Search

Introducing { 10 1 ¯ 2 } 10 1 ¯ 1 > extension twin and their variants in magnesium alloys is one of the cost-effective practical approaches for re-assigning the crystallographic axes distributions in wrought magnesium alloys. The twin lamellae and variants can significantly refine the grain size through twin boundaries. Besides, the synergetic effect of twin boundaries, twining morphology, twinning variants, and textural changes can considerably increase the formability, yielding behavior, and anisotropic mechanical behavior of the magnesium alloys. Therefore, this review article emphasized the different pre-straining techniques and the underlying mechanisms which were responsible for the changes in the crystallographic texture, enhancing the mechanical strength and exacerbation of anisotropic mechanical behavior. In the end, some scientific problems have been proposed for the further development of the magnesium alloys.

International audience; The heterogeneous strain field measured by digital image correlation in the central gauge area of a cruciform specimen permits to characterize the plastic anisotropic behavior of metallic sheets with a unique specimen [1]. Indeed, minor and major strains measured along several paths show a wide range of strain states, from uniaxial to equi-biaxial stress state. Thanks to the recording of forces along the two loading directions and to the strain field measurement, an identification of complex anisotropic yield criteria (like Bron and Besson criterion) is possible by inverse procedure. Then, a unique test is sufficient to evaluate the anisotropic behavior of the sheet. The aim of this work is to evaluate the effect of temperature and strain rate on the anisotropic behavior of AA6061 sheet. For this purpose, a biaxial device equipped with four hydraulic cylinders and accumulators (applied speed until 200mm/s), and with an air flow generator (applied temperature up to 160°C) is used to perform in-plane biaxial tensile tests for different conditions of strain rate and temperature. Numerical simulations of the equi-biaxial tensile test are then performed for these different experimental conditions. The numerical simulations are based on a FE model of the cruciform specimen including temperature and strain rate hardening dependencies while the Bron and Besson yield criterion has been calibrated only at room temperature and under quasi-static conditions. Finally, agreement between experimental and numerical evolutions of principal strains and strain paths ratios along three specific profiles is compared and the relevance of using only one experimental condition (room temperature and quasi-static strain rate) to calibrate the yield criterion is discussed. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https//creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of the 23rd International Conference on Material Forming.

Graphical abstract, Al-Li alloys are attractive for military and aerospace applications because their properties are superior to those of conventional Al alloys. Their exceptional properties are attributed to the addition of Li into the Al matrix, and the technical reasons for adding Li to the Al matrix are presented. The developmental history and applications of Al-Li alloys over the last few years are reviewed. The main issue of Al-Li alloys is anisotropic behavior, and the main reasons for the anisotropic tensile properties and practical methods to reduce it are also introduced. Additionally, the strengthening mechanisms and deformation behavior of Al-Li alloys are surveyed with reference to the composition, processing, and microstructure interactions. Additionally, the methods for improving the formability, strength, and fracture toughness of Al-Li alloys are investigated. These practical methods have significantly reduced the anisotropic tensile properties and improved the formability, strength, and fracture toughness of Al-Li alloys. However, additional endeavours are required to further enhance the crystallographic texture, control the anisotropic behavior, and improve the formability and damage tolerance of Al-Li alloys.

International audience; Depending on processing conditions, ordered microstructures of conjugated oligomers or polymers exhibit variable amounts of grain boundaries, lattice disorder, and amorphous (disordered) regions. These structural details can be determined very precisely. Their correlations with optical or electronic properties, however, are very difficult to establish, because, for example, optical spectra are usually averaged over regions with different degrees of disorder. In an attempt to facilitate the interpretation of optical spectra, we performed systematic studies on thin films and μm-sized single crystals of thiophene-based conjugated molecules, which allowed identifying the relative contributions of ordered and disordered regions in optical emission spectra. A detailed multipeak analysis of the emission spectra showed that the peak positions, the energies of the emitted photons, showed only minor changes, independent if highly ordered or rather disordered samples were examined. However, the relative emission intensity changed significantly between samples. In particular, for highly ordered single crystals the purely electronic 0–0 transition nearly vanished, that is, it was essentially optically forbidden as theoretically predicted. Thus, changes in emission probability are correlated with the degree of structural order in semiconducting conjugated systems and provide a possibility to quantify structural order.

Prior studies on synthesis of nanocrystalline elements have discussed the effect of ball milling on lattice parameter, crystallite size, and micro-strain. For elemental milled powders, the anisotropic peak broadening does not change with increasing milling time. However, the effect of alloying addition on the anisotropic behavior of ball milled nanocrystalline powders remains an unexplored area. Here we report the effect of chromium and aluminum addition on the anisotropic behavior of iron in nanocrystalline Fe-20Cr-5Al (wt%) alloy powders synthesized by ball milling. The experimental results show that the anisotropic behavior of iron changes towards isotropic with milling. This change was also correlated to the theoretically calculated anisotropic factor from the change in elastic constant of iron due to milling. Addition of alloying elements exhibited a monotonic rise in the lattice parameter with crystallite size, which was attributed to the excess grain boundary interfacial energy and excess free volume at grain boundaries. Transmission electron microscopy image confirmed the crystallite size and nature of dislocation obtained using modified Williamson-Hall method. (C) 2016 Elsevier B.V. All rights reserved.

