Ductile to brittle transition of an A508 steel characterized by Charpy impact test, part I., experimental results

International audienceThis study is devoted to the ductile–brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain-rates. Effects of impact velocity on ductile–brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids coalescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile–brittle transition range using fully coupled local approach to fracture is presented

Ductile to brittle transition of an A508 steel characterized by Charpy impact test, part II., Modeling of the Charpy transition curve

International audienceA finite element simulation of the Charpy test is developed in order to model the ductile to brittle transition curve of a pressure vessel steel. The material (an A508 steel) and the experimental results are presented in a companion paper (Part I [Engng. Fract. Mech.]). The proposed simulation includes a detailed description of the material viscoplastic behavior over a wide temperature range. Ductile behavior is modeled using modified Rousselier model. The Beremin model is used to describe brittle fracture. The Charpy test is simulated using a full 3D mesh and accounting for adiabatic heating and contact between the specimen, the striker and the anvil. The developed model is well suited to represent ductile tearing. Using brittle failure parameters identified below −150 °C, it is possible to represent the transition curve up to −80 °C assuming that the Beremin stress parameter σu is independent of temperature. Above this temperature, a temperature dependent Beremin stress parameter, σu, must be used to correctly simulate the transition curve. Quasi-static and dynamic tests can then be consistently modeled

Multiaxial fatigue crack initiation on filled rubbers : statistical aspects

International audienceWhite reinforcement fillers such as precipitated ...

Cyclic loadings and crystallization of natural rubber: An explanation of fatigue crack propagation reinforcement under a positive loading ratio

Natural rubber is known to have excellent fatigue properties. Fatigue crack propagation studies show that, under uniaxial tension loading, fatigue crack growth resistance increases with the loading ratio, even if the peak stress increases. Studies dealing with crack initiation confirm this trend. If strain induced crystallization is believed to play a major role in this reinforcement process, it is not clear yet by which mechanism this reinforcement takes place. Using SEM investigation, it is shown here that the reinforcement process is associated with strong crack branching in the crack tip region. From experimental results it is shown that under particular reinforcing loading condition a cyclic strain hardening process can be observed on the natural rubber which is able to overcome classically observed softening effects. A cumulative strain induced crystallization process is proposed to explain the stress ratio effect on fatigue crack initiation and propagation properties of natural rubber.a LAMEFIP, Arts et Métiers ParisTech, Esplanade des Arts et Métiers, 33405 Talence Cedex, France b Centre des matˇıeriaux P.M. FOURT, Ecole Nationale Supˇıerieure des Mines de Paris, UMR CNRS 7633, Evry Cedex 91003, Franc

Damage and fracture of PVDF at 20°C

International audienceFlexible oil pipelines are multilayered structures used for the transport of crude oil or natural gas from the seabed to the surface in offshore oil fields. Because of severe service conditions, composite structure made of metallic and polymeric layers must be used. PVDF is a good candidate as it accommodates tensile and flexural deformations and guarantees watertightness. The present paper focuses on the mechanical properties of PVDF at 20°C. Several specimens with different geometries were tested: notched specimen and cracked specimen under tension. Fracture surfaces were examined to determine fracture mechanisms. Based on mechanical testing and microscopic observations, a modified Gurson-Tvergaard-Needleman model for semi-crystalline polymers is proposed. The model allows to represent the non-linear behavior and the cracking of polymer structures

COMPORTEMENT VISCO-HYPERELASTIQUE ENDOMMAGEABLE D'ELASTOMERES (SBR ET PU) (PREVISION DE LA DUREE DE VIE EN FATIGUE)

PARIS-MINES ParisTech (751062310) / SudocSudocFranceF

Comportement et fatigue multiaxiale d'un élastomère chargé (NR-CB)

PARIS-MINES ParisTech (751062310) / SudocSudocFranceF

Fissuration fragile lente du Polyamide 11 (Mécanismes et durées de vie en fluage)

PARIS-MINES ParisTech (751062310) / SudocEVRY-MINES ParisTech (912282301) / SudocSudocFranceF

Crack initiation and propagation under multiaxial fatigue in a natural rubber

International audienceThe ever growing use of elastomers and polymers in structures leads to the need of pertinent multiaxial fatigue life criteria for such materials. Thus, the understanding of the fatigue crack initiation micro-mechanisms and their link to the local stress and/or strain history is essential. Scanning electron microscopy and Energy Dispersive Spectroscopy (EDS) have been used to investigate those micromechanisms on a natural rubber. Rigid inclusions were systematically found at the crack initiation. Depending on the type of inclusion (identified by EDS), cavitation at the poles or decohesion are the very first damage processes observed. Cracks orientations are compared to local principal stress orientation history, the later being obtained from finite element calculations (FE). It is shown that if large strain conditions are correctly taken into account, cracks are found to propagate systematically in the direction given by the maximal first principal stress reached during a cycle, even under non-proportional loading. A fatigue life criterion is proposed