Crack blunting

Fig. 8.62 Crack-blunting model of stage II crack progagation (after Laird) ...

Tirosh J. (1973). The effect of plasticity and crack blunting on the stress distribution in orthotropic composite materials. ASME Trans. J. Appl. Mech. 40, 785-790. 

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... 

Yet, for systems A and C, the measured fracture energies remain low compared with the critical fracture energy of the bulk aluminum 10 J Moreover, we do not observe islands of passivation material on the A1 fracture surface and, inversely, we do not observe A1 on debonded surfaces of the passivation films. This suggests that the loss of interfacial adhesion is close to a brittle fracture process despite the influence of plasticity of the A1 substrate and crack blunting at the interface. This sort of brittle mode of interfacial failure, including plastic flow in a ductile material (the substrate), has been observed or discussed for a sapphire/Au interface. ... 

These results demonstrate that the embrittlement of the PE implants accompanies a microhardening of a surface layer and an increase in crystallinity. The two pieces of evidence are complementary and imply a reduction in the crack-blunting ability of the material, i.e. a diminution of the number of interlamellar tie molecules which connect adjacent lamellar stacks. In consequence the elastic properties of the material diminish and cause the material to microharden during wear. The increase in microhardness at the wear surface is partly because the amorphous component decreases in quantity and partly because its chemical nature changes as it undergoes simultaneous microhardening and loss of elasticity. 

Beltz G. E., Lipkin D. M. and Fischer L. L., Role of Crack Blunting in Ductile Versus Brittle Response of Crystalline Materials, Phys. Rev. Lett. 82, 4468 (1999). 

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. 

In addition (see Figure 6), the samples containing 2.2 wt % water showed large crack arrest lines at high values of AK (AK > 3.7 MPa/n). These arrest lines, which were associated with the periodic interruption of the FCP test to read the crack-tip position, imply the occurrence of creep. (If creep is closely confined to the crack-tip, it may well be associated with crack blunting.) Some finer lines were observed as well (for discussion see the following section). 

The cohesive tensile stress is greater than the cohesive strength in shear, in which case shear breakdown will occur (i.e., dislocations will move away from the crack tip) and the crack will lose its atomic sharpness. In other words, the emission of dislocations from the crack tip, as shown in Fig. 11.10a, will move material away from the crack tip, absorbing energy and causing crack blunting, as shown in Fig. II.IOZ). 

Of primary importance in moist environments is the plasticization, or softening, of the adhesive, a process that depresses Jg and lowers the modulus and strength of the elastomer [89-91]. Plasticization of the adhesive may also allow disengagement from a microrough adherend surface to reduce physical bonding and thus reduce joint strength and durability [37]. On the other hand, it may allow stress relaxation or crack blunting and improve durability [92]. 

We compare the three different types of solutions in order to develop a complementary insight into crack blunting and its meaning for understanding the factors that affect ductile crack growth, particularly in polymers. 

---