Study On Mesostructure And Mechanical Properties Of Weft-knitted Biaxial Multi-layer Yarn-bonded Fabric Reinforced Composites

Multi-layer-connected Biaxial Weft Knitted(MBWK)fabric-reinforced composites have been used widely in the field of transport,plane wing,pilot helmet and medicine;the European Union and USA have focused on the development of MBWK fabric-reinforced composites for decades.Excellent in-plane mechanical properties and favourable formability on 3D curved surface make their application field wide,thus the research needs of the geometric structure and mechanical behavior for MBWK fabric-reinforced composites are increasing.The warp yarns and weft yarns have been bound together by the stitching yarns in the MBWK fabric-reinforced composites,the existence of stitching yarns have improved the interlaminar properties.The delamination between warp-yarn layer and weft-yarn layer in the MBWK fabrics has been inhibited effectively.In view of this condition,the elastic performance of MBWK fabric-reinforced composites has been predicted based on mesoscopic structure,which has combined with the classical laminated plate theory.According to the gradually deeping of the"layer-stack-whole",the overall theoretical predicted precess has been developed from the elastic properties of undirectional fiber reinforced composites to the elastic properties prediction of RVE to the elastic properties of the MBWK fabrics reinforced composites via the classtical laminated plate theory.The elastic prediction processes have been completed in three steps.(1)The RVE unit has been decomposed according to fiber bundle orientation in the MBWK fabric-reinforced composites.The warp subunit structure,the weft subunit structure and the stitching sununit structure have been deterimined.(2)The warp subunit structure and the weft subunit structure have been regarded as unidirectional composites;the stitching sununit structure has been regarded as composite that included infinite infinitesimal sections.The infinitesimal sections have been regarded as unidirectional composites.Through establishing the predictive model of sununit structure,the elastic constants of one set MBWK fabric reinforced composites have been determinded.(3)According to certain order of multiple sets of MBWK fabrics in the composites,the whole elastic constants of MBWK fabric-reinforced composites have been determinded by classical laminate thery(CLT).The prediction model has been verified by uniaxial tensile experiments of the MBWK fabric-reinforced composites.The results indicate that there are good consistency between the experimental data and predicted results,which proved the veracity of the predicted model.Secondly,in order to verify functions of stitching yarns in the composites,the MBWK fabric reinforced composites with polyester stitching yarns and aramid stitching yarns have been tested using three point bending test and interlaminar shear test,respectively.In these tests.Digital Image Correlation(DIC)system were used to gain the longitudinal strain(?11)and the interlaminar shear strain(?13)contours of the materials,which qualitatively shown that the interlaminar properties of the materials with aramid stitching yarns were much better than that with polyester stitching yarns.Finally,through the design of inserted yarn combination,the MBWK fabric-reinforced composites with aramid/carbon hybrid fibers have been carried on the tensile and bending properties tests,and the results indicated that the tensile and bending properties improved with the increasing of carbon fiber content,which showed obvious hybrid effects of composites.The positive role of the stitching yarns in the MBWK fabric-reinforced composites and the fiber hybrid effect on the MBWK fabric-reinforced composites were discussed,thus we can comprehensively discuss the broad application of the MBWK fabric-reinforced composites.