Bonding mechanisms and zones

The interaction between the metal and the composite is of upmost importance as it directly influences the strength of the joint and assists understanding of the failure mechanisms. As discussed in Section 3.3, there are essentially two bonding mechanisms that promote the strength of friction riveted joints: mechanical anchoring and adhesion forces. Mechanical anchoring, also termed

macromechanical interlocking, contributes most of the joint’s strength owing to the mechanical interference of the rivet tip widening [147], while reconsolidated polymer against the rivet shaft promotes additional adhesion forces. Although occurring in overlapped friction riveted joints of CFRP, other subcategories of such bonding mechanisms were explored in this work owing to the complex material flow described in Section 6.1.

Besides macromechanical interlocking (Figure 6.13-a), micromechanical interlocking occurs along the length of the rivet shaft, which is considered an adhesion mechanism [217]. Figure 6.13-b shows fiber and polymer embedding in the rough surface of the rivet. The Coulomb friction developed between the solid fiber network and the rivet at the initial stages of the joining process leads to material wear, increasing the roughness of the rivet surface. Figure 6.13-c shows irregular metal debris in the material flash probably the result of this wearing process. Moreover, the hot compressive work imposed in the rivet tip may also create additional irregularities on the rivet surface as the softened metal encounters solid fibers. The low viscosity polymer and broken fibers flow into such asperities on the metal surface, and when consolidated, promotes interlocking on a microscale.

Figure 6.13 a) Overview of a friction riveted joint showing the macromechanical interlocking through the rivet tip widening; b) micromechanical interlocking by embedding of fibers and PEEK matrix; c) detail of the

outward flash material showing metal debris resulting from the wearing process between fibers and rivet.

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

The squeezed material consolidated between the composite parts contributes additional adhesion forces, by means of wetting and molecule interdiffusion over the composite interfaces.

Figure 6.14-b shows the top view of the squeezed material obtained by µCT. During rivet insertion, a low viscosity PEEK molten layer flows through the composite plates, owing to the combined effect of centripetal forces (Fcp) imposed by the rotating rivet and centrifugal forces (Fcf) imposed by axial joining forces. However, the presence of broken fibers in the squeezed material can impair a stable flow and consequently lead to an inhomogeneous consolidation of the squeezed material. By analysing a cross-section of the squeezed material, three regions were observed, as shown in Figure 6.14-c.

Figure 6.14 a) Overview of a friction riveted joint, showing its mid-plane where low viscous polymer and broken fiber flow, forming the squeezed material; b) top view of the squeezed material acquired by X-ray

µCT; c) cross-section of the squeezed material, showing three regions of material consolidation. (Joining parameters - RS: 15000 rpm, FFI: 5 kN, FFII: 10 kN, DF: 7.5 mm, CP: 0.2 MPa)

The flow front, distant from the rivet shaft (Region I, Figure 6.14-c), incompletely wets the inner surfaces of the composites and cools down rapidly. As soon as the viscosity of the material increases, any air entrapped in the highly viscous material can no longer escape, leading to a reconsolidated material with high porosity and therefore low adhesion efficiency. Goushegir, Dos Santos, and Amancio-Filho [208] addressed a similar formation in CF-PPS/AA2024 friction spot joints. The low viscous PPS, molten during the joining process, wetted the surface of the aluminum and, owing to its fast cooling rate, entrapped air in the outer region of the joining area [208]. As the flow front cools down, the flow of still soft polymer is restricted and becomes denser, decreasing the internal flaws. Consequently, a homogeneous intermediate region is formed, without porosity (Region II, Figure 6.14-c). Additionally, the interfaces between the squeezed material and composite plates cannot be distinguished in Region II, which indicates molecule interdiffusion. When two or more polymeric plates are exposed to sufficient energy through temperature and pressure, the polymer chains gain mobility and may diffuse across interfaces to reach a favorable conformation, and thereby decrease the entropy of the system [218]. In Region III of Figure 6.14-c, which is close to the rivet, although there is polymeric interdiffusion the presence of internal flaws indicate unstable material flow and possible thermal decomposition of the composite matrix – addressed in Section 6.3.1. In this region the composite material is exposed to high temperatures for a longer time and it flows through a restricted space, which can exert shear forces on the low viscous material and consequently flow turbulences, as reported with viscous polymer systems [195].

Bearing in mind that interdiffusion of polymeric molecules promotes higher adhesion forces than incomplete wetting [219], leads one to expect that the failure mechanisms governing Region I will be adhesive failure, while in Regions II and III it will more likely be cohesive failure. The failure mechanisms of the friction riveted joints under shear loading are thoroughly investigated in the next chapter. Despite the existence of additional adhesion forces, owing to the squeezed material, it leads to inevitable separation of the composite plates, which can compromise the durability of the joints, as discussed in Chapter 9. The amount of squeezed material and extension of the adhesion regions in Figure 6.14-c is directly dependent on heat generation during the joining process and the plastic deformation of the rivet tip, and in turn these are influenced by the process parameters. With a larger widening of the rivet tip, more softened composite material is displaced from the joining area, and some of this to the composite overlap area. Figure 6.15 shows the linear dependency of the squeezed material area (ASM indicated in Figure 6.14-b) with plastic deformation of the rivet, measured indirectly by volumetric ratio VR (see Section 5.2.5.2). The influence of process parameters on the joint formation, and the bonding mechanisms discussed here, were indirectly addressed in Section 6.5.2 about the DoE for optimizing mechanical performance of the joint.

Figure 6.15 Linear correlation between volumetric ratio (VR) and area of squeezed material (ASM).