芳香族聚酰胺（PPTA）纤维弯曲疲劳的形变和断裂机理

普通PPTA纤维和改性PPTA纤维具有近似的应力应变的曲线，但后者的弯曲疲劳寿命比前者要长的多。在SEM下观察了弯曲疲劳过程的形态变化，发现该过程主要是结构沿纤维长度分裂即原纤化过程。改性PPTA纤维对原纤化的阻抗显著地大于普通PPTA纤维。粉碎降解和低温断裂的结果也得出类似结果。表明大分子间结合力在PPTA纤维弯曲疲劳中起很重要的作用。弯曲疲劳作用被归结为对材料拉伸和压缩的复合作用。拉伸PPTA纤维的动态观察得出，拉伸或压缩纤维将出现剪切带或滑移带，它们引发纤维原纤间的分离。剪切屈服是其主要形变机制。PPTA纤维弯曲疲劳过程如下：重复弯曲引起材料局部区域的剪切屈服，出现剪切带，并随之发生结构分裂，纤维广泛地原纤化，最后原纤和原纤束相继破坏，纤维最终断裂。

Machines of Deformation and Fracture of PPTA Fibers Bending Fatigue

As-received PPTA fibers (fiber A) and modified PPTA fibers (fiber B) are found to have similar tensile stress-strain behavior. But fiber B is found to have a much longer fatigue life than fiber A. The morphology of both fibers during the fatigue process is observed by SEM. It is found that the bending fatigue mainly is the process of the split along fiber length and fiber B is much stronger on resisting the split than fiber A. The TEM observation of the residual of both of the fibers disintegrated by ultrasonic radiation give similar results. It expresses that the coherence of inter-macromolecules plays a veryimportant role in the bending fatigue life of PPTA fibers. The effect of bending fatigue on the materials is considered as the repeated effect of tension and compression in turn. The dynamic observations of the fibers in tension conducted in SEM show that the shear bands on slip bands caused by tension or elastic snap-back respectively often initiate the split of fibers. This suggests that the shear yield is the main deformation mechanism of PPTA fibers in tension and in compression. The process of bending fatigue for PPTA fibers could be expressed as follows. The repeated bending causes the shear yield of local regions and appearance of the shear bends. Then the splitting of the structure takes place and the fiber fibrillates greatly. And finally the fibrils and fibril bundles break down in turn and the fiber failure.