PSI - Issue 60

Ganesh Nigudage et al. / Procedia Structural Integrity 60 (2024) 678–689 Ganesh Nigudage / Structural Integrity Procedia 00 (2023) 000 – 000

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halves of die. At point C of force-displacement curve crack growth occurs. At point D the decrease in thickness increases with punch displacement and failure occurs along thinned portion. The SP test is displacement-controlled, i.e. the punch is pushed with constant velocity of the crosshead through the specimen and the force F required to keep the punch moving is measured as a function of punch displacement v (at the punch tip) or specimen deflection u (measured on the lower side of the specimen). For establishing empirical correlations, characteristic force values from small punch test are used to correlate with yield and tensile strength and displacement values are used to correlate with total elongation obtained from tension test. The characteristic force values are normalized by square of initial specimen thickness while establishing correlations. In region between one and two there is departure from linearity or elastic-plastic transition which suggests association with yield stress. The maximum force of load-displacement curve is used to correlate to ultimate stress of material (Torres and Gordon, 2021). For evaluating tensile properties of a given material, characteristic values of force, displacement are identified from the SPT test record and fed into empirical correlations to obtain estimates of mechanical properties of material under consideration. The aim of the present work is to generate a set of empirical correlation constants by performing SPT of RPV material Cr-Mo-V steel at room temperature and -60 o C and then generate validated correlation constants by comparing characteristic load and displacement values from SPT with the tensile properties obtained from standard tensile test.

2. Experimental details 2.1. SP test set-up

The fixture used for SPT consisted of upper and lower die, punch and ball (2.4mm diameter). The fixture was mounted on screw driven universal testing machine equipped with 100kN capacity load cell and extensometer. The extensometer had knife edges which were used to attach to punch and fixture to record punch displacement during test. The ball is driven through the specimen using punch which is displaced by cross head of test machine. Figure 3 shows the SP test fixture mounted on test machine.

Fig.3. SP test fixture mounted on the test machine with the extensometer for punch displacement measurement

2.2. Material and test conditions Cr-Mo-V reactor pressure vessel steel is the material under consideration whose conventional tensile properties at room temperature and -60 o C in different condition are given in Table 1. This material was used in different conditions like as-fabricated (archive) base, as-fabricated (archive) weld, thermal base and thermal weld. Thermal base and weld are the specimens that were subjected to operating temperature of reactor and are not irradiated. Thermal base and weld specimens were removed from the reactor after 24600 cumulative hours of reactor operation. The SP tests of above four specified specimens were performed at 0.5mm/min displacement rate at room temperature and -60 o C. The SPT specimens were prepared from Charpy V-Notch specimens, (which were earlier subjected to standard impact