Effect of surface finish on strength degradation of glass ceramics

The processing of components manufactured from ceramics, such as machining, induce's cracks on or beneath the surface leading to strength degradation, Usually, four-point bending tests,are used for measuring the strength of c eramic materials, In this paper, a new approach is proposed to evaluate the loss of strength due to machining. First, beam specimens of glass ceramics are prepared by milling, The surface profile of representative areas of th e specimens is determined using a laser profilometer. The surface topograph y of the representative area is reconstructed to characterize the surface i rregularities after machining. A finite element program is then,employed to analyze the stress distribution on a rough surface of a beam in four-point bending. A performance index-the stress concentration factor, is introduce d to quantify the effect of surface irregularities on determining the criti cal crack depth related to the machining-induced damage. The flexural stren gth of machined ceramic specimens is predicted using the fracture mechanics . The strength degradation is compared with the experimentally determined v alues. A close corresponding between the predicted and measured strength is observed. The potential of using a computer-based evaluation to study the strength degradation of glass ceramics by machining induced surface damage has been established. (C) 1999 Elsevier Science Ltd. All rights reserved.