Mr. Akihiro Kojima Profile

Akihiro Kojima

at Chuodaigaku Rikogakubu;Chuo Univ

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Proceedings Article | 13 March 2019 Paper

Strain-induced crystallization to prolong the lifetime of pneumatic artificial muscles

A. Kojima, M. Okui, I. Hisamichi, N. Tadami, T. Tsuji, T. Nakamura

Proceedings Volume 10965, 109650X (2019) https://doi.org/10.1117/12.2513246

KEYWORDS: Artificial muscles, X-ray diffraction, Finite element methods, Actuators, Robots

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Various types of soft actuators have been developed for application in wearable movement-assist devices or soft robots. The authors have developed a straight-fiber-reinforced pneumatic rubber artificial muscle (SF-ARM). The SFARM is composed of rubber that is reinforced with fibers aligned only in the axial direction. When air pressure is applied to the SF-ARM, the reinforced fibers limit the rubber expansion to the radial direction so that the muscle contracts in the axial direction. The SF-ARM contracts by 38% at maximum, and this contraction rate exceeds the contraction rate of the McKibben artificial muscle. However, the SF-ARM is not well-suited for practical use because the strain on the SF-ARM while it is actuated is large which can cause fatigue failure of the rubber. This study focuses on suppressing the growth of cracks using strain-induced crystallization of the natural rubber, to prolong the lifetime of the SF-ARM. Natural rubbers form a crystalline layer in the direction perpendicular to the direction of stretching. This crystal layer effectively suppresses the growth of cracks in the SF-ARM when under strain. Deliberately developing a crystal layer should extend the lifetime of the SF-ARM. First, this study confirmed the formation of a crystal layer under extension of natural rubber (NR) and styrene butadiene rubber (SBR) using wide-angle X-ray diffraction measurements. Next, the strain concentration near the crack was analyzed using finite element method simulations. Finally, fatigue-life tests were conducted with SF-ARMs made of NR and SBR.

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