Eva Héripré

Micro-scale experimental investigation of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock

Abstract An experimental study of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock under hydration is presented. The investigation, which combines environmental scanning electron microscopy (ESEM) and digital image correlation techniques, has been carried out at the micrometric scale of the composite microstructure of the rock. Specimens were hydrated in the ESEM over a wide range of relative humidity and observations conducted on two planes: plane 1 parallel to the bedding plane, and plane 2 perpendicular to it. The observations reveal that the local swelling (which can be quantified at a local gauge length of about 5 μm) is strongly anisotropic in both planes. The global swelling, measured over areas of about 500 μm in width, is also clearly anisotropic in plane 2 (with major swelling direction normal to the bedding plane), but not in plane 1. The global isotropy in plane 1 arises from the uniform distribution of the orientation of anisotropic local strains, while the anisotropic swelling in plane 2 is due to a preferred local orientation.

Microscale insight into the influence of humidity on the mechanical behavior of mudstones

The mechanical behavior of mudstones strongly depends on humidity. In this paper, we present some microstructural insights into this phenomenon gained from a microscale investigation using a novel experimental method. The experimental method consists of combined hydric and mechanical loading tests in environmental scanning electron microscopy, as well as full-field strain measurement by digital image correlation techniques. The sample is subjected to a stepwise wetting (21%, 80%, and 99% relative humidity); for each equilibrium moisture state, a uniaxial compression test is performed. The microscale observation reveals that humidity-induced changes in the mechanical behavior of mudstones are controlled by the deformation and microcracking upon wetting. With increasing relative humidity, expansion of pores causes the clay matrix to be softer. In addition, because of the reduction in shear modulus and the lessening of capillary effect, shear bands are prone to appear at a high humidity state. The microcracking upon wetting, which results in predamage of the material, also affects the mechanical behavior. Finally, the sample with more moisture exhibits a more ductile behavior that involves more pronounced microcracking at failure.

Mechanical Study of Novel VPS-Titanium Coating on Polyethylene Substrates

Thick metallic or ceramic functional coatings onto polymers are of great interest for different domains such as the aerospace and medical industries. A vacuum plasma spray process has been developed to produce coatings on high- and low-temperature melting polymers including PEEK and polyethylene. This study reports the first experimental characterization of the strength and adherence of such titanium coatings on medical grade polyethylene substrates. Four-point bending coupled to microscopic observations show the existence of a critical tensile strain of 1% corresponding to the onset of cracking in the coating. For strains up to 6%, the crack density increases without any noticeable debonding. Fatigue tests over 106 cycles reveal that under this critical strain the coating remains uncracked while above it, the cracks number and size remain stable with no noticeable coating detachment. A protocol for laser shock adhesion testing (LASAT®) was developed to characterize the coating-substrate adhesion and captured the existence of a debonding threshold. These results provide quantitative guides for the design of orthopedic implants for which such a titanium coating is used to enhance anchorage to bone tissues. More generally, they open the way for systematic measurements quantifying the adhesion of metallic coating onto polymer substrates.

Microstrain Analysis of Titanium Aluminides

The aeronautic and automotive industries have shown a renewed interest in TiAl based alloys. The main reasons for such an interest are their low density (~3,8g/cm3), a good stiffness and a high strength for temperatures up to 750°C. However, these alloys exhibit, in their polycrystalline form, a poor ductility at room temperature with widely scattered values. The aim of this study is therefore to characterise their mechanical behaviour with a multiscale methodology, coupling microstructure analysis and strain field measurements. This methodology employs orientation imaging microscopy as well as digital imaging correlation techniques with an intragranular step size of a few micrometers. Two chemical compositions (47 at. % Al and 48 at. % Al) and two processing routes (casting and powder metallurgy) are studied. Thus, four different types of final microstructures are considered, from fully lamellar Ti3Al (a2) + TiAl (g) microstructure to bimodal ones composed of two-phase (a2+g) lamellar grains and monolithic g grains. Firstly, the microstructure is characterised crystallographically and morphologically. This allows the identification of a representative volume element (RVE) inside the analysed volume. Then, uniaxial mechanical tests are performed for each microstructure, and the strain fields are analysed with a multiscale approach, which determines the spatial distribution of the strain field heterogeneity with respect to the different microstructures.

Influence of boundary conditions on strain field analysis for polycrystalline finite element simulations

This paper presents a first validation of a novel methodology for identifying the parameters of a crystallographic elastoplastic constitutive law. This is accomplished by comparing simulation and experimental results at different length scales: the microstructure scale and the representative volume element scale. Experimentally, the microscopic strain fields and the microstrucural characteristics can be obtained only at the surface of the specimen. As a consequence, in finite element simulations only at the surface there is a oneto-one correspondence between the mesh and the experimental observed grain morphology. In this paper, the morphology of the subsurface grains is obtained by a simple extension in the thickness direction of the surface morphology. The aim of this study is then to verify whether the surface data contain sufficient information for the identification of the parameters of the constitutive law.

Stress Corrosion Cracking Initiation of Alloy 82 in Hydrogenated Steam

Experiments in hydrogenated steam were performed on several U-bend specimens extracted from two Alloy 82 welds. Results demonstrated that Alloy 82 is susceptible to SCC in hydrogenated steam at 400 °C and its susceptibility depends on its chemical composition, welding process and thermal treatment. The microstructure was characterized in the apex of the U-bend specimens. Chemical analysis were performed by electron probe microanalyser (EPMA) and secondary ion mass spectrometry (SIMS) on several areas in the weld in order to correlate crack initiation with chemical heterogeneities. It was concluded that there are more cracks in the roots of the weld passes where the impurity content (sulfur, titanium and aluminum) is higher.