Fatigue reversed stress

Alloy Condition R Fatigue Reversed-Stress (MPa) R Rotating Bending Fatigue Strength (MPa)... 

Resistance to fatigue and corrosion fatigue The resistance of titanium to fracture by fatigue, induced by imposition of rapidly reversing stresses. 

Figure 5.40 Variation of stress with time that accounts for fatigue failure by (a) a reversed stress cycle and (b) a repeated stress cycle. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 210. Copyright 2000 by John WUey Sons, Inc.

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. 

Figure 8.17 Variation of stress with time that accounts for fatigue failures, (a) Reversed stress cycle, in which the stress alternates from a maximum tensile stress (+) to a maximum compressive stress (-) of equal magnitude, (b) Repeated stress cycle, in which maximum and minimum stresses are asymmetrical relative to the zero-stress level mean stress cr, range of stress cr and stress amplitude cr are indicated, (c) Random stress cycle.

A cylindrical bar of 1045 steel having the S-N behavior shown in Figure 8.20 is subjected to rotating-bending tests with reversed-stress cycles (per Figure 8.18). If the bar diameter is 15.0 mm, determine the maximum cyclic load that may be applied to ensure that fatigue failure will not occur. Assume a factor of safety of 2.0 and that the distance between loadbearing points is 60.0 mm (0.0600 m). 

Corrosion Fatigue failure by cracking caused by reversing alternating stress in the presence of a corrosive environment. 

---