Jérôme Crépin

A procedure for identifying the plastic behavior of single crystals from the local response of polycrystals

The overall and local tensile responses of an α iron multicrystalline sample are investigated in order to derive the plastic constitutive equations for the constituent single crystals. The macroscopic stress–strain curve and some statistical characteristics of the strain field measured on the sample surface are compared with their simulated counterparts. The optimal values of the material parameters of four types of hardening laws are derived by a minimization procedure. The best results are obtained with a nonlinear anisotropic law which uses the dislocation densities on the slip systems. This procedure is then validated on a fine-grained polycrystalline sample of a similar material by using the measured displacement field on the edge of a selected area as boundary conditions for finite element method (FEM) computation. The resulting optimal material parameters for the single crystal are found to be consistent with the values available in the literature, and the whole simulated strain fields as well as the evolution of the crystallographic texture, is compared satisfactorily with the experimental data.

Cracking behaviour of PVD tungsten coatings deposited on steel substrates

Abstract Tungsten coatings have been deposited on steel substrates by magnetron sputtering. For the same processing conditions, the increase of the coating thickness enhances the (111) component of the crystallographic texture whereas the residual stress level decreases. Tensile and four-point bending tests, associated with an acoustic emission analysis, have been performed inside a SEM chamber in order to study the cracking mechanisms. When the residual stresses are taken into account, an intrinsic critical cracking stress and the associated energy release rate can be determined; the obtained values suggest an intergranular cracking mechanism. No debonding has been observed at the interface despite the large plastic deformation of the substrate at the crack tips. The observed strain localisation modes in the substrate near the interface have been shown to have a major influence on the limit crack density.

Plastic deformation mechanisms of β treated zirconium

This study aims to determine the different deformation mechanisms of grade 702 zirconium under uniaxial tension and at room temperature. The grade 702 zirconium tested had undergone an {alpha} {yields} {beta} {yields} {alpha} cycle at a slow cooling rate ({approximately} 15{degree} s{sup {minus}1}). Three deformation mechanisms were identified: prismatic slip, (10{bar 1}2) twinning and (11{bar 2}1) twinning. The critical resolved shear stress for prismatic slip, (10{bar 1}2) twinning and (11{bar 2}1) twinning was also determined. The effect of the non-uniform redistribution of the hardening elements on location through the grain of the various mechanisms and on the tendency for localized deformation to develop is also discussed.

Cavity growth and rupture of β-treated zirconium: A crystallographic model

Abstract This study aims to understand the damage mechanisms observed in β-treated zirconium. Damage voids are characterized by a tubular morphology with hexagonal cross-section; their growth kinetics is determined experimentally. From these observations, a model based on the principle of creation of free surface by crystallographic slip permits one to explain the stability of the hexagonal shape and to predict a growth rate closer to the experimental value than traditional models. This improvement is due to the sensitivity of the free surface creation mechanism to the stress concentration factor k , which can not be accounted for in models based on continuum mechanics.

Histological and biomechanical study of dura mater applied to the technique of dura splitting decompression in Chiari type I malformation

Many techniques are described to treat Chiari type I malformation. One of them is a splitting of the dura, removing its outer layer only to reduce the risks of cerebrospinal fluid (CSF) leak. We try to show the effectiveness of this technique from histological and biomechanical observations of dura mater. Study was performed on 25 posterior fossa dura mater specimens from fresh human cadavers. Dural composition and architecture was assessed on 47 transversal and sagittal sections. Uniaxial mechanical tests were performed on 22 dural samples (15 entire, 7 split) to focus on the dural macroscopic mechanical behavior comparing entire and split samples and also to understand deformation mechanisms. We finally created a model of volume expansion after splitting. Dura mater was composed of predominant collagen fibers with a few elastin fibers, cranio-caudally orientated. The classical description of two distinct layers remained inconstant. Biomechanical tests showed a significant difference between entire dura, which presents an elastic fragile behavior, with a small domain where deformation is reversible with stress, and split dura, which presents an elasto-plastic behavior with a large domain of permanent strain and a lower stress level. From these experimental results, the model showed a volume increase of approximately 50% below the split area. We demonstrated the capability of the split dura mater to enlarge for suitable stress conditions and we quantified it by biomechanical tests and experimental model. Thus, dural splitting decompression seems to have a real biomechanical substrate to envision the efficacy of this Chiari type I malformation surgical technique.

Mechanical properties of glenoid cancellous bone.

