Study On The Fracture Toughness Of Type I Interlaminar Sutured Composites In Salt Spray Environment

Stitched composites possess a broad application prospect in the field of marine engineering owing to their good delamination resistance,good corrosion resistance and excellent designability.The harsh salt fog conditions in the marine can damage the material matrix and fiber/matrix interface of composites,leading to the deterioration of mechanical properties of composites in service process,which finally cause the failure in advance.In practical marine applications,delamination damage is the most common failure mode in composite structures,which greatly affects the structural integrity and durability of composites.Based on the above points,on the basis of the experiment for mode ? interlaminar fracture toughness of composites,combined with the macroscopic fracture morphology and microscopic damage analysis of the stitched composites,the effects of salt fog aging treatment and stitching parameters difference on the interlaminar property of the stitched composites were studied in this project.Firstly,a new stitching method similar to"Tufting"has been developed,which was suitable for carbon fiber.Four stitched preforms with different stitching densities and patterns were designed and woven.The stitched performs compounded by resin transfer molding process to become the DCB specimens,which required for testing are obtained.Secondly,taking experimental methods on the study of the mode ? interlaminar fracture toughness of stitched composites.The experiment for Mode I interlaminar fracture toughness was carried out on stitched composites and laminated composites,and their differences in macro-mechanical properties and failure morphology characteristics were analyzed.The results show that the critical strain energy release rate(G_IC value)of stitched specimen is 33 times higher than that of laminated specimens.It is mainly due to the energy consumed by destroying the fiber/matrix interface and destroying and pulling out of the stitching yarns.The alternating salt fog aging treatment is used to simulate the erosion effect of the composites in the marine environment.The characteristics of the fracture surfaces and the surrounding area of stitching yarn,the topography of resin-rich area,weight gain curve,load-displacement curve and critical strain energy release rate before and after salt fog aging were compared and analyzed.The results show that salt fog aging treatment damaged the interlaminar property of the stitched composites,the G_IC value of the 5 mm×5 mm stitched specimen which treated with salt fog for 4 days decreased by 3.43%-3.83%,the G_IC value of the specimen which treated with salt fog for 8 days decreased by 11.84%-12.70%.The debonding failure of stitching fibers and resin interface is the main reason for the decrease of G_IC value Furthermore,the effect of stitching parameters on the interlaminar property of the composites after salt fog aging was explored by changing the stitching pattern and density.The results show that the stitching density has a significant influence on the interlaminar property of the stitched composite before and after salt fog aging.The introduction of stitching yarn would make composites suffer more serious erosion during the salt fog aging process,and with the increase of stitching density,the adverse effect of the composites would be intensified.Specifically,after 8days of salt fog aging,the G_IC value of the stitched composites with stitching density of 5mm×5mm,7.5mm×7.5mm,10mm×10mm and laminated composites decreased by 11.84%,7.24%,6.74%and 2.35%,respectively.The stitching pattern has little effect on the interlaminar property of the stitched composites with the same density before and after salt fog aging.Thirdly,the cross section shape of warp and weft yarn inside the satin woven composites and the bending shape on the surface of stitched performs were observed.Based on the observation,the reasonable assumptions of the yarn section and bending shape of fiber were generated,the geometric relationship of each component yarn was deduced and the mesoscopic model was established.The engineering constant of the stitched composites calculated by bridging model,which sets the foundation for further simulation of its interlaminar fracture toughness.