PSI - Issue 64

Angelo Savio Calabrese et al. / Procedia Structural Integrity 64 (2024) 1832–1839 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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As illustrated in Figure 1a and b, in a graphene-oxide-enriched epoxy, Chandrasekaran et al. (2014) and Quaresimin et al. (2016), toughener particles create an environment where the crack tip struggles to penetrate through rigid and firmly bonded particles. Consequently, propagating cracks tend to pin and bow out between the particles, leading to the development of multiple secondary cracks as they navigate around these obstructions (see Figure 1b). Given that the energy required for crack propagation is directly proportional to its length, the presence of toughening agents demands additional energy input, thus enhancing the material's overall toughness and resistance to fracture. The efficacy of crack pinning and deflection toughening mechanisms hinges on several factors, including the type, size, and distribution of additives within the epoxy matrix. Generally, larger and stiffer toughening agents tend to exert stronger pinning effects, whereas smaller or more flexible additives may facilitate enhanced energy dissipation mechanisms due to crack deflection. Crack pinning and deflection can also be caused by the formation of an interphase or immobilised layer of polymer around the particles, attributed to modifications in the polymer chains occurring during matrix curing at particle surface locations, Odegard et al. (2005). 2.2. Crack bridging When the toughened epoxy material experiences stress, cracks may initiate and propagate through the matrix. The addition of particles of higher dimensions than the crack process zone can provide a bridged constraint effect to the cracked surfaces, which, instead of completely separating, are first required to break or pull out the bridging reinforcement. Figure 1c shows the crack tip region within an epoxy nanocomposite containing a 1% weight percentage of graphene nanoparticles, Ozcan et al. (2019). As inferred by Figure 1c, the presence of graphene inserts within epoxy provided a bridged constraint effect that contributed to restrict the growth of the crack tip, and at same time acts as an anchor to the cracks at the bridge point. A similar finding was also found by Wu et al. (2015). The bridging action of the reinforcing agents helps to distribute stress around the crack tip, reducing stress concentrations and preventing or delaying further propagation of the crack. Additionally, the bridging reinforcement absorbs energy as it resists crack propagation, increasing the material's toughness. In toughened epoxy systems with a high density of reinforcing agents, multiple crack bridging events may occur as cracks propagate through the material. Each bridging event contributes to dissipating energy and hindering crack propagation, thereby enhancing the material's toughness and durability. 2.3. Cavitation and plastic void growth The toughening mechanisms associated with micrometer sized particles have frequently been shown to be due to cavitation of toughener particles followed by plastic deformation of the matrix, Yee and Pearson (1986) and Mafi and Ebrahimi (2008). Figure 1d shows the stress-whitened fractured surface of a rubber-toughened epoxy polymer. As shown in Figure 1d, Yee and Pearson (1986), when the material is subjected to stress, such as impact or deformation, localized stress concentrations occur at high-stress regions like crack tips or interfaces between the epoxy and toughener particles. These stress concentrations can exceed the cavitation threshold of the particles, leading to the nucleation of cavities within them. As stress increases, these cavities grow within the toughener particles, inducing a plastic dilatation of the matrix around the cavitated particles, according to the plastic void growth mechanism. This process absorbs energy that would otherwise contribute to crack propagation, effectively toughening the material. The presence of cavities within the toughener particles disrupts crack propagation paths, deflecting cracks and dissipating energy, thus enhancing the material's fracture toughness and impact resistance. 3. Toughening methods of epoxy polymers To overcome the inherent limitations of epoxy adhesives in terms of fracture toughness, incorporating a secondary microphase of toughening agents into the pre-polymer mixture can prove highly effective. These toughening agents can be either organic, such as rubber, polymeric, or inorganic particles like silica, graphene, or alumina particles, whose addition enhances the cured polymer's ability to resist crack propagation.