test were input into the non-linear plastic deformation program. Bending of 90° or 120° angles and subsequent relaxation processes with spring back phenomenon were simulated for the clasp shoulder and the arm. Thereafter, the clasp models were deflected 0.25 mm, 0.50 mm, and 0.75 mm to simulate the denture removal and insertion. Results: Under the bending force, the peak tensile stress over 2,400 MPa appeared at the outside surface of the corner. After relaxation, the residual tensile stress of approximately 50 % of the peak stress was shown at the inside corner. The stresses were insensitive to the bending angles. When under the clasp deflections, the peak stress at the inside corner increased by 17 % to 49 % depending on the deflection magnitude. The increment was larger at the shoulder than the arm. At the shoulder angle, the maximum tensile stress under deflection was 11% lower with the 120° angle than that with 90° angle. As the deflection increased from 0.25 mm to 0.75 mm, the permanent deformation at the clasp tip significantly increased, with the maximum of 19mm. The load required to deflection was 50 % larger with 120° angle at the clasp arm than that with 90° angle. Conclusion: The results suggest that the fracture and the permanent deformation of the wrought-wire clasps are initiated at the clasp shoulder, while the retentive force can be controlled by the bending angle at the clasp arm.