Crack blunting mechanisms

The changes in crack propagation types (from stable to stick-slip) are associated with the crack blunting mechanism, which is favored by high temperatures and low strain rates, conditions that decrease general trends cannot be extended to very high strain rates because a transition from isothermal to adiabatic conditions may... 

The fracture behavior of epoxy thermosets has been of growing interest since the mid-1960 s when investigations by Broutman and McGarry and Mostovoy and Ripling were published. Literature references seem to have peaked in the late 1970 s and early 1980 s when studies on crack blunting mechanisms speculations... 

Fig. 8.23 Diagram of two crack blunting mechanisms [12]. With kind permission of John Wiley and Sons...

Kinloch A J and Williams J G (1980) Crack blunting mechanisms in polymers, J Mater Sci 15 987-996. 

The reduced hardness and improved machinability are attributed primarily to the crack deflection process. It can be seen in Fig. 13.8 that the composite showed obvious particle pullout and significant crack deflection along interphase boundaries due to the weak interface bonding. The crack deflection mechanism (absorbing fracture energy and blunting crack tip) could lead to an increase in machinability. As described above, the thermal expansion... 

Crack Stability. At low test speeds, stable crack growth with an extended stress-whitened plastic zone and crack blunting occur by the same mechanisms as those involved in the kinetics of the plastic zone, namely, rubber cavitation followed by shear deformation of the matrix. The ability of the matrix to shear is controlled by its relaxation behavior, which therefore determines its plasticity and the deformation imposed on rubbery particles distant from the notch. 

A precondition of toughness enhancement is prevention of premature crack propagation there are some crack stop mechanisms including effects of crack tip blunting, reduction of critical stress concentrations, or reduction of crack length and crack propagation velocity (2). 

Toughening epoxy matrices using liquid reactive rubbers (such as carboxyl-terminated butadiene acrylonitrile, CTBN, or the amine-terminated equivalent, ATBN) has been widely reported in the literature. Spherical rubber particles of a proper size distribution (usually 1-5 pm) can effectively enhance the toughness through crack blunting or cavitation mechanisms. However, rubber modification of epoxies becomes... 

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