摘要
纤维材料疲劳性能的表征是认知方法,这对其应用有重要的指导作用。而对材料的认知又促进表征方法的发展。国内外已经有的滑轮式弯曲疲劳、三点弯曲疲劳和钟摆式弯曲疲劳,均不是定点(fix-point)弯曲。考虑到这些仪器存在的不足,本研究在单纤维的表征及其多功能测量上进行了探索,研制了相应的多功能定点弯曲疲劳仪(FipBFAN),并对Keylar49单纤维的弯曲疲劳性能作了表征。具体内容如下:
(1)多功能定点弯曲疲劳仪(FipBFAN)的研制,实现单纤维的定点弯曲及其在线观察与测量:能够完成预张力、弯曲角度、弯曲频率的设置与自动控制;可模拟光热及其复合环境下的疲劳实验;可直接提取弯曲实测过程中的张力的波动曲线疲劳特征值和弯曲疲劳寿命。
(2)该仪器(FipBFAN)山辐照系统、加热系统、摆动系统、数字光学观察系统、空间位置控制系统、控制箱及软件构成。测量原理为组合原位的实时表征,已申请国家发明专利。实验所得结果与以前的仪器相比更为精确、可靠、完善。
(3)使用FipBFAN测试了Kevlar49纤维的单纤维弯曲疲劳性能。实验结果表明,通过预张力—断裂次数(T—N)曲线,证明预张力与弯曲疲劳寿命为指数关系;弯曲疲劳过程的张力因弯曲发生循环波动的变化;弯曲疲劳过程固定应变,Kevlar49纤维明显的存在张力松弛,为保证张力准动态平衡,进行微拉动张力补偿,故纤维存在仲长,即发生蠕变行为的体现:弯曲角度越大,张力循环的幅度越大,弯曲疲劳寿命越短:弯曲角度越小,预张力越小,弯曲疲劳寿命越长:在小的弯曲角度和大的预张力下与大的弯曲角度与小的预张力下的破坏效果是等效的;角度对弯曲疲劳寿命的影响程度与预张力有关,预张力越小,越小的弯曲角下弯曲疲劳寿命增加越大,增加的幅度随着角度的减小而提高:频率越高弯曲疲劳寿命越短;100℃弯曲点温度环境对Kevlar49纤维弯曲疲劳寿命影响不明显:相对于100℃温度的影响,辐照光的影响较大;光热复合环境实验证明,光热复合环境有交互作用且疲劳寿命明显减少。
(4)在实验过程中Kevlar49单纤维的张力随弯曲发生动态变化,基于此相对于一般蠕变和松弛的概念,提出了动态松弛和动态蠕变的概念。
(5)通过对Kevlar49单纤维弯曲疲劳过程中的形貌分析得到:Kevlar49纤维的断裂主要是正常弯曲断裂而不是表面磨损是纤维弯曲点发生弯曲疲劳破坏的过程。在纤维被弯曲的区域(纤维弯曲点),由于被弯曲的各层面受到反复的挤压、且应变不同,故形成层间剪切,使纤维弯曲点首先原纤化,原纤化纤维逐步分散并从外层到内层逐步断裂。由于纤维弯曲点处大量的原纤化纤维的断裂和原纤化纤维中的抽拔导致纤维的断裂。
Characterization of fiber fatigue property as one method of cognition has an important guidance to the application of fiber materials,while the cognition of materials,in turn,accelerates the development of characterization technology. Bending fatigue over sheave,three-point bending fatigue,and swinging fatigue had been developed,while all of three bending methods are not fix-point bending. Considering shortages of these devices,multifunctional fix-point bending fatigue analyzer(FipBFAN) has been developed.Using the apparatus,the bending fatigue property of single Kevlar49 fiber specimen is characterized.The detail as follows:
(1) Development of FipBFAN realized observation and measurement of the fix-point bending for a single fiber;it can carry out the flexible setting and automatic control of fiber pretension,bending angle and frequency;the environment with light, heat,or complex light heat can be simulated;and fluctuation curve of tension eigenvalue and life of bending fatigue can be directly picked up.
(2) FipBFAN consists of light irradiation system,heating system,swinging system, digital optical observation system,fiber position adjusting system,and controlling box and software.The measurement principle is combined,in-situ and real-time characterization.The method and device had been patented.Compared to prior devices,it is more accurate,reliable and perfect in measurement.
(3) Using the FipBFAN,the bending fatigue property of single Kevlar49 fibers was tested.Experiment results showed:T-N curve proved exponent relation between pretension and life of bending fatigue;tensions on single fibers undulated circularly in the process of bending;Kevlar49 fiber evidently relaxed under constant strain;for insuring the balance of tensions,adjusting fine-tuning parts to let tensions compensated,so the fiber was elongated to a certain extent,that is,The fiber creep; the amplitude of tensions became bigger and life of bending fatigue became shorter as bending angle became larger;the life of bending fatigue became longer as pretension and bending angle became smaller;the effect of large pretension and small bending angle on life of bending fatigue was equal to small pretension and large bending angle; the effect of bending angle on life of bending fatigue was related to pretension,the increasement of life of bending fatigue was larger under smaller bending angle as pretension became smaller,and the range of increase was enhanced as bending angle became smaller;the life of bending fatigue became shorter as frequency became higher;the decrease of life of single Kevlar49 fiber bending fatigue was not evident under 100 degrees centigrade;comparing to 100 degrees centigrade,the effect of irradiation light on life of bending fatigue was larger;the experiment of complex light heat environment proved environment of complex light heat had a complex function and bending fatigue life evidently lowered.
(4) Tensions undulated circularly in the process of bending,so based on common concepts on relaxation and creep,the authors brought forward the concepts of dynamic relaxation and creep.
(5) Fractography analysis of Kevlar49 fiber in the course of bending fatigue drew the conclusions that the fracture of Kevlar49 fiber was led due to bending of bending point not surface abrasion;layers of Kevlar49 fiber bending point were extruded repeatedly and strain kept constant,so interlayers shaped shear;fiber of bending point firstly scattered and broke from outer layers to inner layers.Due to fracture and drew-out of many fibrils of bending point,fiber broke.
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