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Chapter VIII Micro-hardness studies… particularly in shear and hydrostatic compression, and thus induces irreversible deformation in localized regions of a test surface. In many brittle materials the mean contact pressure can be as much as a tenth of an elastic modulus [91] which indicates that the intrinsic bond strength of the lattice itself is exceeded. One of the important aspects of indentation damage is cracking. An understanding of indentation fracture is important for many practical as well as academic interests [92]. Brittle materials are used in industries quite commonly. For example, in the aerospace industry, the components of brittle materials face problems of strength degradation and material removal by crack growth under erosive conditions. The contact of erosive particles, including both solids and liquid drops, generates intense local stress concentrations, which give rise to characteristic indentation fracture patterns. Erosion studies of brittle materials are aimed towards understanding of the role of plastic deformation in production of these crack systems [93-95]. The cracks propagate into surrounding elastic hinterland. For crystalline solids, deformation processes may also be required for the propagation of non-cleavage (radial) cracks in ionic solids. Surface micro-flaws formed in an indentation with a sharp or blunt indenter have been studied in contest to evaluations of surface damage resistance, fracture toughness and wear resistance of brittle materials. Such micro-flaws are regarded as similar to typical fracture-strength-controlling flaws produced in manufacturing processes such as forming, grinding, cutting and handling. Therefore, it is important to study the mechanism of flow formation in indentations. In glasses a simple crack (Hertzian Crack) is usually produced by blunt indenter. In sharp indenters, such as Vickers or Knoop diamond pyramid, however, the crack formation mechanism is complicated because an 313
Chapter VIII Micro-hardness studies… irreversibly deformed zone, affecting the stress field for crack formation, appears beneath the indenter. The primary cracks produced by indentation with Vickers diamond pyramid for glasses are radial, medial and lateral types. The radial and median cracks are particularly important because they control the fracture strength of the indented specimen under bending or tensile loading conditions. The different types of cracks are shown in figure (8.9). The cracking mechanism about Knoop and Vickers indenters in Al2O3 and SiC has been reported by Lawn and Swain [93]. With Vickers indented specimens, the cracks were always observed radially extended outwards from the corners of the indentations. Figure: 8.9 Schematic diagram depicting the types of cracks. Basically, two distinct stages of crack formations are proposed [95], which are cracks occurring during loading and cracks occurring during unloading. During unloading, just before removal of the indenter, the new cracks initiate at the deformation zone and extend sideways into a shallow- 314
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Saurashtra University Re - Accredit
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Statements Under O. Ph.D.7 of Saura
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ACKNOWLEDGEMENT The author expresse
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in-law, Mother- in -law and all oth
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of NLO materials in rapidly develop
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doped samples in comparison to the
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micro-hardness decreases. Also, as
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Papers Published in National and In
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Seminar on Recent Advances in Conde
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Chapter I Brief Introduction... Cha
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Chapter II Solution Growth…. Tabl
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Chapter III CHN Analysis, FT IR …
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