Investigation of the Performance of Flow Models for TWIP Steel

WOS: 000440433900058Modeling of metal processing requires constitutive laws able to represent the experimental material behavior. Of the large number of available empirical constitutive equations, only a subset may be fitted accurately to given experimental data. The present work is aimed at identifying the equations that can be used to model the ambient temperature mechanical behavior of high Mn twinning-induced plasticity (TWIT') steels. These are fitted to experimental data for TWIP900 and further compared in terms of their ability to predict springback. The reference springback value is determined experimentally for the same material. The study provides guidelines for the selection of the constitutive model in forming simulations for this type of steel

Effects of aging parameters on formability of 6061-O alloy

WOS: 0002802058000346XXX series aluminum-magnesium-silicon (Al-Mg-Si) alloys are medium strength alloys widely used as automotive body materials. The mechanical properties of these alloys are adjusted by performing age hardening heat treatments. In this research, the effect of aging time on formability of 6061-O alloy is investigated. The formability of the material is evaluated by tensile. Erichsen, and hole expansion tests. Results reveal that formability decreases with increasing aging time. The evolution of the anisotropy, r, with the aging time depends on the direction of probing relative to the texture direction. Yield surfaces predicted using the Hill-90 and Barlat-89 models are plotted using experimental r values for several aging times. Since r changes, these surfaces deform slightly and expand with increasing aging time. (C) 2010 Elsevier Ltd. All rights reserved

Investigating Orientational Defects in Energetic Material RDX Using First-Principles Calculations

Orientational defects are molecular-scale point defects consisting of misaligned sterically trapped molecules. Such defects have been predicted in α-RDX using empirical force fields. These calculations indicate that their concentration should be higher than that of vacancies. In this study we confirm the stability of a family of four orientational defects in α-RDX using first-principles calculations and evaluate their formation energies and annealing barrier heights. The charge density distribution in the defective molecules is evaluated and it is shown that all four orientational defects exhibit some level of charge reduction at the midpoint of the N–N bond, which has been previously related to the sensitivity to initiation of the material. We also evaluate the vibrational spectrum of the crystal containing orientational defects and observe band splitting relative to the perfect crystal case. This may assist the experimental identification of such defects by Raman spectroscopy

Homogenized elastic response of random fiber networks based on strain gradient continuum models

International audienceThe purpose of this work is to develop anisotropic strain gradient linear elastic continuum models for two-dimensional random fiber networks. The constitutive moduli of the strain gradient equivalent continuum are assessed based on the response of the explicit network representation in so-called windows of analysis, in which each fiber is modeled as a beam and the fibers are connected at crossing points with welded joints. The principle of strain energy equivalence based on the extension to the strain gradient of the Hill–Mandel macro homogeneity condition is employed to identify the classical and strain gradient moduli, based on the application of a sequential set of polynomial displacements on windows of analysis of different sizes. The scaling of the first- and second-order moduli with network parameters, such as network density and the ratio of fiber bending to axial stiffness, is determined. We observe a similar dependency of classical and strain gradient moduli on the same network parameters. The internal length scales associated with the gradient coefficients of the constitutive equation are also defined in terms of the network parameters. The strain gradient moduli prove to be size-independent in the affine regime, and they converge toward a size-independent value in the non-affine deformation regime after a rescaling of physical dimensions by the window size. The obtained results show that the strain gradient moduli scale uniformly with the square of the magnitude of the strain gradients applied to the window of analysis

Increasing Ductility and Toughness of Photocurable Thermosets with Microstructures Controlled by Phase Separation

In this work, photopolymerization-induced phase separation (photo-PIPS) is implemented into a polymer network consisting of both acrylic and epoxy-based chemistries to allow for the development of an acrylic network through photopolymerization and for enhanced strength and stiffness to be obtained via thermal cross-linking of the epoxy-based chemistries. Two epoxy-based end-group terminated polydimethylsiloxane (PDMS) polymer additives [hydroxy-terminated PDMS (OH-PDMS) and diglycidyl ether-terminated PDMS (epoxy-PDMS)] are added to enable phase separation, creating a heterogeneous polymeric material. During thermal treatment applied after photopolymerization, these additives can cross link together, concurrently with the cross-linking of the epoxy-based monomers which are part of the network, therefore creating elastomeric, phase-separated subdomains. Mixtures of OH-PDMS and epoxy-PDMS of various molar ratios are used to alter the extent of phase separation and the amount of cross-linking that occurs within the subdomains. It is observed that OH-PDMS does not phase separate when present alone, while it separates in the presence of epoxy-PDMS. Cross-linking within the phase-separated domains containing both types of PDMS leads to enhanced ductility, with the optimal ductility and toughness enhancement being obtained for the PDMS stoichiometry (OH-PDMS:epoxy-PDMS 80:20 by mol %). This composition retains good creep resistance and has only a slightly smaller strength than the neat resin