Assessment of strength dominating near-surface characteristics of machined ceramics
The strength controlling surface effects of different machined ceramics have been assessed by means of experimentally determined bending strengths, depths of micro-plastic deformation and residual stresses, calculated flaw depth and calculated effective fracture stresses. A qualitative information concerning machining induced damage can be evaluated from the deformation depth, measured by advanced nondestructive X-ray diffraction methods. These methods additionally allow to determine machining induced residual stresses and to decide whether damage is the strength controlling effect or if the strength increasing effect of residual stresses has to be taken into account. Using the results of bend tests, a quantitative assessment of the effect of damage and residual stresses on strength can be evaluated by fracture mechanically based calculations. The investigations show, that on the basis of calculated crack sizes and effective fracture stresses also unexpected influences of machining parameters on strength can be clarified if the effect of residual stresses is taken into account. Thus, effective guidelines for optimized machining parameters can be evaluated. The results also show, that correlations between machining parameters and strength are strongly dependent on the type of the machining procedure and on the material. The reasons are machining and material dependent contributions of damage and residual stresses to strength. Additionally, the geometry's of cracks may be different in different materials although comparable machining procedures are used.