Multi-scale Structural Damage Mechanisms Of Three-dimensional Five-directional Braided Composite T-beam Under Bending Fatigue Loadings

Three-dimensional(3-D)four-step braiding technology is an effective way for manufacturing complex cross-section preforms and special shaped parts,such as 3-D braided T-beams.The three-dimensional braided T-beams can not only eliminate the joints and structural defects between the flange and the web,but also do not have the delamination failure mode in the long-term dynamic and static loading,which have the structure advantages that other kinds of T-beams do not have.This project focus on the bending fatigue behaviors of 3-D braided carbon/epoxy composite T-beams.The bending fatigue properties and S-N curves were obtained from the fatigue tests.The bending fatigue damage mechanisms were numerical analyzed from the until cell and the microstructure approach.Then the influence of T-beam geometries on the microstructure and the influence of component materials on the bending fatigue behaviors of the composites were revealed.In the experimental,the load deflection curves and damage morphologies were obtained by testing the quasi-static three-point bending behaviors of the composite.The stress level of cyclic loading was selected according to the peak load of the quasi-static three-point bending results.The three-point bending performance of the composite was tested under 80%,70%,60% and 50% stress level.The fatigue life curve(S-N curves),load deflection curves,stiffness degradation curves and the fatigue damage morphologies were obtained from the fatigue tests.We found that:(1)According to the S-N curves,the fatigue life increases with the decrease of stress level.the fatigue limit is 50% stress level.(2)The load deflection curves are obviously hysteresis,and the area of the hysteresis loops will continue to increase as the cyclic loading continues until the compositefails.(3)The stiffness degradation curves under different stress levels show typical ‘ three stages’ features.The damage mechanism of the composite is different at different stages.The matrix crack occurs on the flange at the initial stage,the resin-yarns interface debonding occurs at the middle stage,fiber breakages occurs in the web at the last stage.(4)The damage areas are both concentrated on the center of the flange and the web under quasi-static and fatigue loadings.However,the damage degree of the composite under fatigue load is more serious than that under quasi-static load.In the finite element analysis,five types of unit cell models,i.e.,interior cell,surface cell,corner cell,interior cell in joint region and corner cell in joint region were constructed according to the microstructure of the 3-D five-directional braided T-shaped composite preform from unit cell approach.Then the FEM model was established.The stiffness degradation curves,the deflection change curves,the stress and strain distribution under different stress levels were obtained from the FEA.The small size FEM model of 3-D five-directional braided T-shaped composite was established from the microstructure approach.The stress degradation and stress distribution of different materials in the composite,energy absorption,interface damage and the fatigue damage morphologies of the model were analyzed.We also found that:(1)The stiffness degradation curves and the deflection change curves show ‘three stages’ features from until cell approach,which are similar to the experimental results.The stiffness decreased sharply while the deflection greatly increased at the initial and the last stage.At the middle stage,the stiffness decreases slowly and the deflection increases slightly.(2)In the until cell model,the stress and the strain concentration areas are the center of the flange and the web.The stress distribution change is small but the strain distribution change is large.The strain changes dramatically at the last loading stage(98% fatigue life).(3)In the microstructure model,the trend of stress degradation in the yarns is consistent with the resin matrix.The stress concentration areas and the damage areas appear at the center of the flange and the web,and the restricted areas of the web.This project investigated the three-point bending fatigue properties of 3-D five-directional braided T-shaped composite by means of experiments and finiteelement analysis to reveal the fatigue damage mechanism of the composite.Such a methodology could be extended applied to the fatigue resistance structural design of other 3-D five-directional braided composites with complex cross section.