Fatigue stress range

The second failure mode to consider is fatigue. The drum will revolve about once every second, and each part of the shaft surface will go alternately into tension and compression. The maximum fatigue stress range (of 2 x 56 = 112 MPa) is, however, only a quarter of the fatigue limit for structural steel (Fig. 28.5) and the shaft should therefore last indefinitely. But what about the welds There are in fact a number of reasons for expecting them to have fatigue properties that are poorer than those of the parent steel (see Table 28.1). 

Fig. 28.5. Fatigue data for a typical structural steel in dry air. Note that, if the fatigue stress range is less than 440 MPa (the fatigue limit] the component should last indefinitely. The data relate to a fatigue stress cycle with a zero mean stress, which is what we have in the case of our tail drum.

Figure 28.6 shows the fatigue properties of structural steel welds. The fatigue limit stress range of 120 MPa for the best class of weld is a good deal less than the limiting range of 440 MPa for the parent steel (Fig. 28.5). And the worst class of weld has a limiting range of only 32 MPa ...

In the case of the ASME codes for nuclear pressurised components, the questions of fatigue design and of flaw evaluation are dealt with separately in ASME Section III and Section XI Appendix A, respectively. The design S-A curve for machined butt welds typical of thick section pressurised components is set at a factor of two on stress range or twenty on cyclic life, whichever is more conservative, below the mean of S-N data developed on smooth cylindrical specimens in air. (A somewhat similar design curve obtained by a different method from experimental S-A data for machined butt welds is given in British Standard 5500.) These safety factors are intended to encompass any adverse influence of minor weld defects, size effects, data scatter and environment. As far as environmental effects are... 

The stress intensification factors in Appendix D of ASME B31.3 have been developed from fatigue tests of representative piping components and assemblies manufactured from ductile ferrous materials. The allowable displacement stress range is based on tests of carbon and austenitic stainless steels. Caution should be exercised when using eqs. (la) and (lb) (para. IP-2.2.10) for allowable displacement stress range for some nonferrous materials (e.g., certain copper and aluminum alloys) for other than low-cycle applications. 

Fatigue lifetime increases with increase of test frequency, v, according to a power law relationship, for both PS and HIPS and the slope of the lop N-log v plot appears independent of stress ampHtude over the stress range investigated. Fatigue endurance also increases with frequency for ABS but its frequency sensitivity is less than that of HIPS. 

Fatigue cracks are initiated at stresses below the conventional yield stress at which bulk plastic flow occurs. In this stress range, surface inhomogeneities and favorably oriented grains allow slip by movement of dislocations, which produce surface offsets that, due to localized work hardening (slip interference), are not reversed as the stress is reversed. As a result, surface intrusions and extrusions of the form proposed in Fig. 7.118 are produced (Ref 162). The intrusions and extrusions,... 

Repassivation processes have become an important subject in stress corrosion studies and also in other forms of corrosion, e.g., pitting corrosion and corrosion fatigue. A range of scratching and straining electrode techniques have been employed. While it is not possible to go into detail, the results have to be examined in relation to the techniques employed, e.g., has repassivation started before the scratching or straining has stopped It is important also to know whether the current measured under potentiostatic conditions is a complete anode current or the difference between an anode current and a cathode current (most commonly due to H" " ion reduction). Typical repassivation rates correspond to an equation (2 1 ) of the type ... 

Fatigue test. Fatigue tests should be carried out at a constant frequency between 1 and 25 Hz, with a sine wave form and using TAST or double overlap joints as described above. The joints should be capable of surviving, without failure, 10 cycles of a mean shear stress range between 1.0 and 10 MN at 20 °C for TAST joints... 

It should be noted that, in the few cases reported, the SCCG exponent for cyclic fatigue is much larger than unity it > 1 [105,108,109]. Thus, like SCCG, the cyclic fatigue life is heavily dependent on the applied stress range and the size of the initial flaws. 

N,f ) stress range (ypf yMf Ao, ypf Yuf Ax respectively) each acting alone (Fatigue life) n Number of stress cycles to which the FRP component being designed will be subjected during its design life... 

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