IRDL - CNRS FRE 3744
Université Bretagne Sud
Lorient - France
Toward virtual forming and design : the challenges of thermo-mechanical characterization and parameter identificationof high strength steels
PhD proposal starting in October 2016 for 3 years
Universidade de Aveiro
Aveiro - Portugal
Thermo-mechanical behavior - material characterization - parameter
identification - high strength steels.
Forming and mechanical design of metallic and composites structures must become more and more virtual, in order to limit the number of prototypes and decrease the time to industrialization of a new product. Moreover, a virtual approach leads to a safer structure with optimized performances. The main point that controls the reliability of the numerical simulations is the pertinence of the representation or modeling of the thermo-mechanical behavior of the material, and in particular the material parameters of the constitutive equations involved in the model.
Nowadays, one of the real challenges concerns the determination of the material parameters, on a low-cost basis but with an optimal quantity of information to guarantee the reliability of the predictions. The aim of this project is to develop an efficient methodology for determining the material parameters of thermo-mechanical models, from dedicated tests that involve non-homogeneous temperature and strain fields. Indeed, this non-homogeneity leads to richer information than more traditional approaches with quasi-homogeneous tests, thus to a decrease of the number of experiments. The Virtual Fields Method (VFM) is an alternative compared to the Finite Element Model Updating (FEMU) for material parameter identification in the field of thermo-mechanical modeling [1,2]. These 2 methods are based on the use of mechanical fields, like the strain field recorded by digital image correlation techniques. Their relevance was shown in numerous applications, especially for measure of the Young modulus of composites in the case of linear anisotropic elasticity and even for non-linear behaviors in plasticity. However, forming and service life in industrial environments can take place under sever conditions, as warm temperatures (i.e. below one half of the melting temperature), with strain and temperature fields involving high gradients.
These conditions will require an extension of both VFM and FEMU to thermomechanical conditions, which was not dealt with previously. Indeed, aside the theoretical formulation of the optimization, the temperature dependence entails an increase of the inputs and of the material parameters.
This works will be split into three main parts :
(i) an experimental part relative to the characterization of temperature and strain fields with non-homogeneous tests e.g. tension beyond necking, biaxial tension and simple shear performed either under several isothermal conditions or with thermal gradients leading then to strain localization,
(ii) a part dedicated to material parameters identification with a classical methodology based on a method coupled with finite element simulations (FEMU), and
(iii) a part dedicated to the development of an original optimizing methodology with the virtual field method (VFM), based on the experimental strain and
The ranges of temperature, strain rate and strain depend of the material. In this work, high strength steels (like dual-phase steels, bake-hardening steels) will be considered.
The PhD will be carried out as a joint supervision between the University of South Brittany (France) and the University of Aveiro (Portugal), leading to a double diploma. The French team is specialized and recognized for its works on the experimental characterization of the thermo-mechanical behavior of materials, its modeling and the determination of the related material parameters. The Portuguese team is recognized for its work on numerical methods for shape and material parameter identification, with the development of a dedicate software Sdl . Experimental testing will be performed in both countries; while the FEMU method will mainly be developed in France and the VFM in Portugal.
The PhD student is expected to spend 18 months in Lorient and 18 months in Aveiro, either by time spans of 6 months or more.
Originality of the work
The characterization of the mechanical behavior of metallic materials at warm and high temperatures under complex strain paths is still an experimental challenge. Indeed, the heating system, the control and measure simultaneously of the temperature and strain fields require specific equipment like thermal camera and digital image correlation systems, and synchronized acquisition.
Moreover, the increase of the number of data, both input data (temperature and strain fields) and parameter to identify, require the use of robust algorithms and to deal with the uniqueness of the solution, which is a central concern in parameter identification.
Finally, the extension of VFM to thermo-mechanical behavior necessitates more academic investigations.
there exists a long and fruitful collaboration between the two research teams, and also the University of Coimbra (Portugal), with a total of 4 PhD in joint supervision, either already defended or ongoing works: Jérémy Coër (2014), Nelson Souto (2015) and Vasco Simoes (2017).
 F. Pieron, M. Grédiac, The Virtual Fields Method. Extracting Constitutive Mechanical Parameters from Full-Field Deformation Measurements, Springer (2012)
 N. Souto, Computational design of a mechanical test for material characterization by inverse analysis, PhD Université Bretagne Sud & University of Aveiro, 2015
 A. Andrade-Campos, S. Thuillier, P. Pilvin, F. Texeira-Dias, On the determination of material parameters for internal variable thermoelastic-viscoplastic constitutive models, International Journal of Plasticity 23 (2007) 1349-1379
They should have a strong knowledge in mechanical engineering and a good level in English.
To apply to this PhD position
Send CV (with the score to an English test) and motivation letter and marks obtained for the master degree by email (cf. addresses below).
Paid by the French University during 18 months and the Portuguese University during 18 months. Classical academic scholarship (around 1400€/month free of charge).