基于瞬时频率的木材声发射事件辨识与损伤监测

为了从能量的角度客观评价木材损伤程度，依据应变能释放产生的声发射（AE）信号特征，提出一种基于信号瞬时频率的AE事件辨识方法，并依据AE事件的发生密度从能量的角度进行损伤动态监测。采用小波分析的方法对原始信号进行降噪预处理，用经验模态分解（EMD）进行波形重构。依据AE信号的频率分布特征将AE信号分为2类，并对应地定义DAE和FAE 2类AE事件。通过Hilbert变换获得AE信号的瞬时频率，依据瞬时频率计算2种AE事件发生的密度，依此判断和预测木材的应力变化及损伤趋势。结果表明，FAE信号频率明显高于DAE信号频率，并且当DAE事件密度维持较高水平时，意味着材料以弹性形变为主；相反当FAE事件密度维持较高水平时，说明材料正在发生频繁的断裂，从而为木材损伤程度的研判提供评价依据。

Wood Acoustic Emission Event Identification Based on Instantaneous Frequency and Damage Monitor

In order to evaluate damage state of wood from the aspect of strain energy,acoustic emission(AE)signal released by strain energy was used to monitor stress level of wood.Specifically,AE events were redefined according to instantaneous frequency of AE signals.And then,the density of AE events was calculated to estimate the damage degree.Firstly,a four-channel signal acquisition system was built for AE signal collection and its software was programmed in LabVIEW.After the raw AE signal was collected by the system,wavelet analysis method was used to filter noise and reconstruct AE waveform.Then,the reconstructed AE signal was dealt with empirical mode decomposition(EMD)to reduce the noise level.Secondly,two types of AE signals were defined by their frequency characters.Two kinds of AE events were defined by two types of AE signals.One was denoted FAE that was caused by fracture,and the other was DAE caused by fibers deformation in this paper.At last,instantaneous frequency of AE signal was obtained through Hilbert transform.Meanwhile,the density of each AE event was used to evaluate the damage degree and its growing trend.From the test results,the frequency of FAE was obviously higher than that of DAE.Higher density of DAE event meant that the damage was mainly caused by elastic deformation.Correspondingly,higher density of FAE event meant that the damage was mainly caused by fracture.With instantaneous frequency,a new definition of AE event was proposed in this paper.Especially,quantity and density of AE event could reflect the stress level and damage degree of wood.