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

小行星9615705 这是一个基本的研究，以了解自然引发的小疲劳裂纹相对于大裂纹的异常行为。 有人怀疑，异常的行为是与三维特征的小裂纹，包括大，孤立的扰动裂纹前端，以及缺乏裂纹尾流闭合。 一种实验方法检查了镍基和铝基合金中重要的微观结构变量的影响，包括晶粒尺寸和形状、变形行为和取向。 采用了一种新的技术，包括激光干涉位移测量系统和显微光学系统。 激光干涉测量系统可以测量小裂纹的裂纹张开位移，显微照相系统可以记录表面裂纹长度的变化。 激光干涉测量系统的独特之处在于可以同时精确测量非常小的裂纹的裂纹柔度和裂纹闭合水平。 这有利于计算裂纹尖端在表面和深度区域的增长率，纳入纵横比的连续变化。 因此，可以将小裂纹行为与大裂纹行为进行准确比较，从而更好地理解小裂纹现象。 据信，这种方法提供了新的见解的异常特性的小裂纹。 这种新的实验技术是有用的先进材料，包括金属间化合物合金，陶瓷和复合材料，其中高精度非接触式位移测量技术需要精确的性能测量的机械行为的研究。 ***

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. ***