ASME B31J-2008 pdf – Standard Test Method for Determining Stress Intensification Factors(i-Factors) for Metallic Piping Components.
requires a new C constant to be developed, since materials such as copper, aluminum, or very high-strength steels exhibit different fatigue life from plain carbon steel. The intent of the test is to develop an S1F that is geometry dependent, not material dependent.
Identifying nominal dimensions and wall thicknesses is important, particularly in the case of branch connections, to ensure extrapolation of results to other sizes is done correctly. The importance of the weld profile is clearly shown in reference [6].
(c) Paragraph 3.3. Markl’s tests were based on linear elastic equivalent moments, i.e., a constant displacement or rotation was applied and the moment at the failure location was based on extrapolation of the M-O (or F-6 elastic curve. This allows agreement with the way linear elastic thermal expansion analyses are used, even though predicted stresses may be above yield.
(d) Paragraph 3.4. The use of a nominal pressure is to ensure a ready means of detecting a through-wall crack. The use of 500 cycles as a minimum is to ensure correlation with the lower bounds of Markl’s work. From reference [4], it can be seen that the preponderance of tests lie above 1,000 cycles. The few that fall below show a fair amount of scatter off the proposed straight line. Until more work is done in the very low cycle range, the lower limit of 500 will remain.
(2) Section 4
(a) Paragraph 4.1. If forces are being applied, it is important to measure the distance from the point of application to the point of failure in order to determine the appropriate equivalent moment.
(b) Paragraph 4.2. The section modulus is used in piping analysis to convert the calculated moments to stresses. Thus, it is important that the section modulus used to calculate the stress in the test agrees with that to be used in the analysis (and described in the Code).
(c) Paragraph 4.3. The equation in para. 4.3 is taken directly from the work by Markl [e.g., reference [4], eq. (4)]. Since Marki’s tests formed the basis of the current i-factors and Code rules, use of Markl’s equation is appropriate for correlation.
(d) Paragraph 4.4. The factor for the number of tests is to provide for uncertainty. The basis for the factors is engineering judgment.
While K55 would produce slightly higher factors, those provided in para. 4.4 are deemed acceptable based on the scatter in Marki’s original testing.
(e) Paragraph 4.6. The equation for variable amplihide tests is the same basic equation as was incorporated in ANSI B31.1-1955 and is still used by the piping Codes to convert different operating condition stress ranges, typically thermal stress ranges, to a single base stress range.
(3) Section 5
(a) Paragraph 5.1. Based on the work in “Thermal Fatigue and Thermal Stress” by Manson, the material exponent, n, for metals stays fairly constant at 0.2, and has been set to that value in the Standard. Based on work done by W. Koves, the material constant, C, can be found from a ratio of the moduli of elasticity, i.e.,
C (other material) = 245,000 E(other material)!
E(carbon steel = 27.8E6 psi)
(b) Paragraph 5.2. Dimensional extrapolations need to be justified based on either elastic-stress theory or tests of additional sizes. Elastic theory was the basis of Markl’s extrapolation work in elbows and straight pipe. It is important that extrapolation be justified.
(4) Section 6. The reason for the test report is to assure owners that the testing was carried out in compliance with B31J. Since the test report must also describe any weld profiles, the owner can also ensure that such profiles/procedures are incorporated into the welding program for the installation. The basis for the i-factor must be able to be reviewed by the owner or his agent.
[1] WRC Bulletin 392, “Standardized Method for Developing Stress Intensification Factors for Piping Components,” E. C. Rodabaugh, June 1994.
[2] Marki, A. R. C., “Fatigue Tests of Welding Elbows and Comparable Double-Mitre Bends,” Trans. ASME, Volume 69, 869—879 (1947).
[3] Markl, A. R. C. and George, H. H., “Fatigue Tests on Flanged Assemblies,” Trans. ASME, Volume 72, 77— 87 (1950).
[4] Markl, A. R. C., “Fatigue Test of Piping Components,” Trans. ASME, Volume 74, 287—303 (1952).
[5] Markl, A. R. C., “Piping-Flexibility Analysis,” Trans. ASME, Volume 77, 127—149 (1955).
[6] WRC Bulletin 392, “Effects of Weld Metal Profile on the Fatigue Life on Integrally Reinforced Weld-On Fittings,” G. E. Woods and E. C. Rodabaugh, June 1994.
[7] WRC Bulletin 329, “Accuracy of Stress Intensification Factors for Branch Connections,” E. C. Rodabaugh, December 1987.