Within the framework of European project MULTIMETAL (Structural performance of multi-metal component), several fracture tests on different types of multi-material specimens have been performed. Present fracture toughness standard methods, e.g. ASTM E 1820 are not directly intended for Dissimilar Metal Welds (DMWs). Therefore further investigations are needed in order to define the best practice in fracture mechanical tests and their analysis for DMWs.
Specimens were taken from welded plates: a narrow gap Inconel DMW junction between ferritic and austenitic stainless steels, designed and delivered by AREVA France. The work is focused on the nickel alloy - ferritic steel interface which is the weakest area of such welded pipes regarding ductile tearing. This paper is focusing on the experimental methodology in this particular case of multi-material fracture test specimens.
The first difficulty consists in machining specimens with a location of crack plane at the interface or close to the interface. Physically, the interface between the nickel alloy weld and the ferritic steel is not a straight plane. Some normalized specimen - geometries like CT (Compact Tension specimen), SE(B) (Single Edge Bend specimen), or SE(T) (Single Edge Traction specimen) - have been machined in the DMW junction. Some recommendations are discussed about this first step of work.
The second difficulty is the procedure of fracture test itself. The present fracture toughness standard method uses the method of compliance for the determination of J-Δa curves. The plasticity, in ferritic and stainless steel parts of specimens, has induced unsymmetrical response. Locally the path of the crack has been influenced by the gradient of properties of the multi-material medium (weld, heat affected zone). The J values, determined at crack initiation, are dependent of the location of crack plane. Results between specimens are compared and discussed.
Based on the current standard, this article highlights some shortcomings of this standard when used in heterogeneous specimens. The well-known problem of transferability is also pointed out (different values of J for crack initiation depending of the geometry and size of specimen).
In conclusion further numerical works are needed to assess experimental results and provide intrinsic material data, in terms of fracture properties of the Nickel alloy-ferritic steel interface.
