Elastic flexural properties of multistranded stainless steel versus conventional nickel titanium archwires

Based on a recent investigation that modeled the elastic properties (ie, strength, stiffness and range) of multistranded wires made from linearly elastic materials, three-stranded (triple) and six-stranded coaxial (coax) stainless steel (SS) wires were compared to single-stranded (single) SS and conventional nickel titanium (NiTi) leveling wires. To measure Young's modulus of elasticity (E), flexural tests were performed with an Instron mechanical testing machine in a three-point bending arrangement having a span length of 8.9 mm or 12.5 mm. A strong correlation between wire stiffness and the area moment of inertia demonstrated that strand interaction was negligible at low activations and that E = 199 GPa was constant even for the heavily drawn coax strands. Using the Instron with an extensometer, the 0.1% yield strengths (sigma(YS)) of the single SS wires and the straight inner strands within the coax wires were tested. The ratio of the sigma(YS) to the ultimate tensile strength averaged 0.81 for the single wires, 0.88 for the coax wires, and was subsequently assigned 0.85 for the triple wires. The average sigma(YS) values were 1.88, 1.83, and 1.78 GPa for the single, triple, and coax SS wires, respectively. For each NiTi wire, both the sigma(YS) and the elastic limit (sigma(EL)) via cyclic loading were measured. The conventional NiTi wires displayed nonlinear elasticity, as the average sigma(EL) values (1.10 GPa) were 50% higher than the average sigma(YS) values (0.73 GPa). Compared to the elastic properties of the conventional NiTi wires, the triple and coax SS wires generally matched the stiffness, but had only one-third to one-half of the strength and range. Since the properties of strength and range are both proportional to sigma(YS), fabrication using alloys with enhanced sigma(YS) values would allow multistranded SS archwires to compete better against conventional NiTi products.