6.5 Conclusions Loading rate effects on z-pinned IM7/8552 carbon/epoxy composite laminates were investigated using a Flying Wedge test method. Two different types of crack propagation were observed for unpinned laminates and z-pinned laminates. The crack was observed to propagate in stable mode in unpinned laminates. The crack in z-pinned laminates was observed to propagate in start-stop mode in which the crack was arrested at certain points for some time followed by sudden propagation. The critical mode I strain energy release rate was observed to increase with increasing wedge velocities for the unpinned laminates. The energy release rates were observed to decrease with increasing wedge velocities for z-pinned laminates for velocities ranging from quasi-static to 40 m/s. This trend was observed because of the dominance of the changes in the energy release rates of the pin pullout due to increasing wedge velocity. The energy release rate increases again from 40 to 50 m/s. This was probably because of the change in the fracture behavior from start-stop to stable crack propagation with increasing wedge velocity. References 1. Dransfield KA, Baillie C, Yu-Wing Mai (1994) Improving the delamination resistance of CFRP by stitching – a review. Compos Sci Technol 50:305–317 2. Frietas G, Magee P, Dardzinski P, Fusco T (1994) Fiber insertion process for improved damage tolerance in aircraft laminates. Acta Mater 25:36–43 Fig. 6.9 Relationship between mode I critical strain energy release rate and wedge velocity for specimens with 0.5 % z-pins Fig. 6.10 Relationship between mode I critical strain energy release rate and wedge velocity for specimens with 2.0 % z-pins 6 Loading Rate Effects on Mode I Delamination of Z-Pinned Composite Laminates 57
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