Essai de traction | EPFL Graph Search

thumb|Essai de traction terminé. Un essai de traction est une expérience de physique qui permet d'obtenir des informations sur le comportement élastique, le comportement plastique et le degré de résistance à la rupture d'un matériau, lorsqu'il est soumis à une sollicitation uniaxiale. Certains objets manufacturés doivent avoir un minimum de solidité pour pouvoir supporter les charges, le poids et bien d'autres efforts. L'essai de traction permet de caractériser les matériaux, indépendamment de la forme de l'objet sollicité, ou la performance d'un assemblage mécanique. Comme tout essai mécanique, l'essai de traction reproduit une sollicitation simple, donc éloignée des sollicitations réelles, mais facilement maîtrisable et reproductible. Cet essai ou expérience consiste à placer une petite barre du matériau à étudier entre les mâchoires d'une machine de traction qui tire sur la barre jusqu'à sa rupture. On enregistre l'allongement et la force appliquée, que l'on convertit ensuite en d

Investigations and numerical modeling of mechanical properties of tempered martensitic steel Eurofer97 at various loading rates, temperatures and after spallation irradiation

Serafin Knitel

The reduced activation tempered martensitic steel Eurofer97 is a reference steel for a structural applica-tions in fusion reactors. The first-wall and blanket materials will be subjected to high thermal and neutron fluxes and will experience very complex, time-dependent mechanical loading. Owing to impinging neutrons in materials, points defects and gasesous impurities (hydrogen and helium) are created and accummulated in the form of small defect clusters in the microstructure altering the mechanical properties. Notably, fracture toughness decreases as strength increases. Eurofer97, being a body centered cubic alloy, presents a ductile-to-brittle transition temperature T0 that shifts to higher temperatures after irradiation. The main objective was to study the loading rate and irradiation effects on plastic flow and fracture properties of Eurofer97. Tensile and fracture tests were performed at temperatures in the transition region, at low and high loading rates on unirradiated material. Tensile properties of irradiated material were assessed after irradiation in the Swiss neutron spallation source SINQ to a dose of about 11 dpa and helium content of 500 appm at about 250 Â°C. The information gathered from the mechanical tests was used either as input in finite element models of fracture specimens, or as benchmark data to calibrate and verify the predictions of the models. Fractographic observations were also conducted. The tensile tests on unirradiated Eurofer97 showed a strong temperature dependence and strain rate sensitivity of the stress at low temperature, consistent with a mechanism of nucleation and propagation of double kink on screw dislocation. At high loading rate, the strain rate hardening was partially compensated by thermal softening due to quasi-adiabatic heating. The tensile properties after irradiation showed strong irradiation hardening (increase of the yield stress) accompanied by a drastic decrease of uniform elongation. The constitutive behavior at large strains, representative of those at crack tip, was obtained with an inverse method, based on finite element (FE) models of tensile specimens, developed to derive the true stress-strain curves from the load-displacement ones. These curves were used as input for the FE models of fracture specimens. Comparative fractographic observations on unirradiated and irradiated specimens did not reveal any strong effect of irradiation or helium on fracture mechanism, which remains transgranular cleavage. This observation indicates that higher helium concentration than 500 appm is necessary to initiate intergranular cleavage by helium bubble precipitation on grain boundaries. Dynamic fracture tests were performed on unirradiated Eurofer97. A shift of about 10 Â°C of T0 by one order of magnitude in loading rate was determined, which was in good agreement with results obtained on different ferritic steels. Static fracture toughness tests were also performed on disk compact tension specimens that can be accommodated in irradiation tubes of SINQ. Different FE models of fracture specimens were developed to include strain-rate sensitivity of flow stress, heating resulting from the plastic work in the fracture process zone and loading rate. The structure of the stress field at the crack tip was studied and the temperature dependence of a lower bound to toughness data in the transition was reconstructed using a calibrated criterion of local approach to brittle fracture.

EPFL2018

Irradiation induced behavior of pure Ni single crystal irradiated with high energy protons

Zhongwen Yao

The microstructure and tensile behavior of pure single crystalline Ni irradiated to low doses at room temperature with 590 MeV protons are investigated. Tensile tests performed at room temperature show radiation induced hardening, which increases with increasing dose. The microstructure of irradiation induced defects observed under transmission electron microscopy consists of about 50% stacking fault tetrahedra, 30-40% dislocation loops and 10% of unidentified features. In addition, the irradiated and deformed specimen microstructure exhibits, at the beginning of deformation, shear bands as straight channels that are free of defects. The macroscopic results of the localized deformation have been observed during tensile test. The surface topography evolution is discussed in terms of localized shear and crystal rotation, which relates to the formation of deformation cells. (C) 2003 Elsevier B.V. All rights reserved.

2003

The microstructure and tensile properties of pure Ni single crystal irradiated with high energy protons

Zhongwen Yao

The microstructure and tensile behavior of pure single crystalline Ni irradiated with 590 MeV protons are investigated. First results have been obtained for a damage level of 0.13 dpa at room temperature and 0.002 dpa at 523 K. The irradiation induced defect microstructure observed under transmission electron microscopy consists of loops and stacking fault tetrahedra in a lower density. Tensile tests were performed at room temperature and show observable radiation induced hardening already at 0.002 dpa. Dislocation channels and cells were observed and the transition from the initial channel formation to the development of the deformation cells is investigated. First results are presented here. (C) 2002 Elsevier Science B.V. All rights reserved.

2002

Investigations and numerical modeling of mechanical properties of tempered martensitic steel Eurofer97 at various loading rates, temperatures and after spallation irradiation

Serafin Knitel

EPFL2018