Toughening mechanisms crack deflection

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. 

FIGURE 2. a) Schematic showing toughening through crack deflection at the fibei/matrix interface, b) SEM micrograph of fracture surface ofNextel 610/monazite/alumina composite tested at 1200°C showing fiber pullout and c) fracture surface ofNextel 610/alumina composite tested under the same conditions. Absence of crack deflection mechanism led to brittle failure in c). 

Figure 7 shows these results schematically for both twist and tilt crack deflections. Thus, for the stress intensity factor required to drive a crack at a tilt or twist angle, the appHed driving force must be increased over and above that required to propagate the crack under pure mode 1 loading conditions. Twist deflection out of plane is a more effective toughening mechanism than a simple tilt deflection out of plane. 

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. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

The influence of the shell thickness has also been reported (Sue et al., 1996a). By varying the shell thickness from 15% to 35% of the total thickness, the dispersion of CSR particles changed from a random to a well-dispersed but locally clustered distribution. The latter produced a higher toughening effect, probably due to the promotion of a crack-deflection mechanism, in addition to the other toughening mechanisms. 

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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