Non-fracture prediction of a C–Mn weld joint in the brittle-to-ductile fracture transition temperature range Part I: Experimental results and numerical study C. Niclaeys a,b,c,⇑ , T.H. N’Guyen d , S. Marie d , S. Chapuliot e , S. Degallaix a,b,c a Université Lille Nord de France, F-59000 Lille, France b EC-Lille, LML, F-59650 Villeneuve d’Ascq, France c CNRS, UMR 8107, F-59650 Villeneuve d’Ascq, France d CEA DEN/DM2S/SEMT/LISN, CE-Saclay 91191 Gif-sur-Yvette Cedex, France e AREVA-NP PEER-F, Tour AREVA, 92084 Paris La défense Cedex, France article info Article history: Received 8 April 2011 Received in revised form 12 September 2011 Accepted 16 October 2011 Keywords: Brittle to ductile fracture transition Weld joint SEM Piping system abstract This paper evaluates the brittle fracture risk for a C–Mn weld in the upper shelf of the brittle-to-ductile transition: the criterion considered is based on a critical stress r th , with the failure probability related to the volume around the crack where the maximum prin- cipal stress exceeds r th . The weld shows a complex microstructure with two types of melted zone. SEM observa- tions showed that the main cleavage sites are located in the coarse grain zone, near an inclusion. In test analyses, the material’s heterogeneity in the weld metal is considered to estimate local fields at the weld pass scale. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Brittle fracture is a catastrophic failure mode of ferritic steel that must be eliminated in nuclear power plant components. This requires using criteria referring to the brittle-to-ductile transition temperature RT NDT , in an attempt to keep metal as far as possible from this temperature, at which a risk of brittle failure exists. However, with strain and thermal aging or neutron embrittlement, the RT NDT temperature increases with time, making it difficult to entirely exclude the potential risk of brittle fracture: critical defects in terms of fast fracture could become extremely small and thus undetectable by in-service inspection. However, the criteria based on RT NDT and envelope toughness curves are pessimistic when applied to structures, in par- ticular thick structures containing small surface defects. These data, fitted on highly constrained and bent fracture toughness specimens such as compact tension (CT) or single edge notched beam (SENB) specimens, are not representative of structures containing small surface defects and locally submitted to tension: considerable loss of constraint is observed and the stress/ strain concentration at the crack tip becomes smoother, thus reducing the risk of brittle fracture. A consequent shift in brittle-to-ductile transition temperature can be observed. To benefit from this shift in the fast fracture analysis of structures, a simple model has been developed based on the principle that the cleavage fracture occurs only if the maximum principal stress exceeds a threshold stress [1–5]. The risk 0013-7944/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2011.10.010 ⇑ Corresponding author at: Université Lille Nord de France, F-59000 Lille, France. E-mail address: Christophe.niclaeys@ec-lille.fr (C. Niclaeys). Engineering Fracture Mechanics 79 (2012) 149–166 Contents lists available at SciVerse ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech