Synchronous belts consist of a rubber, a facing fabric and a helical cord that reinforces the belt stiffness. Therefore, the fatigue failure of the helical cord through the life is important for the stiffness loss of the belts. The purpose of this study is to discuss the mechanism of the extensional stiffness loss of the helical cord during the bending fatigue. The experimental results show that the extensional stiffness reduces up to about 50–60% of the initial value by the accumulation of the bending fatigue damage. The main damage is the de-bonding along the interfaces of the stranded glass filaments in the cord. The three dimensional FEM results show that the extensional stiffness is reduced by the initiation of the de-bonding. Then a simplified mechanical model is proposed to explain the stiffness loss considering the de-bonding. The model reasonably well explains the extensional stiffness loss.

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