Cracks occurring at arbitrary orientations in lamellar composites are studied. The associated boundary value problem is formulated in terms of dual integral equations in which the kernels are found in series form. This series representation of the kernels is shown to converge quite rapidly. Variation of the stress intensity factors with the constituent elastic moduli is presented, and a comparison of calculated values with a Mohr's circle approximation of the stress intensity factors at arbitrary orientations is made. This comparison results in the conclusion that the Mohr's circle approximation is accurate as long as the crack tip is far enough from the bimaterial interface.

The plastic flow at the crack tip is characterized by a model compatible with slip line theory. From this model it is shown that a continuous distribution of dislocations may be derived. Then assuming that these dislocations are emitted from the crack tip and move along slip lines to their final position, the Peach-Koehler force is used to calculate the plastic work involved. Since the plastic zone size is dependent on crack length, two plastic effects are present upon propagation. Primarily the distribution of dislocations present moves along with the crack tip, secondarily new dislocations are emitted to fill the larger plastic zone. These effects yield plastic work which is dependent on both σ2 and σ4, with σ being the applied stress. This dependancy yields a critical stress relationship different from that proposed by either Irwin or Orowan. It also leads to the determination of a subcritical flaw size, i.e., one which will never become unstable.

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The possibility of slip bands providing sites for fracture initiation within composites of lamellar construction is investigated. It is proposed that the tips of these bands, which terminate at the composite's bimetallic interfaces, are the specific locations of fracture initiation. Application of a dislocation intensity factor criterion allowed the applied stress at which cracks may be initiated to be calculated. The predictions compare favorably with experimental data. The dependence of the fracture initiation stress calculated from this model is shown to follow a Hall-Petch relationship.

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