Research On The Influence Of Axial Yarns Distribution Of Braided Composite Tube On The Axial Compression Damage Behavior

The application of carbon fiber composite materials to parts bearing axial loads,such as automobile A-pillar,landing leg of Falcon 9 rocket and the crash-box of CRH highspeed train,has brought many advantages: reducing inertia,emissions,braking distance and kinetic energy during collision.But the cost of carbon fiber composite materials is high,and the failure mode is difficult to control.Therefore,it is necessary to optimize the design of the reinforcement structure of carbon fiber composite materials to meet the requirements of saving material costs and improving collision performance while reducing weight.This article first discussed the preparation and experimental methods of inserting axial yarns into biaxial braided fabrics to form a two dimension triaxial braided composite(2DTBC)tube.Secondly,the geometric and constitutive models of triaxial braided composite material tubes were explored,and a quasi-static crushing finite element model was established.A combination of experiments and simulations was conducted to study the mechanical properties of triaxial braided composite tube surfaces under quasistatic crushing,and the role of axial yarns in this study was evaluated.In order to improve the energy absorption of composite tube fittings and explore the mechanism of axial yarn on damage modes,the following research work was carried out and completed in this paper:The influence of the number and distribution of axial yarns on the compression resistance of specimens was experimentally studied.Axial yarns were more sensitive to axial loads than braided yarns,as the initial stiffness of the specimens increases with the number of axial yarns.More axial yarns would increase the depth of damage,in other words,composite tubes can withstand more compression deformation without causing serious damage.Composite tubes with different quantities of axial yarns exhibit different failure modes: progressive folding,medium length folding,and unfolding.Typical folding appearances were found on three axial yarn specimen(T3),with several protruding corners close to the compression plate.In specimen T9,a convex angle similar to T3 exists near the compression side.However,there were major cracks that penetrated the entire tubes.It caused structural instability and insignificant SEA improvement.For specimen T12 and T18,the characteristic of the failure mode is strong fiber tearing and thus named as splaying mode.Usually,this failure mode is considered to help absorb more energy.Explore the role of axial yarns in the specimen through simulation methods.An algorithm for automatically generating geometric models using braided parameters and minimum repeat elements as input parameters was proposed.The deformation and stress distribution of 2DTBC tubes under quasi-static compression were simulated.The finite element model can effectively simulate the damage morphology of the specimen,providing a theoretical basis for the analysis of composite tube fittings.Combining infrared imaging and micro-CT reconstruction methods,the formation process and mechanism of structural damage leading to composite structure failure were studied.The correlation between the maximum infrared temperature and the peak load coud be used to roughly indicate the location of critical damage(matrix damage and yarn interface damage).In addition,infrared equipment could also observe that the damage area depth of triaxial braided tubes was greater than that of biaxial braided tubes.The damage area of the triaxial composite tube was related to the position of the axial yarn.The microstructure of the specimen is reconstructed into a three-dimensional model through CT reconstruction.The development of inter yarns damage dominates the overall damage morphology of the specimens.After the introduction of axial yarns,the initially separated braided yarn interfaces were interconnected,providing conditions for crack coalescence.At the same time,cracks caused by the buckling of the axial yarn and the competition between the braided yarn blocks could also occur at the junction of the axial yarn and the braided yarn.The above findings provided a theoretical basis and relevant guidance for improving energy absorption of carbon fiber braided composite tube fittings under axial compression behavior.