Global mechanical performance and fracture analysis

The friction riveted joints were weighed shortly after removal of the joints from the aging chamber (within an hour) and then mechanically tested under lap shear testing. The mass change, displacement at break, and residual strength, as a function of days of exposure were compared with unaged samples, as illustrated in Figure 9.6. Up to 14 days of aging, the joint strength stayed virtually constant, but after 28 days an increase of approximately 23 % was observed. The mass change and the displacement at break both increased gradually over exposure time, reaching (2.8 ± 0.3) % and (4.1 ± 0.2) mm, respectively, after 28 days of exposure.

Figure 9.6 Mechanical strength along with displacement at break (Db) and mass change (Mn) of the friction riveted joints as a function of days of aging exposure.

The water uptake in the CFRP composite can either be effected by capillary flow through volumetric defects or by diffusion, which relies on molecular arrangements, and therefore depends on the polymer’s degree of crystallinity [260]. Polymer crystals are usually ordered toward secondary hydrogen bonds which when in the presence of water are partially replaced by water ions [260].

Higher crystallinity provides more potential sites for electrostatic absorption of water molecules [21].

For these reasons, after 28 days of aging the reported increase in crystallinity for the friction riveted joints (Table 9.1) may explain the reason for their increase in Mn. The process-induced defects at the metal-composite interface in the joints (Figure 6.10) may also have accelerated water uptake by capillary flow.

As is well known for polymers, water molecules in the composite work as a plasticizer, softening the composite matrix, which may explain the enhancement of Db in the aged friction riveted joints [260]. An increase in joint ductility was also evidenced by an increase of the bearing area, as shown in Figure 9.7. After 28 days of exposure, the localized plastic deformation at the edges of the composite hole had increased by 47 %.

Figure 9.7 Bearing area of failed friction riveted joints loaded under shear as a function of days of hydrothermal aging exposure.

On the other hand, the water swelling caused by aging exposure can impair the composite, and consequently the joint strength, because it compromises the integrity of the fiber-matrix interface and therefore the material’s load carry ability [80]. By contrast, as shown in Figure 9.6, the aged joints presented a higher strength after 28 days of exposure. As explained earlier, the hydrothermal aging enhanced the mechanical properties of the fiber-matrix interface by transcrystallinity. The nucleation of crystals on the fiber surface effectively decreases the discontinuity in moduli between the fiber and the matrix, leading to better stress transfer across the fiber-matrix interface [124].

Similar to the surface sizing of carbon fibers, transcrystallinity optimizes fiber-matrix compatibility and improves the mechanical performance of the composite [21]. As well as the crystallinity effect, thermal aging can release residual stress remained from the joining process, and may also have a positive effect on the mechanical properties, as reported by Yang et al. [76].

Figure 9.8-a shows a typical cross-section of a failed friction riveted joint after 28 days of hydrothermal aging. Instead of the kink band formation that is typically seem in a lap shear tested friction riveted joint, as described in Section 7.1.2 (Figure 7.6), buckling (Figure 9.8-b) and fiber debonding were identified (Figure 9.8-c) preferentially within a fiber bundle oriented 0°. Due to an increase in PEEK ductility by water uptake, it is expected that the matrix accommodate better the out-of-plane displacement of fiber bundles when under the compression imposed by the rivet in lap shear testing. Particularly for a fiber bundle oriented 0°, its high aspect ratio allowed for higher deflection when loaded under compression. As fiber deflection is accommodated within the ductile matrix, the onset of fiber breakage by transmitted shear is apparently delayed. The ductile matrix has however lower global effect on the secondary bending of the aged joints under lap shear test.

Although the displacement at break increased, the out-of-plane displacement of the neutral line presented similar behavior as unaged joints, and therefore similar peel stresses are expected. The realignment of the overlap area to the neural line of aged and unaged was analyzed through digital image correlation (DIC) and the results were presented in Appendix N.

Figure 9.8 a) Typical cross-section of a friction riveted joint after 28 days of hydrothermal aging, showing high plastic deformation of the composite in rivet proximity; b) microbuckling, and c) intralaminar fiber

debonding of the fiber bundles oriented 0° in the joint bearing area.

Moreover, as any transcrystalline interphase induced by hydrothermal aging is expected to enhance adhesion at the fiber-matrix interface, one may assume that load transferability between these phases increased. That would shift the fiber-matrix interface damage to within the fiber bundle during joint loading, which would explain the fiber debonding in Figure 9.8-c. Authors [22,266]

proposed that moisture uptake in CFRP causes compressive strengthening by an optimized load transfer between fiber and matrix, which in turn increases shear resistance of such an interface.

Figure 9.9 shows the fracture surface of a joint after 28 days of hydrothermal aging. As explained in Section 9.1.1, the exposed surface of the composite exhibited cavitation probably induced by the galvanic coupling and hydrolysis of the PEEK. Similar defect was identified in the composite surfaces, inside the overlap area (Figure 9.9b) and close to the consolidated squeezed material (Figure 9.9-c). Despite the fact that such pit indicates lack of joint sealing, they did not affect the mechanical performance of the joints as shown in Figure 9.6. Thus, the corrosion can be neglected as detrimental effect on aged friction riveted joints. Additionally, the plastic deformed zone showed to fail mainly in a ductile manner (Figure 9.9-d), similar to the unaged joints (see Figure 7.4).

Figure 9.9 a) Typical fracture surface of the friction riveted joints after 28 days of hydrothermal aging; SEM images of the composite surfaces showing corrosion-induced cavitation from b) the lower, and c) the upper

composite parts inside the overlap area; d) detailed view of the ductile failure of the squeezed material.

From a microstructural analysis of the joint fracture surface, an indication of transcrystallinity could be observed by the exposed fibers in the squeezed material, compared to the unaged sample.

Figure 9.10-a shows an unfeatured fiber surface of an unaged friction riveted joint and Figure 9.10-b shows highly oriented fibrils emerging from the fiber surface of a joint hydrothermally aged for 28 days. The radial growth of crystals on the fiber and the formation of elongated fibrils familiar in several polymer composites [124,134,171,267,268] as characteristic transcrystalline morphology.

Figure 9.10 SEM images of the fiber surface of a) unaged; and b) friction riveted joints after 28 days of aging, showing an unfeatured and radial PEEK fibrils growth from the fiber surface; c) illustration of the dependence between the degree of crystallinity, transcrystallinity, and hence fiber-matrix adhesion.

The dependence of bonding mechanisms in the fiber-matrix interface on the degree of crystallinity (Xc) is schematically shown in Figure 9.10-c. Higher crystallinity creates more potential electrostatic sites for adsorption of the matrix into the fiber, which may have created the rougher fiber-matrix interface seen in Figure 9.10-b. As the strong fiber-matrix interface is loaded under shear, less adhesive failure takes place in such an interface and the bulk matrix fails cohesively instead. A deeper understanding of the transcrystallinity morphology and its nucleation and growth kinetics are beyond the scope of the work, but some literature on the topic can be found in [132].

To summarize: It can be concluded that water uptake along with the temperature led to oxidation of the metal nut and washer, as well as to cavitation of the composite surface, which did not play any role in joint performance. Moreover, transcrystallinity was induced in the composite by hydrothermal aging, preferentially in the thermomechanically affected material close to the rivet, which in turn led to better local and global mechanical properties of the friction riveted joints.