EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Structural ceramics are brittle materials with high strength. However, they often contain intrinsic flaws, such as pores, which may act as a stress concentrator during mechanical loading. Cracks usually initiate at such flaws and their unstable propagation follows, causing “catastrophic failure”.

One way of increasing the resistance to the propagation of cracks is combining ceramic materials with different thermo-mechanical properties in a layered architecture. Using the strain mismatch between materials during cooling from sintering, residual stresses may be introduced in the layers. The in-plane compressive residual stresses, which “shield” the propagation of cracks, are counter-balanced with tensile residual stresses. Consequently, edge cracks at the free surface, tunnelling cracks in the bulk, and/or delamination of layers at their interfaces may form. The location, size or shape of such cracks cannot be known a priori, nor predicted with the linear elastic fracture mechanics.

In this work, we demonstrate how edge crack and/or tunnelling crack formation in laminates may be assessed using the finite fracture mechanics (FFM). This approach utilizes a “coupled criterion” (CC), which requires that the stress and energy at the potential location of the crack simultaneously exceed the inherent strength and fracture toughness of the material. A parametric finite element analysis of crack formation in layers of varying thicknesses and residual stresses has been conducted, demonstrating, that for certain combinations of thickness and stress, crack initiation may be prevented.