Characterization Of Single Kevlar 49 Fiber Fatigue Property Based On Multifunctional Fix-point Bending

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.