Effect of different fibre orientations at the interface on fatigue delamination growth

Abstract

Composite materials have multiple failure modes. Most of these failure modes are dedicated to the different layers, e.g., delamination. Unfortunately, these damages can occur internally, causing it to be invisible to the naked eye. A better understanding of how this happens and what causes it, is required to ensure safety. Therefore, for this research, delamination is the most interesting failure mode. Previous researches have primarily been focusing on quasi-statically loaded UD specimen. However, since a layup in a structural part of an aircraft consists of multiple layers with different fibre orientations, it is not guaranteed that a delamination will occur between two 0º plies. Furthermore, an aircraft is designed to be in service for decades and consequently is loaded thousands of times. A structure which is subjected to a cyclic loading will behave differently than one to a quasi-static loading. Hence, in aviation industries, tests with cyclically loaded specimens are more relevant. This research project focused on fatigue delamination crack growth of multi-directional interfaces. In order to generate mode I delamination, DCB specimens were manufactured and tested. Fatigue experiments were conducted to see the influence of the interface angle as well as that of the orientation of the interface angle. To cover most relevant delamination planes, tests with the following interface were conducted: 0º//0º, 45º//45º, 90º//90º, 0º//45º, 0º//90º, 45º//−45º and 30º//−60º. Furthermore, the crack fronts were monitored by means of C-scans and the fracture surfaces were examined. It was found that for all different interface orientations, fibres bridged. This was demonstrated with an increased delamination resistance for a longer crack. The nature of these bridging fibres, however, differed. Fibre bridging regarding to nesting showed a less significant increase in fracture resistance than bridging fibres due to oscillatory crack propagation behaviour. Furthermore, the different interfaces showed various crack fronts. The crack front tends to propagate towards the direction of the fibres of either of the fibre orientations around the interface.