Fracture of bicrystal metal/ceramic interfaces: A study via the mechanism-based strain gradient crystal plasticity theory A. Siddiq a, * , S. Schmauder a , Y. Huang b a Institut fu ¨ r Materialpru ¨ fung, Werkstoffkunde und Festigkeitslehre (IMWF), University of Stuttgart, Germany b Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, USA Received 4 March 2006 Available online 21 November 2006 Abstract Two continuum mechanical models of crystal plasticity theory namely, conventional crystal plas- ticity theory and mechanism-based crystal plasticity theory, are used to perform a comparative study of stresses that are reached at and ahead of the crack tip of a bicrystal niobium/alumina specimen. Finite element analyses are done for a stationary crack tip and growing cracks using a cohesive mod- elling approach. Using mechanism-based strain gradient crystal plasticity theory the stresses reached ahead of the crack tip are found to be two times larger than the stresses obtained from conventional crystal plasticity theory. Results also show that strain gradient effects strongly depend on the intrin- sic material length to the size of plastic zone ratio (l/R 0 ). It is found that the larger the (l/R 0 ) ratio, the higher the stresses reached using mechanism-based strain gradient crystal plasticity theory. An insight into the role of cohesive strength and work of adhesion in macroscopic fracture is also pre- sented which can be used by experimentalists to design better bimaterials by varying cohesive strength and work of adhesion. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Metal/ceramic interface; Mechanism-based strain gradient crystal plasticity; Cohesive model; Macro/ micro fracture analysis 0749-6419/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijplas.2006.08.007 * Corresponding author. Tel.: +49 711 685 62579; fax: +49 711 685 62635. E-mail address: amir.siddiq@mpa.uni-stuttgart.de (A. Siddiq). International Journal of Plasticity 23 (2007) 665–689 www.elsevier.com/locate/ijplas