Papers by Author: Yun Hee Lee

Abstract: Surface roughness is main source of error in instrumented microindentation when it is not negligible relative to the indentation depth. The effect of a rough surface on the results of instrumented microindentation testing using spherical indenter was analyzed by applying the contact depth model, which takes surface roughness into account. Improved variations in hardness and Young’s modulus were shown for W and Ni when the results were analyzed by this rough-surface model, while these values were underestimated with increasing surface roughness when analyzed by the flat-surface model. The deformation state of asperities underneath spherical indenter was also discussed.

Abstract: We tried to apply the nanoindentation technique to yield strength characterization by modifying a previous research. Although the yield strength determining technique developed by Kramer et al. has been successfully demonstrated for large scale indentations on bulky metals, its applicability is still doubtful to nanoscale indentations on thin films with severe roughness, anisotropy, and interfacial constraint. In order to overcome these problems, we combined the nanoindentation technique with a three-dimensional indent visualization technique in this study. Nanoindentation tests were performed for Au and TiN thin films and their corresponding indents were scanned by using an atomic force microscope. From the three-dimensional pile-up morphology, a circular pile-up boundary was measured and input into the yield strength formulation as an effective yielded zone radius. The yield strengths calculated were directly compared with those from the microtensile test.

Abstract: A nondestructive rod compression has been proposed as a new strength characterizing technique for in-service components. Using electrical discharge machining, milling machining, and stamping, it was possible to machine small rods, typically about 0.5 mm in both diameter and height on the target surface. Static compressions of the rods were carried out using a flat punch and their deformation behaviors were recorded as load-displacement curves. Referring to initial dimension of the rods, engineering stress-strain curves implying the yield strengths were calculated from the deformation curves. Surface yield strengths from the rod compressions were directly compared with reference compression results and their slight discrepancies were discussed from the influences of pre-deformation and damage of the rod and additive compliance of the base.