Effect of Secondary Stresses on Pipe Fracture

This paper presents experimental results from pipe-system tests from the International Piping Integrity Research Group (IPIRG) and Battelle Integrity of Nuclear Piping (BINP) programs where the magnitudes of the thermal expansion (secondary) stresses were different in two pipe-system experiments with identical circumferential surface cracks in the same material. The pipe systems were loaded with a single-frequency dynamic forcing function to provide inertial and seismic anchor motion (SAM) moments. In the first tests, the hydraulic ram at the dynamic loading point was locked in place during the heating, so that the natural thermal expansion stresses developed. In the second test, the hydraulic ram at the dynamic loading point was statically offset after the heating to simulate a higher thermal expansion stress. The magnitude of the thermal expansion stress for both cases was within the ASME Section III limits. The results showed that the same total moment was reached in both tests, but the magnitude of the inertial moments at failure was reduced in the second test by the increase of the moment from the thermal expansion stresses. The reason for this effect is that the crack was relatively large, so the failure stress due to the crack was below yield of the uncracked pipe in the pipe system. In this case, the plasticity at the surface crack causes small displacements of the pipe compared to the overall elastic displacements of the pipe system; therefore, the yielding at the crack plane did not relieve the secondary stress, causing them to behave as a primary stress. This behavior is consistent with the B31.1 and ASME Section III Class 2 and 3 piping paragraphs on “Local Overstrain”. The implication from this work is that the safety factor on secondary stresses in the ASME Section XI Code pipe flaw evaluation procedures should be a function of the failure stress. Furthermore, secondary stresses should be included for all piping materials (including wrought stainless steels), and have the same safety factor as the primary stresses for stresses below the yield strength of the material.