金属薄板中导波的模态识别和波速测定
Mode identification of guided wave and velocity measurement on a thin metal plate

按照Lamb波理论, 板内导波由多阶对称波和反对称波组成。当波长远大于板厚时, 利用经典平板理论对Lamb方程进行简化, 此时板内主要有零阶对称波S0和反对称波A0两种模态。认识薄板内声波模态, 不仅有助于分析声源性质, 提取有效信号, 而且可以提高声发射检测中时差定位的准确性。利用PAC公司的Fieldcal标准信号发生器和Nielsen Hsu断铅法作为信号源在0.7 mm薄钢板上进行声波传播实验。通过比较实测波速和理论波速甄别板内的声波模态。实验发现对称波S0实测波速和理论波速比较吻合, 而反对称波A0的波速由于频散而较难精确测量。综合两种模态波的波速情况, 声发射检测中利用对称波速度进行声源定位计算更加接近薄壁结构声波的传播状况。
According to Lamb theory, guided waves contain multiple symmetric and antisymmetric modes on a plate. When the wavelength is much greater than the plate thickness, classical plate theory can be used to understand the wave motion. There are two modes of propagation. One is called the extensional (S0) and the other the flexural mode (A0). In order to research these acoustic modes, a signal source named Fedical and the other named breaking pencil lead were used for a steel plate of 0.7mm thick. Both the fundamental extensional and flexural modes were detected with transducers. Acoustic modes were analyzed and wave velocities were measured. The experimental value of the extensional wave velocity is in close agreement with the theoretical velocity, but it is difficult to measure the flexural wave velocity accurately because of its dispersion. It can be concluded that extensional wave velocity is reliable for source localization in AE testing.