A cyclic load treatment was developed in this study to improve the mechanical properties of natural fiber in preparation of composites. During a cyclic load process under high temperatures, a load was applied on ramie yarns up to a proper value and then unloading, which was repeated for different number of cycles. The tensile properties and microstructural changes were investigated by tensile measurements and X-ray diffraction method (XRD) respectively. The results indicate that the application of cyclic load plays a key role in enhancing the tensile properties of ramie yarns, that is, significant increases of around 18-25% in tensile strength and about 67-106% in Young's modulus were obtained for the cyclic load treated yarns. In contrast, individual heat treatments without cyclic load were performed on ramie yarns, leading to an unchanged tensile strength and a decreased Young's modulus. In addition, XRD analysis showed no significant change in crystallinity and crystalline orientation degrees of ramie yarns after cyclic load treatment. The factors, such as decreased microfibrillar angle, increased amorphous orientation as well as optimized fiber arrangement in yarn structure were considered to contribute the improved tensile properties of ramie yarns.
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