Fatigue 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... 

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. 

The Paris exponent n is in the range 2-4 for most metals and alloys, but is often much larger in ceramics. This implies that crack advance in ceramics after fatigue crack nucleation is much more rapid and occurs over a narrow range of... 

Lukas, P. Fatigue crack nucleation and microstracture. ASM Handb. Fatigue Fract. 19 (1997)... 

Fig. 3.76 Fatigue crack nucleation at subsurface inclusions in two ion-nitrided specimens of 34CrNiMo6 steel (reproduced with permission of [73])...

As to whether any of these defects are critical depends on their extent, position, and the nature of the applied stresses. Their presence is more likely to be indicative of poor joint manufacture than of any impending failure site, especially for short-term loading. Over the long term, these defects may allow faster ingress of water or aggressive substances, or provide the sites for fatigue crack nucleation. It still, therefore, remains necessary to look for these defects. 

F. Determining Cycles-to-Failure Utilizing Fatigue Crack Nucleation Theory... 

Fine, M.E. Stolkarts, V. Keer, L.M. Fatigue crack nucleation assisted by thermal activation. Mater. Sci. Eng. 1999, A272, 5-9. 

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