Korean J Orthod.  2009 Oct;39(5):310-319.

Load-deflection characteristics and plastic deformation of NiTi closed coil springs

• Son AY
• Lim SH

Abstract

OBJECTIVE
NiTi closed coil springs were reported to have relatively constant unloading forces. However, the characteristics of NiTi closed coil springs from various manufacturers have not been elucidated. The purpose of this study was to compare load-deflection characteristics of various NiTi closed coil springs and to find out the optimal range of extension.
METHODS
Seven kinds of NiTi closed coil springs from five manufacturers were tested. Load deflection curves were obtained at extension ranges from 2 mm to 30 mm. Also, springs were kept extended during a 4 week period, and then load deflection curves were obtained again.
RESULTS
Sentalloy (Tomy) and Jinsung blue (Jinsung) showed superelasticity in every extension ranges tested and showed plastic deformation of less than 1 mm. Ni-Ti (Ormco) showed superelasticity only after the springs were extended at or more than 10 mm, thereby meaning that clinicians should extend these springs at or more than 10 mm to utilize the superelasticity. Orthonol (RMO) and Nitanium (Ortho Organizers) did not show superelasticity. After 4 weeks of extension, all springs showed plastic deformation less than 1 mm when the extension was at or under 25 mm.
CONCLUSIONS
The superelastic behavior of NiTi closed springs were different among various NiTi spring products, and some NiTi closed springs failed to show superelasticity.

Keyword

NiTi closed coil spring; Load-deflection curve; Plastic deformation

MeSH Terms

Nickel
Plastics
Titanium
Nickel
Plastics
Titanium

Figure

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  Fig. 1 A, Schematic drawing of universal testing machine and water bath setup; B, Jigs used in this study.

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  Fig. 2 Load-deflection curve illustrating superelasticity of NiTi spring. σSIM, Transformation threshold point; σR, reverse transformation finish point; H, hysteresis (loading force - unloading force at midpoint of extension); PD, plastic deformation.

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  Fig. 3 Load-deflection curves of NiTi springs. X axis, Deflection (mm); Y axis, Load (N). 1st cycle, initial test; 2nd cycle, after 4 week extension.

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  Fig. 4 Change in transformation threshold point (σSIM) and reverse transformation finish point (σR) according to the amount of spring extension. X axis, Deflection (mm); Y axis, Load (N). σSIM L, transformation threshold point load; σSIM E, transformation threshold point extension; σR L, reverse transformation finish point load; σR E, reverse transformation finish point extension. *2 mm, 5 mm σSIM and 2 mm, 5 mm, 30 mm σR of Ni-Ti medium was not measured due to their ambiguity.

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