Prolonging the Fatigue Life of Soft Rubber Actuators by Inhibiting Crack Extension Using the Hysteresis of Strain-Induced Crystallization of Natural Rubber

In this paper, we report the extension of the life of rubber materials that are subject to large deformations. Unlike conventional robots, soft robots, which have been attracting increased attention in recent years, often undergo large deformations of the robot itself; thus, the lifespan of these robots is an issue. Therefore, in this study, we extended the fatigue life of rubber materials by focusing on the hysteresis of the strain-induced crystallization characteristics of rubber. Strain-induced crystallization is a phenomenon in which rubber molecules exhibit crystal structure by elongation. The crystal structure is generated by elongation and disappears with relaxation. However, hysteresis occurs in the generation and disappearance process; therefore, by not completely relaxing the rubber, the disappearance of the crystal layer is slowed by hysteresis, and the formation of cracks is hindered. In this study, we selected two types of rubber: those that have strain-induced crystallization characteristics (NR: natural rubber) and those that do not (SBR: styrene-butadiene rubber). First, wide-angle X-ray diffraction was used to confirm the formation of crystal layers under stretching for the two types of rubber. A fatigue life test was conducted using rectangular test pieces made of NR and SBR by repeated tensile tests. Then, a fatigue life test was conducted for a straight-fiber-type pneumatic artificial muscle prepared by NR and SBR. The fatigue life was approximately 100 times longer than usual, confirming the effects of prolonging the fatigue life using strain-induced crystallization