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Elementary results are applied to the buckling of stitched laminates and woven composites with three-dimensional (3D) reinforcement that contain delamination cracks. The through-thickness fibers are assumed to provide continuous, linear restoring tractions opposing the deflection of the delaminated layer adjacent to the crack. With the boundary condition that the ends of the delaminated layer are clamped and with deflections permitted in one direction only, there exists a characteristic length a0 for buckling: if the length, 2a, of the delamination crack exceeds 2a0, then, when buckling occurs, it will consist of waves of period 2a, and will usually not span the whole delamination. A simple expression is derived for the minimum density of through thickness reinforcement required to suppress buckling of the delamination layer prior to failure by other mechanisms. It is shown that for typical densities of through-thickness reinforcement in current stitched laminates and 3D woven composites, the length 2a0 will rarely exceed the thickness of the delaminated layer by as much as an order of magnitude. Furthermore, the current generation of these materials are probably considerably overdesigned in the quantity of through-thickness reinforcement they contain for suppressing buckling.

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Numerical simulations are used to design test geometries and loading histories that are suitable for probing the large-scale bridging effects of through-thickness reinforcement that is shearing at high strain rates. The bridging effects are represented by a cohesive law and tests are sought that will determine any rate dependence in its parameters. The end-notched flexure test is studied, because it allows easy application of time-dependent loading and has proven to be an information-rich test in the quasi-static case. However, dynamic conditions greatly complicate fracture behavior, with possible regimes of hammering and multiple cracking, which should be avoided when maximum information is sought. Information content is addressed by focusing on regimes within the full computed solution space where crack growth is approximately steady state and the information content of experiments can be most easily assessed. Numerical results show that the hypothetical rate dependence in the cohesive law causes strong and measurable changes in the regime of steady-state behavior, if the tests are properly selected to vary the crack sliding speed. The estimates of information content are conservative, because the information available from all possible tests of specimens designed by analysis of the steady-state regime will necessarily exceed the information deduced by analyzing the steady-state regime alone.