木材裂纹尺寸及分布对声发射横波传播特性的影响研究

Effects of Wood Crack Size and Distribution on the Transverse Wave Characteristics of Acoustic Emission

为探究木材表面裂纹尺寸与分布对声发射横波(transverse wave component of acoustic emission,AE TW)传播特性的影响,通过锯切方式制作不同尺寸的裂纹,研究沿木材顺纹理方向传播的声发射横波的传播特性。首先,在试件表面锯切出4条具有相同长度而宽度与深度不同的规则裂纹,分别采用铅芯折断和信号发生器模拟产生突发和连续的AE源,并且通过等间距分布在试件表面的5个传感器采集AE信号,采样频率设置为500 kHz。然后,在铅芯折断试验的基础上,依据驻波成分的基频,对原始AE信号进行高通滤波处理,截止频率设置为7 kHz,再采用相关性分析的方法确定AE TW在固定距离上的传播时差,进而依据时差定位的原理计算其传播速度。最后,通过信号发生器产生不同电压等级的150 kHz脉冲串作为AE源,研究不同裂纹尺寸下的AE TW能量随距离的衰减规律。结果表明,在无裂纹试件中,AE TW的平均传播速度为824.4 m·s^(-1)。而仅改变裂纹深度时,随着裂纹数量的增加其速度分别从1216.5、1171.1、1220.8 m·s^(-1)减小到913.8、820.6、885.2 m·s^(-1)。无裂纹试件能量衰减速率|K|为2.98,随着裂纹尺寸的增加,衰减速率呈现减小趋势,裂纹尺寸与分布不影响其能量衰减规律,且当能量初始值不同时,所表现的衰减规律也相同,其AE TW速度和能量变化均能反映裂纹的存在。

To investigate the effects of wood surface crack size and distribution on the propagation characteristics of transverse wave component of acoustic emission(AE TW),the propagation characteristics of AE TW propagating along the wood texture direction were studied by making cracks with different sizes.Firstly,4 regular cracks with the same length and different widths and depths were making on the specimen's surfaces.The burst and continuous AE sources were simulated by pencil lead breaking test and signal generator,and the AE signals were collected by 5 sensors with equal spacing on the specimen's surfaces,and the sampling frequency was set to 500 kHz.Then,based on the pencil lead-breaking test,the original AE signal was high-pass filtered according to the fundamental frequency of the standing wave component,and the cutoff frequency was set to 7 kHz.The correlation analysis was adopted to determine the propagation time differences of AE TW at a fixed distance,and the propagation speed was calculated according to the principle of time difference localization.Finally,the decay law of AE TW energy with distance for different crack sizes was investigated by generating 150 kHz pulse strings of different voltage levels as AE sources passing through a signal generator.The results showed that the average propagation velocity of AE TW was 824.4 m·s^(-1)in the crack-free specimens.While changing only the crack depth,the velocity decreased from 1216.5,1171.1,1220.8 m·s^(-1)to 913.8,820.6,885.2 m·s^(-1)with increasing number of cracks,respectively.The energy decay rate|K|of the crack-free specimen was 2.98,and the decay rate decreased with the crack size.The crack size and distribution did not affect the energy decay law,and when the initial value of energy was different,the decay law exhibited was the same,and the AE TW velocity and energy change reflected the existence of cracks.