PSI - Issue 16

Ihor Dzioba et al. / Procedia Structural Integrity 16 (2019) 97–104 Ihor Dzioba, Sebastian Lipiec/ Structural Integrity Procedia 00 (2019) 000 – 000

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temperatures is necessary. Modern production and control technologies do not allow cracks appearance in newly manufactured elements. However, during long-term operation under conditions of cyclic loads and environmental impact, cracks can develop in the elements (Zvirko et al. (2017), Nazarchuk and Nykyforchyn (2018)). Such defects often occur in the areas of welded joints (Dzioba and Tsyrulnyk (2009), Pala and Dzioba (2017)).

Nomenclature J C

critical value of the J integral

σ yld

yield stress function effective plastic strain triaxiality stress factor

ƞ

ε eff_plas

ƞ 0 reference value of the triaxiality coefficient middle stress effective stress ƞ parameter in the Bai-Wierzbicki procedure _0 value of plastic strain at which material weakening beginning occurs _ further plastic strain values H ( ε pl_0 ) Heaviside function SEM scanning electron microscope SENB single edge notch bend specimen

Methods for assessing the strength of elements taking into account the possible presence of crack-type defects are presented in the FITNET procedures (Fitnet (2008)). These procedures are based on the knowledge of standard strength and fracture toughness characteristics, which leads to relatively high conservatism of the results obtained during the assessment. It should be emphasized that high conservatism can be reduced by analyzing at a higher advanced levels and taking into account the influence of in-plane constraint on fracture toughness. Ideas of a local approach to the analysis of the cracking process were proposed in the middle of the 20th century by Barenblatt (1959), Novozilov (1969). However, the development of the local approach took place only with the development of computer techniques and numerical calculation methods (Ritchie at al. (1973), Beremin (1983), Seweryn (1994), Neimitz et al. (2007, 2010)). In this study, the achievement of the critical conditions of the material when brittle or ductile fracture is initiated, is assessed on the basis of analysis of local stress and strain distributions in the most stressed area, i.e. before the crack tip. The analysis was based on the results obtained on SENB specimens made of S355JR steel. Experimental tests were carried out on specimens of S355JR (18G2A) steel with a microstructure of ferrite with coagulated carbide particles (FC). The S355JR steel with a microstructure of FC was obtained from steel with a layered ferritic-pearlite microstructure (FP) (Fig. 1a) by annealing during 150 h at 600 0 C. A laboratory heat treatment was carried out to receive a homogeneous microstructure on the specimen sections. This treatment allowed obtaining a homogeneous FC microstructure (Fig. 1b) instead of a layered FP with similar strength and plasticity characteristics of S355JR steel (Dzioba et al. (2018a). Strength and plasticity characteristics of the material were determined on the basis of uniaxial tensile tests on standard cylindrical specimens with a diameter of Ø = 5.0 mm according to ASTM E8 (2003). Stress-strain diagrams for nominal and true stresses for three representative temperatures are shown in Fig. 2a. The fracture toughness was determined as the critical value of J integral, J C , on standard SENB type specimens according to ASTM E1820-09 (2011) requirements using the change compliance technique. Experimental research was carried out in a thermal chamber in the temperature range from – 120 to +20 0 C. During the experimental tests, all necessary signals were recorded on all specimens, which were then used to determine the material characteristics and in numerical calculations. 2. Material properties and test methods