根据声弹性原理提出了一种新的测量材料表面焊接应力的激光超声方法。利用Nd:YAG脉冲激光在材料表面激发高频率超声瑞利波,采用非线性激光干涉仪对检测焊接应力的超声瑞利波进行探测。探测点的位置保持不变,通过激发源的扫描来改变激发源和探测点之间的距离,干涉仪探测到一系列超声脉冲波形信号。采用波形相关技术计算相邻超声瑞利波的传播时间延迟,得出瑞利波的传播速度,进而根据声弹性理论计算出相应的应力值。通过激光源在焊缝附近的扫描,得到焊缝周围的应力分布。测量了铝焊接平板表面的残余应力,得到了样品表面的焊接应力分布。实验结果表明,这种方法可以实现样品表面焊接应力的快速扫描测量,使其在材料表面焊接应力分布无损检测领域具有一定的应用价值。

A new method based on laser ultrasonic was proposed to survey welding stress distributions on the material surface according to the acoustoelastic principle, The high frequency Rayleigh waves excited by the Nd:YAG pulsed laser were used to measure the welding stress and picked up by a nonlinear interferometer. A serial of ultrasonic pulsed signals can be detected by the interferometer through changing the distance between fixed detecting point and ultrasonic source point by the scanning of the laser source. The waveform cross-correlation technique was applied to compute the propagation time delay of the adjacent Rayleigh waves, and the propagation velocity of the Rayleigh waves was obtained, by which the stress value was figured out via acoustoelastic relation. The residual stress distribution can be measured around welded joint while the laser source scans around the welded joint. This experimental installation was utilized to test the welding stress on the surface of a welded aluminum plate and the stress distribution was measured. The experimental results indicate that this method can scan over the specimen surface and measure the welding stress quickly, which make it have potential in the non-destructive evaluation of the surface welding stress distribution.