Experimental assessment of the damage tolerance of z-pinned aerospace composites

The general aim of this PhD study is to investigate the damage tolerant properties of aircraft carbon/epoxy laminates and bonded joints reinforced with z-pins. Much remains unknown about the efficacy and strengthening mechanisms by which z-pins improve the damage tolerant properties of composite structures, and this PhD aims to advance the current state-of-art by a combined experimental and analytical investigation. Specifically, the PhD examines the barely visible impact damage resistance and post-impact (damage tolerant) properties of z-pinned carbon/epoxy laminates. The project aims to determine the influences of key parameters (such as the impact energy, laminate ply stacking pattern and volume fraction of z-pins) on the impact damage resistance, post-impact compression stiffness, strength and fatigue properties, and interlaminar toughening mechanics of carbon/epoxy laminate panels. In addition, the PhD investigates the effects of tensile and compressive pre-loading on the impact damage tolerance of z-pinned laminates. These research studies provide original findings about the capacity of z-pins to increase the damage tolerance of aircraft laminate panels (such as wing or fuselage skin sections) against barely visible impact damage events. Another important aim of the PhD project is to determine the damage tolerance of z-pinned carbon/epoxy joints. The project investigates the structural properties, impact damage resistance, damage tolerance, and toughening mechanisms of T-shaped bonded joints and single lap joints reinforced with z-pins. This research aims to understand the capacity of z-pins to resist delamination cracking along the bond-line of aircraft joints, which results in improved damage tolerance. A final objective of the PhD is to undertake a holistic assessment of the implications of z-pin reinforcement on the design of damage-tolerant aircraft composite structures. Z-pins increase the delamination and damage tolerant properties of composites, but often at the expense of structural properties such as in-plane stiffness, strength and fatigue performance. The PhD project critically assesses whether the benefits gained by z-pinning out-weight the disadvantages, which is a key topic influencing the introduction of z-pinning as a strengthening and toughening method for composite aerostructures.