Stress relaxation and recovery behaviour of composite orthodontic archwires in bending

The viscoelastic behaviour of prototype composite orthodontic archwires was evaluated using a bend stress relaxation test. Archwires having 10 differe nt volume fractions of reinforcement were subjected to constant bending rad ii in a water bath at 37 degrees C for time periods of up to 90 days. The w ires were subsequently released and left unconstrained for the same testing conditions. Creep-induced changes in the unconstrained bending radii of th e wires were measured at specific times during both phases (stress relaxati on and recovery) of the test. The statistical analysis showed that stress r elaxation behaviour was strongly correlated with the archwire reinforcement level. The final relaxation varied, with decreasing reinforcement, from 2 to 8 per cent. Archwire recovery was not correlated with reinforcement leve l, and revealed a final viscous loss of only 1 per cent. The relaxed elasti c moduli in bending of the composite wires were similar to the elastic modu li in bending of several conventional orthodontic archwire materials. Losse s that were associated with the viscoelastic behaviour varied with decreasi ng reinforcement level from 1.2 to 1.7 GPa. Because these modulus losses we re minimal, each archwire retained sufficient resilience to be applicable t o the early and intermediate-stages of orthodontic treatment.