| Epoxy resin matrix composites have higher strength and stiffness compared with traditional materials,which have been used in national defense,petrochemical,civil transportation and other aerospace fields in recent years.However,the damage mechanism of composites is one of the main problems restricting their utilization.The appearance of invisible internal defects will reduce the stiffness of composite materials,and affect the strength and service life of them.Therefore,it is necessary to monitor,assess and analyze the propagation of different types of damage caused by static loads,cyclic loads and environmental effects.Accurate assessment and prediction of damage mechanisms are essential for the design and implementation of composite systems.Acoustic emission(AE)technology is an innovative method to detect and identify the damage mechanism of epoxy resin matrix composites.The damage evolution behavior inside the material is analyzed from AE parameter diagram,waveform FFT spectrum diagram and location diagram.Matlab is used to decompose the waveform signals at 5layers of Daubechies wavelet basis,and the signal characteristics corresponding to different frequency segments are obtained,which are corresponding to specific damage mechanisms.(1)Carbon nanotubes/epoxy resin composites were prepared to study the application of AE detection technology in the detection of composite structure manufacturing process.Epoxy resin casting was prepared at different curing temperatures and CNTs composites with different contents were prepared,and the effect of AE on acoustic wave propagation inside the materials was investigated.AE parameter diagram and spectrum analysis show that the energy of AE signal collected by epoxy resin at 140 °C is higher than that at 110 °C,and the attenuation time is longer.AE signal amplitude decreases by 68.01% with the increase of CNTs addition amount.The energy value and rising time of AE signals measured are 47.43% and 4.31% higher at the AE signal detection point near the distance.Mechanical results show that the strength and elongation at break of epoxy resin are increased by 19.96% and 16.93% with the addition of CNTs,respectively.The signal frequency of epoxy resin matrix damage is concentrated in(130-190)k Hz.The results indicate that AE parameters and distance attenuation tests have certain applicability in the detection of composite materials,and the failure frequency range of matrix can be used in the subsequent damage pattern recognition.(2)Glass fiber/epoxy resin composites(GFRP)were prepared to study the application of AE detection technology in the study of composite properties.Taking epoxy resin and GFRP widely used in Shengli Oilfield of Sinopec as the research object,the AE signal amplitude attenuation can reach 42.53 d B and 46.75 d B,and the sound velocity decreases by 13.92%and 17.03% after UV aging and high temperature and high pressure mineralized water aging.As the aging time increases,the proportion of signals with amplitude of 70-85 d B and above 85 d B in the failure of the specimen decrease,and the signals with amplitude of 50-70 d B increase,indicating that the matrix damage is serious,and the increased crack growth leads to the increase of signals generated by the matrix damage.The reliability of AE detection method in evaluating the aging residual life of GFRP is proved by tensile test of in-service tubing samples.(3)Carbon fiber/epoxy resin composites(CFRP)were prepared to research the application of AE detection technology in structural integrity detection of composites.Compared with GFRP laminates,the maximum attenuation value in CFRP laminates is only 22.3 d B,and the maximum sound velocity can reach 6263.16 mm/ms.The damage evolution process and types in the experimental failure process of carbon fiber frame fatigue and front fork fatigue after impact are distinguished by the impact number,energy,duration and other parameters,so as to provide acoustic basis for further study of the failure mechanism.The frequency domain waveform FFT diagram and wavelet transform diagram are consistent with the damage characteristics of AE parameters.The test results are verified with the Ansys simulation results,indicating that AE technology is an important means to monitor the integrity of materials. |
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