# Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites
## 3 Results and Discussion  
### Mechanical Response  
Six groups of off-axis specimens were subjected to tension tests. Each group has six samples. Since all specimens fractured in the same mode, a representative 45\({}^{\circ}\) off-axis tension specimen, after failure, is selected and shown in Figure 5a. Final failure occurs with a matrix crack along the fibre direction for all tested specimens. The fracture surfaces are parallel to the \(x-z\) plane. Figure 5b shows the oblique end-tab region after the 45\({}^{\circ}\) off-axis tension test. The clamping dents indicate no sliding or rotation during the test and therefore validates the effectiveness of the oblique end-tabs.  
The extracted axial stress-strain curves are shown in Figure 6. All specimens exhibit significant nonlinear behaviour. The 15\({}^{\circ}\) off-axis tension specimen shows a linear stress-strain relationship before the stress increased to 250 MPa. As the stress mounts, the specimen shows a nonlinear response. The failure strength of the 15\({}^{\circ}\) off-axis tension specimen is prominently higher than the other specimens. An explanation is that when the fibre misalignment angle is smaller than 15\({}^{\circ}\), fibre is the primary load bearing constituent,  
Figure 5: Representative fractured 45\({}^{\circ}\) off-axis tension specimen: (**a**) Failure mode of a 45\({}^{\circ}\) off-axis tension specimen; (**b**) End-tab region of the specimen.  
and the tension/shear coupling effect of the matrix is suppressed. Thus, the 15\({}^{\circ}\) off-axis specimen shows brittle behaviour with an elongation after fracture of 3%. When specimen off-axis angles are between 30\({}^{\circ}\)-60\({}^{\circ}\), the stress-strain curves exhibit a linear relationship before the strain reaches 1%. The mechanical response then changed into a strain-hardening phase as the strain increases rapidly, and the stiffness decreased gradually until the final fracture. The 45\({}^{\circ}\) off-axis specimen has the largest failure strain of over 5%. Other than the brittle thermoset epoxy [17], the thermoplastic matrix demonstrates elastic-plastic behaviour. The 75\({}^{\circ}\) and 90\({}^{\circ}\) specimens have similar stress-strain relationships. The specimen elongation during the strain hardening stage is relatively short, indicating a decreased plastic deformation.  
The off-axis tension stiffness vs. off-axis angle relationship is shown in Figure 7a. The largest value of stiffness appears in the 15\({}^{\circ}\) off-axis specimen. The stiffness decreased sharply as the off-axis angle changed from 15\({}^{\circ}\) to 30\({}^{\circ}\). The stiffness descent gently when the off-axis angle altered from 30\({}^{\circ}\) to 75\({}^{\circ}\). The 75\({}^{\circ}\) and 90\({}^{\circ}\) specimens have nearly the same stiffness values. Figure 7b gives the off-axis tension strength vs the off-axis angle curve. The strength declines sharply from 419.6 MPa to 137.4 MPa with the angle increased from 15\({}^{\circ}\) to 45\({}^{\circ}\). The strength then slowly descends to 102.1 MPa as the off-axis angle rises to 90\({}^{\circ}\).  
Figure 6: Stress-strain relationships of different off-axis tension tests.  
Figure 7: Relationships between material properties and off-axis angles: (**a**) Off-axis tension stiffness vs. off-axis angle curve; (**b**) Off-axis tension strength vs. off-axis angle curve.  
The changing trends of strength and stiffness, with relation to off-axis angles, show that at a small off-axis angle of 15\({}^{\circ}\), fibre is the main load-carrying component, and the material has maximum strength and stiffness. Loads transfer from fibre to matrix when, the off-axis angle increases to 30\({}^{\circ}\), resulting in a rapid decrease in strength and stiffness. Tension and shear combined stress state forms and evolves as the off-axis angle changed from 30\({}^{\circ}\) to 90\({}^{\circ}\) but has fewer effects on the strength and stiffness values.