Papers by Author: Éric Maire

Abstract: During the pre-forming stage of the RTM process, large deformations can occur, especially for double-curved shapes. Knowing the mechanical behaviour and the actual geometry of fibrous reinforcements at the mesoscopic scale is of great importance for several applications like permeability evaluations. As such, forming modeling is particularly demanding on the quality of geometric modeling and of the mesh associated. Indeed, analysis of the internal structure of materials in general, and woven materials especially, recently led to major advances. X-ray Micro Tomography (XRMT or μCT) allows detailed and accurate 3D observations inside the sample, which is not possible with the standard microscopy techniques restrained to surface observations. It distinguishes the yarns and even the fibers that define the directions of anisotropy of the material. A FE model is generated from the processed tomography images. It has been chosen in this study to use hypoelasticity behaviour law. Indeed, the yarns are submitted to large deformations, so that the orientation of the material is significantly modified and the fiber direction has to be strictly followed in order to fulfil the principle of objectivity. A way to retrieve the neutral composite reinforcement axis by skeletonization is proposed in order to know the privileged direction of the yarn and thus implement it in the constitutive law. A comparison between experimental and simulations obtained from μCT and idealized geometry of a transverse compression test on the G0986 is presented.

Abstract: The mechanical behavior of DP980 steel sheets of 1.7 mm thickness has been investigated with both tensile and bending tests. Free bending tests were performed on square samples of 60mm side. The bending tool has a sharp radius of around 0.4 mm and the sample simply lies on two rollers. Scanning electron micrography observations were performed in order to check the occurrence of cracks, that indicate the onset of rupture in bending. Moreover, X-ray microtomography observations were performed on smooth and notched tensile specimen, with a specific small-size geometry, and bending specimen. Maximum void volume fractions of 1.5 10^-3 were recorded and the influence of the triaxiality ratio was investigated, by changing the notch radius. In the case of bending, samples were cut in the bent area and void volume fraction distribution was analyzed along the sheet thickness. Material parameters for Gurson-Tvergaard-Needleman (GTN) model, associated with isotropic hardening and von Mises yield criterion, were identified from the tensile tests. Inverse identification was performed over the different sample geometries, showing that GTN model can not capture the triaxiality ratio influence. Finite element simulations of the bending test were then carried out, in order to compare experimental and predicted void volume fractions in the sheet thickness.

Abstract: Compression tests are carried out at high-temperature on Thermec-master Z, followed by gas quench. Microstructures after deformation are evaluated using SEM-EBSD. Significant grain refinement occurs by dynamic recrystallization for high temperature and low strain rate (T>1100°C, SR<0.1s^-1), and at high strain rate (SR~10s^-1). Dynamic recrystallization is discontinuous and takes place from the grain boundaries, leading to a necklace structure. The nucleation mechanism is most likely to be bulging of grain boundaries. However, recrystallization occurs also by rotation of annealing twins. Thereafter the twin boundaries can bulge as well. The modeling of mechanical behavior gives a fair quantification of flow softening due to dynamic recrystallization indicating the progress of dynamic recrystallization with deformation.

Abstract: Spark plasma sintering has been used for decades in order to consolidate a wide variety of materials and permitting to obtain fully dense specimens. This technique has been mainly applied to ceramics. This paper concentrates on an unusual use of spark plasma sintering system: obtaining innovative materials especially architectured ones. Different applications are presented. Firstly, the SPS technique has been used to elaborate nanometers grain size materials or containing nanoscale microstructure. This is possible since the sintering temperature and the holding time are far lower in the SPS compared to other techniques. Then SPS has been used to realize diffusion bonding. In that case again, bonding can be realized at low temperature and for short time. It permits for example to realize bonding between two copper layers which is of a great importance for microelectronic applications. It is worth noting that this bonding can have the same mechanical strength as pure copper even for diffusion time of a few minutes. Secondly, bonding has been also carried out between a metallic layer and a ceramic one. This could lead to design of new layered materials combining interesting properties in terms of mechanical strength but also in terms of electrical resistance. The SPS machine has also been used to obtain porous materials (cobalt alloys or copper) with an adapted microstructure (porosity, tortuosity,). These structures could open new perspectives for biomedical or for microelectronic applications. All these examples lead to a better understanding of the physical processes which happen during spark plasma sintering.

