Institute for Systems Research

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Author: search.filters.author.Ng, S.J.Subject: search.filters.subject.image processing

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    Characterization of Indentation Impressions on Human Enamel For Hardness Measurement
    (1997) Zhang, G.; Le, Dung T.; Tucker, S.R.; Ng, S.J.; ISR
    This paper presents results from investigating indentation impressions on human enamel under micro-hardness tests. The experiments of hardness testing were performed on a microhardness indentation machine under different loading conditions. Images of indentation impressions were obtained using an environmental scanning electron microscope. Geometrical shapes of hardness indentations were visualized in three-dimensional space using computer graphics. Quantitative information was obtained through atomic force measurements to characterize "pile-up", "sink-in", and elastic recovery of enamel. Special efforts have been made to study the microstrucutual effect of the calcified rods orientations on the fracture patterns formed during the hardness tests. Significant findings include that the occlusal surface demonstrates much stronger resistance to the indentation force than does the buccal surface and shows 40% elastic recovery after indentation. A new formula to determine hardness value has been proposed. By incorporating the reversible deformation into the evluation, a normalized hardness measurement can be made to form a basis for comparison and other investigations where hardness has its unique role to play.

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    Machinability Evaluation of Dental Restorative Materials
    (1996) Ng, S.J.; Zhang, G.M.; ISR
    Ceramic materials are ideal candidates for dental restorative applications for their color, texture, and mechanical properties which closely resemble those of the human enamel. However, due to the inherent brittleness of ceramic material, material processing, especially machining, poses a variety of difficulties. Research efforts of this thesis are directed to the development of a critical guideline for evaluating the machinability of ceramic materials, where human enamel is used as a reference material for comparison.

    Using a systems engineering approach, a computer-based surface integrity assessment methodology is formulated. It combines the most recently developed image processing technology with computer graphics while incorporating the principles of fracture mechanics. Microhardness testing is used to study material properties related to machining. Four types of material selected are human enamel, Dicor-MGC, HCC Dentine, and HCC Enamel. Three- dimensional visualization of the surface impressions is achieved using an environmental scanning electron microscope and an atomic force microscope. Machining experiments are conducted to study the surface integrity, including surface finish, micro- cracking, and edge chipping. Analytical investigation correlates these surface responses to the machining parameters, such as spindle speed, feed rate, and the depth of cut, to seek a parametric region in which quality of machined ceramic components can be ensured. Surface integrity performance indices such as surface roughness, cavity density, and chip aspect ratio are proposed to quantify such evaluations.

    Major contributions of this thesis research include the development of the combined SEM- AFM stereophotography method. The high resolution achieved with this method ensures coverage of rich information on the surface texture formed during machining. Specific findings of this thesis research include the identification of micro-mechanics of fracture occurred during the material removal process, and a good understanding of possible influences of the microstructures on the machining performance.