Fatigue stress cycle

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.

Fatigue stress cycles (a) tension-compression stress cycle. 

Fatigue stress cycles (b) tension-stress cycle, and (c) irregular stress cycle. 

Cumulative Damage. Pressure vessels may be subjected to a variety of stress cycles during service some of these cycles have ampHtudes below the fatigue (endurance) limit of the material and some have ampHtudes various amounts above it. The simplest and most commonly used method for evaluating the cumulative effect of these various cycles is a linear damage relationship in which it is assumed that, if cycles would produce failure at a... 

Fatigue properties in bending are most appropriate for copper aHoys as these are often used as spring contact components in beUows and electrical switches and coimectors. These articles are usuaHy designed for acceptable service Hves at a moderate to high number of stress cycles. 

Here is the number of cycles to fracture under the stress cycle in region i, and Nj/Nf is the fraction of the lifetime used up after N, cycles in that region. Failure occurs when the sum of the fractions is unity (eqn. (15.4)). This rule, too, is an empirical one. It is widely used in design against fatigue failure but if the component is a critical one. Miner s Rule should be checked by tests simulating service conditions. 

Contain tensile residual stresses which are usually as large as the yield stress. Weld liable to fatigue even when applied stress cycle is wholly compressive. Reduce residual stresses by stress relieving, hammering or shot peening. 

Example 2.22 A certain grade of acrylic has a Kc value of 1.6 MN and the fatigue crack growth data as shown in Fig. 2.77. If a moulding in this material is subjected to a stress cycle which varies from 0 to 15 MN/m, estimate the maximum internal flaw size which can be tolerated if the fatigue endurance is to be at least 1(P cycles. 

As approaches the crack can also propagate by hydrogen embrittlement processes during the higher load parts of the stress cycle. This forms the basis of various models which have been developed to describe corrosion fatigue, probably the best-known of which are the superposition models due to Wei"". In its most recent version this model takes the form ... 

The fatigue strength of most TPs is about 20 to 30% of the ultimate tensile strength determined in the short-term test but higher for RPs. It decreases with increases in temperature and stress-cycle frequency and with the presence of stress concentration peaks, as in notched components. 

Endurance limit To develop S-N curves the fatigue specimen is loaded until, for example, the maximum stress in the sample is 275 MPa (40 ksi) (Fig. 2-43). At this stress level it may fail in only 10 cycles. These data are recorded and the stress level is then reduced to 206 MPa (30 ksi). Tliis specimen may not break until after 1,000 stress cycles at this rather low stress level. 

Fatigue tests are normally performed by applying one of the stress cycles described above until the test specimen fractures. The number of cycles to failure, Nf, at a... 

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.

Material Tensile strength, a (MPa) Max. fatigue stress, amax (MPa) max Fatigue cycles Residual strength (MPa)... 

FIG. 25.6 Stress amplitude vs. fatigue lifetime in number of stress cycles. 

Types of tests. Laboratory corrosion fatigue tests can be classified as either cycles-to-failure (complete fracture) or crack propagation (crack growth) test. In cycles-to-failure testing, specimens or parts are subjected to a sufficient number of stress cycles to initiate and propagate cracks until complete fracture occurs. Such data are usually obtained by... 

The number of permissible load cycles, which results from the fatigue stress evaluation, does not necessarily determine the life time expectancy of the pressure vessel, but sets the intervals for fissure tests. 

Fig. 6.20 Relationship between the decay in interfacial shear stress and the fatigue life of fiber-reinforced ceramics, as proposed by Rouby and Reynaud. (a) The interfacial shear stress r is assumed to decrease during cyclic loading, reaching a steady-state value after Ni cycles, (b) The composite strength decreases in parallel to the decrease in r. An infinite fatigue life is predicted if the strength corresponding to t is above the maximum fatigue stress. After Rouby and Reynaud.46...

Fig. 4. Photograph of PS fatigue specimen cycled for 80% of its expected fatigue life at a stress amplitude of 17.2 MPa and at 21 Hz...

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