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采用半解析有限元法计算圆环周向导波频散特性,群速度的计算可以不依赖于相速度。以外径202 mm,内径200 mm的铝环为例,计算了其在2.5 MHz频率时最低阶模态的相速度、群速度和振型。进一步设计了实验方案,对选定模态导波分别经过无裂纹和径向深0.5 mm的内壁裂纹的铝环的回波波形进行了分析,通过回波的时间与声程计算群速度进而确定模态。结果表明,裂纹缺陷处会发生模式转换,通过反射回波可以较精确的定位铝环内壁的单一缺陷。
The semi-analytical finite element method is used to calculate the circumferential dispersion of the guided wave, and the group velocity can be calculated independently of the phase velocity. Taking the aluminum ring with an outer diameter of 202 mm and an inner diameter of 200 mm as an example, the phase velocities, group velocities and mode shapes of the lowest order mode at 2.5 MHz were calculated. The experimental scheme was further designed. The echo waveforms of the aluminum ring with the selected mode guided waves passing through the inner wall cracks without cracks and radial depths of 0.5 mm respectively were analyzed. The group velocities were calculated from the time and the acoustic path of the echoes, Modal. The results show that the mode transition occurs at the defect of the crack, and the single defect of the inner wall of the aluminum ring can be more accurately located by the reflected echo.