ASME STP-PT-027:2009 pdf free download

ASME STP-PT-027:2009 pdf free download.EXTEND LOW CHROME STEEL FATIGUE RULES.
3 CREEP-FATIGUE DATA
Creep-fatigue data have been developed in tests utilizing many combinations of strain range. hold time, temperature and load measurement. For the most part, creep-fatigue tests are run with loads that cycle between compressive and tensile and with tensilc hold periods ranging from seconds to times exceeding a few minutes, but rarely more than an hour. Plastic strain amplitudes typically do not exceed 1-2 percent and are usually only a fraction of 1 percent. The total number of cycles applied before failure or a specific load reduction is reached may extend into the thousands, but because of the high cyclic frequency, the total time of exposure may be only tens or, at most, a few hundreds of hours.
For the purpose of this study, creep-fatigue data on several of the strain softening alloys were gathered from many sources. The data from which the plastic strain range may be estimated are shown in Figure 3. Included in the plot are some data from tests that show the effects of tensile hold times. Most of the data are from relatively high frequency tests where the accumulated time at creep temperature is very short. It appears that longer hold time tests result in fewer numbers of’ cycles. i.e. there is a creep-fatigue interaction. A line provided by a producer of 2 ¼ (‘r-IMo-V alloy, shown in Figure 3, did not include significant hold time effects.
The most widely scattered points in the figure are for hold time tests of one brittle heat of an alloy for which the “no hold time” tests were also mainly outside the scatter band. It is not expected that pressure vessel alloys of interest in this project will behave in a creep brittle manner when tested in uniaxial tension.
4 CREEP-FATIGUE INTERACTION
Creep-fatigue test results are often plotted on an “interaction diagram” of the type shown in Figure 4 for the 9l 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.