Damage Evolution And Statistical Characteristics Analysis Of Acoustic Emission Signals For Carbon Fiber Three Dimensional Braided Composites

Carbon fiber three dimensional(3D)braided composites are advanced composites by combining modern materials and braiding technology,which have been increasingly studied and used in numerous fields such as aviation,aeronautic,military,vehicle and energy industries.This is due to a series of excellent mechanical properties like high specific stiffness and high specific strength,favorable damage tolerance,outstanding impact and fatigue resistance,high load-bearing capacity and low delaminating tendency than the traditional laminate composites.In this study,the tensile behaviors and damage characteristics of 3D braided composites with different preform structures,braiding angles and sizes were investigated systematically.Furthermore,the acoustic emission(AE)signals and surface deformation fields information of 3D braided composites were monitored by AE and digital image correlation(DIC)complementary experimental system.Meanwhile,the lead break tests were performed several times to explore the AE propagation and attenuation characteristics of3 D braided composites in different directions.Moreover,the cluster and statistical analysis methods were proposed to process and analyze AE signals.Besides,the internal microstructure and damage characteristics of the fractured 3D braided composites were observed by micro-computed tomography(Micro-CT)technology.And some important conclusions are drawn as follows:3D braided composites have different load-bearing capacity under various preform structures and braiding angles.It can be found that 3D five directional braided composites possess excellent load-bearing and deformation resistibility capacity with additional axial reinforced yarns as compared to 3D four directional braided composites.Meanwhile,the 3D five directional braided composites with smaller braiding angles own more compact space structures,and the load-bearing capacity improved with decreasing the braiding angles.3D braided composites have anisotropic characteristics,which results in the attenuation of AE amplitude along the braiding direction is significantly lower than that of otherdirections.Besides,the further distance between the lead breaking point and AE sensors,the greater AE amplitude attenuation.In addition,the propagation and attenuation characteristics of AE signals in 3D braided composites provide an important reference for further AE detection and research in tensile tests.The relationship between AE characteristics and damage factors of 3D braided composite is extracted,according to the dynamic variability of the typical AE parameters such as amplitude and cumulative hits.Moreover,the AE signals collected during the tensile tests are processed by FCM algorithm,and the AE signals of 3D braided composites are divided into two clusters.The CLUS 1 with low frequency corresponds to interface damage and matrix cracking,while the CLUS 2 with higher frequency represents fiber and matrix debonding and fiber damage.The damage evolution process of 3D braided composites is closely related to dynamic response behaviors of the corresponding AE signals.Meanwhile,the statistical analysis of AE signals with amplitude spectrum,normalized probability distribution and probability entropy can be used as criterion quantitatively to distinguish and quantify various types of irreversible damage and failure mechanisms in 3D braided composites.The complementary non-destructive testing(NDT)technology combining AE and DIC can monitor the damage evolution behaviors of 3D braided composites from inside to outside,which provides an important reference for the safety evaluation and health monitoring of carbon fiber 3D braided composites.Besides,the Micro-CT technology can be used to observe the internal microstructure information and damage morphology of 3D braided composites,providing novel ideas for the further study of damage and failure mechanisms in braided composite systerm.