Influence of Fatigue Crack Growth on Strength

The material parameters, the factor Aa/AN), and the SCCG exponent h, are each dependent on mean stress and temperature. Here, N is the number ofload cycles and Nf is the numbers of cydes to failure. 

Cyclic crack growth influences lifetime in similar manner to SCCG (see previous section). Under conditions where cydic fatigue is the dominant damage mechanism, the cydic lifetime can be determined in an analogous way to the procedures discussed above. For example, the equation analogous to Eq. (27) is [14]  

It should be noted that, in the few cases reported, the SCCG exponent for cyclic fatigue is much larger than unity it 1 [105,108,109]. Thus, like SCCG, the cyclic fatigue life is heavily dependent on the applied stress range and the size of the initial flaws. 

The action of cyclic fatigue and of SCCG can be assumed to be independent, and the crack advance due to both mechanisms can be simply added. However, as both growth rates depend heavily on the applied load, and since the SCCG exponents are, in general, different, it is quite likely that the growth rates are very different. The simplified model description can then be used whereby only one of the two mechanisms is dominant. 

Proof testing is an often (routinely) used technique to guarantee a component s minimum service life [2, 4, 102, 110]. In order to clarify the basic ideas of proof testing, reference is made again to a uniaxial tensile and homogeneous stress state. 

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