Study On Lateral Crushing Damage Mechanism Of Biaxial And Triaxial Braided Composite Tubes

Carbon fiber braided composite tubes have gradually become the key structural material in automobiles,drones,aerospace and other fields.In order to meet the weight reduction requirements of load-bearing components,it is urgent to design more lightweight structures with excellent energy absorption characteristics.The braided composite tubes are prone to crushing loads in the process of use,resulting in complex internal damage and degradation of material properties.The axial stiffness,strength and damage resistance of two-dimensional triaxial braided composite tube are enhanced by the introduction of axial yarns on the basis of traditional two-dimensional triaxial braided tube.However,due to the complexity of braided structure,the flexural properties of the triaxial braided composite tube is rarely analyzed,especially the progressive damage behavior is not fully understood.Therefore,it is of great engineering significance to study the damage mechanism of biaxial and triaxial braided composite tubes during lateral crushing.In this paper,circular braiding device was used to prepare biaxial braided tube（Z₀）,triaxial braided tube(Z₁₈),biaxial and triaxial mixed structure braided tubes（Z₉up and Z₉down）by setting different number and position of axial yarns.Then,the tubes were cured by vacuum assisted resin transfer molding technology.The lateral crushing of braided composite tubes was studied by three-point bending test,and the temperature distribution of the tubes was monitored in-situ using infrared thermography,and the bending deformation and damage evolution trend of specimen were clearly observed.The Abaqus mesoscale simulation models for lateral crushing of biaxial and triaxial specimens were also established to explore the impact of axial yarns on the fracture morphology and bending damage evolution of braided composite tubes.Especially,an analytical method was proposed to correlate the change of infrared maximum temperature with the damage mechanism.Combined with load-displacement curve and infrared temperature rise-time curve,the critical moments of initial damage fluctuation and final failure are selected.The experiment was terminated at the stress levels corresponding to these key points,the relevant damage locations of the specimen were cut,and the damage morphology of the cross-section was photographed with an ultra-depth of field microscope.The influence of axial yarn on the initial damage of specimens was verified by analyzing infrared thermal imaging characteristics and cross-section damage morphology.The progressive damage evolution and failure mechanism of biaxial and triaxial braided composite tubes subjected to lateral crushing were expounded.The results show that the axial yarns enhance the bearing stress and energy absorption capacity of the braided composite tube,greatly improves the bending stiffness of the specimen,and the more the axial yarns,the greater the bending stiffness.Infrared thermography technology can be used to predict the location of damage failure and monitor the strength of damage and destruction.The change of infrared maximum temperature has a good correlation with the damage initiation and failure mechanism of specimens.The abrupt changes of maximum temperature successfully indicate the occurrence of fiber breakage,especially the axial yarn breakage.In the process of three-point bending,the fibers at the top of the specimen are damaged by compression and extend downward along the circumferential spiral of the inner layer;the outer fibers on the left and right sides are stretched and spread towards the inside of the lower half of the tube.If there are axial yarns on the compression side of the tube,the maximum temperature increases by 0.5℃in the linear elastic stage,and two initial damages of debonding and fiber breakage will occur.The debonding started at the interface of the axial yarn.The axial yarns increased fiber crimp and expanded the axial damage range,resulting in obvious wavy delamination between layers of the specimen.In addition,the biaxial and triaxial hybrid structures with axial yarns only on the compression side can inhibit damage from spreading down the braided tube wall,which meets the requirements of lightweight and shows better damage resistance performance.