PSI - Issue 60

S.K. Chandra et al. / Procedia Structural Integrity 60 (2024) 203–213 Author name / StructuralIntegrity Procedia 00 (2019) 000 – 000

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were always debatable due to large scale plasticity, necking around the crack tip and crack tip tunneling which depending on the material can be quite extensive, and a regime of stable tearing prior to onset of unstable fracture is established (Schwalbe et al., 2005). Because of the extensive plasticity and necking, initiation fracture toughness first increases to a maximum value, then it gradually decreases to the plane strain limit (plate material) with increasing specimen thickness. Materials with thickness corresponding to the first regime are called sheet materials (Pardoen et al., 1999). The essential work of fracture (EWF), , finds use for crack initiation characterization of thin sheets (Cottrell et al., 2005). However, due to the extensive stable crack growth regime in thin sheets, prediction of failure by crack extension is the main concern in the assessment of safety and reliability of different engineering components. Crack tip opening angle (CTOA) has been introduced as the parameter for characterizing crack growth resistance of thin sheets (Newman et al., 2003). Since its introduction, the CTOA parameter has been successfully applied for several metallic sheet materials, both for material qualification (Zhu and Joyce, 2012; Newman et al., 2003; Heerens and Schoedel, 2009; Darcis et al., 2008; Reuven et al., 2008; Xu et al., 2010; Hashemi et al., 2013; Horsley, 2003; James and Newman, 2003; Pussegoda et al., 2000), and for integrity assessment (Seshadri et al., 2003; Wang et al., 2014; Wang and Hwang, 1996; Brocks et al., 2006). For experimental determination of critical CTOA value ( ), the ASTM standard (ASTM E2472, 2012) recommends the use of C(T) and M(T) specimens of large (nearly 200 mm) width with anti-buckling guides, constant monitoring of the test, pre-cracking and rigorous data analysis. Such optical measurement demands sophistication both in terms of skill and measurement. This standard is quite comprehensive and basically recommends two direct (optical microscopy and digital imaging, DIC) and two indirect (microtopography and finite element analysis), methods for CTOA measurements. Among these methods, optical microscopy is the most commonly adopted method for measuring CTOA (Schwalbe et al., 2005, Newman et al., 2003, Heerens and Schoedel, 2009; Darcis et al., 2008; Reuven et al., 2008; Xu et al., 2010; Hashemi et al., 2013; Horsley, 2003; James and Newman, 2003; Pussegoda et al., 2000) due to its relative simplicity. The application of the other methods, DIC (Heerens and Schoedel, 2003; dawicke et al., 1995; Čolić et al., 2017 ], microtopography (Lloyd, 2003; Lloyd and McClintok, 2003) and finite element analysis (Schwalbe et al., 2005 2, Newman et al., 2003) is somewhat limited as these are bit complex both in terms of experimentation and analysis. On the contrary, the δ 5 concept ( δ 5 is the opening displacement at the original crack tip, measured over a gage length of 5 mm by an attached gage) introduced by GKSS (Schwalbe et al., 1994) provides a simpler and faster alternative for crack growth characterization in thin sheets by determining the δ 5 - ∆ curve of large sized C(T) and M(T) pre-cracked specimens. However, the application is only limited to the Al alloy (n = 0.005) (Heerens and Schoedel, 2009; Heerens and Schoedel, 2003, Schwalbe, 1995), also the published reports dealing with thickness lower than 2 mm are very scanty, notwithstanding the extensive use of thin metal sheets in several important industrial applications. The present study is aimed at characterizing crack growth resistance of a 1 mm thick interstitial free steel sheet by optical metallographic determination of critical CTOA, and δ 5 . The optical CTOA ( ) measurements at the growing crack tip on the specimen surface by using a light microscope integrated with a digital camera has been performed by conducting test on both the pre-cracked DENT and SENT specimens (not subjected to buckling) following the essence of ASTM E 2472 at three different ramp rate corresponding to quasi-static nominal strain rate of 10 -4 , 10 -3 and 10 -2 s -1 . Moreover, the notch root radius independence of critical CTOA, has been examined by conducting tests on notched ( :0.1 ) SENT specimens at a nominal strain rate of 10 -4 s -1 . For δ 5 measurements, instead of using a δ 5 gauge optical measurements on both notched ( :0.1 ) as well as pre-cracked the test specimens (DENT & SENT) have been explored at a nominal strain rate of 10 -4 s -1 . The transferability of − crack growth ( ∆ ) and δ 5 − crack growth ( ∆ ) between two geometries has been verified. Also, a comparison has been made between angle ( 5 ) (determined from δ 5 − ∆ curve) with optically measured CTOA, values at advancing crack tip, in order to find whether ( 5 ) is an actual measure of CTOA, .