Research On Low-velocity Impact Mechanism Of Curved Multi-axial Warp-knitted Composites

The multi-axial warp-knitted composites(MWKC)are typical novel fiber-reinforced materials.As the enhancement structure of MWKC,the multi-axial warp-knitted fabric(MWK)is featured with non-crimp,axial symmetry,high orientation and smart designing.This structure can be utilized to fabricate ultra-thin and ultra-thick curved composites due to its characters of non-association and easy infiltration.During the procedure of manufacturing,assembling,transportation and repair,low-velocity impact is unavoidable.Meanwhile,since the MWKC always serves as the components with curved surface in the applications,the study on the low-velocity impact behavior is a significant topic,as well as the research focus of in the present paper.(1)The tension and tear tests for MWK were implemented to analyze the displacement and damage of the binding yarns and the axial yarns after loading.The resulted show that the main failure modes of MWK were binding yarn fracture,axial yarns breaking and tows displacement.The regulation of binding yarns damage could be found.Since the knitting orientation was consistent with 0o yarn layer,they always bear the load toghther with 0o yarn tows.However,because of the weak strength,the binding yarns always got broken before the damage of axial tows.Furthemore,the diaplacement of the axial tows followed certain rules as well.Each yarn layer played a typical role,but cooperate with each other as well.The study in this part lays a foundation for the later research on the low-velocity impact mechanism of curved multi-axial warp-knitted composites.(2)The combination of quasi-static impact and low-velocity impact was employed to strdy the failure modes and damage propagation of the curved MWKC with different curvatures and sequences.The main failure modes were matrix crack,yarn fracture,fiber pull-out,delemination and yarn debonding.One of the high-light point of this research was to empoly the Phase analysis method into the study of the curved MWKC to reveal the damage evolution.The relationship among stress propagation,damage and energy absorption was discussed.Moreover,the numercial simulation was achieved focusing on the two kinds of impact mechanism of the curved MWKC.Then the stress distribution and energy absorption capacity were discussed.The performance of the curved MWKC was eveluated by combining the research results obtained from experiments and simulation.(3)Curvature C(kC)was used in the designing of car door impact area.After geometrical model building and material setting,the FE simulation of curved MWKC car door under low-velocity impact was implemented.SAPH440 steel car door was utilized as the comparative item.Under the comparison,the advantages of curved MWKC car door was found,involved energy absorption,safety performance and environment protection.Through the tests,simulations,analysis and verification towards the low-velocity mechanism of curved MWKC,the theoretical foundation was well established for the application field explanation of this material.