Non-linear finite element stress analysis of plastic deformation in Co-Cr wrought-wire clasps

The purpose was to assess the influence of plastic deformation by bending on stress, flexibility and permanent deformation in wrought-wire clasps.

A three-dimensional finite element model of a straight wire (120 mm in length and 1.0mm in diameter) was created. The non-linear stress-strain relationship of a commercial Co-Cr alloy straight wrought-wire, measured by means of tensile test (n = 5) was put into the program. Bending to an angle of 90 degrees or 120 degrees and subsequent unloading processes with spring back phenomenon, were simulated in the clasp shoulder and arm. The stress distributions were analyzed at loading and unloading. Thereafter, the clasp models were deflected outwardly 0.25, 0.50, and 0.75 mm at the clasp tip, to simulate the removal and insertion of a denture.

Under the bending force, the maximum tensile stress was recorded at the outside surface of the bending corner; while after unloading, the maximum tensile stress appeared at the inside of the bending angle. By deflection of the clasp tip, this stress increased up to 203% of that before deflection. The change of stress by deflection was larger at the shoulder than at the arm. The load required for deflection was approximately 43% larger in the models with the arm angle of 120 degrees than those with an angle of 90 degrees .

The results suggest that the permanent deformation of wrought-wire clasps is likely to initiate at the clasp shoulder, while clasp flexibility is dependent on the bending angle of the clasp arm.