Application of a Novel Experimental Technique to Probe the Effect of Microstructure on Small Cracks in Fatigue

9615705拉维坎德兰这是一项基础性研究，旨在了解自然引发的小疲劳裂纹相对于大裂纹的反常行为。据推测，这种反常行为与小裂纹的三维特征有关，包括裂纹前沿的大而孤立的扰动，以及缺乏裂纹尾迹闭合。一种实验方法检验了重要的微观结构变量的影响，包括镍基和铝基合金的晶粒度和形状、变形行为和取向。采用了一种新技术，包括激光干涉位移测量系统和光学显微镜系统。激光干涉系统可以测量小裂纹的裂纹张开位移，而光学显微镜系统可以记录表面裂纹长度的变化。激光干涉系统的独特之处在于能够同时准确测量极小裂纹的裂纹顺应性和裂纹闭合程度。这便于计算裂纹尖端在表面和深度区域的扩展速率，同时考虑了长宽比的连续变化。因此，可以准确地比较小裂纹和大裂纹的行为，从而更好地理解小裂纹现象。相信这一方法为研究小裂纹的反常特征提供了新的思路。这项新的实验技术适用于研究先进材料的力学行为，包括金属间化合物合金、陶瓷和复合材料，这些材料需要高精度的非接触位移测量技术来精确测量性能。***

9615705 Ravichandran This is a fundamental study to understand the anomalous behavior of naturally initiated small fatigue cracks relative to large cracks. It is suspected that the anomalous behavior is associated with the three-dimensional characteristics of small cracks, including large, isolated perturbations in crack front, as well as a lack of crack wake closure. An experimental approach examines the effects of important microstructural variables, including grain size and shape, deformation behavior ,and orientation in nickel- base and aluminum-base alloys. A novel technique is employed involving a laser interferometric displacement measurement system and a photomicroscopic system. The laser interferometric system allows the measurement of the crack opening displacements of a small crack and the photomicroscopic system records changes in the surface crack length. The unique aspect of the laser interferometric system is the simultaneous accurate measurement of crack compliance and crack closure levels of very small cracks. This facilitates the calculation of growth rates of crack tips at both the surface and depth regions, incorporating the continuous changes in aspect ratio. An accurate comparison of small crack behavior with that of large cracks is therefore possible, leading to a better understanding of small crack phenomena. It is believed that this approach provides new insights into the anomalous characteristics of small cracks. %%% This novel experimental technique is useful in studies of mechanical behavior of advanced materials, including intermetallic alloys, ceramics and composites, where high precision non-contact displacement measurement techniques are needed for accurate property measurements. ***