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

Ah-young Son; Sung-hoon Lim

doi:10.4041/kjod.2009.39.5.310

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

Original Article

Korean Journal of Orthodontics

Korean Journal of Orthodontics 2009; 39(5): 310-319.

Published online: 31 October 2009

DOI: https://doi.org/10.4041/kjod.2009.39.5.310

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

Ah-young Son, DDS, MSD^a, Sung-hoon Lim, DDS, MSD, PhD^b

^aPostgraduate Student, Department of Orthodontics, School of Dentistry, Chosun University, Korea.

^bAssociate Professor, Department of Orthodontics, School of Dentistry, Chosun University, Korea.

Corresponding author: Sung-hoon Lim. Department of Orthodontics, School of Dentistry, Chosun University, 421, Sosuk-dong, Dong-gu, Gwangju 501-825, Korea. +82 62 220 3874; shlim@chosun.ac.kr

Received 6 March 2009 Revised 21 August 2009 Accepted 23 August 2009

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.

Keywords: NiTi closed coil spring, Load-deflection curve, Plastic deformation

Figures and Tables

Fig. 1

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

Fig. 2

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

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.

Fig. 4

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

Table 1

NiTi closed coil springs used in this study

^*Inner diameter of the spring; ^†Force level described by the manufacturer.

Table 2

Characteristics of load/deflection curves of NiTi springs at 15 mm extension

^*Loading and unloading forces were measured at the 7.5 mm extension point of the load/deflection test of the 15 mm extension.

Table 3

Plastic deformation of NiTi springs (unit: mm)

Table 4

Plastic deformation after 4 weeks of extension (unit: mm)

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