Morrow correction

To cope with mean stress effect and its continuous reduction as the material enters the low cycle fatigue region. Morrow [17, 18] has proposed the following correction that applies only to the elastic component of Eq. (6.10) 

The graphic representation of (6.17) is offered in Fig. 6.12. The fatigue life reduction due to mean stress is actually affecting only the elastic component Sg of the Sa-N curve moving toward higher life the transition point N,. However, the Morrow correction offers a drawback that clearly results from Fig. 6.13. 

The ratio between the elastic component of strain and the plastic one, zjzp, depends on the mean stress. It is clear that this cannot be necessarily true. Nothing prevents from having two hysteresis loops with the same ratio J p, but different mean stress. For example, the two cycles 1 and 2 of Fig. 6.13 imposed inside the 

If the independence of the ratio EjEp of is restored it introduces an effect of mean stress in the plastic domain that experience does not confirm. As to the Morrow correction (6.17), Walcher, Gary and Manson [20] observed that in alloys like Ti-6Al-4 V the fatigue strength coefficient a fCm be so high as to mask the mean stress effect. Therefore, they proposed a correction to the fatigue strength coefficient of the type 

6 Strain-Based Fatigue Analysis Low Cycle Fatigue 

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Fig. 6.12 Down-shift of fatigue strength curve according to Morrow correction that takes into consideration the mean stress only in the elastic component of total response...

Let s now use the Morrow correction (6.17) to take into consideration the mean stress. Table 6.13 lists the data relative to the three cycles to use in Eq. (6.17). Again, the number of cycles A has been calculated by iteration. Total damage is a little lower than that evaluated with the SWT method, therefore the allowable number of times that the time history of Fig. 6.37 can be applied is higher... 

Song H, Lim H, Paria BC, Matsumoto H, Swift LL, Morrow J, Bonventre JV, Dey SK (2002) Cytosolic phospholipase A2alpha is crucial [correction of A2alpha deficiency is crucial] for on-time embryo implantation that directs subsequent development. Development 129 2879—2889... 

Ince, A., Glinka, G., 2011. A modification of Morrow and Smith-Watson-Topper mean stress correction models. Fatigue Fract. Eng. Mater. Struct. 34, 854-867. 

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