Fine Measurement And Numerical Simulation Of Deformation And Internal Damage Of Carbon Fiber Braided Composites

Compared with traditional metal materials,carbon fiber braided composites have many outstanding mechanical properties,such as high strength,high specific modulus,high damage resistance,light quality,which are used as the material of load bearing part.However,due to the complexity of composites structure,its mechanical properties will be affected by the original defects caused by process limitations and the mechanical bearing direction.In order to avoid accidents caused by the decrease of load-bearing capacity of composites in service,it is of great significance to study the influence of voids on mechanical behavior and damage characteristics of composite materials and the mechanical behavior and damage characteristics of three-dimensional braided composite materials under transverse tensile loading..Firstly,on the basis of vacuum assisted resin transfer molding process,different external pressure was applied to change the void content of composites specimens.Besides,Micro-Computed Tomography(Mirco-CT)was used to observe the internal structure and calculate the void content of the specimens.At the same time,Acoustic Emission(AE)and Digital Image Correlation(DIC)are used to collect the AE signals and deformation field information of plain weave carbon fiber braided composites with different porosity during the process of three point bending.Furthermore,the specimens with the highest and the lowest void content are chose for progressive loading experiment.The damage morphology and voids evolution are observed by Mirco-CT.Secondly,the transverse tensile damage behaviors and mechanical properties of three-dimensional five-directional braided composites is studied.The transverse tensile process of specimens is simulated by Finite Element Method(FEM).Besides,AE,DIC and Infrared Thermal imaging(IRT)are used to monitor the transverse tensile damage behavior of three-dimensional five-directional braided composites specimens in real-time and the fracture morphology are observed by Scanning Electron Microscopy(SEM).In addition,the obtained knowledge and detection methods were applied to engineering practice,and AE and IRT were used to study the tensile damage behavior of type IV gas cylinder liner materials under different loading rates.Following conclusions are summarized from research results:(1)Applying additional pressure lower than 0.3 MPa can reduce the porosity of composites during vacuum assisted resin transfer process,but additional pressure higher than0.3 MPa can reduce the flow rate of resin,inhibit the resin from impregnating carbon fibers,and lead to the increase of porosity content.For the specimens prepared in this paper,the porosity can be reduced to 0.1145% and the bending strength can reach 414.420 MPa when0.3 MPa is applied.If the vacuum assisted resin transfer process is not completely sealed,the porosity will greatly increase to 21.34%,and the bending strength will greatly decrease to305.172 MPa.(2)At the process of the the bending test,it is found that the kind band along the width of the specimens occurs at the parts contacting loading head,and various damages including matrix cracking,interface damage and fiber damage are concentrated near the kind band.(3)The damage modes of three-dimensional braided composites under transverse tensile test are classified into matrix deforming and cracking,fiber debonding and shear fracture of fiber.The stress and strain distributions calculated by FEM can effectively predict the damage behaviors.The damage and fracture processes of three-dimensional braided composites under transverse tensile test can be divided into four stages: undamaged stage(corresponding to hardly any acoustic emissions signals),initial internal damage stage(producing several AE signals below 60 dB),damage developed stably stage(relating to a number of signals with the amplitude under 60 dB)and damage developed rapidly stage(correlating with plenty of signals with amplitude over 80 dB accompanied by heat increasing 1.5 ?).(4)There is no obvious change in the load displacement curve of the inner liner material of type ? gas cylinder at the loading rates of 4 mm/min and 20 mm/min.When the loading rate increases to 40 mm/min,the maximum load of the specimen rises and the ultimate failure displacement reduces,and the toughness of the specimen decreases.