Isotropic to distortional hardening transition in metal plasticity

The present paper aims to discuss the transition from isotropic to distortional hardening behavior of metallic materials, based on the Homogeneous Anisotropic Hardening (HAH) model. Furthermore, the effect of yield locus distortion on the evolution of the strain increment, under the assumption of associated flow, is theoretically discussed and exemplified. Special cases, such as coaxial and orthogonal stress states, are analyzed to provide better insight into the model. Particular emphasis is put on the monotonic loading case, which is compared to isotropic hardening. Finally, the evolution equations of the state variables are examined and their properties are discussed. (C) 2014 Elsevier Ltd. All rights reserved.111512Ysciescopu

A dual-mesh strategy for the 3d simulation of fineblanking processes

Fineblanking technology is used to produce blanked metal components which show outstanding surface quality and part flatness. The defining characteristics of the process are, besides the use of a counter punch and a V- Ring, the tiny die clearance and a rounded cutting edge. The 3D FE simulation of the process proves to be thus very challenging. This is mainly because in comparison to the part dimensions (which are of the order of 10mm) a very small mesh size needs to be chosen on the cutting edge (~0.01mm), which leads to a very big number of elements and also tiny time steps. This paper aims to show a solution to the problem using the Arbitrary Lagrangian Eulerian FE formulation, applied on two different levels of refinement. First a relatively coarse mesh (element size of about 0.1mm around the cutting edge) is applied to solve the full size 3D problem. The flow information is subsequently used on a much finer mesh (size ~0.005) defined around a small region on the cutting line to accurately compute the stress-strain distribution around the radi

Numerical investigation of the post-necking behavior of aluminum sheets in the presence of geometrical and material inhomogeneities

The proper numerical treatment of strain is an indispensable prerequisite of an accurate localization prediction. The plastic deformation in the post-necking regime is, however, challenging to simulate as factors with marginal influence in the stable range can sensitively affect results in the localized regime. The present study aims to clarify the role of intrinsic inhomogeneities in aluminum alloy AA6016-T4 sheet sample in its post-necking deformation behavior. This is carried out through the FE simulation of a tensile test with randomized thickness and yield stress properties. The inhomogeneous thickness distribution of the sheets has been characterized by a 3D optical digitization system, whereas the crystallo-graphic texture has been measured using X-ray diffraction analysis. The results show that the presence of inhomogeneities allow for a more realistic description of the localized necking phenomenon, especially concerning the initiation and development of shear bands. Also, the variable shear angle observed on failed specimens is better captured numerically in presence of inhomogeneities. (C) 2016 Elsevier Ltd. All rights reserved.1142sciescopu

An extended Modified Maximum Force Criterion for the prediction of localized necking under non-proportional loading

Strain localization is one of the main sources of failure in sheet forming processes. State of the art forming limit curves allow the prediction of localization for linear strain paths but fall short in case of non-proportional loading. The aim of this contribution is to revisit the Modified Maximum Force Criterion (MMFC) and extend it to accommodate distortional hardening models. This is accomplished by uncoupling its formulation from any particular yield function and thus enabling its use as a generic framework for the prediction of forming limits under arbitrary loading conditions. Furthermore a novel approach is proposed for considering strain rate sensitivity, which substantially improves the predictive capabilities of the model under plane strain tension conditions. The method is applied to steel and aluminum materials and the role of phenomena such as Bauschinger effect, latent hardening and cross-loading contraction on localization are discussed. (C) 2015 Elsevier Ltd. All rights reserved.111611sciescopu

On the mechanics of edge cracking and the reliable determination of edge formability limits

Blanked edge surfaces are rough and hardened. They therefore lead to inhomogeneous deformation on the edge, which can trigger localization within the shear affected zone (up to few mm from the edge). The size and extent of these phenomena are primarily a function of the shearing process and are only marginally coupled to the global/homogeneous deformation behavior of the blank A direct numerical simulation of such local deformation effects would require a prohibitively high resolution to capture the microgeometry of the edge and thus remains unfeasible in the current industrial practice. A predictive model can therefore only be achieved by determining limit strains on the edge, which are compatible with the homogeneous numerical framework used. The present contribution aims discussing the basic mechanics of edge cracking based on tensile tests with edges blanked with different die clearances. The local and global strain evolutions in the vicinity of the edge are analysed and a new evaluation procedure is proposed for the reliable determination of limit strains. The application of this method in industrial context is also discussed.open access</p