Damage process of shale is of great importance when considering the wellbore stability and reservoir stimulation during the work of shale gas recovery. As shale has typical transversely isotropic structures and may be under different stress states in the reservoir, the damage process should be studied while considering both loading directions and confining pressures. A series of cyclic uniaxial and tri-axial compressive tests have been carried out on the shale samples with different oriented weak planes and confining pressures. The dissipated strain energy and its ratio to the releasable elastic strain energy have been studied comparing with the fracturing patterns of the samples. Based on the strain energy dissipation, damage variable is defined and the damage evolution equation is built to describe the damage process of shale samples dependent on loading directions and confining pressures. The damage equation shows that the damage of the shale samples increases as a power function of the axial stress. Under higher confining pressures, the damage is limited in the early loading stage, while it increases significantly with the formation of macro shear fractures when the peak strength is approaching. The change of increasing rate is weak for the shale samples (β = 60°) as the main failure pattern is slip along the weak planes. This study is helpful for understanding the damage process and failure of wellbore, as well as the stimulation effect of the shale gas reservoir.

Understanding hydraulic fracturing mechanisms in heterogeneous laminated rocks is important for designing and optimizing well production, as well as for predicting shale gas production. In this study, a micromechanics-based numerical approach was used to understand the physical processes and underlying mechanisms of fracking for different strata orientations, in-situ stresses, rock strengths, and injection parameters. The numerical experiments revealed a very strong influence of the pre-existing weakness planes on fracking. Geological models for rock without weakness planes and laminated rock behave very differently. Most simulated fractures in the rock without weakness planes were caused by tensile failure of the rock matrix. In an intact rock model, although a radial damage zone was generated around the injection hole, most of the small cracks were isolated, resulting in poor connectivity of the fracture network. For rock models with pre-existing weakness planes, tension and shear failure of these structural planes formed an oval-shaped network. The network was symmetrically developed around the injection well because the strength of the pre-existing weakness planes is generally lower than the rock matrix. The research shows that the angular relations between the orientation of the structural planes and the maximum horizontal stress, as well as the in-situ stress ratios, have significant effects on the morphology and extent of the networks. The strength of the pre-existing weakness planes, their spacing, and the injection rate can&nbsp, dramatically&nbsp, influence the effectiveness of hydraulic fracturing treatments.

In this paper a study is presented of the tensile fracture behavior of progressively-drawn pearlitic steels obtained from five different cold-drawing chains, including each drawing step from the initial hot-rolled bar (not cold-drawn at all) to the final commercial product (pre-stressing steel wire). To this end, samples of the different wires were tested up to fracture by means of standard tension tests, and later, all of the fracture surfaces were analyzed by scanning electron microscopy (SEM). Micro-fracture maps (MFMs) were assembled to characterize the different fractographic modes and to study their evolution with the level of cumulative plastic strain during cold drawing.

International audience; This chapter depicts the advantages and drawbacks of knits as composite reinforcements and gives a nonexhaustive overview of some properties and applications. First, the knitting process capabilities are described, focusing on the opportunity given to knit net-shape 3D preforms with holes and various local properties. Then, the main characteristics of knit permeability are discussed. Finally, the mechanical behavior of dry knits and knit-reinforced composites are investigated. The improvement of the composite mechanical properties by adding inlaid yarns is particularly emphasized, from both experimental and modeling points of view.

WOS; International audience; Components made of short glass fiber reinforced (SGFR) thermoplastics are increasingly used in the automotive industry, and more frequently subjected to fatigue loadings during their service life. The determination of a predictive fatigue criterion is therefore a serious issue for the designers, and requires the knowledge of the local mechanical response under a large range of environmental conditions (temperature and relative humidity). As the cyclic behavior of polymeric material is reckoned to be highly nonlinear, even at room temperature, an accurate constitutive model is a preliminary step for confident fatigue design. The injection molding process induces a complex fiber orientation distribution (FOD), which affects both the mechanical response and the fatigue life of SGFR thermoplastics. This paper presents an extension of the constitutive behavior proposed by the authors in a previous work [Launay et al., Int J Plasticity, 2011], in order to take into account the influence of the local FOD on overall anisotropic elastic and viscoplastic properties. The proposed model is written in a general 3D anisotropic framework, and is validated on tensile samples with various FOD and loading histories: monotonic tensions, creep and/or relaxation steps, cyclic loadings. In Part II of this paper [Launay et al., Int J Fatigue, 2012], this constitutive model will be applied to the simulation of different fatigue samples subjected to multiaxial cyclic loadings.