Abstract
Synchronous belts are used for a cum shaft drive of automobile by the advantage of their superior synchronous characteristic. These belts are one of the fiber reinforced rubber (FRR) composite materials. That is to say, they are constructed from rubber, facing fabrics and helical cords. Especially, the helical cords keep the extensional strength of the belts. It is known that the helical cords get easily damaged by a bending fatigue around pulleys when the belts are driven. Moreover, the extensional strength decreases with an increase in driving time. This phenomenon is known through experience and is one of the most important problems in the practical use. In this paper, the reduction of extensional strength in helical cords because of the bending fatigue, which is related to the tensile member breakage, was investigated with some experiments, FEM analysis and mechanical analysis. Firstly, we observed the fatigue failure morphology of belt-cords, and confirmed that the de-bonding is initiated and extended at the interface between stranded glass filaments. Moreover, we obtained the result that the extensional strength was reduced as a consequence of the bending fatigue. Secondly, the stress distributions on the cross section of cords were analyzed by using a three-dimensional FEM. The FEM results have shown that the stress of fibers increases with the initiation of the de-bonding. Moreover, it was confirmed that the reduction of tensile strength could be explained by the initiation and extension of the de-bonding. Finally, a simplified mechanical model was proposed to explain the reduction of extensional strength of cords. The analytical results had a good correlation with the FEM and experimental results.
