Fatigue Tests of Welding Elbows and Comparable Double-Mitre Bends

Abstract It has long been known that the increased flexibility of curved pipe, resulting from its ovalization under the influence of bending moments, is reflected in increased stresses which are customarily evaluated by applying intensification factors to the stresses evaluated by the common beam theory. No similar information has heretofore been published regarding mitre joints. On the basis of comparative full-scale fatigue tests, the present investigation shows that, for double-mitre bends of given proportions, the stress intensifications (at the corners) are 20 per cent greater than for long-radius welding elbows when bending moments are applied tending to open or close the bend; and 70 per cent greater when bending is done sidewise, out of the plane of curvature. Expressed in terms of fatigue life under identical conditions of pipe-line expansion, this means that welding elbows will withstand 2½ times as many stress reversals in bending in the plane, and 14 times as many cycles in bending transverse to the plane of curvature, as compared with mitre bends. Indications were obtained also that internal pressure has no marked effect on the magnitude of the stress intensifications encountered. Stress-relieving and grinding the welds likewise produces no significant change in the endurance strength.