Bonded layered structures are widely used to meet high performance requirements that a single layer material cannot satisfy. All layered structures have a finite service life due to inevitable failures caused by chemical, thermal or mechanical loadings during operation. The inevitability of failure in a bonded layered structure demands the prediction of the service life for layered structures, and this requires understanding of the failure mechanism. The failure of bonded layered structures often initiates from a crack at surface, a crack at interface, or interfacial delamination. Failures due to surface fracture or interfacial delamination have been widely studied. However, brittle fractures originating at the interface have not been extensively investigated.
In this work, the failures of bonded layered structures due to cracks/flaws initiating at the interface are investigated. A cracked bilayer domain analysis is first presented for estimation of the SIF for a 3D half-penny shaped crack originating at a bonded interface in idealized bilayer geometry subjected to remote constant tensile and proportional bending loadings. Handbook-type curve-fitted equations are obtained for the SIF as a function of modulus ratio of bonded dissimilar materials through extensive finite element parametric studies. The cracked bilayer domain analysis is then combined with macro-level stress calculations in a structure without a crack (uncracked body analysis), and a simplified method is proposed for accurate estimation of the SIF.
The cracked bilayer domain analysis is extended to estimation of the SIF of a half-penny shaped crack normal to the interface in the top layer of a three-layer bonded structure. To obtain a simple estimate of the SIF, the method of reduction of an idealized cracked trilayer domain to that of a corresponding bilayer domain has been introduced based on the notion of an equivalent homogeneous material for the two bottom layers.
Based on the cracked bilayer/trilayer domain analysis, the effect of adhesive layer on the probability of cracks initiation from the interface in bonded layered structures is quantitatively investigated. The notion of a critical stress density function is introduced to account for the bridging mechanism. The failure probability of glass-ceramic disks bonded to simulated dentin subjected to indentation loads is predicted. The theoretical predictions match experimental data suggesting that the bridging mechanism plays an important role for accurate prognostics to occur.
The developed method is useful in predicting brittle failure initiating from interfacial flaws in a layered structure.