Microstructure and local mechanical properties of joined CF-PEEK

When CF-PEEK is exposed to high temperatures and shear rate, an intense flow of broken fibers and low viscosity matrix is induced in the vicinities of the rivet shaft, which is enhanced by the

plastic deformation of the rivet tip, as discussed in Section 6.3.3. Therefore, the final microstructure of the composite is highly dependent on heat input, joining conditions, and the geometry of the rivet tip. Figure 6.10 illustrates a typical microstructure of CF-PEEK in the friction riveted joints where the rivet tip assumed a bell shape (S2, as described in Section 6.3.2).

Figure 6.10 a) Typical cross-section of Ti6Al4V/ CF-PEEK friction riveted, showing the CTMAZ and the squeezed material, indicating three regions of microstructure interest as detailed in b) to d); b) and c) reoriented fiber bundles and reconsolidated composite material; d) voids; and e) fiber-matrix debonding

underneath the rivet tip. (Joining parameters RS: 15000 rpm, FFI: 5 kN, FFII: 10 kN, DF: 7.5 mm, CP: 0.2 MPa)

A composite thermomechanically affected zone (CTMAZ) was identified surrounding the rivet (Figure 6.10-a) and this is characterized by reoriented fiber bundles, volumetric flaws, and reconsolidated material that was formed during the unsteady state viscous dissipation phase (II, Figure 6.1). Throughout this phase, broken fibers embedded in molten polymer flow partially outwards (forming a flash) and between the composite parts (squeezed material), as shown by the yellow dotted lines in Figure 6.10-a and -b. The circular pattern assumed by the composite flow – as reported in Section 6.1 – may realign the broken fibers in vicinity of the rivet, resulting in their near-circular appearance (Figure 6.10-c). Altmeyer et al. [26] defined this region as a composite stir zone (CSZ), where short fibers are stirred by the rotating rivet during FricRiveting. The rivet penetration reoriented the fiber bundles upwards in the proximity of the rivet, as indicated by the white arrow in Figure 6.10-d, which may have been enhanced by the upward flow of the composite. Internal flaws, including voids and delamination, could also be detected in the CTMAZ. The low viscosity of the molten polymer assists the entrapment of air pockets and gases evolved from the thermal degradation of the matrix, as addressed in Section 6.3.1, which remain as voids in the joint due to the fast cooling rate (voids are highlighted by yellow arrows in Figure 6.10-b and -d). Figure 6.10-e depicts matrix-fiber debonding in the composite underneath the rivet tip. Two effects could contribute to its formation: a significant difference in coefficients of thermal expansion between fiber and matrix,

which with a fast cooling rate generates residual thermal stresses, or by an extensive shear imposed by the rivet, which compromises the fiber-matrix interface leading to delamination. The first contribution would occur during the cooling phase, whereas the second would happen during the joining process.

By increasing the heat input the rivet tip over deforms, which has an influence on the composite flow and the extent of internal flaws in the friction riveted joint. Figure 6.11 shows an example of a friction riveted joint that formed an inverted bell shape (S3). Owing to greater rivet plastic deformation and the higher process temperature (1064 ± 159) °C localized in the vicinity of the rivet for a longer time, more of the composite is thermomechanically affected, thereby enlarging the CTMAZ, as outlined by the white dashed line in Figure 6.11-a. Consequently, more voids were formed (Figure 6.11-b). The overdeformation displaced more fiber bundles upwards and led to turbulence within the composite in its vicinity, shown by the formation of a vortex (dashed arrows in Figure 6.11-b). Additionally, the volume of molten polymer and broken fibers that was forced to flow outwards increased, leading to composite through-thickness delamination, where composite flowed within a fiber bundle, as shown in Figure 6.11-c. Therefore, the shape of the rivet tip compromises the integrity of the composite, which may degrade the mechanical properties of the friction riveted joints.

Figure 6.11 a) Cross-section of Ti6Al4V/ CF-PEEK friction riveted where the rivet tip was overdeformed, showing the CTMAZ along with two regions of microstructure interest, as detailed in b) and c); b) the reoriented fiber bundles and formation of a vortex in the reconsolidated composite material; c) composite

delamination. (Joining parameters - RS: 15000 rpm, FFI: 5 kN, FFII: 15 kN, DF: 7.5 mm, CP: 0.26 MPa)

The local mechanical properties of the inner and outer regions of the CTMAZ were evaluated using nanohardness measurement, as described in Section 5.2.8.2. The nanohardness method has been widely used to describe the viscoelastic and dynamic mechanical properties of polymer composites, particularly of the matrix, where a small indentation size is required to avoid interference of the fibers with the measurement [174]. Figure 6.12 illustrates the dynamic indentation modulus

and hardness of the PEEK base material (BM) and PEEK CTMAZ. The load-displacement curves obtained from the nanohardness test are presented in Appendix E. At shallow indentation depths, up to 100 nm, any increase of the modulus and hardness is reported to be a combination of errors during the surface determination, indentation size effects, and near-to-surface modifications such as localized oxidation of the sample surface [216]. The average values of these responses are near constant, between 200 nm and 800 nm for all replicates, and therefore were used for the evaluation of properties. A 33 % decrease in elastic modulus from (5.4 ± 0.04) GPa to (3.6 ± 0.03) GPa and a 53 % decrease in hardness from (0.34 ± 0.006) GPa to (0.16 ± 0.002) GPa was observed by comparing the BM to the CTMAZ. This result complies with the physicochemical changes of the composite, as discussed in Section 6.3.1. The thermomechanical decomposition of PEEK, mainly by chain scission and possible oxidation, results in a lower molecular weight, which softens the polymer and therefore decreases its strength. In addition, any strengthening effect from the spherulites in semicrystalline polymers such as PEEK [128,130,216] is also impaired by a decrease in the degree of crystallinity caused by the thermal degradation of the friction riveted joints.

Figure 6.12 Dynamic indentation for a) modulus and b) hardness of PEEK as a function of displacement into the surface of BM and the CTMAZ of the friction riveted joint. (Joining parameters RS: 15000 rpm,

FFI: 5 kN, FFII: 10 kN, DF: 7.5 mm, CP: 0.2 MPa)