International audienceIn this paper, a non unified elasto-viscoplastic behavior model based on internal state variables, is investigated in order to describe the thermo-mechanical stress-strain fatigue response of 55NiCrMoV7 tempered martensitic steels (AISI L6). This model considers a quadratic yield criterion to define the elasticity domain. It allows the determination of two inelastic strain mechanisms resulting from two stress states which can be related to the typical continuous cyclic softening of the tempered martensitic steels. This cyclic softening is reproduced through an isotropic component (drag stress). A memory effect is also introduced to take into account the influence of the plastic strain range on the amount of the cyclic softening. The kinematic component (back stress) of the model allows the description of complex load conditions to which tool steels are subjected. Strain recovery (Baushinger effect), time recovery terms (cyclic behavior including tensile dwell times) and ratcheting effects are considered. The numerical implementation is addressed and two integration methods (explicit and implicit) of the constitutive equations are presented. Moreover, the identification methodology of the model parameters from only two sets of experimental data is presented; the coefficients of the kinematic and isotropic parts are determined successively. The main difficulty consists in reaching a good description both of the cyclic behavior for different strain rates and the ratcheting effect. Last, a validation stage of the three dimensional model is investigated from low cycle fatigue tests performed on different notched specimens

To cite this version:D Garcia, Jean-José Orteu, Luc Penazzi. A combined temporal tracking and stereo-correlation technique for accurate measurement of 3D displacements: application to sheet metal forming. Journal of Materials Processing Technology, Elsevier, 2002Elsevier, , 125-126, pp.736-742. 10.1016 AbstractOptical methods that give displacement or strain fields are now emerging significantly in the mechanical sciences. Much work has been done on two dimensional (2D) displacement/strain measurement from a single camera but the proposed methods give only in plane strains. A binocular correlation based stereovision technique has been developed: (a) to measure the three dimensional (3D) shape of a static object or (b) to measure the strains of an object undergoing some 3D mechanical or thermal stress.In this paper, the application of the stereo correlation technique to measure accurately the 3D shape of a stamped sheet metal part or the surface strain field undergone by the part during the stamping process is presented.

a b s t r a c tIn this study, the high temperature wear behavior of hot forming tool steel grades is investigated by successive sliding of a pre-alloyed Usibor1500P s strip heated at high temperature. Experimental tests are performed at high temperature on an instrumented Deep-Drawing Process Simulator (DDPS). This laboratory pilot is employed to rank different steel grades used as tool materials in the hot-stamping process. The wear damage of the tool (die radius) is characterized by profilometry and SEM observations, and three quantitative criteria are determined from 2D profile measurements to assess adhesive and abrasive wear. Under examined conditions at high temperature, a predominant transfer mechanism is observed, while abrasive wear appears as minor damage. When the surface hardness of the tool material is not great enough, the sub-surface of the die radius can exhibit a plastic shear deformation of about 10 mm in depth. This leads to emission of wear debris coming from the cumulated cyclic plastic deformation of the sub-surface. In contrast, for high surface hardness, the adhesive wear rapidly reaches an asymptotic state.

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