Characterization Of Weak-Links Of High-Performance Fibres

Fibre weak-link (FWL) is one of the most important flaws affected on the processing and end-use performance of fibres. The theory about fibre weak-links had been put forward early, but it is still difficult in applied characterization. Up to now, there are not effective methods published to characterize FWL. Some fibres, such as cotton, wool, and carbon fibre have been investigated referred to the FWL theory. There has, however, been little study of this theory for high-performance fibres (HPFs) because of its uniformity on the appearance.Taking the PBO fibre, Nomex fibre and nylon as the studying, the weak-links of the three fibres have been systematically analyzed by the tensile test, morphological observation and the combination of the two methods in situ. The detail as follows:(1) A CCD digital camera combined with an optical microscope was used to measure the minimal diameter and the diameter at the breakpoint of every fibre. Then the tensile properties of each corresponding fibre was measured by the XQ-1 fibre tensile tester. On the basis of this experiment, variations of such tensile properties as tenacity, 'real' stress, strain at break, specific work of rupture and their distributions can be abtained. On the traditional method, people often combine the tensile properties to the average linear density of a fibre, this is imprecise obviously. In this paper, the morphological observation and the tensile curves are combined together to analyse the fibre weak-links of these high-performance fibres in situ.(2) Adopt the distribution simulation method to discuss the fitted distribution of the fibres by means of LogNormal, Gauss, Lorentz and four-parameter weibull functions, and to get the relative coefficients and applicability of these functions to the tensile eigenvalue. As the results, Lorentz distributions generally provide the best fit to the distribution of the fibre tensile properties, when compared with other functions. The distribution of PBO's tenacity is fittes to Weibull distribution, but that of the real stress is fitted to Lorentz distribution.(3) The critical value for the calculation of the rate of fibre weak-links, namelyWeak-link Probability, has determined. By the value of the weak-link ratio, we can judge the relative strength or weakness of each kind of fibre. For one kind of fibres, the tendency of the value of the Weak-link Probability is consistent with different critical values. That is, the nylon is the weakest, the PBO fibre is the strongest, and the Nomex fibre is in the middle. On the contrary, it also can determine the critical value under a fixed Weak-link Probability, especially when compare the critical value with its average or the maximal value, that the specific value can be determined, thus the quality of different kinds of fibres is clear.(4) The inner structural defects of the fibres, i.e. fibre weak-links, are difficult to be characterized generally. In this paper, however, the combined method of single fibre tensile curve test and fibre fracture morphology by SEM observation has been developed. The fibre weak-links of this three fibres have been characterized in accurate and reliable approach respectively. On the basis of this study, the FWLs can be characterized in simplified way. The experimental results show that the PBO fibre is usually laminated and splitted into fibrids , and the fibrids have a high tenacity, so the tensile behavior of PBO fibre is excellent. The weak-link of Nomex fibre is mainly arosed by the step-stage flaws. The FWL of the nylon are almost the large and linear defects on the fibre surface.(5) The measurement of the tensile strength of single fibres is not commonly practiced in the industry, perhaps because of its slowness. The algorithms of the frequency density function of fibre strain at break have been achieved by means of the modulus curve, BS1 '(e), differentiated from the fibre-bundle tensile curve, BS(e), and the fibre weak-link probability can be found out based on BS1 '(e) curve accurately and easily. By BSJ '(e) curve, can also...