G. de Portu et al. /Composites: Part B 37(2006)556-567 I mm Fig. 3. Example of delamination among layers of dissimilar materials the geometry of the layered structure, in particular on layer 4.1. Mechanical approach thickness 19, 28 The overall stress field, affected by different shrinkage during sintering and CTE mismatch between The magnitude of surface compressive stress can be constituent phases/layers, mismatch in elastic constants estimated from the difference in indentation crack length between different phases/layers, and layers'geometry, may between a stressed surface and stress-free surface [58]. For a be rather complex and thus difficult to predict by theoretical stress-free material the fracture toughness can be related to the calculations. In order to avoid cracking and delamination, a indentation load and length of the relative cracks emanating precise control of both magnitude and distribution of residual from the comers of the impressions through the following tresses is mandatory. In multilayered ceramic components, equation the development of a reliable experimental procedure for the evaluation of residual stresses is highly desirable. The Klc=X (4) development of such a technique may also help to substantially reduce the computational time required for complete three- where dimensional finite-element calculations Few techniques are available for assessing residual stresses KIc toughness of the stress free material in ceramic materials, including X-ray diffraction, neutron x dimensionless constant (experimentally deter- diffraction, indentation method and piezo-spectroscopic and- lyses of photo-stimulated fluorescence or Raman bands. In this P indentation load paper, we focalize our attention on the latter ones Co crack lengththe geometry of the layered structure, in particular on layer thickness [9,28]. The overall stress field, affected by different shrinkage during sintering and CTE mismatch between constituent phases/layers, mismatch in elastic constants between different phases/layers, and layers’ geometry, may be rather complex and thus difficult to predict by theoretical calculations. In order to avoid cracking and delamination, a precise control of both magnitude and distribution of residual stresses is mandatory. In multilayered ceramic components, the development of a reliable experimental procedure for the evaluation of residual stresses is highly desirable. The development of such a technique may also help to substantially reduce the computational time required for complete three￾dimensional finite-element calculations. Few techniques are available for assessing residual stresses in ceramic materials, including X-ray diffraction, neutron diffraction, indentation method and piezo-spectroscopic ana￾lyses of photo-stimulated fluorescence or Raman bands. In this paper, we focalize our attention on the latter ones. 4.1. Mechanical approach The magnitude of surface compressive stress can be estimated from the difference in indentation crack length between a stressed surface and stress-free surface [58]. For a stress-free material the fracture toughness can be related to the indentation load and length of the relative cracks emanating from the corners of the impressions through the following equation KIc Zc P c3=2 0 (4) where KIc toughness of the stress free material c dimensionless constant (experimentally deter￾mined) P indentation load c0 crack length. Fig. 3. Example of delamination among layers of dissimilar materials. 560 G. de Portu et al. / Composites: Part B 37 (2006) 556–567