Mechanism Of Weak-link Inducement And Restraint For PAN-Base Carbon Fibres In Formation And Surface Treatment

The failure of a fiber occurs only at the position where the mechanical property is the weakest. Therefore, the fracture mechanical property of a fiber depends not on the whole or normal structure parameters of the fiber, but on the structure of the fiber weak links (FWL). The structure of fiber weak links (FWL) is the subject of this study. Both the effect of the structure of fiber weak links on the strength and its variation and the change of the structure of fiber weak links during the surface treatment of carbon fibers were discussed. Thus, the technical parameters in the manufacture process which are benefit to control the weak links structure can be found according to the probable reasons for the exsistence of all kinds of weak links structures. In whole, this dissertation is a basic theoretical research to enhance the strength of carbon fiber and stabilize the tensile property of carbon fibers.Firstly, the test result of the tensile property of carbon fibers should be obtained accurately. The factors of the tensile test of carbon fibers were discussed in this paper. Carbon fibers are brittle, all kinds of damages may be induced during the tensile test process, which result in the errors, and the tested results can not be used to characterizing the strength and its variation of carbon fibers actually at this rate. All probable factors, such as, the pretension during stretching, the position of adhesive point and adhesive effect, the angle between stretching direction and the axis direction of fiber, were discussed in this study respectively. The effect of the above-mentioned factors on the tested results was given, and the ways to calibrate the errors were presented to get rid of the effect of the irregular operation on the tested strength and its variation of carbon fibers. Thus, the accuracy of the measurement of the strength and its variation of carbon fibers can be ensured, and the actual mechanical characteristics can be related to the structure of the fiber weak links.To observe the structure of the FWL of carbon fibers, the obtainment of thereserve rate of the fracture-ends of carbon fibers were discussed.Two ways to express the FWLs of carbon fibers, the variation of strength and size effect were discussed. The structures of the FWL of carbon fibers were different one another, so the tensile property of carbon fibers were different one another. That's the basic reason of the existence of the strength variation. The correlativity of the fracture force distribution, the fracture stress distribution, the fracture strain distribution and the modulus distribution with the Weibull and Gaussian distribution were discussed one by one. Besides, there exists another way to express the FWL of carbon fibers, i.e. size effect. The size effect means that the longer the testing gauge length is and the bigger the diameter is, the lower the strength of carbon fiber is in the tensile property experiment of carbon fibers. The effects of the size effects caused by both the gauge length and the diameter of fibers on the tested strength of carbon fibers were discussed, respectively. The influence of the volume variation on the strength variation of fiber was also discussed.Secondly, the structure features of the FWLs of carbon fibers were discussed. The carbon fibers were divided into three groups according to the fracture tensile stress of the corresponding carbon fibers. The structure features of FWLs were discussed according to the groups by the SEM observation, the analysis on fiber tensile properties and the corresponding discussion of the two sorts of results. The characteristics of tensile curves of each group were discussed, too. The following information can be gotten in the SEM observation: the origination of the failure, the propagating direction of the crack, the possible reasons for the failure and the flaws which resulted in the failure of the fiber and their importance in the failure process. Some flaws often presented in the FWLs' structure of carbon fibers failed at low stress, and some flaws were important in the fracture process. The possible reason of the formation of these flaws was discussed and the way to control these flaws was put forward, too.Thirdly, if the surface treatment of carbon fibers will result in the enlargement and worsen of the FWLs was characterized. Carbon fibers often act as the reinforcement of composite, but the surfaces of carbon fibers are inert chemically, soreinforcement of composite, but the surfaces of carbon fibers are inert chemically, so the interface adhesion between the reinforcement and base of composite is not strong enough. Therefore, the surface of carbon fibers must be treated before their application. However, improper surface treatment will result in the lost of the tensile strength of carbon fibers. Because plasma surface treatment and electrochemical oxidation are the most popular and developable ways in the fields of manufacture, the carbon fibers were treated by plasma and oxidized electrochemically respectively. The result of the surface treatment of carbon fibers were analysed by comparing the wettability of carbon fibers after the two surface treatments and the time effect of that;the effect of the two surface treatments on the FWLs were proved by comparing the tensile strength of carbon fibers after the two surface treatments and the change of tensile strength with time after the surface treatment;the mechanism of the effect of surface treatment on the enlargement and control of the FWLs were verified by observation of the change of the mechanical behavior and structure of FWLs after the surface treatment. It turned out that the plasma treatment is more advantageous in treatment effect and the control of the FWLs, and it should become the main method to enhance the adhesive property of carbon fibers.