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Part 1: Fatigue of Butt-Welded Pipe and Effect of Testing Methods: Fatigue of Butt-Welded Pipe

This report combines the results of two bending fatigue studies on butt-welded pipe: one on 1 " schedule 80 pipe and the other on 4" schedule 40 pipe. The main objectives of these two investigations are to extend the fatigue database of butt-welded pipe from Markl's tests into the low cycle range (100 to 2,000) and to compare the measured fatigue strength with formulas developed by Markl on stress amplitude versus cycles to failure. Markl's paper covered the range from 2,000 to 4 X 10 cycles.

Part 2: Fatigue of Butt-Welded Pipe and Effect of Testing Methods: Effect of Testing Methods on Stress Intensification Factors

Markl used cantilever-type testing, as indicated in Figure 1a, to experimentally establish the relationship for carbon steel: Equation (1): i=245000/(S N0.2) where S= (W/) L/Z. The stiffness, (W/), was determined prior to the fatigue tests. As used in Equation (1), S is an elastic-bases stress amplitude in psi; corresponding to the fatigue-test controlled displacement amplitude, , inch. N is the cycles-to-failure from the fatigue test.

Stress intensification factors (i-factors), based on Equation (1), have been used in the B31 series of Piping Codes since about 1956. They are also used in ASME Section III for Class 2 and Class 3 Piping.

For B31.1 and NC/ND of Reference [3], the dependence on anticipated cycles during operation is included via an "f-factor" which varies from 1.00 for 7000 or fewer cycles to 0.5 for 100,000 or more anticipated cycles. B31.3- 1996, "Process Piping" is similar except that it extends the f-factor down to 0.3 for 200,000 or more cycles.

Relatively recently, Scavuzzo, Srivatsan and Lam from the University of Akron ran controlled displacement tests using 4-point-bending as indicated in Figure 1b. Figure 2 shows the comparison between Equation (1) and these results for NPS 1-1/2 pipe. Figure 2 also shows a point at N=200 from Markl tests of "straight pipe" (discussed in more detail in Section 3.1).

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Published: 1998 Number of Pages: 90 File Size: 1 file , 2.8 MB