Mixed mode delamination of polymer composite laminates reinforced through the thickness by z-fibers

The mixed mode delamination behavior of through-thickness reinforced carbon–epoxy laminates was investigated using two different test specimens, a T-stiffener and a mixed-mode bending (MMB) specimen. Small quantities of titanium or carbon z-fibers (short rods) substantially improve delamination resistance in both types of specimen. Reinforcement raises the ultimate strength of the MMB specimen by a factor of three. However, the failure sequence and therefore the ultimate load in the T-stiffeners depend strongly on the test configuration. No change in ultimate load is seen in some cases but up to 40% improvement is observed in others. Improved delamination resistance results from crack bridging by the z-fibers, which reduces the driving force for crack growth. Mode I crack displacement is suppressed more effectively than mode II displacement, resulting in purely mode II cracking in what without z-fibers would be a mixed mode or primarily mode I loading situation. This important consequence of so-called large scale bridging effects confirms recent theoretical results for delamination specimens. The mechanisms of bridging and crack propagation are described here in detail.