Research On Recognition And Evaluation Method Of Damage Modal Acoustic Emission Of Fiber Reinforced Composites

In recent decades,Fiber Reinforced Composites(FRC)have been widely used in many fields because of their excellent properties.However,because the damage mechanism of FRC is complex and diverse,including a variety of damage forms,especially in the process of use,many kinds of damage failures mix and overlap with each other,which causes many difficulties to the damage detection and evaluation of materials.Acoustic emission detection technology has been widely used in FRC damage detection,but the traditional acoustic emission signal analysis methods are often difficult to accurately extract the waveform details directly related to the sound source mechanism,and the damage identification rate is low.Therefore,the research on the corresponding relationship between the damage physical mechanism and the acoustic emission source and the extraction of damage modal features from the sound source signal waveform are of great significance to the realization of FRC damage identification and damage evaluation.In this paper,based on Lamb wave theory,typical OP and IP excitation sources were simulated on the upper surface and side of 7057-T6 aluminum plates and composite laminates by lead-breaking excitation.Combined with the waveform characteristics of acoustic emission signals and the energy spectrum of Continuous Wavelet Transform,the propagation characteristics of Lamb waves in different materials were analyzed and the modal characteristics of typical sound sources were studied.At the same time,the S₀ and A₀ modes of acoustic emission signals were separated by Empirical Mode Decomposition and Digital Filter respectively.Secondly,based on the FRC damage mechanism,the control experiments of fiber fracture,matrix cracking and delamination were designed,and the main modal characteristics and frequency distribution of the three damage signals were analyzed,which verifies the rationality of the frequency range of different filters set in the modal separation method.Based on this,the corresponding damage type of signal was determined by comparing the amplitudes of different modal component waveforms,and an algorithm for FRC damage identification was established.Its identification accuracy in three single FRC damage signals is more than 82%.At the same time,its effectiveness in FRC mixed damage identification was verified,which shows that the algorithm has a good ability to characterize the damage process of FRC complete structure.Finally,based on the waveform characteristics of typical S₀ and A₀ modal signals,the characteristic parameter Window Mean Square(WMS)voltage for extracting damage information from modal components of FRC damage signals was constructed.Through the analysis of the acoustic emission signals of three kinds of single damage,it is preliminarily shown that WMS is more sensitive to the change of material damage,and the violent oscillation of WMS value can forewarn the serious damage in the material.In addition,the graded loading test of NOL ring was designed to study the evaluation method of FRC damage Modal Acoustic Emission(MAE).Firstly,the FRC damage identification algorithm was used to analyze the distribution of three typical damage signals generated by the NOL ring in different stages,and the changing trends of the three damage signals in different stages were compared.The results show that matrix cracking and fiber fracture are the main damage forms in NOL ring,and with the increase of load,the proportion of matrix cracking signal in the three damage types gradually decreases from 99.35%to 62.65%,while fiber fracture increases from 0.21%to 36.81%,indicating that the damage severity of NOL ring is gradually increasing.Secondly,the WMS parameters of three kinds of damage signals obtained by damage identification algorithm were extracted and analyzed.By comparing the changes of WMS values of different damage signals at different stages,it is found that WMS values generally oscillate violently at about 90%of the fracture load of materials,and three rules for evaluating the damage state of FRC were summarized.Finally,it is found that the combination of FRC damage identification algorithm and WMS parameter analysis can initially establish a FRC damage MAE evaluation method,which can effectively evaluate any damage type evolution process in FRC,which lays a good foundation for the establishment of FRC damage MAE evaluation system.