Bulk Fracture of Networks

The fracture energy of unsaturated polyesters (UP), vinyl esters (VE), and phenolic resins, is less than 200 at room temperature. Epoxy networks can exhibit higher values but always lower than those of thermoplastics of similar Tg, as polycarbonate, polyetherimide, or polyphenylene ether. 

The fracture energy cannot be related to the failure of chemical bonds which may contribute only with a few Jm . Furthermore, the possibility of crazing is not allowed in thermosets because fibrils cannot exist due to the high crosslink density. So, in the case of high-Tg cross-linked materials the main source of energy absorption before failure is the yielding of the network. This assumption is obviously valid only above the ductile-brittle transition temperature (Fig. 12.5), where yielding is temperature-dependent. 

The basis for the relationship between fracture and yielding is related to crack-tip blunting which causes a decrease of the local stress concentration. So, higher applied loads are required to produce failure. A decrease of (Ty will increase the values of Kjc and Gj. This is the general trend that is observed therefore, it is not possible to pretend to increase both the yield stress and the fracture resistance of a neat thermoset. However, this may be accomplished by including a modifier in the formulation (Chapters 8 and 13). The resulting heterogeneous structure promotes new deformation mechanisms that increase the fracture resistance. 

Every factor that increases the value of Oy will produce a decrease in the fracture resistance. Physical aging acts in this direction (Lin et ai, 1986 Cook et al., 1999). 

The effect of structural parameters on and Gjc can be summarized as follows any change in the chemical structure (use of monomers with different molar masses, use of nonstoichiometric formulations, etc.), will produce a variation in Tg this will directly affect the value of Oy, which, in turn, will modify the fracture resistance. An increase in Tg will lead to an increase in Oy, measured at constant temperature, and to a decrease in the fracture resistance. This is why high-Tg epoxy networks exhibit very low values of fracture resistance. 

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