Marion Martiny

Limits to the ductility of metal sheets subjected to complex strain-paths

Abstract Localized necking in sheets under biaxial tension is analysed by using a Marciniak-Kuczynski localization approach along with a new plane-stress yield criterion. The determination of the forming limits under linear, bilinear and curvilinear strain-paths, shows that a simple effective strain approach defining the limit to ductility as a function of the current strain-rate ratio applies with good accuracy for arbitrary strain-paths. Analytical expressions of the forming limits are proposed, for either positive or negative values of the strain-rate ratio. These expressions are then implemented in finite element simulations of sheet-metal forming processes, so as to be used as a detector of necking. The pertinence of this approach is discussed in the case of the hemispherical punch test and of the cup drawing test.

Analytical modelling of drawbeads in sheet metal forming

Abstract An analytical model is developed to account for the bending/stretching process experienced by a metal sheet passing along a drawbead. The geometrical evolution of an elementary length of the sheet is described by the Love–Kirchhoff assumption. The material is assumed to be elastic–plastic, and the assumptions of either isotropic or non-linear kinematic hardening are considered. First, the stress–strain history for any point within the thickness of the sheet is derived from the evolution of curvature imposed by the tooling, together with the evolution equations relating the tensile force N and the bending moment M . The external forces exerted by the tools are then estimated from equilibrium equations expressed for different parts of the sheet along the tooling. The results are compared with the predictions of finite element simulations and with experimental results of the literature. A detailed analysis is finally performed to delineate the influence of geometry, material parameters and friction on the restraining force, the thickness strain and effective strains at the exit of the tooling.

Finite element simulations of sheet-metal forming processes for planar-anisotropic materials

Sheet-metal forming simulations are made using a plane-stress yield function proposed by Ferron et al. for orthotropic sheets. Following the general method for the integration of rate-independent plasticity models, the yield function has been implemented in the implicit Abaqus/Standard finite element code. Simple forming operations are simulated, such as the hemispherical punch test and the cup drawing test. The combined effects of rheological and tribological parameters on the strain distributions and the limits to formability are analysed, and discussed in relation with improved descriptions of the yield surface. Also, the magnitude of ears predicted for the cup drawing test is compared with experimental values on tin-plate steel.

Thermo-mechanical simulation of PCB with embedded components

Abstract In recent years, in order to increase density and performance of electronic boards, components are embedded in internal layers of printed circuit boards (PCBs). The reliability of this new technology has to be investigated to ensure the working of the electronic boards submitted to harsh environment and long mission profiles. To study the thermo-mechanical behavior of these boards, finite element simulations have been performed. It is observed that embedded passive chips are subjected to complex loading during the lamination process, due mostly to shrinkage of the resin, differences in material properties and also because of temperature excursion. The effects of material parameters and of the geometrical configuration are investigated in details. It will be shown that the generated stresses are not critical for the passive chip size considered in the present work.

Strain-Induced Martensite Formation of AISI 304L Steel Sheet: Experiments and Modeling

Metastable austenitic stainless steels are prone to strain-induced martensite transformation (SIMT) during deformation at room temperature, as in the case of sheet metal forming processes. The SIMT is influenced by the chemical composition, grain size, temperature, deformation mode or stress state and strain-rate. In this work, interrupted and continuous uniaxial tensile tests were performed in AISI 304L sheet to evaluate the SIMT as a function of strain and strain-rate effects. The SIMT was evaluated by feritscope and temperature in-situ measurements and both XRD and optical microscopy techniques. The SIMT kinetics was also investigated by means of thermo-mechanical finite element simulations using a phenomenological model. In the small strain range, the yield stress increases with the strain-rate whereas in the large strain domain a cross-effect in the stress-strain curve is observed given that the SIMT is inhibited due to the specimen heat generation. A very good correlation between XRD and feritscope measurements was found from the interrupted uniaxial tensile testing. The finite element numerical simulations allowed to identify the parameters of a phenomenological model which describes the SIMT kinetics of AISI 304L steel sheet as a function of plastic-strain, strain-rate and temperature effects.

Heterogeneous Biaxial Tensile Tests For The Characterization Of Sheet Metals Plastic Anisotropy

Two types of cruciform specimens designed for the inverse identification of anisotropic yield surfaces of sheet metals are presented. A particular emphasis is given on the specimens design and on the sensitivity of strain fields to plastic anisotropy. The relations between experimental strain fields and identified yield surfaces are outlined for 4 materials with widely different anisotropies.