Papers by Author: Monique Gaspérini

Abstract: This paper focuses on the analysis of the microstructure and of the texture through the sheet thickness after temper rolling of very thin ferritic steels. The study uses EBSD and X-Ray diffraction. Comparison is made between an interstitial-free (IF) steel and of some industrial low carbon ferritic steels used after ageing. The experimental results are discussed with respect to the anisotropy of the mechanical behaviour after temper rolling during simple shear tests.

Abstract: Although the Taylor-type models gives reasonable texture prediction of the monotonic cold deformation of annealed aluminum alloys both qualitatively and quantitatively, results are less satisfactory for the simple shear test when the alloy is heavily pre-deformed by cold rolling. The reason for this less good prediction originates from strain localization. A virtual stress-strain curve is proposed in which the texture aspects are dealt with by the FC Taylor simulation and the microstructure aspects by a model for the development of intragrain dislocations structure. This virtual yield law is used in a finite element simulation. A strain localization behavior is observed during the finite element simulation similar to that observed during experimental simple shear test. The strain profile of a specific global strain is discretized into a series of strain and the volume fractions of the regions deformed to these strain levels, using the statistical method of histogram. A secondary FC-Taylor simulation is performed, in order to generate the deformation textures, corresponding to this series of deformation strains. The global texture is generated by merging these textures with consideration of these volume fractions. Using this procedure of multi-level modeling, quite satisfactory texture prediction is observed, compared with the measured texture at this strain.

Abstract: In-situ SEM shearing tests were performed on samples from the heavily cold rolled (Extrahard) aluminium alloys, where the parallelepiped test sample was cut as to let shear direction (SD) have an angle α with the rolling direction (RD). This shear angle ranges from 0o to 165o with an interval of 15o. These include three heavily cold rolled non heat-treatable aluminium alloys AA1200, AA3004 and AA5182. During these tests, strain localization (macro-shearbands) was bserved. This phenomenon is found to be anisotropic and depends on the angle α. The strain localization or macro-shearbands are believed to be related with strain softening, where the flow stress decreases with strain. According to the crystal plastic theory, the strain softening is considered as resulting from the joint effects of texture and evolution of microstructure, in particular the dislocation patterns. Focusing on texture softening, simple and advanced Taylor type micro-mechanical simulations (Full-constraint Taylor (FC Taylor) and Advanced Lamel models (Alamel)) are performed to calculate the texture and average Taylor factor evolution with the increment of shear strain, on the basis of the measured rolling textures. After the simulations, the shear strain at which texture softening happens is recorded for each alloy and each shear angle. For alloys AA3004 and AA5182, it is found the texture-softening trend is similar to the experimental observations, which showed that the strain localization starts at smaller strains at shear angles of around 30-60o and 120-150o, finally leading to early failure. On the contrary, for alloy AA1200, the calculated average Taylor factor evolution does not resemble the flow behaviour. Furthermore the conclusions for alloys AA3004 and AA5182 are only qualitative, as the value of texture-softening strains predicted by simulation seems different from the observations. This shows that the importance of other effects such as possible microstructural softening mechanisms, especially the one due to the change of strain path (rolling/shear). Then for future models, it will be necessary to incorporate both the texture effects and microstructural effects comprehensively in order to precisely predict the strain localization behaviour of materials.