The Study Of Damage And Failure Of Braided Composites Based On Mirco-CT And Acoustic Emission Technique

Although traditional two-dimensional braided composites have excellent characteristics such as high specific strength and high specific modulus,and are widely used.However,due to their layered structure,in the long-term service process,there are often occur cracks between layers,causing structural safety risks.To address this issue,a new type of three-dimensional braided composite,which is enhanced along the thickness by combining three-dimensional braiding technology with modern composite material,has emerged and is favored in the field of structural engineering because of its integrity and designability.However,due to the complexity of the three-dimensional braided structure,many factors affect its mechanical properties.Among them,the damage mechanisms of the three-dimensional braided composite materials with different braided structures,different braided angles,and different bearing directions is different to some extent,so there is still a lack of systematic analysis on this problem in current research.Therefore,the study of mechanical properties and damage mechanism of braided composites under bending load,especially the analysis of 3D braided composites and progressive damage process,is of great significance to ensure the structural health of composites in service.This paper first conducts attenuation tests on two types of sensors.Acoustic Emission(AE)and Digital Image Correlation(DIC)were used to monitor the carbon fiber plain-weave composite laminate test piece in real time under the four-point bending loading conditions,and the damage evolution mechanism of composite during bending loading was deeply understood.Secondly,using this complementary technology to monitor the damage evolution and deformation process of three-dimensional braided composite materials with different braided structures(three-dimensional four-direction and three-dimensional fivedirection)and different braided angles(20 ° and 30 °)under three-point bending loading.The micro-computed tomography(mirco-CT)was used to observe the internal characteristics and fracture damage of composite samples.Thereafter,the progressive bending damage test was performed on the longitudinally loaded specimen and the transversely loaded specimen,respectively.Simultaneously,the changes of the AE signal generated by the damage are acquired in real time,and tomography(micro-CT)technology is used to identify the initiation / evolution and distribution characteristics of the internal damage of the 3D braided composite material.After that,the performance characterization and damage monitoring of the plastic liner full-wound composite gas cylinder were performed by AE technology,and the obtained academic knowledge and testing skills were further applied to engineering practice.The above research results are summarized as follows:(1)The AE signal on the surface of the carbon fiber braided plain weave composite material has an exponential decay characteristic.With the increasing load of the specimen,the counts of the AE signal increased sharply,the relative energy gradually increased,high amplitude and high duration signals appeared,and the corresponding displacement and strain fields also changed significantly,clearly showing the state of the loading and the form of the force at different locations.(2)The bending load-deflection curve of three-dimensional braided composites is similar to linear when initial damage occurs,but non-linear after cracks occur.Compared with threedimensional four-direction braided composites,three-dimensional five-direction braided composites have a large number of AE signals with amplitudes of 70 to 100 dB.In addition,the three-dimensional five-direction-20 ° specimen has more high-frequency signals than the threedimensional five-direction-30°specimen.Combining the changes in the strain and displacement fields,it can be found that the bending mechanical properties of the threedimensional five-directional braided composite material are significantly enhanced due to the addition of axial yarns.More fiber bundles can bear the load,and the carrying capacity is stronger.(3)The internal characteristics and fracture damage of 3D braided composite materials are observed from Mirco-CT tomographic images.Near the damage center of the specimen,matrix cracking,fiber debonding,and delamination between and within layers are observed.The observation results of microscopic damage morphology are in good agreement with AE characteristics and out-of-plane deformation,and the damage mechanism and material properties of 3D braided composites with different braided structures and angles are verified.(4)From the bending load-deflection curves of the longitudinally and transversely loaded test specimens,it can be seen that the failure load and bending strength of the longitudinally loaded test specimens are much greater than those of the transversely loaded specimens.According to the distribution characteristics of acoustic emission parameters such as energy,count,amplitude,duration and frequency,the evolution of progressive damage of the two specimens can be reflected.It can be found that the damage resistance of the longitudinally loaded specimen is better than that of the transversely loaded specimen.Moreover,the presence of the Kaiser effect(KE)is also observed.(5)Micro-CT is used to observe that the progressive failure mechanism of the longitudinally and transversely loaded specimens is significantly different.For longitudinally loaded specimens,slight cracks and void defects are found in the initial stage,and then the void defects evolve into cracks,and the original cracks further expand.After the last loading,a distinct bending band is formed and some serious damage occurred,such as tow debonding and fiber breakage.For transversely loaded specimens,more cracks appeared after the first load.Subsequently,significant matrix fracture is observed in the region of tensile stress.In particular,the shear fracture of the fiber in the thickness direction is found.Finally,debonding of the tow,cracks propagated in the width direction and delamination are observed(6)AE technology can perform real-time monitoring of the damage of plastic liner fullwound composite gas cylinders,verifying the effectiveness of AE technology in engineering applications for the safety inspection of special equipment and containers with composite materials as the main structure.Using the distribution characteristics of AE signals,they can be subjected to health evaluation and failure analysis.