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The nucleation and growth of microcracks in elastic lamellar microstructures is studied numerically. The analyses are carried out within a framework where the continuum is characterized by two constitutive relations: one relating the stress and strain in the bulk material and the other relating the traction and separation across a specified set of cohesive surfaces. In such a framework, fracture initiation and crack growth, including micro-crack nucleation ahead of the main crack, arise naturally as a consequence of the imposed loading, without any additional assumptions concerning criteria for crack growth, crack path selection or micro-crack nucleation. Full transient analyses are carried out and plane strain conditions are assumed. The specific problem analyzed is a compact tension specimen with two regions of differing lamellar orientation separated by a fracture resistant layer of finite width d, which is small compared to the physical dimensions of the specimen. An initial crack, normal to the applied loading, is assumed to exist in the first region whose lamellar orientation is fixed. The lamellar orientation of the second region, beta, is varied, as is the thickness of the fracture resistant layer. It is found that microcrack nucleation in the second region is highly sensitive to the lamellar orientation in that region for small values of d. However, microcrack nucleation becomes rather insensitive to beta with increasing d. It is also shown that a linear elastic fracture mechanics model with one adjustable parameter gives good agreement with the numerical results for fracture initiation.

In-situ compact tension tests on binary lamellar titanium aluminide (TiAl) possessing the colony "polycrystalline" microstructure illustrate a range of damage phenomena and toughening mechanisms including crack nucleation across colony boundaries, plastic deformation of bridging ligaments, and multiple cracking within colonies. Here, the effects of relative lamellae misorientation and offsets between neighboring colonies on crack growth are investigated computationally through an idealized microstructure of multiple colonies. Within each colony, the brittle Ti3Al lamellae are represented as parallel planes of comparatively low toughness embedded in a matrix of ductile TiAl lamellae that are collectively modeled as an elastic-viscoplastic solid with higher fracture toughness. Plane strain calculations of crack growth are carried out on a compact tension geometry. The calculations are in good qualitative agreement with the in-situ observations, capturing many features of crack growth such as multiple microcrack nucleation and plastic deformation of residual ligaments. Experiments and numerical analyses show that changes in lamellar orientation and alignment across a colony boundary can contribute significantly to the fracture resistance. The numerical results demonstrate that the fracture resistance of these alloys is determined by an intricate interplay between matrix ductility, Ti3Al and TiAl fracture toughnesses, and colony boundary toughness. This suggests the possibility of computationally-guided material optimization through microstructural control of these material properties.