Pascale Balland

Compartmentalized Model for the Mechanical Behavior of Titanium

As close as you watch them, the materials (especially metals) present discontinuities that can easily be qualified as strong. Dislocations, structures formed by these dislocations, phases and grains are all discontinuities, also sources of heterogeneity, with effects on material behavior that are not really well reproduced by a model based on a continuity assessment. Consequently, the materials should be considered as a set of compartments with different behaviors. This promotes an alternative way to define models. A coherent modeling process is probably the integration of the different behaviors of the material compartments within the global model. The objective is here to build an efficient elasto(visco)plastic model of the mechanical behavior of titanium combining compartmentalized behaviors. After setting the frame of the study, which is of primary importance, the proposed modeling process is running as follows (i) choose a local behavior, (ii) identify the parameters of crystalline texture that must be integrated into the simulation and (iii) finally formulate a way of combining local compartments behaviors. The intrinsic properties of Finite Element codes are used to achieve the integration of the whole system.

Numerical study of the micro-formability of thin metallic materials: virtual micro-forming limit diagrams

This paper presents the determination of virtual micro-forming limit diagrams from two types of numerical simulations based on the finite element method: one with modelling of the full tool for microdeep drawing and a thin blank with geometric imperfections, based on a defined roughness; and a second called “reduced simulation” where different deformation paths were simulated with appropriate boundary conditions by introducing the same type of geometric imperfections on aluminium 1050A (99.5%). A new test for detecting the onset of necking, called the “change of slope” criterion has been defined. Several methods based on histograms to represent the distributions of major and minor strains, have been used to determine the strain at the onset of necking. The numerical micro-forming limit diagrams (MFLD) were then compared to one obtained experimentally.

Numerical Study of the Impact of Constitutive Modelling on the Evolution of Necking in the Case of a Tensile Test on C68 Grade Steel

This article deals with numerical simulation of necking. It draws the attention onto the importance of the description of strain-hardening and the effects on the evolution of necking. In order to compare necking evolution in relation with different plasticity models, a tracking procedure which consists in determining the evolution over time of discharged volumes of the sample is adopted. Models that take into account physical phenomena at the microscopic level and especially the heterogeneities of materials from a mechanical point of view seem well suited to fit experimental evidence connected to necking.

Numerical determination of micro-forming limit diagrams: introduction of the effect of grain size heterogeneity

This paper presents the determination of virtual micro-forming limit diagrams by a behaviour law adapted to thin materials introducing the effect of microstructure heterogeneity. The observed size effect, i.e. the decrease in flow stress when the grain size increased, was modelled by a specific phenomenological behaviour law. A reduced numerical simulation by a finite element method rendered it possible to carry out very fast simulations of the different modes of deformation and to determine the virtual micro-forming limit diagrams from the onset of necking. This methodology was compared with experimental results on aluminium 1050A (99.5%) of thickness 0.2 mm. The comparison of the experimental and numerical data demonstrated good agreement between the real and virtual results obtained by such a methodology.

Physically Based Kinematic Hardening Modelling of Single Crystal

This paper suggests a new method to take into account the kinematic hardening in constitutive behaviour of metallic face centred cubic single crystal. The keystone of this model is that kinematic effect arises from a description of the crystal dislocation microstructure based on a heterogeneous distribution of slip strength. In this paper, these concepts are applied to crystalline plasticity models. In particular, simulations of monotonous tensile test and cyclic loading are analysed to show that implementation of the proposed concepts, without any additional complexity, gives the current model a wide range of applications from small cycled strains to large monotonous ones.

An original prediction of localisation during tensile and biaxial expansion tests on copper by the compartmentalized model

This paper aims to show that necking in metallic sheets subjected to forming operations strongly depends on intrinsic heterogeneity factors of the material. This study is applied to the case of localisation occurring in ultrathin sheets of copper subjected to forming operations. Ultrathin sheets of metal have a far more heterogeneous nature than thicker ones due to their rather small number of grains in thickness. The results of numerical simulations during tensile test and biaxial expansion of these ultrathin sheets until necking are compared firstly by using a conventional phenomenological model and secondly by using a modelling in which spatial heterogeneities are introduced through a distribution of hardening parameters. Indeed, to simulate this latter, heterogeneities are introduced in the modelling of material behaviour by means of a Rayleigh’s distribution of the yield stress for each finite element of the numerical model. The comparison between the experimental and numerical results obtained with ...

Characterization and Modeling of the Elastic Behavior of a XC68 Grade Steel Used at High Strain Rates and High Temperatures

This article discusses the characterization and modeling of the elastic behavior of a semi-hard steel used in incremental forming operations which implies great loading speeds at high temperatures and large springback after each passage of the roller. The knowledge of the elastic behavior is essential to correctly predict these springbacks during forming. The objective is therefore on the one hand the characterization of the elastic response of the material under different conditions and on the other hand the definition of a model that describes the material behavior with as much precision as possible. To this end, two models, one phenomenological and the other built on more physical basis, are considered.

FE simulation of a steel thin‐wall short‐tube forming process

This article concerns the forming of a steel thin‐wall short‐tube by an innovative small surface contact deformation process. A Finite Element model was built to predict the forming of the workpiece. The model controls the complex kinematics of the process. The problems associated with contact control and computation time are also investigated. Compared with measurements taken by the industrial partner, the first results obtained are highly promising, with regard to predicting both the workpiece geometry and the forces acting on the tools.

Comparative study of modelling efficiency regarding localization development

The determination of limit loadings that induce strain localization during material forming operation is always an important industrial concern, especially when it appears during the forming operation during which the material is highly stressed. Several recent studies have demonstrated that the intrinsic structural heterogeneity of metallic materials plays an important part in their mechanical behavior. Nevertheless, the role of heterogeneities on strain localization is not yet really broached. The objective of this study is to determine if the structural heterogeneity of materials has an impact on the localization of ductile materials. A corollary action is then to validate the use of models that integrate this heterogeneity to achieve better results. The study is composed of two parts: experimental and numerical. Concerning the experimental part, in order to obtain different degrees of heterogeneities, titanium samples were prepared to have materials with structure hexagonal and cubic centered which pr...

A standard framework for mechatronics

This research aims to highlight the levers that are within the reach of Small and Medium Organizations (SMO) who have the purpose to engage in mechatronics. Subsequently, we consider to gradually build up a framework so as to enable these SMO, presently involved in the general mechanical industry, to evaluate their ability to make mechatronic products, adopt scalable evolution strategies and access the promising mechatronic markets by challenging novel business models, necessary to evolve towards mechatronics.