Experimental results for the cyclic pressure fatigue of several pressure vessels with flat heads are reported and compared against various fatigue design methods. The geometry is based on a welded joint presented in a recent paper by Kalnins, Bergsten, and Rana (2005). Weld designs between the head and shell include full penetration welds completed from a single side and both sides of the shell. Unique characteristics of this test include thin plate design, a large D/t ratio, and a low membrane-to-bending ratio. These are aspects of flat head geometries which have not been widely reported in the literature. Allowable fatigue cycles for ASME Section VIII, Division 2, IIW, PD 5500, EN 13445, API 579, and the Master S-N method (proposed for the ASME Section VIII, Division 2 rewrite project) are presented. Results show that several of these design method produce non-conservative fatigue life predictions. In addition, the fatigue results demonstrate that size effects and plate thickness effects have diminishing influence for thin plates failing at less than 100,000 cycles. Finally, the fatigue strength of stainless steel is compared to carbon steel and the lack of a unified approach to stainless steel fatigue design is discussed.
Part 1 of this paper was presented by Mr. Chris Hinnant at the 2008 PVP Conference in Chicago. Since 2008 additional fatigue test data has become available to the Paulin Research Group (PRG) which includes tests on unreinforced fabricated tees intended to support the fatigue curve approach established in the 2008 paper, and twenty-five additional straight pipe cantilever fatigue tests on carbon, stainless, duplex, CuNi, P91, X42 and X65 materials. These more recent experimental results confirm the fatigue slope recommended for cantilevers in the 2008 paper, but suggest that the original Markl slope may be more suited for some configurations of branch connections. Comparisons of the B31 Code allowable with failure data, tests, reviews of ratcheting behavior in pipe systems, and crack growth monitoring help draw conclusions about design equations in the B31 Codes and fatigue test procedures.
Experimental results for the fatigue testing of several welded flat head geometries are reported. These tests are similar to those previously reported by Hinnant (2006) [1] and focus on the fatigue behavior of full penetration welds with cover fillet welds. Fatigue calculations according to several fatigue design methods are compared against the experimental results, as are the mean fatigue curves of several of the design methods. Of particular interest for these new tests is the effect of plate thickness, testing environment, and geometric effects. Nominal plate thickness values ranging from 0.0625" (1.59 mm) to 0.1875" (4.76 mm) have been tested and correlated. Four additional fatigue tests were conducted using air to determine if previous testing in room temperature tap water resulted in decreased fatigue life.
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