Transition Regime Fracture Toughness-temperature Properties of two Advanced Ferriticmartensitic Steels
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Transition regime fracture toughness-temperature properties of two advanced ferriticmartensitic steels Philippe Spätig, Eric Donahue1, George R. Odette1, Glenn E. Lucas1 and Max Victoria Fusion Technology, Centre de Recherches en Physique des Plasmas, Ecole Polytechnique Fédérale de Lausanne, CH-5232 Villigen PSI, SWITZERLAND 1 Department of Mechanical and Environmental Engineering, University of California, Santa Barbara, CA 93106-5080, U.S.A. ABSTRACT Advanced martensitic steels are leading candidate materials for fusion reactor structural components due to their resistance to void swelling, and good balance of physical and mechanical properties. However, irradiations at temperatures below about 400Û&UHVXOWLQ increases in the cleavage-to-microvoid coalescence transition temperature, as well as reductions in the upper-shelf tearing toughness. This paper reports on the transition regime fracture toughness properties of two unirradiated 7-9Cr martensitic alloys, F82H and T91. Effective fracture toughness-temperature curves, Ke(T), were measured in the transition regime using relatively small pre-cracked specimens. Three sets of data obtained on different specimen sizes and geometries for the two steels are analyzed. All the data can be placed on a single so-called master-curve, when shifts due to size/geometry and material are taken into account. The results from mechanical tests, finite element method (FEM) simulations of crack tip fields and confocal microscopy-fracture reconstruction observations are presented. The link between the different mechanisms taking place at various length scales, resulting in the complex process of cleavage fracture, is discussed. INTRODUCTION While tempered martensitic steels are among the most promising candidate materials for structural components of fusion reactors [1], they experience a so-called ductile to brittle fracture mode transition from high-temperature microvoid coalescence to low-temperature cleavage [2]. As a consequence of neutron irradiation, the transition between the ductile and brittle regime is shifted to higher temperature, possibly limiting the use of these steels in some applications to fusion first wall and blanket structures. It is therefore of primary importance to characterize both the fracture properties of the unirradiated material and to develop a methodology, based upon an understanding of the fundamental mechanisms, to assess the changes in fracture toughness under irradiation in order to safely manage reactor structures. The macroscopic failure in real structures results from the interplay and succession of a series of atomic-, microscopic-, mesoscopic- and macroscopic-scale mechanisms which eventually lead to fracture. Studies and efforts to link the information obtained at an atomic level to macroscopic continuum mechanics scales have been undertaken, e.g. [3, 4, 5] The goal of this paper is to link our results and investigations performed at different length scales. First, macroscopic measurements of effective fracture toughness data are obtained
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