PSI - Issue 71

Shreebanta Kumar Jena et al. / Procedia Structural Integrity 71 (2025) 103–110

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Gao et al. (2010), Gladskyi et al. (2013) and Sakane et al. (2011) that the extent of additional hardening is mainly governed by shape, sequence, amplitude of the load path and microstructure of the material. In the recent past, large numbers of multiaxial fatigue tests have been conducted on primary heat transport piping material (C-Mn steel) of Indian Pressurized Heavy Water Reactor (IPHWR) on un-notched tube specimens by Arora et al. (2016). These un notched tube specimens were used to quantify the extent of material damage (in terms of cyclic material hardening) under non-proportional loading as compared to corresponding proportional load scenarios. This study brought out significant additional material hardening under non-proportional conditions due to rotation of maximum shear stress (or principal) planes causing slip activity on large sets of material planes. Due to this reason, the fatigue life of un notched tube specimens under non-proportional loading condition was observed to be shorter than that of corresponding proportional loading as brought out by Arora et al. (2016). Further, some literature suggests that the fatigue life under remote multiaxial loading conditions is also governed by type of notches present in the test specimen as demonstrated by Gates et al. (2014). It has been observed from the test investigation carried out by Gates et al. (2016), when a thin tube specimen with single transverse hole subjected to nearly same remote In-phase and 90 ° Out of phase loading scenario, the fatigue life in case of In-phase loading comes out be lesser than that of the 90 ° Out of phase. The test fatigue life in case of thin notched tube specimen (with hole) is complete contrary to that of the previous understanding on unnotched tube specimen (without hole). In order to have a clear understanding on how the fatigue life of notched tube specimens get influenced under remote multiaxial loading condition a greater need of test and analyses data are required. In view of this, the present study is aimed at carrying out a test on tube specimens with a single sided hole under remote multiaxial strain-controlled condition. The objective is to understand the effect of proportionality and non-proportionality on fatigue life in the presence of notch. Additionally, Digital Image Correlation (DIC), a contactless optical surface strain measuring technique has also been employed to study the evolution of strain filed ahead of the hole tip as well as to monitor how the maximum principal direction changes when subjected to remote multiaxial loading condition. Further, the same technique is also used for capturing the fatigue crack initiation event (corresponding to ~1 mm crack size on visible surface) precisely for curved notched geometries under cyclic loading and low strain amplitude conditions. This type of extensive study on notched tube specimens under remote multiaxial loading scenario is hardly reported in the literature. 2. Material, specimen geometry and test setup details Low C-Mn steel (SA333 Gr.6) is generally used as the primary piping material of the Indian Pressurized Heavy Water Reactor (IPHWR). This grade of C-Mn steel has been selected as the material for present fatigue test studies.

Tube specimens with single sided though thick circular holes in the middle of gauge region have been machined using Electro-Discharge-Machining (EDM) process (refer Fig. 1). Typical Ra value on the whole surface was measured and it is varying between 3.3 μ m to 5.5 μ m. Two different sizes of hole diameters have been used to understand the effect strain gradient on fatigue life. These fatigue tests have been carried out on tension torsion machines

Fig. 1. Schematic of test specimen used for remote proportional/ non proportional axial-torsion tests

with maximum/ minimum axial load and torque carrying capacities of +/-100 kN and +/-1000 Nm, respectively. The testing setup consists of two independent actuators for axial and torsional load applications. Typical test set up with an extensometer for remote strain control and DIC for local (ahead of hole) strain measurement is shown in Fig. 2. The remote strains have been controlled using axial-torsion extensometer having measurement ranges for axial strain and angle of twist as ± 10 % and \ ± 2.5°, respectively.