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

1.研制了一种基于线阵图像传感器的干涉式应变/位移计，并与液压伺服疲劳试验机联动。该系统称为A系统，通过分步加载得到了裂纹张开位移和载荷的滞回曲线。系统A的测量长度为100微米时，分辨率可达LNM。采用半导体激光器作为光源，实现激光与载荷波的同步斩波，并测量循环加载过程中的位移。这种称为系统B的系统最高可应用到30赫兹的频率。机械快门还配备了He-Ne激光系统，以机械地与负载波同步斩波。该系统称为C系统，适用于在3赫兹以下的加载频率下测量裂纹张开位移。用A系统测量了低碳钢光滑试件表面裂纹的张开位移。由裂纹闭合测量得到的有效应力强度范围在小裂纹不扩展时是恒定的。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疲劳小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。粗糙度引起的裂纹闭合起着重要的作用。该系统同样适用于碳化硅颗粒增强铝合金等金属基复合材料。在裂纹闭合测试的基础上，讨论了疲劳小裂纹的扩展行为。

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.