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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estimate FT for in-service industrial structures. Indentation fracture toughness (IFT) tests have emerged as alternatives to overcome some of the deficiencies of the standard techniques for FT estimation. The factors behind the development of IFT are manifold like these: (i) are simpler, faster, semi nondestructive type and less expensive than the standard methods, (ii) can be adopted to estimate fracture toughness of in-service structures at localized points of interest, and (iii) bear the potential to evaluate variation of FT due to gradient microstructural changes like across weldments, apart from the major advantage that the tests require limited amounts of material to achieve FT. The initial development of IFT was primarily based on the consideration of fracture stress, indentation energy to fracture (IEF) or mean contact pressures (Byun et al., 2000; Haggag et al., 1989; Murty et al, 1998) and the adopted principles encompassed usually stress based criteria. Several empirical formulations are suggested over the years for estimating IFT for brittle materials using Vickers indentation (Ray and Dutta, 1999), but the reliability of the estimations by Vickers indentation has been questioned (Quinn and Bradt, 2007) An empirical approach for the assessment of fracture toughness of ductile materials was earlier suggested by Haggag et al (1990, 1998). But the principle for determination of IFT of ductile materials should encompass strain-based criterion. Some recent works on IFT of ductile materials are emerging (Lee et. al., 2006, Ghosh et. al. 2010, Jeon et. al., 2017, Chatterjee et. al., 2016) based on indentation energy to fracture (IEF) with occasional attendant modeling or simulation, but additional efforts are required to achieve feasible solution and this report is aimed in this direction. The major objectives of this investigation are: (i) to develop a suitable methodology for determining fracture toughness of ductile materials using ball indentation test (K CBIT ) considering a new procedure based on determining IEF using a critical strain to fracture criterion, and (ii) to validate the development with comparative assessments of K CBIT for different structural steels with those obtained by standard fracture toughness tests or with reported data for similar materials. 2. Experimental Materials and Microstructures : The selected steels for this investigation are: (a) AISI 304LN stainless steel (b) SA333 Gr.6 steel, (c) AISI 316LN stainless steel, (d) 0.14% C steel and (e) Interstitial Free (IF) steel. The chemical compositions of the steels are given in Table 1. The microstructures of the steels were revealed by standard metallographic practice of grinding, polishing and etching. The IF and the carbon steels were etched with 2% Nital solution whereas the stainless steels were etched using aqua regia (1:3 HNO 3 :HCL). The average grain size of the steels was measured by linear intercept method as per ASTM standard E-112 with the help of an image analyzer. Table 1 Chemical compositions of the investigated steels (in wt %) Material C Si Mn P S Cr Ni N Fe IF steel 0.003 0.009 0.13 0.012 0.009 - - 0.0034 Bal. 304LN SS 0.017 0.54 1.80 0.028 0.014 18.55 9.55 0.1 Bal. 316LN SS* 0.021 0.45 1.75 0.03 0.015 16.5 11.8 0.08 Bal. 0.14% C steel 0.14 0.32 1.10 0.018 0.009 0.02 - 0.005 Bal. SA333 steel 0.18 0.25 0.90 0.004 0.002 0.06 0.04 0.0064 Bal. *2.4% Mo Tensile and Hardness Tests: Cylindrical tensile specimens of 6mm diameter and 30 mm gage length were fabricated from the available steel plates following ASTM standard E8/E8M-11. The tensile tests were carried out at a cross-head velocity of 1 mm/min using a Universal testing machine (Shimadzu, model: AG-5000G) at the room temperature of  300 K. The used cross-head velocity corresponds to nominal strain rate of 2 x 10 -3 s -1 . Samples for hardness tests were cut from the as received blanks with approximate dimensions of 15 x 15 x 15 mm 3 . Opposite surfaces of these specimens were made parallel and were polished up to 1000-grade emery paper. Vickers hardness measurements were carried out at a load of 10 kgf using a standard Vickers hardness-testing machine. At least five readings were taken for estimating the average hardness value. Fracture Toughness Test: Measurements of fracture toughness were carried out using standard J integral tests on fatigue pre- cracked 1″ CT specimens following ASTM standard E 1820-11. The loading sequence consisted of: (a) ramping in actuator control through 0.15 mm, (b) holding at that point for 20 seconds to allow for relaxations, (c) unloading through 0.8 mm, (d) immediately reloading through 0.8 mm and then repeating the whole cycle till the specimen reached well beyond the maximum load bearing capacity of the specimen. All loading-unloading