Materials Structure & Micromechanics of Fracture V

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Authors: Ladislav Kander, Karel Matocha, Jaromír Sobotka
Abstract: This article is focused on optimization of fundamental utility properties and technological process of advanced low alloy steel 15NiCuMoNb5 used for power engineering mainly for production of large capacity steam boilers. Consequently problem critical locality in weld after post weld heat treatment is studied. Optimization of welding technology is proposed. Fracture behaviour of welded joint made from boiler plates 80 mm in thickness of 15NiCuMoNb5 type steel after N + T PWHT regime after welding procedure were studied. Stable crack growth measurement and evaluation of fracture toughness of base material as well as weld metal in temperature region from 0 up 350°C were carried out. Unstable cleavage fracture and fracture toughness KJc of both parts of welded joint at test temperatures cover the transition region were evaluated. The reference temperature T0 approach was used for evaluation of temperature dependence of fracture toughness in transition range.
Authors: P. Pérez, S. González, G. Garcés, G. Caruana, P. Adeva
Abstract: The microstructural and mechanical characterization of two alloys within the Mg-Ni-YRE system prepared by casting and subsequent hot extrusion at 400°C have been carried out. The microstructure of both materials consists of a fine-grained magnesium matrix embedding a high volume fraction of second phases; coarse Mg12RE and long period ordered stacking structure (LPS phase) and fine Mg2Ni particles. Both alloys show high strength values up to 250°C. The yield stress values at room temperature are 295 and 405 MPa for low- and high-alloyed magnesium alloy, respectively. Load transfer from the magnesium matrix to coarse Mg12RE and LPS particles account for the high strength of both alloys at temperatures below 250°C. Above this temperature both alloys exhibit a superplastic behaviour at low stresses with elongations of 700 and 450 % for the low and high-alloyed magnesium alloy, respectively.
Authors: Marie Svobodová, Jindřich Douda, Jiří Kudrman
Abstract: This paper deals with changes in mechanical and structural properties of Steel T23 during long-time annealing at high temperatures. The research is focused on the degradation of the base material (steel T23), where the samples of steel, after the initial heat treatment, were annealed at temperatures of 600, 650 and 700 °C for 10 to 10 000 hours in a furnace with air atmosphere. This contribution summarizes the experimental results of mechanical and structural measurements and gives the relations between them.
Authors: Gianni Nicoletto, Radomila Konečná, P. Baicchi, V. Majerová
Abstract: The sand casting process usually generates porosity that is highly detrimental especially to the fatigue behavior of Al-Si alloys. Since pores favor early fatigue crack initiation, the total fatigue life is crack-propagation-dominated and, for a given stress level, strongly depends on the initiating pore size. Here long life fatigue strengths were estimated using specimens extracted from different AlSi7Mg castings. Detailed metallographic characterization of specimen sections was carried out to determine pore size distributions and correlate it to test results. Murakami’s experimental procedure for applying the largest extreme value statistics to small defect characterization is applied and discussed.
Authors: A.J. McEvily, Masahiro Endo, S. Cho, J. Kasivitamnuay, Hisao Matsunaga
Abstract: A study has been made of the striations and fissures developed in the aluminum alloy 2024-T3 during fatigue crack growth. Fissures were found to form on inclined facets. They were uniformly spaced as the result of a shielding process. Striation spacings were in accord with da/dN values at the higher levels of K investigated, but at low K levels striation spacings were larger than the corresponding da/dN values. The percentage of the fracture surface containing striations varied with the K level, ranging from less than 1 % at low K levels to 80 % at higher K levels. The reason for the discrepancy between the spacing of striations and the corresponding da/dN values is discussed.
Authors: Jaroslav Polák, Martin Petrenec, Jiří Man
Abstract: Austenitic stainless steel was cycled at a series of temperatures in the interval from 296 K to 113 K. Constant plastic strain amplitude loading at different levels of plastic strain amplitude and testing similar to multiple step test method were applied at different temperatures. The stress amplitude was continually recorded and selected hysteresis loops were stored and later analyzed using statistical theory of the hysteresis loop. Effective stress component and probability density function as a function of temperature were evaluated. The results were discussed in terms of the temperature dependence of the cyclic yield stress and its sources.
Authors: Josef Křišt'an, Jan Kratochvíl
Abstract: To estimate the fatigue endurance limit at the micro-scale two unlike dislocations gliding in a channel of a persistent slip bands are considered. The dislocations are modeled as moving planar flexible curves. The objective of the simulations is to determine the averaged stress in the channel needed for the dislocations to escape one another using statistics of encounters of such dislocation pairs. We employed an iso-strain approach, i.e. the assumption that the total strain everywhere in the channel is the same.
Authors: Keiko Shishime, Masanobu Kubota, Yoshiyuki Kondo
Abstract: Hydrogen is considered to be a possible energy source in the coming future. However, it has been recognized that hydrogen has a detrimental effect on the fatigue strength of metal. The fatigue crack growth characteristic is an important property for the integrity assessment of hydrogen utilization machine. In this report, the effect of hydrogen on the fatigue crack propagation characteristic was studied using low alloy steel, carbon steels and A286 alloy. Especially in this study, very short pre-cracked specimen as small as 0.03 mm deep was used and the near threshold fatigue crack behavior was studied. As a result, materials whose Vickers hardness was higher than 300 were found to be susceptible to absorbed hydrogen.
Authors: Ivo Dlouhý, Ladislav Jurášek, Miloslav Holzmann
Abstract: In pearlitic steels the fracture initiation of pre-cracked specimens is strongly affected by Lüders’ flow and higher strength properties. There is a gap in fracture toughness data of pre-cracked Charpy (PC) and compact tension (CT) specimens between 60 and 90 MPam1/2 due to plastic hinge formation. The distribution of maximum principal stress ahead of the crack tip in PC specimen loaded into fully plastic regime was found to be identical with that of reference stress-strain field in plane-strain SSY condition. 3D FEM simulations have also shown that there is no constraint loss in PC specimens.
Authors: Wei Dong Song, Hai Yan Liu, Jian Guo Ning
Abstract: The plate impact experiments have been conducted to investigate the dynamic behavior of 91W-6.3Ni-2.7Fe with three kinds grain sizes of 1− 3μm, 10 −15μmand 30 − 40μm . The stress-time history curves at different Lagrangian positions were obtained for tungsten alloys at different impact velocities. Lagrangian analysis technique was adopted to discuss the mechanical properties of the tungsten alloys under high strain rate. SEM was introduced to analyze the microstructure properties of tungsten alloys. The influence of grain size on the dynamic behavior of tungsten alloys under high strain rate was obtained and the stress-strain curves ( 4 5 1 10 ~ 10 s− ) of the tungsten alloys were given.

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