Crack bridging mechanism

Furthermore, the model makes it possible to separate the contributions of the three toughening mechanisms as a function of temperature (Fig. 13.6). At high temperatures, the crack-bridging mechanism plays a minor role the void-growth mechanism is very sensitive to temperature and can be completely suppressed at low temperatures. Shear yielding is the main mechanism, except at very high test temperatures where cavitation plays the major role. The contribution of shear yielding depends on the difference between the test temperature and Tg, as discussed in Chapter 12. 

A threshold of interfacial adhesion between both phases is needed to (a) promote the cavitation mechanism and (b) activate the crack-bridging mechanism. For rubbery particles, the former contributes much more than the latter to the total fracture energy. Adhesion is achieved by the use of functionalized rubbers that become covalently bonded to the matrix. Higher toughness values have been reported by the use of functionalized rubbers (Kinloch, 1989 Huang et al., 1993b). However, these experimental results also reflect the effect of other changes (particle size distribution,... 

From a mechanical point of view, a critical level of adhesion (chemical or physical) is required between TP and thermoset TS phases to insure stress and strain transfers and to favor ductile stretching of TP particles in the crack-bridging mechanism (Pearson, 1993 Hodgkin et al., 1998). 

For the in-situ phase-separation process, an increase in the TP molar mass modifies the phase diagram (it lowers cnl), increases the viscosity - which is a drawback for processing and impregnation of composites - but increases the fracture energy associated with the crack-bridging mechanism. 

B. N. Cox and L. R. F. Rose, Time or Cycle Dependent Crack Bridging, Mechanics of Material, submitted. 

FIGURE 18.19 Illustration of a crack bridging mechanism with... 

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