Approximate Models for Ductile Metals Containing Non-spherical Voids—Case of Axisymmetric Prolate Ellipsoidal Cavities,"

The aim of this paper is to extend the classical I Introduction Metals often contain microvoids which significantly alter their mechanical behavior. The growth of such microvoids was studied in the pioneering works of The choice of axisymmetric ellipsoids can serve to model a wide range of void shapes, ranging from penny-shaped cracks to needles. Such a choice was made in several recent papers. In a recent paper (Gologanu et al., 1993), we proposed a constitutive model for ideal-plastic materials containing aligned or randomly oriented prolate ellipsoidal cavities. This model was obtained by extending The aim of the present work is to extend this analysis to the case of oblate cavities. Two fresh difficulties will be encountered. The first one is that while in the prolate case, both of Gurson's classical models for spherical and cylindrical voids could be used as references, only the first one is relevant to the oblate case. The second difficulty arises when one looks for an approximation of the two-field criterion leading to an analytic, Gurson-like expression. While in the prolate case it was possible to neglect the "crossed term" proportional to the product of the two velocity fields in the expression of the plastic dissipation without significantly altering the accuracy of the estimate, the same approximation reveals impossible for oblate voids, as will be detailed below. As in the prolate case, we shall consider an ellipsoidal cavity embedded in a finite volume (this allows to study arbitrarily great porosities), namely, a confocal ellipsoid. Such a geometry is primarily chosen because of mathematical tractability, but it is not a completely unreasonable approximation of an elementary cell in a periodic medium, just as Gurson's hollow sphere. Attention will be restricted to ideal-plastic behavior and axisymmetric loadings. Conditions of homogeneous boundary strain rate (v = D.x, where v denotes the velocity, D the overall strain rate and x the current position) will be enforced on the outer surface. The reason for imposing a homogeneous strain rate rather than a homogeneous stress is that the space of admissible velocity fields is smaller in the former case, so that the search for a

Advanced models for the prediction of forming limit curves

edition: 2ndstatus: publishe

An Electrochemical Oxygen Pump Model – A Tool for Sensor Optimization

AbstractElectrochemical oxygen pump sensors are increasingly important in a range of industrial sensing applications. However, their development has traditionally been based on inefficient empirical approaches. We have built a detailed finite-element model of an oxygen pump electrochemical sensor that is able to simulate the distribution of oxygen within sensor components under a range of conditions. This has been used to predict key performance parameters such as the steady state output current as a function of oxygen concentration, the startup characteristics and the transient response to a step change in oxygen concentration. The model is a powerful tool enabling multiple design concepts to be compared without the need for time consuming prototype sensor construction

Esophageal Pressure Measurement in Acute Hypercapnic Respiratory Failure Due to Severe COPD Exacerbation Requiring NIV—A Pilot Safety Study

Esophageal pressure (Pes) measurements could optimise ventilator parameters in acute respiratory failure (ARF) patients requiring noninvasive ventilation (NIV). Consequently, the objectives of our study were to evaluate the safety and accuracy of applying a Pes measuring protocol in ARF patients with AECOPD under NIV in our respiratory intermediate care unit (RICU). An observational cohort study was undertaken. The negative inspiratory swing of Pes (ΔPes) was measured: in an upright/supine position in the presence/absence of NIV at D1 (day of admission), D3 (3rd day of NIV), and DoD (day of discharge). A digital filter for artefact removal was developed. We included 15 patients. The maximum values for ∆Pes were recorded at admission (mean ∆Pes 23.2 cm H2O) in the supine position. ∆Pes decreased from D1 to D3 (p < 0.05), the change being BMI-dependent (p < 0.01). The addition of NIV decreased ∆Pes at D1 and D3 (p < 0.01). The reduction of ∆Pes was more significant in the supine position at D1 (8.8 cm H2O, p < 0.01). Under NIV, ∆Pes values remained higher in the supine versus upright position. Therefore, the measurement of Pes in AECOPD patients requiring NIV can be safely done in an RICU. Under NIV, ∆Pes reduction is most significant within the first 24 h of admission

An evolving plane stress yield criterion based on crystal plasticity virtual experiments

This paper presents a new hierarchical multi-scale framework that allows taking into account evolution of the plastic anisotropy during sheet forming processes. The evolution of crystallographic texture, which is identified as the main source of the plastic anisotropy, is predicted by the ALAMEL crystal plasticity model. An extension to the phenomenological anisotropic plane-stress yield criterion BBC2008 is proposed, which provides adaptive updates of the local anisotropy in the integration points of the macroscopic finite element model. To this end, the BBC2008 is systematically recalibrated to data provided by the crystal plasticity virtual experiment framework (VEF). An enhanced identification algorithm is proposed. The new algorithm exploits comprehensive material characterization delivered by the VEF. The deep drawing of cylindrical cups is used as a benchmark case. An industrial-grade aluminium alloy AA6016 is chosen for the test case. The experimental part of the study includes X-ray diffraction measurements of the texture as well as mechanical testing, which comprises uniaxial tensile tests, a bulge test and the deep drawing of cylindrical cups. A noticeable through-thickness gradient in terms of both the texture and the plastic anisotropy is identified in the initial material. This issue is taken into consideration in the study. Three groups of simulations have been performed, in which (i) the BBC2008 was calibrated by means of the mechanical tests, (ii) the BBC2008 was calibrated by means of the VEF based on the initial texture, but kept constant throughout the simulation, and (iii) the BBC2008 was systematically recalibrated by means of the VEF to reflect the effects of texture evolution. The earing profiles measured in the experimentally drawn cups are compared to the ones predicted by the simulations. It is found that the cup profile predictions by (i) are clearly inferior compared to both (ii) and (iii). However, in the considered process (iii) provides only a moderate improvement over the results of (ii). The explanation of this limited impact of the anisotropy evolution is based on the analysis of the evolved texture and associated plastic anisotropy.publisher: Elsevier articletitle: An evolving plane stress yield criterion based on crystal plasticity virtual experiments journaltitle: International Journal of Plasticity articlelink: http://dx.doi.org/10.1016/j.ijplas.2015.02.011 content_type: article copyright: Copyright © 2015 Elsevier Ltd. All rights reserved.status: publishe