# Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites
## 1 Introduction
Generally, tensile loads are applied along one of the principal axes of unidirectional composite laminates to measure the fibre tension or transverse tension properties (Figure 1a). In the off-axis tension test, the tensile loading direction is neither parallel nor perpendicular to the fibres. Hence the stress states correspond to the materials' principal axes are tension and shear mixed (Figure 1b). The off-axis tension test of fibre reinforced composites can be used for the characterization of the mechanical behaviour under multiaxial stress states. [PERSON] et al. [15] conducted on-axis and off-axis tension tests of a UREKE 5893 woven composite. Rectangular specimen and end-tab designs were selected. The DIC collected strain fields of 45\({}^{\circ}\) specimens show that maximum strain appears at the edge corner, due to the unbalanced off-axis stress state. [PERSON] et al. [16] studied the fracture behaviour of a moso bamboo veneer composite subjected to off-axis loadings. Dumbbell-shaped off-axis tension specimens were subjected to tensile loadings. The observed tension/shear coupling failure mechanisms include fibre bridging and plastic deformation. [PERSON] et al. [17] studied the off-axis tension responses of woven glass/epoxy composites experimentally. Both specimens show a nonlinear stress-strain relationship. Degraded elastic moduli were reported as the off-axis angle increases. [PERSON] et al. [18] carried out off-axis tension tests on E-glass/PP composites. Matrix plastic deformation and cleavage were observed under off-axis loading. However, investigations on off-axis loading have reported in-plane rotation moment and stress concentration, due to rigid clamping force and anisotropic material properties [19]. [PERSON] et al. [20] and [PERSON] et al. [21] used the end-pinned fixture to eliminate stress concentration within composite off-axis tension specimens. Test results show higher shear strength and strain compared to clamped off-axis tension test. A drawback of this method is stress concentration in the pinhole area could lead to specimen bearing or buckling failure. Some researchers use modified end-tabs to produce uniformly distributed stress states and eliminate bending in off-axis tension specimens. [PERSON] et al. [22] proposed a tab design which has the same length as the specimen to restrict the out-of-plane deformation during the off-axis tension test of fibrous composites. [PERSON] et al. [23] conducted 10\({}^{\circ}\) off-axis tension tests on AS4/3501-6 thermoset composites, using three different clamping conditions: add sandpaper between gripping jaws and specimen, attach glass-epoxy end-tabs to specimen ends, and clamp specimen ends directly. A comparison of the test results shows that the most reliable method is to use end-tabs. [PERSON] et al. [24] investigated the effect of aspect ratio on stress distribution by the finite element method. Stress results indicate non-uniform distribution along the width of the specimens. Increasing the specimen aspect ratio would relieve the stress fluctuation. [PERSON] and [PERSON] [25] proposed a new oblique end-tab design manufactured using glass fibre-reinforced silicon rubber. [PERSON] and [PERSON] [19] conducted 10\({}^{\circ}\) off-axis tests using both regular and oblique end tabs. Finite element analysis shows oblique end-tabs produce a uniform deformation field. Tension stress was also detected, and the tension/shear coupled effect needs further research. [PERSON] et al. [26] carried out off-axis tension tests on PVC coated fabrics using dog-bone specimens with regular end-tabs. Images taken during the test show significant transverse deformation due to the bending moment. [PERSON] and [PERSON] [27] conducted a 10\({}^{\circ}\) off-axis tension test on thermoset composites using rectangle glass fibre end-tabs. Testing results and DIC full-field strain analysis show a non-uniform axial and shear strain distribution. The tension/shear coupling effect brings error to the measured shear strength.