Toughening mechanisms crack bridging

The microstructural observation on cracked samples shows that three toughening mechanisms, crack deflection, crack bridging and pullout caused by lamellas Ti AIC grains operate both in the monolithic material and in the composite, accounting for the high toughness of the matrix and the composite. 

Deflection rarely operates as the sole toughening mechanism in a system, although its contribution in some systems may be significant. Crack deflection, however, is a major aspect of bridge formation processes that leads to toughening via bridging ligaments. 

Toughening for whisker-reinforced composites has been shown to arise from two separate mechanisms frictional bridging of intact whiskers, and pullout of fractured whiskers, both of which are crack-wake phenomena. These bridging processes are shown schematically in Figure 13. The mechanics of whisker bridging have been addressed (52). The appHed stress intensity factor is given by ... 

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. 

Sue et al. (1997) reported results for the same LC epoxy monomer cured with various hardeners. KIe values could be increased up to 1.89 MPa m1/2. Observations of fracture surfaces indicated that crack bridging, crack branching, and crack deflection were the main toughening mechanisms. 

Research into the toughening behavior responsible in the composite materials shows that crack-whisker interaction resulting in crack bridging, whisker pull-out and crack deflection are the major toughening mechanisms. 

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