Fatigue fully reversed stress

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... 

While fatigue data collected in the laboratory are generated using a fully reversed stress cycle, actual loading applications usually involve a nonzero mean stress. The mean stress can be compressive or zero and it affects the strain-life curve as shown schematically in Eig. 1.33. Mean stress has its largest effects in the high-cycle regime. Compressive means extend life and tensile means reduce it. 

Fracture surface morphology varies with stress amplitude and test frequency as well as with composition and molecular weight. Even under fully reversed cycling, discontinuous crack growth bands have been observed on the fatigue fracture surface in PS, SAN and ABS but they have not been seen in HIPS. 

The new S-N curve is the full line of Fig. 8.48 whereas the dotted line represents the S-N curve under fully reversed condition R — —1). It can be seen how the two lines merge at 10 cycles since mean stress is losing its effect in low cycle fatigue. The slope Hb of the line is equal to —1/0.129 = —7.75. Since there will be stress amplitudes below the fatigue limit that can have an effect because of the... 

This criterion respects the hnding that a constant torsion stress does not affect fatigue strength while a constant normal stress does. The application to a fully reversed bending leads to... 

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