PSI - Issue 43

Arnab Bhattacharyya et al. / Procedia Structural Integrity 43 (2023) 35–40 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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operations were done using the displacement rate of 3×10 -3 mm/s. The data from the J-integral tests were used for constructing the J-R curve following the ASTM E1820 for subsequent determination of their J Q values. Ball Indentation Test: Ball indentation tests were performed using a fabricated fixture (Fig.1) fitted to the Shimadzu machine. These tests were done at a crosshead velocity of 0.02 mm/min. A hardened steel ball indenter of 1.59 mm was used for these tests. Specimens of approximate dimension of 10 x 10 x 25 mm 3 were cut and polished up to 1  m surface-finish. The polished surface of each material was indented at least at 10 different locations with increasing magnitudes of load. The maximum loads attained were approximately in the range of 50 kgf to 150 kgf. The variation of load versus indentation-depth (i.e. load line displacement) for each test was automatically recorded by the machine software. A dial gage was employed to crosscheck the maximum total indentation depth during each test. All indentations were photographed for measuring indentation diameters by an image analyzer.

(a) (b) Fig.1 Ball indentation set-up (a) A fabricated assembly attached to Shimadzu universal tensile testing machine. (b) Details of the ball indenter inserted with in the fabricated assembly. (A fractured compact tension specimen is shown underneath the ball indenter). 3. Results Microstructural analysis: Typical representative microstructures of the five investigated steels were recorded. The IF steel exhibits a ferritic microstructure; but austenitic microstructures with annealing twins are observed in 304LN SS and 316LN SS. The 0.14% C and SA333 steels (0.18% C) exhibit banded ferrite pearlite microstructures. The average ferrite grain size of IF, 0.14% C and SA333 steels and that of the austenite grain size of 304LN and 316LN stainless steels were determined. The estimated average grain sizes of IF steel, 304LN and 316LN stainless steels are 60  m, 86  m and 96  m respectively. The average ferrite grain sizes of 0.14% C and SA333 steels are found to be 12  m and 18  m respectively. The volume fractions of pearlite in the later steels, measured by point counting method, are found to be ~15% (0.14% C steel) and ~21% (SA333 steel) respectively. Analyses of Tensile behavior: Typical engineering stress-strain plots for the five investigated steels are shown in Fig.2. The 0.14% C and SA333 steels show distinct yield phenomenon. The upper yield points of these materials are considered for estimating the yield strength (YS) values, whereas YS of the other steels are determined using 0.2% strain offset procedure. The ultimate tensile strength (U.T.S.), uniform elongation (e u ) and total elongation (e t ) of the materials were estimated from Fig.3. The values of reduction in area (R.A.) of the investigated steels were estimated from the measured gauge diameters of tensile specimens before and after the tests. The estimated average tensile and hardness properties of the materials are given in Table 2 Table 2Tensile and hardness properties of the investigated steels Material Y.S.(MPa) U.T.S.(MPa) e u % e t % RA % HV 10 IF steel 94 238 34.6 45.2 82.1 94 + 1.3 316LN SS 272 575 48.2 65.4 84.9 182 + 2.1 304LN SS 352 687 44.6 67.3 86.2 190 + 1.6 0.14%C steel 325 464 25.2 41.7 63.4 166 + 0.8 SA333steel 348 432 30.5 47.5 77.5 135 + 1.2 Analyses of J-integral Fracture toughness: The Load-Crack mouth opening displacement curves for AISI 304LN SS and SA333 Gr.6 steel were recorded during the tests. The inverse of the slope of each unloading part of the curves yields the compliance (C i ) of the specimen corresponding to the load from which the unloading has been carried out and the slopes are calculated with linear regression coefficient (R 2 ) higher than 0.9999. The obtained C i values have been corrected (C ci ) for the specimen rotation following ASTM-E1820. The instantaneous crack lengths (a i ) of a CT specimen are determined from the corrected compliance values, and the elastic and plastic components of J was estimated following the standard equations as given in ASTM E-1820.