Improving the delamination bridging performance of Z-pins through the use of a ductile matrix

This paper presents a characterisation of the effect of varying the polymer matrix in Z-pin through-thickness reinforcement in pre-preg based laminates. Four matrix systems of increasing elongation at break are considered, namely: 1) a low glass-transition temperature (LTG) epoxy; 2) a high glass-transition temperature (HTG) epoxy; 3) a bismaleimide triazine (BT); 4) a bismaleimide (BMI). The last matrix is that used in commercially available Z-pins. The manufacturing of T300 carbon-fibre Z-pins employing the first three matrices via micro-pultrusion is discussed. The BT resin is considered as a benchmark for the manufacturing process. A preliminary screening of the mode II bridging performance of through-thickness reinforcement manufactured using the three matrix systems is carried out. A novel experimental set-up based on an acrylic glass carrier, which allows the failure mode of the through-thickness reinforcement to be visualised, is introduced. The preliminary tests reveal a 7-fold increase of work-to-failure for the candidates with the highest elongation at break, LTG Z-pins, compared to their baseline BMI-based counterparts. LTG Z-pins were then inserted in quasi-isotropic E-glass epoxy laminates and their bridging performance characterised across the full mode-mixity range. The experimental results indicate that LTG Z-pins provide a peak mode I interlaminar fracture toughness of the order of 40 kJ/m², compared to the 28 kJ/m² yielded by BMI Z-pins. Moreover, the transition from full pull-out to rupture for the LTG pins occurs at a mode-mixity of 0.55, whereas BMI Z-pins start failing at a mode-mixity of 0.2. The superior bridging performance of the LTG Z-pins is correlated with the enhanced ductility and toughness of the constituent matrix via detailed fractographic observations.