Sampling And Assessment For Surface Topography Of Carbon Fibre Composite Workpieces

Carbon fibre composites are widely used in manufacturing key parts in aeronautics & astronautics for their many prominent properties like high specific modulus, high specific strength, heat and abrasion resistance etc. Because of the inhomogeneous structure and anisotropy characteristic the surface machined has many imperfections and the topography is complicated, which need 3-D measurement to obtain surface information correctly and 3-D assessment to evaluate the quality properly. For this reason, the determination of proper sample conditions for surfaces of carbon fibre composite workpieces and the assessment of topography are studied by means of theoretical analysis combined with experimental research in this dissertation.The microscope topography of C/Ph workpiece machined is observed by SEM, and the contents including relation between evaluate area and sample area, choosing of sample length, determination of graded series for sample space, characteristics of wavelength and frequency are analyzed. As determination of sample condition has great influence to the surface assessment of microscopic geometry error, aiming at the uncertainty of selecting sample conditions depending on user's own experience in reality, one method that can determine appropriate sample conditions objectively is developed. Based on 2-D spectral analysis and considering the Nyquist sampling theorem and aliasing effect, a normalized proximity factor is proposed to determine proper sample condition. After comparing the roughness parameters obtained with proper sampling, initial sampling and one improper sampling, it finds that the method applies for the measurement of surfaces of carbon fibre composites and can determine sample conditions objectively and properly. For the complexity of workpiece surface, one suggestion which can measure the surface of carbon fibre composite more properly is proposed after measuring several areas. After studying proper sample conditions of C/Ph workpieces machined by different rotational velocities, it can conclude that measure all surfaces with one unique sampling space is not correct. Then power spectrum is used to analyze the frequency components of C/Ph workpiece surface, and obtain the general situation of topography.The roughness topography filtered by TalyMap software is observed. Five roughness parameters are chosen to assess the topography of carbon fibre composite workpieces after studying the C/C surfaces machined by different feed rates. The applicability of other parameters and the relation between parameters and performance of workpiece need further study. The variation rules including values of RMS variation S_q, topography characteristics, height distributions, wavelengths of components and cutting mechanism are studied, and the cutting parameters to machine the workpiece rather better are confirmed.