Effect of pore microstructure on interlaminar shear properties of thick section composites

Short beam shear tests conbined with digital image correlation were carried out for thick-section glass fiber reinforced resin matrix composites，and the variation of interlaminar shear behavior of unidirectional composites with different thicknesses with thickness was obtained. In order to explore the size effect mechanism of interlaminar shear mechanical behavior of materials，the microscopic characteristics of pores of samples with different thicknesses were observed through scanning electron microscope photos of sample slices. Image processing was used to obtain the contours of irregular pores，and parameters for quantitative characterization of microscopic characteristics were proposed. A three-dimensional representative volume element（RVE）model containing fibers，matrix，fiber/matrix interface and pores was randomly generated，and the effect of irregular pores of different sizes and distributions on the interlaminar shear strength of composites was studied by numerical analysis. The results of short beam shear tests showed that the interlaminar shear mechanical behavior of unidirectional composites was independent of the thickness of the specimens，but the shear strength decreased with the increase of the thickness of the specimens. The image analysis results showed that the distribution law，size，concentration and irregularity of the pores in samples of different thicknesses were significantly different. The numerical analysis results of RVE showed that the damage was caused by the destruction of the fiber-matrix interface close to the pores，and the interlaminar shear strength of thick-section composites decreased with the increase of the maximum pore size under the same porosity. At the same time，pore concentration and porosity also have a significant effect on the interlaminar shear strength of the material. The above experimental and analytical results show that the size effect of the interlaminar shear strength of thick-section composite materials is related to the microscopic characteristic parameters of the material pores. As the thickness of the composite material increases，the porosity，maximum pore size and concentration increase，and the shear strength decreases. Therefore，the difference in the microscopic characteristics of pores in materials of different thicknesses is one of the important mechanisms that lead to the decrease in the interlaminar shear strength of the material.