采用机电阻抗法监测三跨桁架结构加载状况,并利用BP神经网络方法进行结构载荷的定位和定量研究.首先,研究激励频率对压电陶瓷片感知结构变化的灵敏度的影响,并选取最佳敏感频段为190~200 k Hz.然后,测得2个监测节点独立性良好,有助于实现载荷定位监测.最后,将采集到的桁架结构加载时的部分阻抗虚部数据进行合理的数据压缩后作为输入样本.压缩前后的数据对比显示这种压缩方法具有可靠性.建立并训练神经网络,剩余部分数据经过相同处理后作为测试样本对训练好的BP网络进行测试.实验结果表明：基于机电阻抗法,利用神经网络可有效实现桁架结构中载荷的精确定位与定量.
It is difficult to quantitatively detect defects by using the time domain or frequency domain features of Lamb wave signals due to their dispersion and multimodal characteristics.Therefore,it is important to discover an intrinsical parameter of Lamb waves that could be used as a damage sensitive feature.In this paper,quantitative defect detection in aluminium plates is carried out by means of wavenumber analysis approach.The wavenumber of excited Lamb wave mode is a fixed value,given a frequency,a thickness and material properties of the target plate.When Lamb waves propagate to the structural discontinuity,new wavenumber components are created by abrupt wavefield change.The new wavenumber components can be identified in the frequency-wavenumber domain.To estimate spatially dependent wavenumber values,a short-space two-dimensional Fourier transform(FT)method is presented for processing wavefield data of Lamb waves.The results can be used to determine the location,size and depth of rectangular notch.The analysis techniques are demonstrated using simulation examples of an aluminium plate with a rectangular notch.Then,the wavenumber analysis method is applied to simulation data that are obtained through a range of notch depths and widths.The results are analyzed and rules of the technique with regards to estimating notch depth are determined.Based on simulation results,guidelines for using the technique are developed.Finally,experimental wavefield data are obtained in aluminium plates with rectangular notches by a full noncontact transceiving method,i.e.,laser-laser method.Band-pass filtering combined with continuous wavelet transform is used to extract a certain frequency component from the full laser-induced wavefield with wide band.Shortspace two-dimensional FT method is used for further processing full wavefield data at a certain frequency to estimate spatially dependent wavenumber values.The consistency of simulation and experimental results shows the effectiveness of proposed wavenumber method for quantitative
In the study,an improved time difference mapping(I-TDM) method is proposed to locate the acoustic emission(AE) source in a composite plate.The acoustic velocity in a composite plate with fiber fractures and matrix crack defects largely varies with the propagation direction.In the proposed method,the prebuilt database is adopted in order to avoid the consideration of the inconsistent propagation speed of waves in a plate.In addition,according to the proposed method,the requirements for wave propagation modes or the sensor layout are not strict in the experiment.Firstly,in order to quickly and accurately select and delete the pseudo AE events in a large number of AE data,the clustering algorithm was used to automatically identify and select the highly correlated AE events on each grid node and construct a time difference training database.Secondly,the interpolation compensation algorithm was adopted to increase grid density and improve grid resolution.Finally,the actual location of the source in the test area was verified by using the weighted difference algorithm.Theoretical analysis and experimental results showed that the average location error of the sound source obtained by the I-TDM method was 18.0 mm,which was lower than that obtained by the traditional time difference mapping(TDM) method,26.5 mm.After the I-TDM method was adopted,the corresponding average relative error was reduced from 5.3% to 3.6% and the overall running time was reduced from 5 h to 16.87 s.The I-TDM method can effectively improve the location precision of the sound source in a composite plate.
