Research On Effect And Evaluation Of Voids On Properties Of Carbon/epoxy Composite Laminates

With the wide application of carbon fiber reinforced epoxy composites in the field of aviation and aerospace, void defects produced during the manufacturing in composites are receiving greater attention. Voids are detrimental to the mechanical properties of composites, so it is significant to study the void content, void size and distribution for the application of composites. A large number of researches have been done on the void problem by overseas researchers. However, little work has been reported from domestic researchers. Therefore, in this dissertation, studies on void problems of composite laminates those are typically used in an aircraft. Mechanics of void formation of polymer composites in autoclave process was discussed. Specimens with different void contents were fabricated through varying autoclave pressures. Quantitative characterization of voids in terms of void size, shape was carried out. The effect of voids on the tensile, compressive, interlaminar shear, low-velocity impact, interlaminar shear fatigue, hygrothermal properties was studied.Mechanics of void formation of polymer composites was analyzed. Theoretical prediction and experimental analysis of effect of autoclave process on porosity for T300/914 composite were done according to voids growth model. The relationship between the magnitude of autoclave pressure and void content was obtained.Optical microscope (OM) and image analysis method were used to study the characteristic of void morphology of composite laminates. The results show that voids tend to locate between the plies, and grow in the direction parallel to the plies. Some voids are observed to occur in the plies when the autoclave pressure is low, and the size and aspect ratio of these voids are small. The area, length, aspect ratio and the number of large voids increase with the decreasing autoclave pressure. Stacking sequence of compsote laminates affects the distribution and morphology of voids.The relationship between voids and the tensile strength and modulus, compressive strength and modulus and interlaminar shear strength of composite laminates was built up. Failure mechanism of composite laminates was examined. The physical model and artificial neural network were used to model the effect of voids on the mechanical property of composite laminates. The results show that the effects of void contents on compressive strength and modulus, interlaminar shear strength (ILSS) were significant, while the effects on tensile strength were small, and the effect on tensile modulus was different because of the difference of stacking sequence. The effects of voids on the initiation and propagation of cracks, microbuckling and kinking under compressive load and breaking of fiber under interlaminar shear load were found.The relationship between voids and the post-impact interlaminar shear strength of composite laminates was built up. OM and X-rays were used to study the effect of voids on the failure mechanism of impact. The effects of voids on the initiation impact cracks were found. The post-impact ILSS of composite laminates decreased significantly with the increasing void content.The relationship between voids and the interlaminar shear fatigue properties of composite laminates was built up. OM and X-rays were used to study the effect of voids on the failure mechanism of fatigue. The results show that the interlaminar shear fatigue life decreases with the increasing void content. The effects of voids on the initiation and propagation of fatigue cracks were found.The effects of voids on the moisture absorption and hygrothermal aging interlaminar shear strength of composite laminates were investiged by carrying different hygrothermal tests on laminates with different void contents. The relationship between voids and the interlaminar shear strength of composite laminates with different moisture uptake and desorption was built up. The rate of water uptake and maximum level of moisture absorption in the composite laminates increase with the increasing void content. The ILSS of aged composite laminates variated complicated because of the existing of post cure. The voids affect the initiation and propagation of cracks under the hygrothermal circumstance.