BACKGROUND Loosening of the glenoid component in total shoulder arthroplasty is the main late complication of this procedure; it may be assumed that it is highly dependent on the quality of the glenoid cancellous bone. Very little is known about the mechanical properties of this cancellous bone. The aim of this study was to determine these properties (Youngs modulus and strength) as well as bone density in different parts of the glenoid cancellous bone to assess their variations. METHODS Eleven scapulas were obtained from six fresh-frozen, unembalmed human cadavers. Eighty-two cubic cancellous bone specimens were extracted and tested using a uniaxial compression test; then the specimens were defatted and correlations with bone density were determined. FINDINGS The study showed significant differences in the mechanical properties with anatomic location and directions of loading. Youngs modulus and strength were found to be significantly higher at the posterior part of the glenoid with the weakest properties at the antero-inferior part. Cancellous bone was found to be anisotropic with higher mechanical properties in the latero-medial direction perpendicular to the articular surface of the glenoid. The apparent density was on average equal to 0.29 g/cm(3) with the higher values at the posterior and superior part of the glenoid. Good correlation between apparent density and elastic modulus was found only in the sagittal planes but not in the coronal and axial plane. INTERPRETATION The mechanical properties determined in this study showed the anisotropy of the glenoid cancellous bone; values of these properties could provide input data for finite element method analyses in shoulder prosthesis designs.

Low cycle fatigue behaviour of β treated zirconium : Partial irreversibility of twinning and consequences for damage

Abstract The aim of this study is to understand the low cycle fatigue behaviour of β treated zirconium. It focuses especially on the contribution of twinning mechanisms. In situ fatigue tests performed inside a scanning electron microscope allow the observation of the activation of twins and their partial reversibility, depending on the applied stress. Moreover, the acoustic signature (shape factor) of the different twinning systems has been characterized allowing the following of inception and evolution of twins during the fatigue tests. Finally, the consequence of such a partial irreversibility on damage and crack localization is shown.

Identification of Crystalline Behavior on Macroscopic Response and Local Strain Field Analysis: Application to Alpha Zirconium Alloys

The purpose of this paper is to present an identification method of the crystalline behavior of a material from a mechanical test performed on a polycrystalline sample. Because of the lack of knowledge about its crystalline behavior, this method is applied to a Zirconium alloy. This identification is based on a finite element modeling of the microstructure, and the results are compared to both the macroscopic and the microscopic experimental results. On the microscopic scale, the plastic strains are obtained using a micro-extensometry technique and the crystalline orientation using an EBSD technique. In order to validate the method, an identification is performed with only two free parameters: the evolutions of the macroscopic and microscopic errors appear to be regular and exhibit a well-defined minimum so that the parameters can be clearly identified.

Characterization of Local Strain Distribution in Zircaloy-4 and M5 ® Alloys

Zirconium alloys with low alloying content are mainly used in the nuclear industry as structural materials because of their superior properties in terms of neutron transparency, mechanical strength, and corrosion resistance. In order to further improve the corrosion resistance as well as the integrity of Zr based cladding tubes under severe thermomechanical loading, the M5® alloy was developed to replace stress-relieved Zircaloy-4. An experimental study conducted at the macroscopic scale between 20 and 500°C shows that the mechanical behavior of the studied Zr based alloys depends on the metallurgical state (stress-relieved or recrystallized) rather than on the chemical composition. To try to understand these mechanical differences, an experimental multiscale investigation was devised at ambient temperature (20°C) in order to characterize the strain distribution at the scale of the grains and at that of the representative volume element. Local strain fields were measured by means of a microscale digital image correlation technique, based on microgrid deposits and scanning electronic microscopy (SEM). Tensile tests were performed inside the SEM chamber. Here, the original method of strain distribution quantification based on statistical strain field analysis is used. First, this analysis reveals a particular strain distribution consisting of bands with an orientation greater that 45° with regards to the direction of macroscopic tension, and second, shows that these interaction lengths are much greater than the average size of the grains, which clearly demonstrates that local investigations cannot be limited to a few grains. Therefore, the macroscopic mechanical response of these materials is not only governed by intragranular heterogeneities but by the local deformations which become organized between the grains in a pattern of bands at a mesoscale, which is determined by medium to long-range interactions. The difference of values in the band characteristics could partly explain the anisotropic global behavior of these materials linked with their microstructure.

Microstructural study of β treated grade 702 zirconium. Scanning electron microscopy and transmission electron microscopy, complementarity of two observational scales

In this study we focus on the precipitation of betagenic elements (Fe, Cr, Ni) that occurs during the β → α allotropic transition in zirconium. In particular its structural characteristics, morphology and their consequences for the mechanical behaviour of the material are probed. The observations are made by means of a scanning electron microscope and a transmission electron microscope. The results reveal the occurrence of a quasi-continuous single crystalline phase, produced by the precipitation of the betagenic additional elements which constitute the so-called lath boundaries. This phase forms a layer surrounding the laths in the bulk of the material. Its morphology could be assimilated to a net, created by precipitates with a size of several micrometres and linked by narrow bridges with a size of several hundred nanometres. The chemical composition of the precipitates are of the Zrx(Cr, Fe2)1 - x type with x ≤ 40%.