# Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites
## 1 Introduction  
Polymer composites have drawn a vast amount of interest in the aerospace and aeronautical industries. The main advantage of polymer composite over metals is the high strength/weight ratio. The development of polymer materials has provided a wide range of matrix choices [1; 2; 3]. These polymer composites can be elastomer composites, thermoset composites or thermoplastic composites. The newly developed carbon fibre reinforced thermoplastic composites (CFRTP) have a semi-crystalline thermoplastic matrix which increased the performance and moulding processes compared to thermoset composites. The thermoplastic polymer is in the form of linear chains that shows ductile mechanical behaviour. [PERSON] et al. [4] evaluated the tension and compression behaviours of open hole glass/PA6 thermoplastic cross-ply and quasi-isotropic laminates. A prominent nonlinear response was reported. [PERSON] et al. [5] use the finite element (FE) method to study the transverse tension failure behaviour of unidirectional carbon fibre reinforced PEEK thermoplastic composites. Simulation results show that the thermoplastic matrix around the fibre/matrix interface exhibits a strengthening effect due to plastic deformation. [PERSON] et al. [6] compared the deformation modes of twin-hole specimens of three woven thermoplastic composite systems: glass fibre/polypropylene (PP), self-reinforced PP, and glass fibre/polycarbonate (PC). The measured strain fields show that the induced shear strain has a load-transfer effect on the deformation of the specimen and results in a lower fibre direction strain level. [PERSON] et al. [7] compared the mechanical behaviours of three woven thermoplastic composites under \(\pm\)45\({}^{\circ}\) quasi-static and high strain-rate tensileloading conditions. Results show that the materials are strain-rate sensitive and exhibit nonlinear responses. [PERSON] et al. [8] conducted 0\({}^{\circ}\), 45\({}^{\circ}\) and 90\({}^{\circ}\) tension and compression tests on short fibre reinforced thermoplastic composites. Different failure mechanisms were observed as the loading direction varies. [PERSON] and [PERSON] [9] conducted in-plane shear tests on an AS4/PEKK thermoplastic composite and reported nonlinear response and plastic deformation. The effect of high toughness thermoplastic matrix on the mechanical response and failure mechanisms of CFRTP requires additional investigation. Several biaxial test methods of fibre reinforced polymer composites were developed including biaxial tension of cruciform specimens, tension/torsion of tube specimens, and in-plane tension/shear of off-axis specimens [10]. [PERSON] et al. [11] carried out biaxial loading tests on AS4D/PEKK-FC thermoplastic quasi-isotropic laminates by subjecting cruciform specimens to a biaxial test rig with a suspended horizontal loading axis. Specimen buckling under compression loading was reported. Strain concentrations along the \(-45^{\circ}\) and \(45^{\circ}\) directions were recorded due to induced shear stress. [PERSON] et al. [12] investigated the mechanical behaviour of a 3D woven T700/TDE-86 composite using short beam shear specimens with three different off-axis angles. Failure modes were changed from yarn tensile and delamination failure, to interface debonding and yarn pull-out. [PERSON] et al. [13] employed a modified Arcan fixture (MAF) to conduct tension/shear and compression/shear combined loading tests on E-glass/RP528 epoxy composites. Comparing the stress-strain curves shows that the specimens fail at much higher stress and strain under compression-shear mixed loading conditions. The use of MAF would cause specimen pinhole shear-out failure when the off-axis angle is smaller than 30\({}^{\circ}\). [PERSON] et al. [14] also employed the MAF to experimentally study the tension/shear and compression/shear behaviours of quasi-isotropic open-hole carbon/epoxy composites. The ply thickness effect was reported with thin ply specimens showing higher strength.