Publications

2015

• Competition between microstructure and defect in multiaxial high cycle fatigue
  
  • Morel Franck
  • Guerchais Raphaël
  • Saintier Nicolas
  FRATTURA ED INTEGRITA STRUCTTURALE, 2015, 9 (33), pp.404-414. This study aims at providing a better understanding of the effects of both microstructure and defect on the high cycle fatigue behavior of metallic alloys using finite element simulations of polycrystalline aggregates. It is well known that the microstructure strongly affects the average fatigue strength and when the cyclic stress level is close to the fatigue limit, it is often seen as the main source of the huge scatter generally observed in this fatigue regime. The presence of geometrical defects in a material can also strongly alter the fatigue behavior. Nonetheless, when the defect size is small enough, i.e. under a critical value, the fatigue strength is no more affected by the defect. The so-called Kitagawa effect can be interpreted as a competition between the crack initiation mechanisms governed either by the microstructure or by the defect. Surprisingly, only few studies have been done to date to explain the Kitagawa effect from the point of view of this competition, even though this effect has been extensively investigated in the literature. The primary focus of this paper is hence on the use of both FE simulations and explicit descriptions of the microstructure to get insight into how the competition between defect and microstructure operates in HCF. In order to account for the variability of the microstructure in the predictions of the macroscopic fatigue limits, several configurations of crystalline orientations, crystal aggregates and defects are studied. The results of each individual FE simulation are used to assess the response at the macroscopic scale thanks to a probabilistic fatigue criterion proposed by the authors in previous works. The ability of this criterion to predict the influence of defects on the average and the scatter of macroscopic fatigue limits is evaluated. In this paper, particular emphasis is also placed on the effect of different loading modes (pure tension, pure torsion and combined tension and torsion) on the experimental and predicted fatigue strength of a 316 stainless steel containing artificial defect. (10.3221/IGF-ESIS.33.45)
  DOI : 10.3221/IGF-ESIS.33.45
• A New Method Combining Finite Element Analysis and Digital Image Correlation to Assess Macroscopic Mechanical Properties of Dentin
  
  • Wang Wenlong
  • Roubier Nicolas
  • Puel Guillaume
  • Allain Jean-Marc
  • Ingrid C. Infante
  • Attal Jean-Pierre
  • Vennat Elsa
  Materials, MDPI, 2015, 8 (2), pp.535 - 550. A literature review points out a large discrepancy in the results of the mechanical tests on dentin that can be explained by stress and strain assessment during the tests. Errors in these assessments during mechanical tests can lead to inaccurate estimation of the mechanical properties of the tested material. On top of that, using the beam theory to analyze the bending test for thick specimens will increase these experimental errors. After summarizing the results of mechanical tests on dentin in the literature, we focus on bending tests and compare the stress assessment obtained by finite element analysis (FEA) and by beam theory application. We show that the difference between the two methods can be quite large in some cases, leading us to prefer the use of FEA to assess stresses. We then propose a new method based on coupling finite element analysis and digital image correlation (DIC) to more accurately evaluate stress distributions, strain distributions and elastic modulus in the case of a three-point bending test. To illustrate and prove the feasibility of the method, Materials 2015, 8 536 it is applied on a dentinal sample so that mean elastic modulus and maximum tensile stress are obtained (11.9 GPa and 143.9 MPa). Note that the main purpose of this study is to focus on the method itself, and not to provide new mechanical values for dentin. When used in standard mechanical testing of dentin, this kind of method should help to narrow the range of obtained mechanical properties values. (10.3390/ma8020535)
  DOI : 10.3390/ma8020535
• Microplasticity in Polycrystals: A Thermomechanical Experimental Perspective
  
  • Charkaluk Eric
  • Seghir Rian
  • Bodelot Laurence
  • Witz Jean-Francois
  • Dufrenoy Philippe
  Experimental Mechanics, Society for Experimental Mechanics, 2015, 55 (4), pp.741-752. In this paper, thermomechanical couplings at the grain scale in metallic polycrystals are studied during the deformation process through an original experimental setup and improved calibration tools and full-field treatments. In order to perform intragranular thermomechanical analysis in a metallic polycrystal at the grain scale, a crystallography-based technique for the projection of the temperature and displacement fields on a polynomial basis is proposed. It enables intragranular coupled analysis of strain and temperature full-field data. Macroscopic, mesoscopic and granular analysis are then conducted and it is shown that the determination of a macroscopic yield stress as well as a critical resolved shear stress in grains is possible. Early local microplastic activity is therefore thermomechanically confirmed. (10.1007/s11340-014-9921-z)
  DOI : 10.1007/s11340-014-9921-z
• A model for ductile damage prediction at low stress triaxialities incorporating void shape change and void rotation
  
