Micromechanical estimates of the critical values of J -integral for the steel of steam pipelines
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MICROMECHANICAL ESTIMATES OF THE CRITICAL VALUES OF J-INTEGRAL FOR THE STEEL OF STEAM PIPELINES B. Medjo,1 M. Rakin,1, 2 M. Zrilic,1 S. Putic,1 and A. Sedmak 3 We analyze the level of damage to 14MoV63 steel for steam pipelines under the conditions of crack-growth initiation according to the mechanism of ductile fracture. The state of the material after service for about 100,000 h at the power plant is estimated by using a combined experimental and numerical procedure based on the local approach to fracture. The level of damage is determined by analyzing the material taken from the pipeline (aged) and new (virgin) material of the same grade (for the sake of comparison). The critical values of the J-integral are determined on single-edge notched bend (SENB) specimens both experimentally and by using a simplified procedure of local approach without knowing the microstructural parameters. Two micromechanical models are applied: the Rice – Tracey models modified by Beremin (uncoupled) and the complete Gurson model (coupled). Keywords: heat-resistant steels, structure, microdamage.
The process of ductile damage develops through three stages: nucleation, growth and coalescence of voids resulting in the final failure of the material [1]. Voids are nucleated around nonmetallic inclusions and secondphase particles when the so-called critical stress within the inclusion or on the inclusion–matrix interface is exceeded. Under loading, materials of different ductile behaviors exhibit both strain hardening and softening due to the presence of voids. The local approach is often used for the analysis and prediction of ductile-fracture initiation in alloys by modeling the nucleation, growth, and coalescence of voids. It is based on the micromechanical models developed to describe the fracture process in accordance with the actual phenomena in the material. The application of these models should not have specific requirements related to the tested geometries, since the model parameters depend only on the material, which enables one to attain the so-called transferability of parameters to different geometries. The main idea of the present work is to apply the local approach to ductile fracture in the analysis of the level of damage of steel taken from exploitation expressed via the ratio of the critical values of the J-integral for the aged and new materials. We study 14MoV63 steel (according to DIN). The crack-growth initiation in SENB specimens is analysed by using the micromechanical models. Micromechanical Models According to the uncoupled modeling, the presence of voids does not significantly alter the behavior of the material [2]. Hence, the damage parameter does not appear in the yield criterion. The frequently applied un1 Faculty of Technology and Metallurgy, Karnegijeva 4, 11120 Belgrade, Serbia. 2 Corresponding author; e-mail: [email protected]. 3 Faculty of Mechanical Engineering, Kraljice Marije 16, 11120 Belgrade, Serbia.
Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 45, No. 4, pp. 53 – 59, July – August,
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