Crack nucleation

The complexity of these chemical and mechanical interactions is such that each metal-environment system must be examined on an individual basis to determine the important processes influencing corrosion fatigue crack nucleation and growth rates. Thus, in the ensuing sections, examples are quoted to illustrate commonly occurring phenomena or establish more general principles with reasonably wide applicability for particular classes of metal/environment combinations. It should be noted, however, that when... 

Much more has been published over the last 50 years concerning the crack growth approach" than the crack nucleation approach for rubber. Consequently, the crack growth approach enjoys a relatively more mature position than the crack nucleation approach. In recent years, the development of equivalence parameters that work in cases involving multiaxial loading has renewed interest in the crack nucleation approach. 

In order to apply the crack nucleation approach, the mechanical state of the material must be quantified at each point by a suitable parameter. Traditional parameters have included, for example, the maximum principal stress or strain, or the strain energy density. Maximum principal strain and stress reflect that cracks in rubber often initiate on a plane normal to the loading direction. Strain energy density has sometimes been applied as a parameter for crack nucleation due to its connection to fracture mechanics for the case of edge-cracked strips under simple tension loading. ... 

Due to the plane-specific namre of crack nucleation under multiaxial tests. Mars and Fatemi proposed the cracking energy density as an equivalence parameter that represents the portion of strain energy density available to be released as crack growth on a specific material plane. The form of the cracking energy density Wc is... 

In a recent study, Saintier et al. ° investigated the multiaxial effects on fatigue crack nucleation and growth in natural mbber. They found that the same mechanisms of decohesion and cavitation of inclusions that cause crack nucleation and crack growth in uniaxial experiments were responsible for the crack behavior in multiaxial experiments. They studied crack orientations for nonproportional multiaxial fatigue loadings and found them to be related to the direction of the maximum first principal stress of a cycle when material plane rotations are taken into account. This method accounts for material rotations in the analysis due to the displacement of planes associated with large strain conditions. 

The relationship between load level and fatigue crack nucleation lives is clearly evident from the e-N and S-N plots for the material. A sample e-N plot for natural rubber is presented in Figure 25.4. An increase in the load level of the applied cycles results in a shorter fatigue life. Strain levels below the fatigue life threshold produce inhnite fatigue lives. The relationship between the load and the fatigue life follows a linear relation when plotted on a log-log scale. 

Mars, W.V. and Fatemi, A., Fatigue crack nucleation and growth in fllled natural rubber. Fatigue Fract. Eng. Mater. Struct., 26, 779, 2003. 

Hagan J. T., 1979, Micromechanics of crack nucleation during indentations, J. Mater. Sci., 14, 2975-2980. 

Brittle Surface Degradation and Mechanism of Crack Nucleation under Indentation... 

In order to start the multiscale modeling, internal state variables were adopted to reflect void/crack nucleation, void growth, and void coalescence from the casting microstructural features (porosity and particles) under different temperatures, strain rates, and deformation paths [115, 116, 221, 283]. Furthermore, internal state variables were used to reflect the dislocation density evolution that affects the work hardening rate and, thus, stress state under different temperatures and strain rates [25, 283-285]. In order to determine the pertinent effects of the microstructural features to be admitted into the internal state variable theory, several different length scale analyses were performed. Once the pertinent microstructural features were determined and included in the macroscale internal state variable model, notch tests [216, 286] and control arm tests were performed to validate the model s precision. After the validation process, optimization studies were performed to reduce the weight of the control arm [287-289]. 

M.F. Horstemeyer, A.M. Gokhale A void-crack nucleation model for ductile metals. Int. J. Solids Struct. 36, 5029-5055 (1999)... 

M.F. Horstemeyer et al Using a micromechanical finite element parametric study to motivate a phenomenological macroscale model for void/crack nucleation in aluminum with a hard second phase. Mech. Matls. 35, 675-687 (2003)... 

Schematic diagram illustrating brittle fracture. It is typical of the temperature range below 0.87g. A crack nucleates at a pre-existing flow or craze and propagates unstably. 

---