Comparison of bending and tension tests for orthodontic wires

Stainless steel, cobalt-chromium-nickel (Elgiloy), nitinol, and beta titanium wires with diameters from 0.010 to 0.040 inch and in rectangular sizes from 0.017 X 0.025 to 0.019 X 0.025 inch were evaluated with the American Dental Association specification no. 32 bending test and the conventional tension test. The modulus of elasticity (E) and yield strength (YS) obtained with the two mechanical testing procedures are reported for as-received wires and after heat treatment at 900 degrees F for the stainless steel and selected Elgiloy wires. Measured values of E and YS in bending were almost invariably higher than the corresponding values obtained in tension. The only general agreement between the modulus of elasticity in bending and tension was for the 0.040-inch diameter stainless steel wires. Anomalously high values of E in bending were also found for most of the heat-treated stainless steel and Elgiloy groups. Difficulties with the bending test are attributed to loading effects at both ends of the test span and deviation of the bent specimen from a circular arc. Improved procedures are described for performing the tension test on the relatively small cross-section orthodontic wires.