| Due to the wide application of carbon fiber braided composites in aerospace,automobile and industry area,However,damage mechanism of braided composite is complex.Understanding the damage evolution of carbon fiber braided composites in real time is the most important aspect with respect to safe operation and service life.In this paper,the damage of carbon fiber braided composites,carbon / glass hybrid plain weave composites and carbon fiber braided composite indentation specimen were monitored in real time by acoustic emission(AE)and digital image correlation(DIC).The acoustic emission signal,the displacement field and the strain field are recorded by this complementary detection technique during the damage of the specimens.In addition,the fracture behavior of the specimen was measured by scanning electron microscope(SEM),and the strain and displacement of the specimen were analyzed by finite element analysis(FEA).The damage process of carbon fiber braided composites is divided into three stages: initial stage,damage accumulation stage and damage failure stage.The load-time curves and the stress-strain curves of the carbon fiber braided composites are basically linear.In the damage stage,the 80-100 dB high amplitude signals and the acoustic emission signals with the peak frequency of 300-500 kHz are obviously increased.The acoustic emission signals with the peak frequency of 200-300 kHz and the amplitude of 50-70 dB is related to matrix damage.The acoustic emission signals with the peak frequency of 300-500 kHz and the amplitude of 80-100 dB is related to fiber breakage.The displacement field and strain field of the specimen are measured by DIC.For the same load increment,the displacement tends to decrease gradually and the maximum tensile strain shows the trend that increase and then decrease during the damage process of the carbon fiber braided composite specimen.The finite element analysis method was used to simulate the damage of the specimen during tensile test.The results were in good agreement with the experimental results of strain-strain curve and DIC results.However,the damage of carbon / glass hybrid plain woven composites is divided into two stages: damage accumulation stage and failure stage.The amplitude of the acoustic emission signal is roughly the same as that of the carbon fiber braided composite.The displacement and strain field changes in the direction of carbon fiber are the same as those of carbon fiber braided composite and the displacement field of the glass fiber shows the trend that decrease and then increase by digital image correlation method.According to the finite element analysis,it is found that the displacement field and the strain field distribution obtained by numerical simulation are in good agreement with the DIC measurement result and the stress-strain curve.The process of indentation damage is divided into two stages: damage accumulation stage and failure stage for the indentation of carbon fiber braided composites.The 50-70 dB low amplitude signal is the main stage in the damage accumulation stage,and the 80-100 dB high amplitude signal is obviously increased during the failure stage.The compression process of the braided composite indentation specimen is monitored by the digital image correlation method.The displacement field and strain field of the specimen are analyzed to show the delamination damage process.Combined with the specimen fracture diagram,it is found that the delamination damage is related to the degree of indentation damage.The combination of acoustic emission technology,digital image correlation method,scanning electron microscopy and strain measurement technology is an effective complementary detection technology.Combined with finite element analysis,this technology can effectively describe the deformation and damage evolution of braided composites to analyze the failure mechanism.In addition,the combination of AE,DIC,SEM and FEA can detect early damage signs and damage evolution in structural health monitoring system and contributes to the structural health monitoring of carbon fiber braided composites. |
