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5 results on '"Jun-Hee Hahn"'

4. Characteristics of indentation cracking using cohesive zone finite element techniques for pyramidal indenters

Author

Jun Hee Hahn, Hyungyil Lee, Felix Rickhey, Hong Chul Hyun, and Jin Haeng Lee

Subjects
Materials science, Indenter geometry, Indentation cracking test, Nanoindentation, Brittleness, Materials Science(all), Modelling and Simulation, Indentation, General Materials Science, Composite material, FEA, business.industry, Applied Mathematics, Mechanical Engineering, Structural engineering, Condensed Matter Physics, Cohesive zone model, Finite element method, Cracking, Mechanics of Materials, Modeling and Simulation, Crack initiation, Fe model, business
Abstract

During indentation of brittle materials, cracks may be generated around the impression, depending on load conditions, material and indenter geometry. We investigate the effect of indenter geometry (centerline-to-face angle and number of edges) on crack characteristics by indentation cracking test and finite element analysis (FEA). Considering conditions for crack initiation and propagation, an FE model is employed featuring cohesive interfaces in zones of potential crack formation. After verification of the FE model through comparison with experimental results for Vickers and three-sided pyramidal indentation, we study the crack morphology for diverse indenter geometries and establish a relation between the crack length c and the number of indenter edges n c . Together with a relation between indenter angle ψ and crack length c , we can predict the length of the crack induced by other types of indenter from the crack size obtained with a reference indenter.

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5. Room-temperature control of the residual stress gradient in titanium micro-cantilever beams by helium ion implantation.

Author

Tae June Kang, Taewoo Kim, Sang-Hyun Park, Lee, Jae S., Lee, Jae H., Jun-Hee Hahn, Ho-Young Lee, and Yong Hyup Kim

Subjects
*TEMPERATURE control, *RESIDUAL stresses, *HELIUM ions, *TITANIUM, *MICROCANTILEVERS, *ION implantation, *CHEMICAL reduction
Abstract

Characterization and the reduction of residual stress and its gradient in thin films are important for improving the reliability of microelectromechanical systems (MEMS) devices. This study examined the room-temperature control of the residual stress gradient in sputtered titanium (Ti) films by helium (He) ion implantation. Ti micro-cantilever beam arrays were fabricated by surface micromachining and used to evaluate the residual stress gradient of the film by measuring the tip deflection of the cantilever beams. The as-fabricated Ti cantilever showed downward bending deformation as a result of the negative residual stress gradient in the film. To release the residual stress gradient, helium ions were implanted at various doses and energies into the Ti cantilever before releasing it from the substrate. The residual stress gradient could be controlled effectively by adjusting the implantation conditions. Almost straight Ti micro-cantilever beams with no residual stress gradient were achieved using the optimal implantation condition of He ions with a dose of 2.0 x 1015 at 80 KeV. The effects of He implantation on the surface properties, such as surface roughness and optical reflectance, were also investigated. [ABSTRACT FROM AUTHOR]

Published
2014
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