Novel damage mitigation concepts for composite structures

Composites reduce structural mass, which is vital to achieve net zero aviation, however the vulnerabilities of composite structures to unstable brittle failure limit the weight-saving potential currently achieved. In this PhD thesis, we develop novel damage mitigation strategies for CFRP structures to address such vulnerabilities and demonstrate significant improvements in mechanical performance can be obtained to a range of loading scenarios by adopting damage mitigation concepts in the design of CFRP structures. We focus on the development of solutions achievable with industrially relevant manufacturing techniques. Firstly, we address the stress concentration at the edge of metal to composite adhesive joints which causes premature failure in components such as composite fan blades. We demonstrate profiling the edge of the metal adherend can provide significant increases of at least 27% in strength, whilst simultaneously providing a more stable failure, and that increasing the profile amplitude and complexity results in further significant increases in the performance. We develop a design, inspired by tree branch-trunk attachments, which we show removes the vulnerability of composite stiffened panels to unstable stiffener debonding by embedding the stiffening laminate within the skin, achieving drastic increases in strength, energy absorption, and failure stability. We identify Automated Fibre Placement (AFP) as an industrially relevant manufacturing route for bio-inspired composite stiffened panels and demonstrate successful manufacture of the concept via AFP. Finally, we focus on a new concept to resist delamination. Composite laminates are often vulnerable to delamination failure between the plies, which can cause a sudden loss of stiffness and unstable failure. We develop the novel concept of Repeated Segment Stacking (RSS) via AFP which we demonstrate introduces fibre undulations within the laminate to interlock the plies and successfully resist delamination growth. We demonstrate successful manufacture of the RSS concept and the ability to control the fibre undulations obtained.