ASME STP-PT-027-2009 pdf free download.EXTEND LOW CHROME STEEL FATIGUE RULES.
I INTRODUCTION
The impetus for this activity arises because the new ASME B&PV Code. Section VIII. Division 2 nilcs permit high strength materials of the type enumerated to be used to temperatures above 7OOF and into their respective creep ranges. A life limiting failure mode is potentially the phenomenon of “creep-fatigue.” We shall define a “creep-fatigu&’ failure as one in which life is shorter than that expected due to either creep or fatigue acting on a structure independently. This occurs in those regimes of stress, strain-rate, time and temperature where the damage mechanisms due to creep and fatigue can be expected to damage the same microsiruclure and properly characteristics. Creep- fatigue is of concern especially where there may be time-dependent straining and where viuying stresses (loads, including start-up and shut down) are among the design conditions.
Comprehensibe and correct creep-fatigue design rules are needed now for the aforementioned alloys because, under the new Section VIII. Division 2 rules, as the respective creep ranges of the materials are approached, in many cases the allowable stresses are significantly higher than those for which there is applicable service experience that would permit exempting design details from fatigue analysis based on documented ‘years of relevant experience.” The same must be said for any new alloys and applications for which there is literally no relevant service experience.
In summaiy then, the combination of new materials and applications for advanced energy systems with higher allowable stresses and increased design temperatures requires an undentanding of creep- fatigue not now available, analytical models to explain and express damage accumulation and relevant test data in order that new, justifiable and correct rules may be developed.
3 CREEP-FATIGUE DATA
Creep-fatigue data have been developed in tests utili?ing many combinations oF strain range, hold time, temperature and load measurement. For the most part, creep-fatigue tests arc run with loads that cycle between compressive and tensile and with tensile hold periods ranging from seconds to times exceeding a few minutes. but rarely more than an hour. Plastic strain amplitudes typically do not cxcccd 1-2 percent and are usually only a fraction of I 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 sccral 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 eflects of tensile hold times. Most of the data are from relatively high frequency tests where the accumulated time at creep temperature is ‘ery short. It appears that longer hold time tests result in fewer numbers of cycles. i.e. there is a creep-fatigue interaction. A line pros ided by a producer of 2 Cr-IMo-V alloy, shown in Figure 3. did not include significant hold time effects.
The most widely scattered points in the figure arc for hold tune 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.