ASME STP-PT-027-2009 pdf download

ASME STP-PT-027-2009 pdf download

4 CREEP-FATIGUE INTERACTION Creep-fatigue test results are often plotted on an “interaction diagram” of the type shown in Figure 4 for the 91 alloy. The data supporting the ASME NH line for the 91 alloy lie close to the horizontal axis and suggest that short hold time fatigue loading severely negatively influenced creep life. However, such test data emphasize fatigue loading whereas high-temperature pressure vessel service would normally be expected to be creep dominated, i.e. representative data would be more closely aligned with the Y axis. Little data of that type is available because of the long test durations required and the corresponding increase in cost of data acquisition. Specifically, pressure vessel service can be better simulated by tests to demonstrate how a relatively small number of cyclic loads would shorten creep life (i.e test results would plot close to the Y axis). The severe effect of fatigue damage on creep life indicated by the NH lines in Figure 4 might be attributed in large part to a high level of strain softening that occurs with these alloys in hundreds or thousands of strain cycles which reduce the tensile strength and thereby degrade the creep properties. Data reported from creep-fatigue tests of the type shown in Figure 3 seldom, if ever, include post mortem information on the material properties or microstructural changes due to cycling. However, acceleration of softening behavior associated with creep straining and the life reduction that goes along with it are well known from interrupted stress-rupture tests of the subject alloys. What is needed is modeling and quantification of the effects.
The above are convenient, powerful, compact and simple relations showing how, under fatigue conditions, creep-fatigue life at a location is related to hold time (cycling rate), plastic strain range, the stress rupture life absent fatigue and a material constant, β, and how to deal with variable amplitude conditions. The first three of these terms can be determined or specified by design activity. The material constant, β, can reasonably be inferred by examining the performance of the alloy (or similar materials) in fatigue tests of sufficient hold-time duration that most of the associated creep- fatigue process damaging microstructural interactions can occur. Another very useful feature of the equations is that the designer may calculate life absent fatigue and then apply a histogram of strain cycling to calculate in a simple step the corresponding life with fatigue. Such life should be computed taking into account multiaxial creep effects. Finally, by benchmarking β against actual test results it will take into some account any other damage interaction mechanisms in addition to softening that may occur in the subject alloy at the temperature of interest.
Most of the fatigue test data on pressure vessel steels in Figure 3 did not include the desired effects of hold times. However, alloys of the 1Cr-1Mo-V type have long been studied by electrical equipment manufacturers for turbine rotor applications and by MPC (in creep-fatigue interspersion tests). For example, D c values for Endo’s experimental creep-fatigue test results are shown in Figure 3 for hold times of 1, 6 and 24 hours and several plastic strain ranges. They were fit to the above equation to solve for β numerically using the corresponding creep rupture times. The β values obtained were in the range of 2.0 and the life results were not very sensitive in that range. Calculation of the corresponding values of the reduction in fatigue strength, D f , in the hold time tests showed a strong creep-fatigue interaction depicted in Figure 6.

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