Effect of residual stresses on the fracture behaviour of notched laminated composites loaded in flexural geometry

Laminated composites in the Al2O3-ZrO2 system were prepared by superimposing ceramic laminae of different compositions produced by tape-casting. Due to the different coefficients of thermal expansion (CTE) between the different layers, these structures exhibit pronounced residual stresses. In the layers with lower CTE, the stress is compressive while layers with higher CTE undergo tensile stress. Distribution of the residual stresses, estimated a priori according to lamination theory, was confirmed using fluorescence and Raman spectroscopy to measure the residual stresses in each layer. The apparent fracture toughness of such a structure was measured by the SEVNB method. Very good agreement was found with the predictions of fracture mechanics according to which the apparent fracture toughness was strongly dependent on the position of the notch tip inside the compressive or tensile layer. Moreover, when the notch tip was located in the compressive layer, the laminated composite showed an unstable fracture, while, when the notch was located in the tensile layer, the laminated composite showed a semi-brittle behaviour.