# Off-Axis Tension Behaviour of Unidirectional PEEK/AS4 Thermoplastic Composites
## 3 Results and Discussion
### Full-Field Displacements and Strains Analysis  
Full-field analysis of the displacements and strains is obtained based on the DIC technique and VSG method. DIC calculation, using subsets limits that the edges of the specimen and end-tabs, should be excluded.  
Figure 8a,b depicts the representative displacement fields in the X and Y directions before the failure of the 15\({}^{\circ}\) off-axis tension specimens. The deformation in the Y direction is significantly higher than that in the X direction. The distribution of displacements has a high degree of uniformity. No distortion is generated along the width of the specimen compared to the report by [PERSON] [27]. The strain fields show non-uniform distribution with scattered strain concentration points. The highest strain in the Y direction appears in the bottom left of the specimen and coincides with the final fracture location. The shear strain fields show homogeneous distribution. Few shear strain concentration points appear at the right edge of the specimen. The shear damage process of the matrix is affected by the coupled tensile stress, leading to discontinuously distributed strain fields. The displacement and strain fields of the 15\({}^{\circ}\) off-axis tension specimen, validate that by using oblique ted-tabs, the shear-induced in-plane bending moment is erased and a homogeneous stress state is ensured.  
The displacement and strain fields of specimens with off-axis angles of 30\({}^{\circ}\), 45\({}^{\circ}\) and 60\({}^{\circ}\) are alike. Therefore, a representative displacement and strain field of a 30\({}^{\circ}\) off-axis tension specimen is illustrated in Figure 9. The displacement fields have a uniform distribution along the fibre direction. The highest Y direction displacements emerge at the upper right corner of the specimen. The tension strain level is higher than that of the 15\({}^{\circ}\) off-axis tension specimen while the shear strain decreases. Both tension and shear strain fields show non-uniformity over the specimen, indicating mixed tension/shear stress states within the specimen [31]. The coupled stresses induced higher axial deformation compared to the 15\({}^{\circ}\) off-axis specimen. A comparison of the stress curves shows that the combined tension/shear stresses improve the damage and plastic deformation of the matrix.  
Figure 8: Representative DIC displacement and strain fields of 15\({}^{\circ}\) off-axis tension specimen: (**a**) X direction displacement field; (**b**) Y direction displacement field; (**c**) Shear strain field; (**d**) Tension strain field.  
The 75\({}^{\circ}\) and 90\({}^{\circ}\) off-axis tension specimens have similar deformation patterns. A representative 75\({}^{\circ}\) off-axis tension specimen is chosen to analyse the deformation and strain behaviours. Figure 10 shows the displacement and strain fields of a 75\({}^{\circ}\) off-axis tension specimen. The displacements are evenly allocated through the specimen width. The highest displacement level is at the top of the specimen and gradually declines along the specimen length. The shear strain is significantly lower than the tension strain. The strain fields show regularity over the specimen width. Shear strain concentration points dispersed over the specimen surface. A tension strain concentration area is located at the right edge of the specimen but not coincident with the final failure position. The 75\({}^{\circ}\) and 90\({}^{\circ}\) off-axis specimens have the smallest deformation before failure, indicating that the matrix shows less ductility under pure tension stress.