Development of Automatic and High-Precision Displacement Gage of Laser Interferometry for Measuring Opening Displacement of Small Fatigue Cracks

1. 研制了一种带线性图像传感器的干涉式应变/位移计（ISDG），并与伺服液压疲劳试验机相连接。该系统称为系统A，通过逐级加载得到裂纹张开位移与载荷的滞回线。系统A在100mm的量规长度下，分辨率可达1nm。采用激光二极管作为光源，实现了激光与加载波的同步切割，并测量了循环加载过程中的位移。这个系统被称为系统B，它的工作频率最高可达3hz。在机械快门上安装He-Ne激光系统，实现激光束与负载波的机械同步切割。该系统称为系统C，用于测量载荷频率低于3Hz.4时的裂纹张开位移。采用A系统测量了某低碳钢光滑试样表面裂纹的张开位移。在小裂纹不扩展时，裂纹闭合测量计算的有效应力强度范围是恒定的。在a系统裂纹闭合测量的基础上，讨论了马氏体和珠光体等第二相对小疲劳裂纹扩展行为的影响，发现裂纹闭合是裂纹扩展速率不规则行为的原因。粗糙度致裂纹闭合起着重要的作用。ISDG系统也适用于金属基复合材料，如SiC颗粒增强铝合金。在裂纹闭合测量的基础上，讨论了小疲劳裂纹的扩展行为。

1. An interferometric strain/displacement gage(ISDG)with linear image sensors was developed and linked with a servo hydraulic fatigue testing machine. This system is called System A. A hysteresis loop of crack opening displacement and load was obtained by step-wise loading. The resolution of System A can reach lnm for the gage length of 100mum.2. A laser diode was used as a light source in order to chop the laser light synchronously with the load wave and to measure the displacement during cyclic loading. This system called System B is applicable up to a frequency of 3OHz.3. A mechanical shutter was also equipped with He-Ne laser system to chop the laser beam mechanically and synchronously with the load wave. This system called System C is useful for measuring the crack opening displacement at load frequencies below 3Hz.4. System A was used measure the crack opening displacement of surface cracks in smooth specimens of a low carbon steel. The effective stress intensity range calculated from the crack closure measurement was constant at the non-propagation of small cracks.5. The effect of the second phase such as martensite and pearlite on the crack propagation behavior of small fatigue cracks was discussed on the basis of the crack closure measurement by System A. The crack closure was found to be responsible for irregular behavior of crack growth rate. The roughness-induced crack closure plays a significant role.6. The ISDG system was also applicable to metal matrix composites such as SiC particulate reinforced aluminum alloy. The propagation behavior of small fatigue cracks was discussed on the basis of crack closure measurement.