The cutting of brittle materials using the precision crack-off process
Abstract
When a cylindrical rod made of a brittle material and containing surface flaws is subjected to lateral fluid pressure over its curved surface only, then it is observed that the rod fractures cleanly along a plane perpendicular to its axis if the pressure exceeds a critical value. Fracture occurs from a dominant surface flaw due to fluid penetrating into the flaw and applying pressure on its faces. This process termed the precision crack-off process has been investigated in this thesis. An experimental and theoretical study of the precision crack-off process has been carried out. In the experiments, a median/radial crack produced by a Vickers or a Knoop indentation is introduced onto the cylindrical surface of a rod made of a brittle material. The rod is then subjected to lateral fluid pressure. The pressure (p) required to cause fracture has been measured as a function of crack size and orientation for a wide variety of ceramics and glasses. By combining linear elastic fracture mechanics (LEFM) with the assumption that fluid pressure is exerted on the crack faces, the fracture pressure has been analytically estimated as a function of crack size, crack orientation, and fracture toughness. The analytically predicted fracture pressure is found to be in good agreement with the experimental measurements. The fracture pressure is also found to have only a minor dependence on the diameter of the rod. Surface roughness measurements using a profilometer have shown that the cracked-off surfaces have a surface finish often better than that of surfaces produced by diamond sawing or coarse grinding. Furthermore, the fractured surfaces are devoid of residual stresses and deformation damage unlike diamond sawed surfaces. An experimental investigation of the crack shapes produced by Vickers and Knoop indentations in ceramics and glasses has also been carried out. By using a serial sectioning procedure as well as sectioning of the indentation by crack-off, the subsurface shape of the median/radial crack system in soda-lime and borosilicate glasses, $\rm Al\sb2O\sb3$ and $\rm Si\sb3N\sb4$ is shown to be half-penny shaped for indentation loads in the range of 100 N to 500 N.
Degree
Ph.D.
Advisors
Farris, Purdue University.
Subject Area
Industrial engineering|Mechanical engineering|Materials science
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