Development of non-contact NDE method by high performance laser AE system

本文研究了利用激光干涉仪非接触测量材料表面绝对速度的声发射换能器。我们已经证明了这种方法可以在没有接触的情况下检测声发射信号，并将其应用于涂层材料在高温下的断裂行为评估。与传统的压电弹性元件声发射传感器相比，该方法存在灵敏度较低的问题。研究的目的是为了改进声发射测量系统，提高检测灵敏度。改进是通过两种方式完成的。(1)激光多普勒振动器的解调是最初设计的。通过对系统内部电路的重新设计，降低了系统的噪声分量。(2)干涉仪的各个通道都受到电磁噪声、机械噪声等的影响。由于声发射信号本质上是瞬态现象，这些噪声基本上无法消除。我们尝试通过对同一点的声发射信号进行多通道检测，并在记录后对信号进行数值平均来降低噪声成分。该方法可以在不损失声发射波形的上升点和峰值振幅特性的情况下，减少唯一的噪声分量。将这些激光声发射技术应用于涂层材料的热循环测试。通过多通道声发射测量分析了声发射源的位置，并基于有限元法模拟弹性波的传播，对断裂模式进行了评估。最后对涂层材料的断裂机理进行了研究。由于等离子喷涂过程产生的初始缺陷会影响涂层的强度，采用激光声发射法监测等离子喷涂后冷却过程中的声发射行为，并考虑工艺条件对缺陷产生的影响。少

In this, research the method using laser interferometer, which can measure the absolute velocity of surface of materials in non-contact, was developed for acoustic emission (AE) transducer. We have demonstrated that this method could detect acoustic emission signals without contact and also applied it to the evaluation of fracture behavior of coating materials at elevated temperature. One of the problems of this method is to have less sensitivity compared with the conventional AE transducer using piezo elastic elements. The purpose of the research is to improve the system for AE measurement and increase the sensitivity for detection... The improvements were done in two ways. (1) The demodulator has been originally designed for laser Doppler vibrator. The frequency' characteristics of demodulator were focused for AE frequency and the noise components of systems were reduced by redesign of internal circuit. (2) Each channel of interferometer is affected by electromagnetic noise, mechanic … More al noise and so on. These noises can not be eliminated basically because AE signal is essentially transient phenomenon. We tried to reduce the noise components by the multi-channel detection of AE signals at the same point and numerical averaging of signals after recording. The method is possible to reduce the only noise component without loss of the characteristics of rising point and peak amplitude of AE wave form. These techniques of laser AE were applied to the thermal cycle tests of coating materials. Location of AE source was analyzed from multi-channel AE measurement and fracture mode was evaluated based on the simulation of elastic wave propagation by finite element method. Finally fracture mechanisms of coating materials were investigated. As the strength of coatings is affected by the initial defects due to plasma spray process, AE behavior was monitored during cooling process after plasma spray using laser AE method and the effect of process conditions on generation of defects was considered. Less