An experimental study of rate effects on Mode I delamination of z-pinned composite laminates

Andrew M Schlueter, Purdue University

Abstract

Z-pinned laminates are designed to suppress interlaminar fracture. Much interlaminar fracture damage is due to impact, so an experiment was devised to examine loading rate effects on z-pinned composite laminates. Loading rate effects on the Mode I fracture of IM7/8552 carbon/epoxy composite laminates with and without pultruded carbon/epoxy z-pins were investigated using a Flying Wedge test method. Unpinned laminates were found to fracture in continuous stable crack propagation, with a positive correlation between critical Mode I strain energy release rate, GIc, and wedge velocity. Two different fracture regimes were found in the z-pinned laminates. From quasi-static to 40 m/s wedge velocities fracture occurred in a series of unstable crack propagations followed by complete arrest of the crack. At high velocity, 250 m/s, steady-state crack propagation was observed, where there was continuous crack propagation and the crack tip and wedge traveled at identical velocities. The possibility of a third transitional fracture regime was noted, as it appears that the 50 m/s wedge velocity resulted in a continuously propagating crack whose velocity oscillated about the wedge's constant velocity. These z-pinned laminates show that both p0, the critical peak crack closure traction exerted by the z-pins, and GIc decrease with increasing wedge velocity in the stop-and-go fracture regime which holds from quasi-static through a wedge velocity of 40 m/s. At a wedge velocity of 40 m/s, there is a maximum reduction from the quasi-static values of over 30% in p0 and over 35% in GIc. The rate effects on p 0 were also found to dominate those directly on the base laminate GIc when determining the total strain energy release rate of the z-pinned laminate. The ratio of GIc for the 2% z-pin specimens toGIc for the 0.5% z-pin specimens was found to remain relatively constant over the range of wedge velocities studied; no discernible relationship was found between wedge velocity and that ratio. These results suggest that the current widely held modeling assumption of rate independent z-pin tractions is unjustified, and therefore models should be refined to incorporate rate effects. Additionally, designers should consider using additional safety factors when designing z-pinned composite parts that are planned to endure impact loading until rate effects on fracture properties are better understood.

Degree

Ph.D.

Advisors

Chen, Purdue University.

Subject Area

Aerospace engineering

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