Controls on the failure mode of brittle inclusions hosted in a ductile matrix

Plane strain deformation experiments were performed on elliptical inclusions of cohesive sand embedded within a slab of pitch, with the aim of investigating the mode of fracturing of brittle inclusions within a ductile matrix. Under pure and simple shear, the inclusions failed in three different modes: tensile fracturing (Mode 1), shear fracturing (Mode 2a) and extensional shear fracturing (Mode 2b). Jeffrey’s (1922) theory of the flow of a viscous medium around an ellipsoidal body was applied to the experimental results to determine the principal tensile and compressive stresses within an inclusion, and analyze the failure modes using Griffith’s Criterion. The analysis reveals that the aspect ratio (R) and the orientation (θ ) of the inclusion control the principal tensile and compressive stresses within it, and in turn govern the mode of brittle deformation. At a particular inclusion orientation, the tensile stress increases, whereas the compressive stress decreases monotonically with increasing aspect ratio of the inclusion. The principal stresses also vary with inclusion orientation for a given aspect ratio, but not monotonically. The analysis delimits the fields of each mode of brittle deformation of inclusions in R- θ space under pure shear and simple shear.