Lateral Bending Behavior Analysis And Failure Mode Control Of Braided Composite Tube

In recent years,CFRP tubular braided composites are frequently used in the structural parts of the commercial aircraft fuselage,automobile bodies,sports equipment,and other fields,such as bumper bars,B-pillar,and other thin-walled structures.In traffic accidents,the frame structural parts are prone to bending deformation and failure due to lateral action.Under bending loads,tubular braided composites are prone to complex failure modes,such as matrix cracking,fiber fracture,and delamination,leading to significant deterioration of their properties.In the structural design of tubular braided composites,the braided Angle directly affects its mechanical properties,and the ring braided technology allows the design of a specific braided Angle for each layer of braided fabric,so the radial-gradient braided structure can be formed.However,there are few types of research on the bending response of this kind of structure.In this paper,the effects of different gradient structures on failure modes of tubular braided composites were studied to optimize the structural design and improve the flexural behavior.In this paper,two structures with different braiding angles are combined to study the influence of the two structures on the bending properties of tube fittings.Firstly,the carbon fiber braided composite tubes[~I40/~O60]and[~I60/~O40]with two gradient structures were prepared according to the different positions of the inner and outer layers of the tube fitting.Secondly,a three-point bending test was carried out on the tube fittings to obtain the load-displacement curve of the test,and the acoustic emission system was used to monitor the time when the damage occurred during the test.Then,micro-CT 3d scanning technology was used to detect the damage distribution inside the sample and analyze its failure mode.Finally,a macro finite element model was established for numerical simulation to analyze the influence of radial gradient structure on the failure behavior of braided composite tubes.The mechanical response curves and damage morphology show that the braided composite tube with a descending gradient can maintain better bending stiffness and energy absorption characteristics under bending load,while the braided composite tube with ascending gradient has a large area of delamination at the bottom under load,which reduces the bearing capacity of the tube.Although it is almost impossible to observe significant differences from the sample surface,the bottom stratification can be predicted by acoustic events of low frequency,high energy,long duration,and high amplitude.The numerical simulation results show that the gradient structure has an important effect on the bending resistance and failure mode of pipe fittings.The braid structure in contact with the indenter in the outer layer distributes the load along the braided yarn,so the fitting is prone to failure at the top,characterized by fiber compression damage in the contact area between the indenter and the tube.The outer layer of the[~I40/~O60]tube with a large braided angle transfers more load downward,resulting in large layers at the bottom,causing the load of the tube to drop sharply.In contrast,the outer layer of the small braided angle of the[~I60/~O40]tube mainly distributes the load along the axial direction,so that the damage is concentrated at the top and prevents stratification at the bottom.Therefore,the braided pipe fitting with a descending gradient can avoid the emergence of large area stratification at the bottom,and effectively improve the bending resistance and energy absorption of braided tubes.