Fabrication of a new composite orthodontic archwire and validation by a bridging micromechanics model.

A new technique based on tube shrinkage is proposed for the fabrication of composite archwires. Compared with a traditional pultrusion method, this new technique can avoid any fiber damage during the fabrication and can provide the archwire with a required curvature in its final clinical usage. The present paper focuses on the technique development and mechanical design and validation in terms of constituent materials by using a micromechanics bridging model. Prototype archwire has been fabricated using fiberglass and an epoxy matrix, with a wire diameter of 0.5mm and a 45% fiber volume fraction. Tensile and three-point bending tests have shown that the mechanical performance of the prototype composite archwire is comparable to that of a clinical Ni-Ti archwire. Another purpose of the present paper is to provide an efficient procedure for a critical design of composite archwires. For this to be possible, the ultimate load especially flexural load carrying ability of the composite archwire must be assessed from the knowledge of its constituent properties. However, difficulty exists in doing this, which comes from the fact that the failure of the utmost filament of the composite archwire subjected to initially the maximum bending stress does not imply its ultimate failure. Additional higher loads can still be applied and a progressive failure process is generated. In this paper, the circular archwire was discretized into a number of parallel laminae along its axis direction, and the bridging micromechanics model combined with the classical lamination theory has been applied to understand the progressive failure process with reasonable accuracy. Only the constituent fiber and matrix properties are required for this understanding. Nevertheless, the ultimate bending strength cannot be obtained only based on a stress failure criterion. This is because neither the first-ply nor the last-ply failure corresponds to the ultimate failure. An additional critical deflection (curvature) condition must be employed also. By using both the stress failure and the critical deflection conditions, the predicted load-deflection up to the ultimate failure agrees well with the measured data. Thereafter, different mechanical performances of composite archwires can be tailored before fabrication by choosing suitable constituent materials, their contents, and the archwire diameters. Several design examples have been shown in the paper.