# Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites
## 3 Results and Discussion
### Fractography  
To further investigate the damage and failure mechanisms of the material under different combined stress states, the fractured surfaces were tested through the scanning electron microscope (SEM) method. Images were taken using a TESCAN VEGA II XMU  
Figure 10: Representative DIC displacement and strain fields of 75\({}^{\circ}\) off-axis tension specimen: (**a**) X direction displacement fields; (**b**) Y direction displacement fields; (**c**) X direction strain fields; (**d**) Y direction strain fields. Figure 9: Representative DIC displacement and strain fields of 30\({}^{\circ}\) off-axis tension specimen: (**a**) X direction displacement fields; (**b**) Y direction displacement fields; (**c**) Shear strain field; (**d**) Tension strain field. scanning electron microscope (SEM) manufactured by TESCAN ORSAY HOLDING a.s., Brno-Kohoutovice, Czech Republic. Figure 11a shows the fracture surface morphology of the 15\({}^{\circ}\) off-axis tension specimen. The shear stress dominated fracture surface is smooth and led to a small part of fibre fractures. The fibre surfaces are sleek due to fibre/matrix debonding under shear loading. The surface of the matrix reveals riverline, textured microflow and matrix plastic deformation features, implying the plastic behaviour of the matrix. Figure 11b is the SEM image of the fractured 30\({}^{\circ}\) off-axis tension specimen. As the off-axis angle increased, the tension stress component within the matrix increased. The fractured surface exhibits scarps, riverline and a larger amount of matrix plastic deformation under a tension/shear combined stress state. The tension/shear coupling effect leads to increased plastic fracture features. Fibre fractures can also be identified owing to tension loading. Figure 12 compares the different fracture features of 45\({}^{\circ}\) and 60\({}^{\circ}\) off-axis specimens which have similar failure strengths. The increased tension stress generates a step-shaped feature on the fractured surface of the specimen with a 45\({}^{\circ}\) off-axis angle as shown in Figure 12a. Scraps features produced by tension/shear coupled stresses are all over the image. Matrix plastic deformation can also be spotted near the fibre surface. An uneven fractured surface was created under 60\({}^{\circ}\) off-axis tension loading conditions as shown in Figure 12b. The size and distribution of the scarps feature on the matrix surface  
Figure 11: Fracture surface morphology of different off-axis tension specimens: (**a**) 15\({}^{\circ}\); (**b**) 30\({}^{\circ}\). Figure 12: Fracture surface morphology of different off-axis tension specimens: (**a**) 45\({}^{\circ}\); (**b**) 60\({}^{\circ}\). enlarged under decreased shear stress and inclined tension stress. The matrix around fibre surfaces developed plastic deformation features as a result of ductile fracture behaviour. Figure 13 depicts the SEM images of the fractured 75\({}^{\circ}\) and 90\({}^{\circ}\) off-axis tension specimens. The surfaces of the failed specimens show similar patterns. Rough fractured surfaces are produced by matrix tension failure. The thermoplastic matrix exhibits ductile-brittle mixed fracture mode under tension. Some riverlines, after brittle fracture and plastic deformation, can be observed on the fractured surface of the polymer. The material shows a non-uniform fracture of the fibres due to resin-rich areas. The amount of fibre pull-out increases compared to the other off-axis specimens as a result of the effect of elevated tensile stress level. The fractographic study of the off-axis specimens shows that under a small off-axis angle of 15\({}^{\circ}\), a high level of shear stress led to shear plastic damage of the matrix, and the fibres limited the material deformation. For specimens with off-axis angles of 30\({}^{\circ}\), 45\({}^{\circ}\) and 60\({}^{\circ}\), the tension and shear combined stresses promote the damage process in the matrix. The thermoplastic resin demonstrates ductile fracture behaviour under the tension/shear coupling effect.