PSI - Issue 13

Letícia dos Santos Pereira et al. / Procedia Structural Integrity 13 (2018) 1985–1992 Author name / Structural Integrity Procedia 00 (2018) 000–000

1987

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ductile fracture process in such materials – the failure in this case is governed by plastic collapse rather than by the original fracture assumptions. As a step to better understand such phenomena and energies related to ductile fracture of high toughness steels, this work investigates Charpy and DWTT geometries in details considering a high toughness API-5L X80 steel. The main goal is to employ refined finite element models including damage and crack propagation to accurately describe the stress states and energy fractions with less simplifying assumptions than those considered in Fig. 1(a) .

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 0 50 100 150 200 250 300 350 400 Energy component fraction Charpy Plateu value [J] deformation initiation propagate

600 650 700 750 800 850 900 950

API-5L X80

σ [MPa]

0 0,2 0,4 0,6 0,8 1

ε

(a) (b) Fig. 1. (a) Shift in Charpy dissipation for the initiation, plastic deformation and propagation components as total energy increases; (b) True stress logarithm strain curve for API-5L X80 steel considered for the simulations. Source: (a) Leis (2015); (b) Scheider et al. (2014) 2. Theoretical background 2.1. Gurson-Tvergaard-Needleman model The GTN (Gurson-Tvergaard-Needleman) damage model is widely applied for ductile fracture simulation. It models the ductile fracture by the nucleation, growth and coalescence of microvoids that initiated in hard or second phase particles (Gurson, 1977; Tvergaard, 1982; Tvergaard and Needleman, 1984). This model is based on the von Mises yield criterion, and the damage evolution is evaluated by the parameter f *, known as modified void volume fraction, using equation (1). The � is the von Mises stress, � the hydrostatic stress, � , � and � are coefficients obtained empirically (Tveergaard and Needleman, 1984). Other parameters of this model are the porosity at the onset of void coalescence (critical porosity - f c ), damage acceleration factor , initial porosity f 0 and fraction of particles where new voids can nucleate f N (Nonn and Kalwa, 2013). These new voids are created based on a mean strain and standard deviation � . These parameters are mesh dependent, thus the element height l y is important. The disadvantages of this model include the mesh dependency and the number of calibrated parameters needed. Nonn and Kalwa (2013) recommended parameters for the GTN model for materials used in pipelines, as shown in Table 1. � � , � , ∗ � � � � � � 2 � ∗ cosh � 3 � � 2 � � �� � � ∗ � � � � (1) Table 1. Recommended parameters for GTN model [%] (%) (mm) 0.01- 0.03 0.1-0.5 0.3 0.1 0,02 4,0 1,5 1,0 2,25 0,1-0,3 Source: Nonn and Kalwa (2013)