Evaluation of toughening concepts at structural features in CFRP—Part I: Stiffener pull-off

The objective of this work was to evaluate two damage tolerance concepts; improved toughness matrix and Z-pinning, under structural conditions using stiffener pull-off tests, which is a typical element test used to characterise structural performance of full-scale components. Both concepts had a negligible effect on initiation of failure, but Z-pinning did stiffen the element, leading to lower strains at damage initiation. However, the Z-pins promoted additional energy absorbing mechanisms which resulted in stable growth and a considerable increase in load carrying capacity. The effect of increasing matrix toughness was less significant, leading to an increase in scatter but some stable damage growth. Combining concepts did not lead to a net improvement due to the parent architecture being modified by the Z-pins, which led to defects such as resin-rich zones, fibre waviness and voids. This suggests that Z-pins may not be compatible with multiphase toughened systems. The results have important implications for damage tolerant design of composites; concepts such as Z-pinning offer the opportunity to design a structure that can withstand significant damage whilst still being fit for purpose.