  • Cao Trong-Son
  • Mazière Matthieu
  • Danas K.
  • Besson Jacques
  International Journal of Solids and Structures, Elsevier, 2015, 63, pp.240-263. Ductile fracture at the high triaxiality regime is well-known to be controlled by void nucleation, growth and coalescence. However, under low stress triaxiality conditions and general three dimensional finite deformations, damage is still poorly predicted due to the complex loading state and microstructural changes under such a condition. Experimental results have revealed not only void growth, but also important void shape change and void rotation under shear-dominated loading. The ability of ductile damage models to predict both void growth with shape change and void rotation is thus crucial for complex loading applications. In the present study, a Gurson-like nonlinear homogenization-based model (namely GVAR) is proposed and compared with the constitutive models for elasto-plastic porous materials developed in Kailasam and Ponte Castañeda (1998) (VAR model) and Danas and Aravas (2012) (MVAR model). The proposed model is based on ad hoc modifications of the VAR model, to give sufficiently accurate results for void growth at both low and high stress triaxialities and keeping the functional form of the original Gurson model. The VAR and MVAR models were based on rigorous linear comparison composite (LCC) homogenization methods, which can describe the evolution of microstructure of porous materials, represented by the void volume fraction, the aspect ratios and the orientations of general ellipsoidal voids. The proposed GVAR model thus inherits these characteristics and provides a sufficiently accurate void growth formulation (and simple at the same time). In addition, the loading direction is not necessary aligned with the ellipsoidal void axes. These models are implemented in an object-oriented finite element (FE) code. The identification of model parameters and the assessment of the proposed model are then carried out via 3D periodic unit-cell computations subjected to different stress states. Comparative results show that the present model predicts relatively accurately the evolution of void volume fraction, void aspect ratios and void rotation for different initial void shapes, void volume fractions and under different stress triaxiality levels. A qualitative application to a tensile test on a notched round bar shows the efficiency of the model to predict microstructure evolution (i.e. voids volume, shape and orientation) in a real-scale model simulation. This model with few parameters to be identified is thus promising to predict damage under complex loading paths and ready to be applied to complex FE simulations. (10.1016/j.ijsolstr.2015.03.003)
  DOI : 10.1016/j.ijsolstr.2015.03.003
• Displacement Reconstructions in Ultrasound Elastography
  
  • Bal Guillaume
  • Imperiale Sébastien
  SIAM Journal on Imaging Sciences, Society for Industrial and Applied Mathematics, 2015, 8 (2), pp.1070-1089. We consider the reconstruction of internal elastic displacements from ultrasound measurements , which finds applications in the medical imaging modality called elastography. By appropriate interferometry and windowed Fourier transforms of the ultrasound measurements, we propose a reconstruction procedure of the vectorial structure of spatially varying elastic displacements in biological tissues. This provides a modeling and generalization of scalar reconstruction procedures routinely used in elastography. The proposed algorithm is justified using a single scattering approximation and local asymptotic analysis. Its validity is assessed by numerical simulations. (10.1137/140988504)
  DOI : 10.1137/140988504
• Fourth order energy-preserving locally implicit time discretization for linear wave equations
  
  • Chabassier Juliette
  • Imperiale Sébastien
  International Journal for Numerical Methods in Engineering, Wiley, 2015. A family of fourth order coupled implicit-explicit time schemes is presented as a special case of fourth order coupled implicit schemes for linear wave equations. The domain of interest is decomposed into several regions where different fourth order time discretization are used, chosen among a family of implicit or explicit fourth order schemes. The coupling is based on a Lagrangian formulation on the boundaries between the several non conforming meshes of the regions. A global discrete energy is shown to be preserved and leads to global fourth order consistency in time. Numerical results in 1d and 2d for the acoustic and elastodynamics equations illustrate the good behavior of the schemes and their potential for the simulation of realistic highly heterogeneous media or strongly refined geometries, for which using everywhere an explicit scheme can be extremely penalizing. Accuracy up to fourth order reduces the numerical dispersion inherent to implicit methods used with a large time step, and makes this family of schemes attractive compared to second order accurate methods. (10.1002/nme.5130)
  DOI : 10.1002/nme.5130
• Improved numerical integration for locking treatment in isogeometric structural elements. Part II: Plates and shells
  
  • Adam Cédric
  • Bouabdallah Salim
  • Zarroug Malek
  • Maitournam Habibou
  Computer Methods in Applied Mechanics and Engineering, Elsevier, 2015. Highlights • We model Reissner–Mindlin isogeometric plates and shells. • We examine membrane and shear locking in bending dominated problems. • Higher continuity elements exhibit superior accuracy when no locking occurs. • We extend one-dimensional reduced quadrature rules to two-dimensional rules. • We assess the performance of the schemes using the shell obstacle course problems. Abstract (10.1016/j.cma.2014.07.020)
  DOI : 10.1016/j.cma.2014.07.020