Issue34

L. Náhlík et alii, Frattura ed Integrità Strutturale, 34 (2015) 116-124; DOI: 10.3221/IGF-ESIS.34.12 122 Figure 5 : Scheme of stages of crack propagation in the compressive layer of ceramic laminate with strongly bonded interfaces. The crack can relatively easy propagate in the outside layers from some flaws, scratches or initial notches under external bending load of the composite. In the tensile layers the crack propagation corresponds to the mode I and cracks propagate perpendicularly to the material interface. Due to this fact it is necessary to prepare the composite with low tensile stresses, i.e. with wide tensile layer (ATZ in studied case) and thin compressive layer (AMZ) with high compressive stresses. The resistance to the crack propagation in the tensile layer is lower due to tensile residual stresses than in the case of crack propagation in the homogeneous ceramics, see e.g. [28]. When the crack passed through the interface to the compressive layer (AMZ) acting compressive residual stresses change the mode of crack propagation from mode I to shear mode II. This causes strong crack deflection or can lead to bifurcation of the propagating crack. Crack propagation parallel to the material interface in the compressive layer is evident from experiments published in [37]. This phenomenon is supported by results obtained as well. The parallel crack propagation is controlled by specific stress distribution in the compressive AMZ layer and influenced by the vicinity of tensile layers. After some propagation the bending nature of loading, existence of material imperfections (flaws, pores) and the vicinity of material interface lead to the other change of crack propagation direction close to AMZ/ATZ interface [32]. These effects contribute to the characteristic stepwise crack propagation in the strongly bonded ceramic laminates. C ONCLUSIONS he paper presented focuses on the crack behaviour in the multilayered ceramic composite subjected to the bending load. Crack propagation in the compressive layer responsible for higher apparent fracture toughness of the composite was investigated by means of finite element method. Sih’s criterion based on strain energy density factor was used for estimation of the crack behaviour in the layers and for the estimation of crack path. It was shown that the crack after passing through the first material interface between tensile and compressive layer retards due to acting of strong residual stresses and the crack is sharply deflected. An additional external load is necessary for further crack propagation at this moment. The depth of crack deflection was determined for different thicknesses of the composite layers. The stress distribution allows in this moment bifurcation of the crack as well, like was shown e.g. in the works [29,31]. Further crack propagation in the compressive layer is more or less parallel to the material interface. The stages leading to the stepwise crack propagation were described in detail. Finite element method implemented in commercial system Ansys with author’s routines was used for numerical simulations. The paper contributes to the better understanding of damage of strongly bonded ceramic laminates and their toughening mechanism. A CKNOWLEDGEMENT his work was supported through the Grant No. 15-09347S of the Czech Science Foundation and the specific academic research grant (K. Štegnerová) No. FSI-S-14-2311 provided to Brno University of Technology, Faculty of Mechanical Engineering. T T