OPTIMUM DESCRIPTION OF YARN AXIS GEOMETRY IN WOVEN COMPOSITES

摘要

Quantitative composite structure description is an important task from both the theoretical and practical point of view. Among all in principle applicable methods for yarn axis description or approximation the Discrete Fourier Transform (DFT) appears as the best suitable one, since it respects yarn periodicity or quasi-periodicity and its output, the spectrum, has a simple and straightforward physical meaning. It can be applied at least in two ways for quasi-periodic function and general closed curve description. On the other hand, the DFT should be used carefully; otherwise unwanted effects appear, such as spectrum leakage that complicates simple and efficient use of outputs. This effect takes a place, when non integer number of periods is sampled. Main application of DFT are presented in the paper: efficient yarn axis approximation, spectrum filtering for removing of experimental errors (like sample bending), effective approximation of general form of closed curve that is a contour of void cuts in composite structure. Also method for leakage reduction is shown. Practical results are mainly in graphical form.