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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material becomes insoluble and infusible. This irreversible nature of cross-linking distinguishes thermosetting resins from thermoplastics, which soften when heated and can be reshaped. This network structure is responsible for the unique properties of thermosetting resins, such as high strength, resistance to heat and an elevate brittleness. Furthermore, during the curing process, volume shrinkage determines the onset of internal stress state in the matrix, which further limit the polymer resistance against crack initiation and propagation. To overcome the fracture toughness limitations of traditional epoxy adhesives, a second microphase can be incorporated into the brittle epoxy matrix, comprising dispersed toughening agents such as fibers or particles. This changes the internal crack propagation process, thereby enhancing the adhesive toughness and increasing the fracture energy. It should be noted that, good particle-matrix bonding is a fundamental condition for enhanced fracture toughness efficiency, Kinloch et al. (1985). Similarly, the even dispersion of tougheners particles is a crucial factor in determining the final properties of the toughened adhesive. In this section, the main toughening mechanisms induced by the presence of disperse agents are described, with particular reference to non-homogenous toughening methods, i.e. those involving phase separation between the modifier and the epoxy matrix. Toughness enhancements can alternatively be achieved through homogeneous methods if tougheners and matrix are interpenetrated and interlocked to produce a homogeneous structure. Four such types of toughening methods, the reader is referenced to Wang et al (2018) and Zhang et al. (2018).

(a)

(b)

(c) (d) Fig. 1. Toughening mechanisms of modified-epoxy polymers. (a) and (b) crack pinning and deflection, Chandrasekaran et al. (2014) and Quaresimin et al. (2016); (c) crack bridging, Ozcan et al. (2019); (d) cavitation and plastic void growth, Yee and Pearson (1986).

2.1. Crack pinning and deflection

The crack pinning and deflection mechanisms, first proposed by Lange (1970) and Kinloch et al. (1985), play a crucial role in enhancing the toughness of epoxy matrices. Within these mechanisms, toughening agents embedded into the epoxy matrix act as obstacles against crack propagation.