PSI - Issue 60

M.K. Sahu et al. / Procedia Structural Integrity 60 (2024) 390–401

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M. K. Sahu et.al./ Structural Integrity Procedia 00 (2019) 000 – 000

Keywords: leak-before-break; cyclic loads; cracked pipes; fracture properties; J-R curve.

1. Introduction The structural integrity assurance of the Primary Heat Transport (PHT) piping system is very important for the safe operation of nuclear power plants (NPPs). These PHT piping system carry coolants under high temperature and high pressure. Leak-Before-Break (LBB) qualification is a well-established approach by NRC, U., (2007), a NUREG-0800 report for design of PHT piping system which require fracture stability demonstration of pipelines under design basis earthquake loads. To address this issue, Reactor Safety Division (RSD), Bhabha Atomic Research Centre (BARC) has been pursuing a comprehensive fracture test program. Initially, the monotonic fracture tests have been done to predict the load bearing capacity of the piping specimens under primarily bending load by Chattopadhyay et. al. (2000) and Chattopadhyay et. al. (2004). This load capacity is utilized for LBB qualification as a maximum critical load. In this conventional approach of fracture stability demonstration under LBB qualification, equivalent maximum load associated with only one cycle of an earthquake event is considered. The deleterious effect on the material due to significant number of reversible cyclic loadings because of earthquake events are not explicitly considered in this methodology. The nuclear components are designed by Boiler, ASME code (2007) based on seismic investigation for at least 10 cycles of equivalent maximum induced load during an earthquake event. Hence, for LBB qualification, the reasonable number of cyclic loadings and related cyclic tearing failure mode should be considered for more realistic assessment. To address this issue, a systematic experimental study was performed. It was aimed at quantifying the deleterious effect of cyclic loadings on load carrying capacity of large sized pipe components of PHT piping system of Indian NPPs by Gupta et.al. (2014) and Gupta et. al. (2015). For singling out the cyclic loading effect, the complementary monotonic fracture tests were also conducted on identical straight pipe specimens. Based on the reduced load bearing capacity due to applied loading cycles, a Cyclic Tearing Failure Assessment Diagram (CTFAD) was proposed. Using this CTFAD for a given number of loading cycles, load reduction can be estimated. This reduction in load value is due to the damaging effect of cyclic loadings. The reduced load value should be used for LBB qualification to rule out the unstable failure in case of cyclic loadings. All these investigations were performed at room temperature. In the present work, the cyclic tearing investigation is extended to the reactor operating temperature i.e., high temperature environment, 300 o C. Under this program, total six pipes have been tested under four-point bend cyclic/monotonic loadings on straight pipe specimens of carbon steel and welded SS304LN. Salient experimental results are presented and compared with already available cyclic tearing test data related to room temperature. 2. Pipe-specimens and test setup details Total six straight pipes with through wall circumferential cracks have been tested under four-point bend loadings at reactor operating temperature i.e., 300 o C. Initially, the straight pipes have been fabricated with circumferential notches by EDM (Electrical Discharge Machining) tool. The machined notches have been converted to sharp cracks by fatigue pre-cracking, which is appropriate for fracture mechanics investigation. Special heating arrangement has been designed for keeping the crack region around 300 o C. An additional arrangement has been made to capture the images of both crack tips for prediction of crack growth during entire fracture/ cyclic tearing experiments. 2.1. Fatigue pre-cracking All pipes have been fabricated with circumferential notches using EDM tool. These notches have finite radius of approximately 1.25mm. Hence, these notches are not very sharp for fracture studies and need to be converted to very sharp cracks. Hence, fatigue pre-cracking is pre-requisite for producing sharp crack for the application of fracture mechanics. All pipes were subjected to cyclic four-point bending under load control using servo-hydraulic actuator. All 8-inch pipes were loaded under ±500 kN capacity actuator while remaining two 12-inch pipes under ±2000 kN capacity actuator. The maximum cyclic load was around 20% of the Theoretical Plastic Collapse Load (TPCL) of the