Matrix dominated failure in continuous carbon fibre reinforced Poly(ether ether ketone)

In recent years, there has been a rising demand for polymers in structural load bearing applications aimed at reducing carbon dioxide emissions. Poly(ether ether ketone) (PEEK) based composites have emerged as potential substitutes to traditional metals and alloys due to their remarkable mechanical and thermal properties. Consequently, carbon fibre reinforced PEEK (C/PEEK) composites have been increasingly adopted in several load bearing applications with a targeted service life spanning several decades. Therefore, understanding their mechanical behaviour under anticipated conditions becomes crucial from a design perspective.
This thesis aims to investigate the role of matrix in the failure of C/PEEK composites. To achieve this, the first step involves capturing the true stress-strain response of the neat PEEK matrix over a wide range of strain rates and temperatures. A novel constitutive model is proposed in Chapter 2 to describe the observed complex non-linear viscoelastic behaviour of PEEK and is validated against the experimental data. Subsequently, the focus shifts towards identifying the two matrix dominated, time-dependent failure mechanisms in off-axis loaded UD C/PEEK composites: Plasticity and Crack growth controlled failure. Investigating UD composites provides a valuable opportunity to examine the impact of several parameters on the two failure mechanisms in a more simplified manner.
Chapter 3 investigates the deformation and failure of UD C/PEEK under quasi-static and creep loading. The observed equivalence in kinetics between these experiments is utilized for describing creep lifetimes. Chapter 4 examines the influence of creep and fatigue loading on the two failure mechanisms in UD C/PEEK, while accounting for the effect of applied stress levels, temperature and loading angle. In Chapter 5, the influence of fibre content on the time dependent failure of UD and cross-ply C/PEEK composites is analysed. Additionally, thermal residual stress build-up is quantified using finite element analysis and 2-D classical laminate theory and its impact on the mechanical behaviour of C/PEEK composites is discussed.
Therefore, this thesis contributes to the understanding of the mechanical behaviour of C/PEEK composites through an expansive experimental approach covering a wide range of temperatures, loading angles, loading types (quasi-static, creep and fatigue) and fibre volume fraction, which further highlights the significance of the role of the thermoplastic matrix in composite structures.