[en] An explicit ductile-brittle transition of molybdenum occurring in both tensile and low cycle fatigue tests was investigated. Tests were performed on several sorts of molybdenum and its alloy TZM, and effects of heat treatment, fabrication method and alloying on the transition behavior and fracture mode are described in detail. All the materials exhibited a brittle failure with degraded fatigue behavior at room temperature, while they became ductile as temperature increased up to 573 K. The tendency of fatigue results was qualitatively in accordance with that of reduction of area in tensile tests. Differences among the materials were minor on the ductile-brittle transition temperature (DBTT), but major on the fatigue life for the embrittled materials. (orig.)

[en] The results of a survey of methods for evaluating fracture toughness characteristics for semi-brittle and brittle materials are presented in this report. These methods differ considerably from those used for ductile materials by the specimen configurations, the methodology of the experiments and by the problems occurring while using these methods. The survey yields several important findings A. It is possible to create steady state crack growth by cyclic loading in several semi-brittle materials. B. The need for pre-cracking is not yet clear, nevertheless it is recommended to evaluate fracture toughens with pre-cracked specimen. C. As crack length and ligament size may effect fracture toughness results it is necessary to define minimum specimen dimensions to avoid this effect. D. The specimen thickness hardly affects the fracture toughens. E. Loading rate for the test is not well defined. It is commonly accepted to end the test in one minute. F. The main mechanism that causes inelastic deformation in semi-brittle materials is related to the generation of micro-cracks

[en] A stacking fault plays an important role in dislocation behavior. In particular, the ratio of an unstable stacking fault (USF) energy and a free surface (FS) energy is an important parameter to describe the brittle versus ductile behavior of materials. The sacking fault energy problem has been extensively studied for a long time, however, the energy values calculated with different empirical potentials are scattered over a wide range depending on the selected potentials. Especially, materials involving α-Fe show the largest discrepancies in the results for the USF energy. In this study, we calculated the USF energy and the FS energy of α-Fe which is used for nuclear reactor vessel materials, by using quantum mechanical ab initio methods

[en] It is proposed the quantitative measure of steel ability to resist a transition from ductile to brittle state based on an analysis both of the strength and ductility characteristics for a wide range of structural steels. It has been shown that the brittleness of steel is determined not the value of a residual strain, but the relation between the level of brittle strength RMC and yield strength value, taking into account work hardening exponent. A possibility of optimizing the set of properties of strength and plasticity to achieve the best indicator of steel resistance to brittle fracture has been substantiated.

[en] The authors investigate the brittle fracture resistance of RPV 15H2NMFA grade 1 steel. The study used sets of small-sized specimens and a set of standard specimens. The paper demonstrates the need for a large-scale process simulation of conditions arising in weld joint zones that are inaccessible for direct testing

[en] The effect of a structural state on the TsM-6 molybdenum alloy cold brittleness is investigated using the method of static bending tests and electron diffraction microscopy. It is found out that the cald brittleness threshold is the highest in a recrystallization state of the TsM-6 alloy and is conditioned by an intergrain fracture, which occurs due to achievement of initial ageing stages in near-to-boundary alloy volumes. The lowest magnitudes of the cold brittleness threshold in a recrystallization state of the TsM-6 alloy are reached in the cases when solid solution decomposition takes place in the whole grain volume without preferred formation of precipitations in near-to-boundary volumes

[en] The strain energy density fracture criterion for crack initation under mixed mode static loading is extended with respect to brittle and ductile fracture taking into account results of metal-physical investigations. The energy density factors Sd (distortion), Sv (dilatation) of the inhomogeneous (cracked) continuum are correlated with the equivalent stresses sigmad, sigmav of the homogeneous (non-cracked) continuum. Recent experimental results are used for comparison. (orig.)

[en] Highlights: • A multilayered structure was developed to overcome the brittleness of quasicrystals. • In-situ TEM nanopillar compression was conducted to observe deformation behavior. • The single-layered amorphous structure deformed via shear banding and fractured. • The ductile multilayered structure deformed via confined dislocation activities.

[en] This study deals with the fracture behaviour of welded thin structures in the ductile to brittle transition range. It aims to propose a criterion to define the conditions for which the risk of fracture by cleavage does not exist on a cracked structure. The literature review shows that the difficulties of prediction of the fracture behaviour of a structure are related to the dependence of the fracture probability to the mechanical fields at the crack tip. The ductile to brittle transition range thus depends on the studied geometry of the structure. A threshold stress, below which cleavage cannot take place, is defined using fracture tests on notched specimens broken at very low temperature. The finite element numerical simulation of fracture tests on CT specimens in the transition range shows a linear relationship between the fracture probability and the volume exceeding the threshold stress, thus showing the relevance of the proposed criterion. Moreover, several relations are established allowing to simplify the identification of the criterion parameters. The criterion is applied to a nuclear structural C-Mn steel, by focusing more particularly on the higher boundary of the transition range. A fracture test on a full-scale pipe is designed, developed, carried out and analysed using its numerical simulation. The results show firstly that, on the structure, the transition range is shifted in temperature, compared to laboratory specimens, due to the low plasticity constraint achieved in thin structures, and secondly that the threshold stress criterion allows to estimate simply this shift. (author)

[en] The Charpy impact test is used to identify the transition between ductility and brittleness. The percentages of ductile and brittle fractures in steel can be evaluated based on each fracture area, which is presently determined by an analyzer with the naked eye. This method may lead to subjective judgement, and difficulty accurately quantifying the percentage. To resolve this problem, a new analysis method based on image processing is proposed in this study. A program that can automatically calculate the percentage of the ductile and brittle fractures has been developed. The analysis is performed after converting an RGB fracture image into a binary image using image processing techniques. The final binary image consists of 0 and 1 pixels. The parts with the pixel values of 1 correspond to the brittle fracture areas, and the pixel values of 0 represent the ductile fracture areas. As a result, by counting the number of 0 pixels in the entire area, it is possible to automatically calculate the percentage of ductile fracture. Using the proposed automatic fracture analysis program, it is possible to selectively distinguish only the brittle fracture from the entire fracture area, and to accurately and quantitatively calculate the percentages of ductile and brittle fractures.