A mechanism-based approach to modeling ductile fracture.

Ductile fracture in metals has been observed to result from the nucleation, growth, and coalescence of voids. The evolution of this damage is inherently history dependent, affected by how time-varying stresses drive the formation of defect structures in the material. At some critically damaged state, the softening response of the material leads to strain localization across a surface that, under continued loading, becomes the faces of a crack in the material. Modeling localization of strain requires introduction of a length scale to make the energy dissipated in the localized zone well-defined. In this work, a cohesive zone approach is used to describe the post-bifurcation evolution of material within the localized zone. The relations are developed within a thermodynamically consistent framework that incorporates temperature and rate-dependent evolution relationships motivated by dislocation mechanics. As such, we do not prescribe the evolution of tractions with opening displacements across the localized zone a priori. The evolution of tractions is itself an outcome of the solution of particular, initial boundary value problems. The stress and internal state of the material at the point of bifurcation provides the initial conditions for the subsequent evolution of the cohesive zone. The models we develop are motivated by in-situ scanning electron microscopy of three-point bending experiments using 6061-T6 aluminum and 304L stainless steel, The in situ observations of the initiation and evolution of fracture zones reveal the scale over which the failure mechanisms act. In addition, these observations are essential for motivating the micromechanically-based models of the decohesion process that incorporate the effects of loading mode mixity, temperature, and loading rate. The response of these new cohesive zone relations is demonstrated by modeling the three-point bending configuration used for the experiments. In addition, we survey other methods with the potential to provide more detailed information about the near tip deformation fields

FCS-MPC upravljačka strategija novim trorazinskim izmjenjivačem otpornim na kvarove

In order to meet the high reliability of aviation inverters, the paper established a new three-level inverter which can increase the reliability in safety-critical applications, what\u27s more, the new topology adding assistant leg to control neutral-point voltage independently. On the basis of the new topology, a mixed logic dynamic (MLD) model was established for the new inverter circuits, and takes finite control set model predictive control (FCS-MPC) for the new inverter. The method takes a discrete-time model of inverter to predict the future value of the all possible voltage vectors generated by the inverter. The vector which minimizes objective function in finite control set is selected as the control of inverter, the objective function used in this work evaluates the voltage error and the switch frequency at the next sampling time. The paper explicitly researched the solving algorithm and realization procedure of the new inverter circuit, its feasibility and validity is verified by the experiment.Za postizanje visokog stupnja pouzdanosti avijacijskih izmjenjivača, u radu je postavljena nova topologija trorazinskog izmjenjivača za primjene u sigurnosno kritičnim sustavima koja ima dodatnu pomoćnu granu za nezavisno upravljanje naponom neutralne točke. Zasnivajući se na ovoj novoj topologiji, dinamički model s mješovitom logikom (MLD) postavljen je za električne krugove novog ispravljača koji za upravljanje pretvaračem koriste konačni skup upravljačkih signala dobivenih modelskog prediktivnog upravljanja (FCS-MPC). Metoda koristi vremenski diskretni model izmjenjivača za predviđanje budućih vrijednosti svih mogućih vektora napona koje generira izmjenjivač. Za upravljanje pretvaračem koristi se upravljački vektor iz konačnog skupa upravljačkih signala dobiven minimiziranjem funkcije cilja koja u obzir uzima grešku napona i frekvenciju sklapanja u sljedećem koraku diskretizacije. U radu je izravno razvijen algoritam za rješavanje problema i procedura za realizaciju nove topologije izmjenjivača, a izvedivost i validnost provjereni su eksperimentalno

Excessive checking behavior during an image comparison task in schizophrenia

International audienc

Experiments and Modeling of Fatigue Behavior of Friction Stir Welded Aluminum Lithium Alloy

An extensive experimental and computational investigation of the fatigue behavior of friction stir welding (FSW) of aluminum–lithium alloy (AA2099) is presented. In this study, friction stir butt welds were created by joining AA2099 using two different welding parameter sets. After FSW, microstructure characterization was carried out using microhardness testing, scanning electron microscopy, and transmission electron microscopy techniques. In particular, the metastable strengthening precipitates T1 (Al2CuLi) and δ’(Al3Li) seen in the base metal were observed to coarsen and dissolve due to the FSW process. In order to evaluate the static and fatigue behavior of the FSW of the AA2099, monotonic tensile and fully-reversed strain-controlled fatigue testing were performed. Mechanical testing of the FSW specimens found a decrease in the ultimate tensile strength and fatigue life compared to the base metal. While the process parameters had an effect on the monotonic properties, no significant difference was observed in the number of cycles to failure between the FSW parameters explored in this study. Furthermore, post-mortem fractography analysis of the FSW specimens displayed crack deflection, transgranular fracture, and delamination failure features commonly observed in other parent Al–Li alloys. Lastly, a microstructurally-sensitive fatigue model was used to elucidate the influence of the FSW process on fatigue life based on variations in grain size, microhardness, and particle size in the AA2099 FSW