PSI - Issue 60

S.S. Satpute et al. / Procedia Structural Integrity 60 (2024) 525–534

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S. S. Satpute et. al./ Structural Integrity Procedia 00 (2023) 000–000

1. Introduction To ensure the safety of piping components like pipes and elbows, which are used in primary heat transport system, it is required to demonstrate Leak-Before-Break (LBB) design concept as explained by NRC, U., (2007), a NUREG- 0800 report and Bourga, et. al. (2015). This concept aims at the application of fracture mechanics principle to demonstrate that pipes are very unlikely to experience sudden catastrophic break without prior indication of leakage. This, however, requires detailed fracture mechanics analysis of different piping components. Over the years, fracture toughness property has been evaluated by conducting tests on compact-tension (CT) specimens. Transferability of the property generated using specimens (in the laboratory) to component level (occurring in real life piping) is an important issue in fracture mechanics explained by Clausmeyer et. al. (1991) and Brocks et. al. (1993). LBB qualification methodology is employed by Chattopadhyay et.al. (1997) in Indian PHWR PHT system for demonstration of safety against unstable ductile tearing for cracked pipes using fracture mechanics analyses. For secondary sodium piping system of prototype fast breeder reactor, LBB qualification is demonstrated by extensive experimental and consequent fracture mechanics analyses by Chellapandi et.al. (2003). Experiments have also been conducted on 90-degree elbows for demonstration of LBB qualification under cyclic loadings on PFBR sodium piping system by Nagapadmaja et.al. (2008). LBB criterion was demonstrated by way of a large number of fatigue/fracture tests by Vaze (2013). Experimental investigations were carried out on six numbers of narrow gap welded stainless steel straight pipes having circumferential through-wall notch under cyclic loading by Vishnuvardhan et. al. (2019). Recently, The LBB and crack growth stability have been analysed extensively in terms of allowable stresses for surface cracked pipe governed by limit loads by Hasegawa et.al. (2022). Similar to LBB, similar beak exclusion methodology have been developed by many countries. All these break exclusion methodologies have been reviewed recently by Blasset el. (2022). Cofie et.al. (2021) examined the application of LBB to very small diameter stainless steel piping in PWRs by determining the leak rate requirements to meet the criteria for both circumferential and axial flaws. The components will be of sizes 8 inch and 12 inch in outer diameter, with thickness of 17mm and 25mm respectively. The materials of the pipes are Carbon steel (CS) and Stainless-Steel Weld (SSW) pipe weld joints. During these tests, different parameters e.g., load, load line displacement (LLD), crack growth and crack mouth opening displacement (CMOD) will be measured. Conventionally, techniques like Direct Current Potential Difference (DCPD) and visual or video camera (manual), have been used for crack growth studies. It is not possible to use DCPD technique for measurement of out-of-plane growth of the cracks and, other techniques are manual with less accuracy. Different experimental methods like compliance method (Schwalbe et. al. (1985)), accoustic emission method and eddy current method (Mackerle, J, (2004)) and Ultrasonic method (Satyarnarayan et.al. (2007)) have been employed as per requirements. Potential techniques like ACPD and DCPD have been extensively used especially for surface cracks by Aliberti et al. (2017). However, these experimental methods have limitations in the high temperature environment and under cyclic loadings with high range of displacements. In Bhabha Atomic Research Centre (BARC), an image acquisition system (IAS) is developed in house for online synchronized imaging of Load and LLD value, and two Crack Tips of piping components. The design considerations for the imaging system were size of the piping components of 8 inch to 12 inch outer diameter pipes of 3 to 4 m length,expected maximum crack propagation in upward direction is 50 mm to 100 mm, expected pipe bending in downward direction is 50 mm to 250 mm. Image processing technique has an advantage over the conventional techniques (e.g., ACPD or DCPD) of measurement of crack-growth, as it can also capture the angle at which the crack has grown out-of-plane in addition to the actual amount of crack growth. One of the requirements for the test is that all the parameters should be recorded simultaneously so that they can be correlated with each other. Imaging technique makes possible recording of all the parameters such as crack growth at different crack tips, load, displacement, and CMOD simultaneously, by the click of one button. Using Imaging system, images of cracks are captured by CMOS scientific imaging cameras at a specified load / LLD interval from the beginning to the end of the experiment. The images thus acquired are subsequently manually analyzed to measure the crack growth. The paper is laid out as follows. After introducing the subject, the paper narrates image acquisition system set-up. This is followed by a discussion on the software on image acquisition and analysis of the images acquired during fracture experiments, respectively. Next, the results of analysis of fracture experiments on pipes are mentioned. The paper ends with a conclusion. 2. Online Image Acquisition System