Low Cycle Fatigue Behavior and Seismic Assessment for Elbow Pipe Having Local Wall Thinning

One of the concerned technical issues in the nuclear piping under operation is pipe wall thinning caused by flow accelerated corrosion. Recently, it has been reported that the elbow section is more suspicious on pipe wall thinning by erosion–corrosion. Some researchers including authors have been studied static and fatigue strength of elbows with local wall thinning. However, still more experiment and analysis data are needed to clarify the technical issues. Accordingly, further experiments and their evaluations were carried out by the authors. This paper presents the influences of size and location on fatigue life. Also as one of the application of the test results, safety margin of elbows with wall thinning against seismic loading is discussed. Low cycle fatigue tests were conducted using elbow specimens made of STPT410 steel with local wall thinning. The local wall thinning was machined on the inside of elbow specimens in order to simulate erosion/corrosion metal loss. The local wall thinning areas were located at three different areas, called extrados, crown, and intrados. Eroded ratio (eroded depth/wall thickness) is 0.5 and 0.8 and eroded angle is 90 deg and 180 deg. The elbow specimens were subjected to cyclic in-plane bending under displacement control (±20 mm) without and with internal pressure of 3 MPa using a universal testing machine. Fatigue life was defined as fatigue crack penetration through the thickness and crack penetration was watched by naked eyes during the test through the protection window made of a transparent plastic plate. Obtained main conclusions are as follows: (1) Existence of local wall thinning in extrados does not have an important effect on fatigue life. Especially, fatigue crack does not initiate at the extrados where the extreme local wall thinning exists (eroded ratio = 0.8 and eroded angle = 180 deg). (2) Regardless of existence of internal pressure, fatigue crack initiates at the crown where local wall thinning does not exist for an elbow with local wall thinning at extrados. This conclusion should be confirmed using eroded elbow specimens under more high pressure. (3) Even if the eroded ratio and the eroded angle reached up to 0.8 and 180 deg, the elbows with local wall thinning have high safety margin against seismic loading, comparing to ASME Boiler and Pressure Vessel Code Section 3 allowable seismic stress criteria.