2022 Edition ASME B31.3: A Paradigm Shift in Stress Range Factor

During the 1950s, Markl conducted a series of tests that have since become the foundation of calculation procedures in modern ASME B31 (and other) piping codes. Today, stress calculations of majority of piping codes are largely based on intensified nominal stresses (using stress intensification factors, or SIFs). SIFs, in turn, are based on a fatigue life of girth butt-weld fatigue strength curve (S-N curve).

While Markl’s efforts were extraordinary, over the years, new tests and research have revealed that the slope of the S-N curve differs from that derived in his tests (see ASME VIII-2 structural stress procedure or EN 13445-3 curves up to 1 million cycles, for example).

A comprehensive overview of this issue, along with test data to support this fact, has been presented by Hinnant and Paulin in ASME PVP paper no. 61871. To align these curve slopes, equation (1c) from ASME B31.3 (stress range factor f) has been revised in the 2022 edition.

Although this modification raises some outstanding questions, such as whether SIFs should be readjusted, aligning these slopes is an important initial step in standardizing fatigue evaluation procedures across various codes.