Institute for Systems Research
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Item Hardness Assessment of Human Enamel(1997) Zhang, G.; Ng, S.J.; Le, Dung T.; Young, D.; ISRThis 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-dimentional space using computer graphics. Quantitative information was obtained through atomic force measurements to characterize ﲰile-up , ﲳink-in , and elastic recovery of enamel. Special efforts have been made to study the microstructual effect of the calcified rods orientations on the fracture patterns formed during the hardness tests. Significant finding 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 evaluation, a normalized hardness measurement can be made to form a basis for comparison and other investigations where hardness has its unique role to play.Item Assessment of Non-Linear Dynamics of Material Removal on Surface Integrity(1997) Zhang, G.; Ng, S.J.; Le, Dung T.; ISRIn this chapter, we present our research on the study of non- linear dynamics observed during the machining of ceramic materials. We will focus on both innovation in machining technology and development of non-destructive evaluation methods to assess machining performance. Three aspects of our work are presented in the following sections. They are, 1) Submerged Precision Machining, and 2) Scanning Electron Microscopy Analysis. Aspect 1 relates to an innovative approach to machining, while aspects 2 relates to evaluation methodologies for surface characterization. These methods and techniques have been developed to achieve cost-efficient machining as well as high-quality surface finish in ceramic material.Item Characterization of Indentation Impressions on Human Enamel For Hardness Measurement(1997) Zhang, G.; Le, Dung T.; Tucker, S.R.; Ng, S.J.; ISRThis 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.Item Machinability Evaluation of Dental Restorative Materials(1996) Ng, S.J.; Zhang, G.M.; ISRCeramic 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.
Item Control of Machining Induces Edge Chipping on Glass Ceramics(1996) Ng, S.J.; Le, Dung T.; Tucker, S.R.; Zhang, G.M.; ISREdge chipping is a phenomenon commonly observed during the machining of ceramic material. Characterization of edge chipping, both in macro and in micro scale, and correlating its formation to machining parameters form a basis for developing new and innovative technologies for controlling in machining induced damage. An experimental-based study using glass ceramic material is performed. Three types of edge chipping are identified. The SEM-sterephotography method and the finite element method are used to evaluate the edge chipping effect under a set of machining conditions. Significant findings are obtained and guidelines for controlling edge chipping during machining are suggested.Item Characterization of the Surface Texture Formed During the Machining of Ceramics(1995) Zhang, G.M.; Ng, S.J.; Le, Dung T.; Job, Lenox S.; ISRThis paper presents the development of a computer-based system to perform characterization of the surface texture formed during the machining of ceramic material. Techniques of image processing and computer graphics are employed to display vivid pictures of micro-scale details of the machined surfaces. Special attention is given to establishing the interrelations between the surface texture formation and the grain facet fracture induced by machining. Results obtained from microscopic analysis of the formed surface texture reveal that grain pullouts and cleavage, as a result of the progressive development of grain boundary micro-cracking, are two mechanisms responsible for the material removal process. The surface texture is an aggregation of macro and micro-scale fractured facets. A new performance index cavity density is introduced to quantify the combinational effect of material microstructure and the dynamic loading during machining on the micro-mechanisms of the material removal process.