Mean stress effect

5 Stress-Based Fatigue Analysis High Cycle Fatigue 

Alternatively, introducing the slope G of the Goodman line. Fig. 5.37, the reduced stress amplitude or fatigue limit can be expressed as 

For instance, in the case of the aluminum alloy considered in Fig. 5.31, for R — 0.6 and N — 10 cycles the mean stress is 262.5 MPa so that the term between parenthesis in (5.48) is equal to 0.45. Since for / = —1 the fatigue limit oy is equal to 135 MPa, the new fatigue limit oy will be 61 MPa, in practice the one experimentally obtained. Note that in this way Eq. (5.48) defines a limit oyor any other stress amplitude as the equivalent fully reversed stress amplitude OequiN) that produces the same fatigue effect, i. e., results in fatigue fracture after the same N cycles of a stress amplitude (Ta,m(N) having a mean value 

Shear stress holds a treatment of its own. At variance with traction and bending stresses that have a sign, whether positive or negative, to which a physical state is associated, traction or compression respectively, shear stress has no sign at all, but simply a direction conventionally assigned that has no physic meaning. 

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Fig. 5.35 Compressive mean stress effect. Experimental results obtained for a 2024-T4 aluminum alloy b mild steel c 41/2 %Cu aluminum alloy d 5 A %Zn magnesium alloy (data from [42 ])...

Fig. 5.39 Goodman and Morrow models to take into consideration the mean stress effect...

Sample Problems on Mean Stress Effect and Fatigue Strength Diagrams... 

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)... 

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... 

Under load history A material fails prematurely relative to load history B. The rational is to be found in the mean stress effect. In case A, in fact, the overload has generated a hysteresis loop of initial path 0-1-2 ending with stress reversal in 2 that... 

Koh, S.K., Stephens, R.I. Mean stress effects on low cycle fatigue for high strength steels. Fatigue Fract. Eng. Mater. Struct. 14(4), 413 28 (1991)... 

Mean Stress Effect on Fatigue Limit of Notched Members... 

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