Improving the delamination bridging behaviour of z-pins through material solutions

Abstract

This thesis presents the development of through thickness reinforcement through the design, fabrication and characterisation of novel Z-pin architectures that exhibit an improved delamination-bridging response. A micro-pultrusion process is developed and optimised to produce small diameter rodstock. Combinations of Z-pin constituents, i.e. matrix and fibre, as well as reinforcement architectures, are investigated to achieve a balanced bridging performance.
Initially, the effect of tailoring the matrix properties is evaluated. Three matrix systems with increasing elongation at break are considered. The experimental results indicate that carbon fibre Z-pins made with a highly ductile matrix provide a peak interlaminar fracture toughness of the order of 40 kJ/m2, compared to the 28 kJ/m2 yielded by commercial CF/BMI Z-pins. Moreover, the transition from full pull-out to rupture occurs at a mode-mixity of 0.55, whereas BMI Z-pins start failing at a mode-mixity of 0.19.
The effect of the fibre constituent is then explored. Z-pins are fabricated using polybenzoxazole (PBO) fibres. Single pin bridging tests reveal that unidirectional fibre PBO-based pins can withstand complete pull out throughout the full mode-mixity range, regardless of the ductility of the pin matrix. These novel Z-pins provide a 20-fold increase in energy dissipation compared to “traditional” carbon fibre/bismaleimide (CF/BMI) pins at load mode-mixity values higher than Φ = 0.9. The effectiveness of these Z-pins is controlled by the higher strength and toughness of the PBO reinforcement, which prevent premature fibre failure under large shearing displacements.
The final single pin bridging study is focused on the employing hybrid fibre architectures, carbon, steel and PBO are hybridised, considering two types of architectures: core/shell and over-braided core. Core/shell Z-pins exhibited a balanced delamination bridging behaviour, experiencing core pull-out in conditions close to mode II. Over-braided core Z-pins tend fail at lower mode mixity values. Single fibre Z-pins based on PBO are shown to perform better than any hybrid-architecture alternatives when the dissipated energy is normalised with respect to a reference 2% areal density.
Finally, the effectiveness of the PBO-fibre Z-pins is characterised via end-loaded split (ELS), mixed mode bending (MMB) and dual cantilever beam (DCB) tests. ELS tests reveal a small improvement in GIIC provided by PBO-based through-thickness reinforcement, when compared to commercial CF/BMI pins. Most importantly, in the ELS tests it is observed that, while all CF/BMI pins fail, all the PBO pins remain intact. MMB tests show better crack stabilisation and progressive failure at 75% mode II loading and up to a two-fold improvement in interlaminar fracture toughness at 25% mode II loading. Regarding mode I loading, the use of the novel Z-pins results in an interlaminar fracture toughness of 4-5 kJ/m2 at a pinning density of 0.5%. This compares to a 2-3 kJ/m2 achieved by samples pinned with standard CF/BMI Z-pins.

Date of Award	3 Oct 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Giuliano Allegri (Supervisor), Bing Zhang (Supervisor), Ian Hamerton (Supervisor) & Stephen R Hallett (Supervisor)