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. ***
