Contrainte (mécanique) | EPFL Graph Search

vignette|Lignes de tension dans un rapporteur en plastique vu sous une lumière polarisée grâce à la photoélasticité. En mécanique des milieux continus, et en résistance des matériaux en règle générale, la contrainte mécanique (autrefois appelée tension ou « fatigue élastique ») décrit les forces que les particules élémentaires d'un milieu exercent les unes sur les autres par unité de surface. Ce bilan des forces locales est conceptualisé par un tenseur d'ordre deux : le tenseur des contraintes. La contrainte va de pair avec une déformation de la matière dans laquelle elle s'exerce. La contrainte mécanique est homogène à une pression (Pa). Notes et références Voir aussi Articles connexes

Mécanique (technique)
Matériau
Essai mécanique
Pression

Liens externes Catégorie:Mécanique des milieux continus Catégorie:Résistance des matériaux

Mechanics of Growth and Coalescence of Pre-existing Voids in a Ductile Matrix

Tomasz Jodlowski

This research project experimentally and numerically investigates the growth and coalescence of pre-existing voids for a wide range of stress states in a ductile matrix with a special focus on shear dominated loading conditions and presents quantitative kinetics data for these processes. The results obtained contribute to further development of current fracture models by generating reliable experimental data for void growth and coalescence, which can be used to evaluate the existing fracture models. A lead-containing copper alloy has been selected as model material, in which lead inclusions act as pre-existing voids – as demonstrated by unit cell numerical simulations for a wide range of stress states. A second copper alloy of similar composition, but without lead allows us to obtain mechanical properties of the matrix. By comparing the behavior of the two alloys it is possible to evaluate the effect of the voids on the macroscopic mechanical behavior. The model materials are deformed and fractured in combined torsion / tension tests and notched tensile tests to subject them to the desired wide range of stress triaxialities. Quantitative fractography and metallography provide the desired kinetics data on void shape, size and orientation, that can be correlated with numerical simulations, as well as existing void growth and coalescence models. The influence of stress-state and micro-structural characteristics on void growth and void coalescence – particularly for zero nominal stress triaxiality conditions – is illustrated at the hand of the results of the investigation. Contrary to the predictions of current micromechanical models, significant growth of small voids resulting in moderate increases in the total void volume is observed in torsion specimens. This growth may be triggered by the developing texture of the matrix material and particularly by twinning. By means of FEM computations with 3D single cell simulations comprising initially ellipsoidal voids, the effect of the initial texture and texture evolution is emphasized. The numerical results showed that the experimentally observed texture of the material accelerates void growth under relatively high stress triaxialities, and possibly allows growth of small voids under low and zero stress triaxialities. The initial shape of the lead inclusions and possibly their growth, was found to be dependent on their initial size. These results emphasize the need for a complete and rather detailed modeling of microstructural and deformation features in order to model damage process correctly. The presented work contributes to a better understanding of void growth and coalescence fracture processes, particularly under shear dominated loading conditions.

EPFL2011

SnSb binary alloy induced heterogeneous nucleation within the confined nanospace: Toward dendrite-free, flexible and energy/power dense sodium metal batteries

Dou Du, Yujie Liu, Min Zhang

Rechargeable sodium (Na) metal batteries (SMBs) provide an alternative energy-dense, low-cost energy storage system beyond prevailing Li-ion technologies. However, their practical deployment requires the performance evaluation of Na anodes on the device level and mitigation of safety hazards, i.e., mechanical stress, electrode pulverization and dynamic interfacial properties. Here, we propose a lightweight N-doped carbon nanofiber substrate with SnSb nanocrystallites monodispersed within the mesopores (SnSb@NCNF). The alloying-induced Na15Sn4 and Na3Sb intermediates act as the heterogeneous sodiophilic "magnets" to homogenize Na ion flux and confine Na deposits within the nanospace. Additionally, the nucleation theory of metal solidification bridges the density functional theory calculations, elucidating the oriented Na propagation that maximizes the anode utilization. With the proper Na plating regulation, the SnSb@NCNF substrate (pre-stored 1 x excess Na) integrates with the NaVPO4F cathode in 5 mA h single-layer pouch cell, the prototype of which exhibits the cycling endurance (96.3% capacity retention for 500 cycles) and high specific energy/power densities even upon the repetitive mechanical flexing scenarios. The nanoconfinement of the heterogeneous nucleation process affords a feasible approach to mitigate the dendrite formation upon the geometry deformation or high areal-capacity loading, which enlightens the further exploration of the energy-dense, mechanical flexible battery system.

ELSEVIER2021

Mechanisms of void initiation at small hard particles and implications for ductile fracture

Daniele Cannizzaro

Ductile fracture in steels is essentially due to cavity nucleation, growth and coalescence. Once voids are initiated in the metallic matrix, and the deformation continues to increase, voids grow in size and finally coalescence to fracture. This dissertation investigates ductile fracture initiating in the bulk of material for relatively low stress triaxialities. Particularly zero stress triaxiality conditions, i.e. under shear dominated loading, provided for an isolated study of single mechanisms such as void nucleation, and shear localization. In this dissertation, we show that the idealized division of ductile fracture in a sequence of elementary processes is not appropriate for the selected quenched and tempered VAR steel. Ductile fracture is nucleation-controlled, i.e without meaningful void growth. Void nucleation occurs by debonding of tempering carbides from the surrounding metallic matrix. The carbide-matrix interface strength is estimated by a dislocation-based stress decohesion model. By means of FEM computations with a unit cell simulations comprising a single carbide, the role of the carbide-matrix interface strength is emphasized. The results of FE computations demonstrate that the carbide-matrix interface strength is the key parameter controlling damage initiation and its evolution to fracture.

EPFL2006

Mechanics of Growth and Coalescence of Pre-existing Voids in a Ductile Matrix

Tomasz Jodlowski

EPFL2011