Abstract: Ti alloys are widely utilized for industrial applications due to their excellent mechanical properties combined with low density. In general, Ti alloys are classified as , + and  alloys, with further subdivision into near  and metastable  alloys. Quite recently, we have presented new type structural ’ martensite (H.C.P.) Ti alloys with low Young’s modulus, high strength and excellent ductility at room temperature. In this work, we examined the microstructure and mechanical properties of ’ martensite type Ti-V-Al alloy after cold- or hot working process. Then, we found that deformation behavior of ’ initial microstructure as compared with (+) initial microstructure was different based on the results of stress-strain curves and Processing Maps under the hot working process. Further, cold rolled ’ martensite microstructure exhibited the refined equiaxed dislocation cell structure, thereby resulting in high strength. This result suggests the new type deformation processing (for both cold- and hot work processing) utilizing ’ martensite in industrial Ti alloys.

Abstract: In this study, Spark Plasma Sintering has been used to sinter pure iron with an initial crystallite size around 100 nm. The process parameters for sintering pure iron have been optimized in order to obtain fully dense materials and avoid excessive grain growth. Archimede's method has been used to calculate the relative density of the sintered samples. It appears that almost fully dense materials can be obtained (95%). X Ray diffraction applied to the sintered samples shows the presence of iron and of the wustite oxide FeO (around 6% wt) formed during the sintering process. Peak enlargement measurements show that the grain size after sintering is around 200nm. This is confirmed by TEM observations showing a dual distribution of grain size. Finally, mechanical characterization has been carried out. The sintered compact exhibits a very high hardness of about 400 Hv. Compression test reveals a very high maximal stress of about 1.2 GPa and that the ductility in compression is non negligible. Using the Hall and Petch law, the calculated grain size should be around 450 nm which is in accordance with direct observations.

Abstract: This paper is concerned with an investigation of the deformation behaviour of an Al-Cu alloy during tensile testing in the semi-solid state. It was carried out by fast in-situ X-Ray microtomography at ESRF, Grenoble. Deformation was performed at constant velocity, which was chosen to be small enough so as not to affect the acquisition of the images. It is observed that deformation is accompanied, initially, by some liquid flow from the adjacent regions towards the deformed zone. Then pores form in the liquid films and grow until they occupy a significant part of the cross section of the specimen. Quantification of this phenomenon was carried out thus leading to a better understanding of pore formation in semi-solid mixtures.

Abstract: A three-dimensional damage percolation model, which captures the effect of microstructural heterogeneity on damage evolution, has been developed to model damage initiation and propagation in materials containing second phase particles. It considers the three phenomena preceding ductile rupture of the material: void nucleation, growth, and coalescence. Threedimensional X-ray tomography is used to obtain measured three-dimensional second phase particle distributions in aluminum alloy sheet. Material damage evolution is studied within a tensile test simulation and compared to measured damage from an in situ tensile test utilizing X-ray tomography. Experimental and simulation results for material damage initiation and evolution are in good agreement.

Abstract: X-ray tomography allows the microstruture of aluminum alloys to be imaged non destructively in three dimensions (3D). This paper shows different examples of the use of this technique for the quantification of damage in model and industrial Al based materials. The model materials are used to setup the technique. The spherical shape of their inclusions makes it easy to compare the measurements with the prediction of standard model for damage. The industrial materials are characterized during in situ tensile but also ex situ bulging and plane strain tension tests. The respective contribution of initiation and growth of damage is measured separately and discussed. The 3D data are also used to quantify the anisotropy of the effect of damage.

Abstract: This paper generally presents different techniques available to image the microstructure of materials in three dimensions (3D) at different scales. It then focuses on the use of the more versatile of these techniques for aluminum alloys : X-ray tomography. The paper describes the recent improvements (spatial and the temporal resolution, grain imaging). Electron tomography is also presented as a promising technique to improve the spatial resolution.