Fatigue usage

Fatigue life estimates include both crack initiation and crack propagation. Crack initiation is estimated by determining the fatigue usage at a specific location that results from either actual or design-basis cyclic loads. Time to initiation can be predicted only if the sequence of the applied loads and recurrence frequency is well known. Such estimates are uncertain if the cyclic loading is random. 

Once a crack has initiated, either by fatigue or SCC, continued application of cyclic stresses can produce sub-critical crack growth. Fatigue crack growth calculations are based on the Paris crack growth relationship  

Kmax = maximum and minimum stress intensity factors during the loading cycle. 

The time dependent crack growth resulting from cyclic loading can be determined by 

The effect of R ratio (Kmin/Kmax) is also influence on crack growth. Increasing the R ratio increases cyclic crack growth. 

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In summary, the addition of the dynamic, thermal and fatigue effects of a natural convection cooldown on the System 80+ reactor vessel does not result in the vessel stresses or fatigue usage factor exceeding the allowable limits specified in the ASME B PV Code, Section III. Therefore, this issue is resolved for the System 80+ Standard Design. 

For consecutive application of cyclic loadings, experimental results indicate that there is a fatigue usage, or the fraction of fatigue life expended as measured by the ratios n /Ni or M2/N2 where Mi, M2 are the number of... 

The cumulative fatigue usage is 0.9, a value that is less than unity. Therefore fatigue failure is unlikely to occur. 

A stainless steel pressure vessel is subjected to 2000 pressure cycles where the stress alternates between zero and 200 MPa. This is followed by 10,000 cycles of thermal stresses that alternate between zero and 800 MPa. Determine the cumulative fatigue usage. 

Consider a stress discontinuity in the vessel of Problem 3 that gives rise to an effective stress concentration factor of 1.5 at which the nominal stress ranges from the table of Problem 3 is applicable. Determine the cumulative fatigue usage factor. 

Load combination Stress intensity range (MPa) Ratio (linearized to surface SI) Re factor (based on linearized SI) Stress amplitude (MPa) = Vi (SI range)(RRe) No. of cycles in) Allowable no. of cycles (A/) Fatigue usage factor (n/N)... 

Transient Alternating stress (MPa) Number of cycles in) Allowable no. of cycles (N) Fatigue usage (n/N)... 

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