Study On Topography Of CFRP Machined Surfaces Based On Fractal & Wavelet Theory

Carbon fiber reinforced plastic composite (CFRP) has many excellent properties, such as light weight, high specific strength, high temperature resistance, corrosion resistance and so on, which is widely used in aeronautics & astronautics. Because of the inhomogeneous structure and anisotropy performance for CFRP, it appeared easily with the unique surface topography characteristics such as fiber pull-out, fiber crushing, fiber tearing, et al. The traditional 2-D evaluation standard is not suitable for CFRP anymore. At present, 3-D topography roughness is the research focus, but the corresponding 3-D evaluation standard has't been established internationally until now, and roughness parameters are relevant to scales, the measured values change with the change of the instruments's resolution and the sampling length, so that it can't reflect the intrinsic characteristic of the surface topogryphy.Research indicates that machined surfaces have fractal characteristics of randomness, irregularity, self-similiarity and self-affinity, and fractal dimension is a kind of parameter irrelevant to scale, so it is possible to apply fractal theory for surface topography. The author of this paper goes at the 3-D surface topography of CFRP, applies the fractal & wavelet theory, takes the SEM image of the cutting surface as the research object, studies the surface topography characteristic of CFRP, the main studies are as follows:(1) The fractal Brownian motion (FBM) is introduced to the research of surface topography, the characteristic of FBM and the physical meaning of fractal parameter H are analyzed, and fractal curves and fractal surfaces are simulated with different H. And the Discrete Fractal Brownian Random Field (DFBRF) model which based on SEM image is studied.(2) The surface fractal dimensions are calculated with the differential box-counting (DBC) method. The results show that the fractal dimension D are closely related 3-D topography of the machined surfaces of CFRP, the rougher the surfaces, the bigger the D, fractal dimension can synthetically evaluated the whole surface topography characteristic. In order to analyze the local topogryphy characteristic of CFRP cutting surfaces, multi-fractal is introduced, the multi-fractal spectrums of the images are calculated, and the multi-fractal spectrum parameters are used to analyze the partial topography change characteristic of the cutting surfaces. (3) The study on the surface texture feature of CFRP is based on DFBRF, the gray values of the SEM images are changed into the fractal parameters H & b, and then fractal parameters H are classified, the same gray value is endowed with when H is in one class, the image surface texture is segmented into different blocks by the new different gray value, so that the local topography characteristics of CFRP cutting surfaces are extracted.(4) The study on the change of the surface roughness texture for CFRP is based on wavelet transform. The SEM image is decomposed by using Daubechies wavelet, the energy proportions of the decomposed subimages are calculated, and the energy proportions of the decomposed high frequency subimages are regarded as the change basis of the surface roughness texture in defferent directions.