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.
Mixed mode delamination of polymer composite laminates reinforced through the thickness by z-fibers
MASSABO', ROBERTA
2002-01-01
Abstract
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.
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