Study On Wood Damage Identification Based On Acoustic Emission Technology

Wood is an environmentally friendly production resource with a wide range of applications in furniture engineering,interior decoration and wood-frame construction.However,during the actual processing and use of wood,various kinds of damage will inevitably occur,resulting in a significant decline in the physical and mechanical properties of wood,reducing its use value.The non-destructive dynamic evaluation and monitoring of the health status of wood has been one of the hot research topics in wood and wood material damage detection.Acoustic Emission Technology(AET)is an applied science and technology to objectively assess the "health" of material structures based on Acoustic Emission(AE)source information without damaging the test object and material properties.The use of AET enables dynamic monitoring of the stress changes in the material in real time and thus effectively evaluates the damage of the material,which provides a proven method to identify damage and study the evolution of property degradation of wood.In this paper,common Pinus sylvestris var.Mongolica Litv and Zelkova schneideriana were used as the research objects,and the multi-channel AE signal acquisition system constructed by NI acquisition equipment and Lab VIEW software was used to obtain AE signals of the specimens under different damage conditions.The AET theory was used to investigate the AE characteristics of wood surface crack damage and dynamic damage process,and then to characterize and evaluate the damage and performance status of wood.The main research and results of the paper are as follows.(1)To address the problem of AE signal acquisition and storage with a long time span generated by wood in the damage process,the paper builds a 4-channel AE signal hardware acquisition platform based on NI USB-6366 high-speed data acquisition card and AE sensors and other peripherals,and uses Lab VIEW software to design the human-machine interface and software control system.The results of three-point bending test on Cunninghamia lanceolata show that the multi-channel AE signal acquisition system developed in the paper can effectively acquire and automatically store the AE signal waveforms of Cunninghamia lanceolata throughout the damage destruction process.As a wood AE signal acquisition platform,the acquisition system can provide basic assurance for the acquisition and analysis of wood AE signals.(2)In the study of the generation of simulated AE source signal of wood and its acoustic characteristics,the paper adopts breaking the lead core and thin wood strips on the end face of wood to generate simulated AE sources,respectively,and investigates the propagation characteristics of AE signals generated by different simulated AE sources on the surfaces of camphor pine and beech specimens through wavelet processing and signal correlation analysis.The results show that there is no significant difference in the frequency distribution characteristics of the signals generated by the two AE sources.The results show that there is no significant difference in the frequency domain characteristics of the signals generated by the two AE sources.In terms of propagation velocity,the average propagation velocity of AE signals generated by fracturing thin wood strips on the end faces of Pinus sylvestris var.Mongolica Litv and Zelkova schneideriana test-pieces were 786.5,1085.1 m/s and 1145.2,1557.6 m/s at the early and late stages of fracture,respectively,while the average velocities of AE signals generated by fracturing lead cores were 760.9 m/s and 1120.5 m/s in Pinus sylvestris var.Mongolica Litv and Zelkova schneideriana test-pieces,respectively.According to the propagation velocity and frequency domain characteristics,the simulated AE source generated by breaking the lead core can better reflect the frequency characteristics of the AE signal generated by wood at a specific damage stage,but the method of using impact force to break thin wood strips proposed in this paper is more objective when studying the AE signal propagation law of wood.(3)In order to explore the AE discrimination of wood surface crack damage,three different sizes of crack damage were artificially created on the surface of Pinus sylvestris var.Mongolica Litv test-pieces,and the propagation law of AE signal on the surface of camphor pine specimens was studied by lead break test.And repeat experiments were conducted using test-piece with natural crack damage to further investigate the effect of actual crack damage on the wood surface on the AE signal propagation characteristics.The results show that the main frequency component of AE signal increases with the increase of the depth of artificial crack,but the crack depth has no significant effect on the propagation velocity of AE signal.The attenuation of AE signal velocity along the same direction is 11.9%,6.0%,17.3%,and 10.1% for four test-pieces,respectively,indicating that there is no clear linear pattern between the artificial crack depth and the change of AE signal propagation velocity.In the natural crack damage test-piece,the AE signal propagation velocity is not significantly different from that of the artificial cracked test-pieces,and its spectral characteristics are closest to those of the test-piece with 6-mm deep cracks.The results provide a nondestructive detection method of wood surface cracks based on the difference of AE propagation velocity,which is of reference value for the identification of natural crack damage on wood surfaces.(4)To address the problem of damage identification and evaluation of wood during bending damage,the paper proposes a wood damage identification method based on the instantaneous frequency of the AE signal generated during the release of strain energy of the material,and dynamic monitoring of wood damage based on the density statistics of AE events.The results show that three types of AE signals are mainly generated during the damage of wood: Deformation AE(DAE)signals,Micro-fracture AE(MAE)signals and Fracture AE(FAE)signals.The frequency range of DAE signal is 30 k Hz?60 k Hz,that of MAE signal is 90 k Hz?115k Hz,and that of FAE signal is 115 k Hz?135 k Hz.By counting the occurrence density of the three AE events in the frequency domain,the stress level of the material can be better reflected,so as to objectively evaluate and predict the degree of wood damage and the damage trend.