OBJECTIVE: The aim of the present study was to evaluate color changes and force degradation of orthodontic elastomeric ligatures in different stretching patterns during a 8-weeks period. METHODS: Two elastomers with the minimum and two with the maximum color changing, and gray elastomers of two brands (American Orthodontics and Ortho Technology) were selected according to an opinion poll with clinicians and color changes after 4 weeks of intraoral use were evaluated. These elastomers were mounted on special jigs fabricated using a CAD-CAM technique, underwent different stretching patterns and the force was measured in 0, 24 hours, 2, 4 and 8 weeks. During in vivo part of the study, force levels of elastomers were measured after 4 weeks on a material testing machine. Data were analyzed with four-way ANOVA and Tukey post hoc tests.RESULTS: All the elastomers showed color changing but the degree of color stability was significantly different. The mean force degradation was higher in 1-mm stretch groups. After 8 weeks, the average residual force of elastomers was 1.45 ± 0.18 N and the maximum force decay was seen in the elastomers that exhibited the maximum initial force.CONCLUSION: There is significant relationship between the stretching pattern and the amount of residual force of elastomers. Elastomers with higher initial forces exhibited higher percentages of force loss after 8 weeks. It seems that there is a relationship between initial color and color changing of elastomers.

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The length-scale free vibration analysis of a rectangular microplate coupled with piezoelectric layers is presented. The modified couple stress theory is used to describe the size effect of the system. The governing equations of motion are obtained using Hamilton’s principle based on the classical plate theory. The transverse part of the electric potential for the piezoelectric layers is considered to satisfy the Maxwell’s equation and the electrical boundary conditions. A new procedure is introduced to decouple the governing equations and then an analytical Levy-type solution is obtained. The exact natural frequencies are established for a wide range of length scales, various plate dimensions, several piezoelectric layer thicknesses, and different boundary conditions. The results show that the effect of length scale parameter is decreased by the piezoelectric electrical field.

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Impact wear can be defined as the wear of a solid surface due to percussion, which is a repetitive exposure to dynamic contact by another solid body. It has not been studied as extensive as other wear mechanisms and the information on causes and actual data is quite scarce. In the present work an experimental wear tester is designed and used to study the impact wear phenomenon experimentally. It creates the consecutive impacts between balls and flat plate. The different ball sizes (up to 30 mm), impact velocities (up to 6 m.s −1 ) and impact angle (10 • −80 • ) can be tried by tester. The mass loss of plate is measured as the wear parameter. These measurements for different specimens revealed some aspects of their wearing behaviors. The obtained results could be helpful in optimal practical designs and explanation of some phenomena associated with impact wear.