PSI - Issue 60

Rosy Sarkar et al. / Procedia Structural Integrity 60 (2024) 75–92 Rosy Sarkar/ StructuralIntegrity Procedia 00 (2019) 000 – 000

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Ratcheting occurs during asymmetric stress cycling but not in asymmetric strain cycling because there is no strain accumulation in strain -controlled loading conditions. Among different forms of cyclic plastic deformations, ratcheting has the highest detrimental effect on fatigue life. Ratcheting and creep vastly differ in the deformation and damage mechanisms [Paul et al. (2010)]. In ratcheting, there is a movement of dislocations, their interaction, and cell formation, whereas, in creep, there is diffusion controlled glide and climb of dislocations, grain boundary sliding, and void formation. There is a considerable effect of temperature on ratcheting strain. With rising thermal ageing temperature and high thermal ageing time, the ratcheting strain accumulation increases, and the ratcheting life decreases [Paul (2019)]. Ratcheting can be classified as material ratcheting and structural ratcheting. Material ratcheting can occur in the absence of structural effects, i.e., if stress is distributed homogeneously in a structure. It is purely a material-related effect and can be observed in material tests. It depends on various factors like mean stress, stress amplitude, frequency, loading history and micro-structural characteristics. It must be accounted for in constitutive equations. Structural ratcheting can occur due to inelastic material behavior under cyclic loading, even if no material ratcheting exists. It is produced by the inhomogeneity of a structure's stress state. It should be reflected in design codes. It is not seen in material tests [Hubel (1996)]. A large number of ratcheting studies have been carried out in the literature. The effect of mean stress and stress amplitude on ratcheting is studied by [Paul et al. (2010)], uniaxial stress-controlled ratcheting experiments have been carried out by [Sivaprasad et al. (2010)], and biaxial ratcheting experiments have been carried by [Ashutosh et al. (2014)] . Ratcheting studies on Type 304LN stainless steel straight pipes and elbows subjected to steady internal pressure and cyclic bending load showed that the specimens underwent significant ratchet swelling, ovalization, and consequent thinning of the cross-section during ratcheting. It is also observed that ratcheting was generally more pronounced in straight pipes than in elbows [Vishnuvardhan et al. (2013b)]. The use of acoustic emission and ultrasonic techniques for monitoring crack initiation/growth during ratcheting has been demonstrated by [Mukhopadhyay et al. (2014)]. ‘Fatigue Ratcheting’ is a phenomenon that leads to a reduction in the fatigue life of a component due to loss of ductility caused by the accumulation of plastic strain. Constant internal pressure (primary load) and cyclic bending stress variation (secondary load) are some of the basic characteristics of a typical Fast Breeder Reactor (FBR) piping system. This type of loading can result in fatigue ratcheting in the case of a power plant pipe bend [Suresh Kumar et al. (2020)]. From the ratcheting studies carried out on a 168 mm OD pipe bend with internal pressure and in-plane bending, it is observed that there is ratcheting strain at intrados, crown, and few other locations except extrados. Intrados and extrados are the inner and outer locations of the pipebend, and the crown locations are at an angle of 90 degrees from the intrados or extrados [Vishnuvardhan et al. (2013a)]. The ratcheting behaviour will lead to strain accumulation and an early exhaust of the ductility, thereby prone to crack initiation [Suresh Kumar et al. (2019)] and leakage [Suresh Kumar et al. (2023)]. Hence, the ratcheting studies on pressurized piping components of power plants give valuable inputs for designing the components and assuring their structural integrity [Vishnuvardhan et al. (2010)]. The SGDHR piping system is designed to operate in the creep regime. Hence the material selected for these piping systems is SS 316 LN. The SGDHR piping system consists of 200 NB (OD 219.1 mm) pipe bends that are 4 mm thick [Suresh Kumar et al. (2010)]. The details of the pipe bend manufactured for conducting the test are shown in Figure 3. The top flanges are taken as rigid. Pressure and weight are the mechanical loads acting in the piping system. The weight of the test specimen is neglected, and the pressure is 0.7 MPa, which is the design pressure of the SGDHR main and fast dump lines [Sudhakar et al. (2007)]. A 3D solid pipe bend has been modeled in ABAQUS software, as shown in Figure 4. The rigid flange on the right side is fixed at Location-B (Figure 3), i.e., the three translations and two rotations (UR1 and UR2) are restricted. Displacement is given in the direction of the X-axis on the left side at Location-A (Figure 3), and all the other degrees of freedom, except UR3, are restricted. 180-degree symmetric sector model is considered for the analysis (Figure 4). 3. Finite element analysis of SGDHR pipe bend