T700/BMI Porosity Detection And Fractal Research Of Pore Morphology Feature

With the rapid reduction of carbon fiber manufacturing costs,carbon fiber reinforced resin matrix composites（CFRP）with excellent comprehensive properties have been widely used in various fields.However,pore is unavoidable in the preparation process of composites,the porosity and pore morphology（length,width,shape and area）have different effects on the mechanical properties of composites.In this study,composite laminates with different laying methods and thickness（the number of layers）were prepared by moulding method.The sound attenuation of composite laminates was detected by ultrasonic C-scan method,and the homogeneous attenuation region was selected as the region containing only pore defects to prepare the pore detection samples.The porosity of composite laminates was examined by microphotography.The pore morphology was observed by optical microscope and field emission electron microscope,and the pore morphology was analyzed by statistical principle and fractal theory.The attenuation coefficient of composite laminates was measured by ultrasonic C-scan,and the porosity of composite laminates was measured by microphotography.It was found that the laying method and the thickness of the layers had some effects on the attenuation coefficient and porosity.The attenuation coefficient and porosity of orthonormal layer composite laminates are higher than unidirectional layer laminates.The porosity of composite laminates is positively correlated with attenuation coefficient and thickness.The relationship between porosity（P）and attenuation coefficient（α）is P=-9.21+5.34α-0.55α²,porosity（P）and the thickness of laminates（f）the relationship is P=0.12+5.88×10^-4f ².The pore morphology of composite laminates was observed by optical microscopy and field emission electron microscopy.It was found that long and irregular pores were mainly distributed at the interface of fiber and resin,small round or elliptical pores were formed in the pits of resin enrichment area and fiber bundles.With the increase of ply thickness of composite laminates,the number and size of pores constantly increase.Statistical analysis of pore morphology under optical microscope shows that the 20⁷5 orthonormal layers composite laminates pore lengths are 13.5²77.4μm,the pore widths are 12.9¹91.5μm,the small pore size（S<1×10³μm²）is reduced from 57%to 13%,the large pore size（S>1×10⁴μm²）is increased from 1%to 21%,and the circular and elliptical pores decreased from 17%to 6%.The pore morphology of composite laminates has fractal characteristics.The pore morphology fractal box dimension D_B was calculated to indirectly judge the complexity of pore morphology and porosity.The results show that the D_B is between0 to 2,and the D_B is closer to 2,the pore tends to distribute in the whole space.When0≤D_B<1,the size of D_B is determined by the complexity of pore morphology and porosity,and the linear relationship between porosity and fractal box dimension is not obvious.When 1≤D_B≤2,the size of D_B is mainly determined by porosity,and the greater the porosity,the greater the D_B.The relationship between fractal box dimension and the thickness of laminates is D_B=0.51+2.2×10^-3f+8.75×10^-5f ².