Effects of residual stress by EB welds on assessment of crack arrest temperature (CAT)

Abstract

The concept of brittle crack arrest has recently become an internationally focused issue for container ships. The International Association of Classification Society (IACS) also prescribed the unified requirement (UR) for brittle crack arrest design, and brittle crack arrest design has been internationally authorized. As one of the methods to evaluate brittle crack arrestability, the crack arrest temperature (CAT) concept, by isothermal crack arrest test, has been proposed since the 1990s. The concept has been applied mainly for tank design. However, no standard has been specified to describe the detailed evaluation procedure. This means that only limited organizations can evaluate CAT and it is considered to be a problem when arrest evaluation is mandated as an international standard. In the background of such circumstances, Japanese research groups including the Japan Welding Engineering Society (JWES) and Nippon Kaiji Kyokai (ClassNK) started the standardization for CAT test in 2016. In the research programme, various aspects of control factors have been investigated based on the test results from many experiments and numerical calculations. The CAT test shall include the embrittled zone to initiate a brittle crack. Either electron beam (EB) line remelting or a local temperature gradient (LTG) can be applied to the embrittled zone. Even if we focus on EB welding only, welding defects in the embrittled zone can be an influencing factor. In this report, we investigate the effects of residual stress by EB welding on the crack driving force, which is quantified as the K value using a 3D finite element method (FEM). As a result, we confirmed the existence of the residual stress which cannot be ignored that is formed on the surface of the EB-welded portion; however, the influence of that on the K value is considered to be small if the CAT test conditions can sufficiently secure the arrest crack length. This result shows that the driving force at the arrested point in the CAT test can be simply evaluated by the LEFM formula without consideration of the residual stress of the EB weld for embrittlement.