Vibration Behavior and Fatigue Strength of Mocked-Up Piping System

For the purpose of investigating vibration characteristics and fatigue strength of a socket-welded joint, a piping system was mocked-up by assembling a straight pipe of 350 mm nominal diameter and a long pipe of 20 mm nominal diameter consisting of straight pipes and elbows. The one end of the small-diameter piping is connected to the large-diameter pipe at its longitudinal midpoint by socket welding, and the rest is supported at several supporting points, the locations of which are changed as an experimental parameter. The materials of the small-diameter piping are carbon and stainless steels. The small-diameter piping was subjected to nearly resonant vibration with the frequency of about 11 Hz by a sinusoidal vibration load applied to the large-diameter pipe. The vibrating displacement amplitude measured on the actual piping was basically 0.05 mm in resonant condition; but the displacement was changed to obtain an S-N curve for the socket-welded joint in the fatigue life range of 10⁴ to 10⁷ cycles. In the mocked-up specimens, fatigue cracks were initiated from the toe of the socket-fillet welds at higher stress amplitude, but from the fillet root at lower stress amplitude. Comparative fatigue tests of straight shape socket-welded specimens fabricated with 20 mm nominal diameter pipe of the same material as used in the mocked-up specimen were carried out under four-point bending condition. The fatigue strength of the socket-welded joint in the mocked-up specimen was about 15 percent lower than that of the simple specimen fatigued by four-point bending load. The reason for this difference is probably due to the triaxial stress condition and three-dimensional restraint condition. The strain gage measurement showed that the shear stress was about 40 percent of the bending stress in the case of the mockedup specimen. In addition, vibration tests of the piping system showed good agreement between experimental and analytical results of vibration behavior.