Geotechnical measurements at Izmir LRT system tunnels

The geotechnical monitoring played an important role in guiding the tunneling applications during the construction of IZMIR LRTS (Light Rapid Transit System) 1st stage. The purpose of this paper is to review the deformation measurements and results gathered during the construction of LRTS 1st stage tunnels.The New Austrian Tunneling concept (NATM) was used for the construction of Ucyol–Konak conventional tunnels of the system. The EPBM (Earth Pressure Balance Method) concept used in the construction of Konak–Basmane precast segmental tunnels. Both tunneling concepts required monitoring of deformations during construction as well as for a period after its completion.     

Molecular mechanics in the context of the finite element method

In molecular mechanics, the formalism of the finite element method can be exploited in order to analyze the behavior of atomic structures in a computationally efficient way. Based on the atom‐related consideration of the atomic interactions, a direct correlation between the type of the underlying interatomic potential and the design of the related finite element is established. Each type of potential is represented by a specific finite element. A general formulation that unifies the various finite elements is proposed. Arbitrary diagonal‐ and cross‐terms dependent on bond length, valence angle, dihedral angle, improper dihedral angle and inversion angle can also be considered. The finite elements are formulated in a geometrically exact setting; the related formulas are stated in detail. The mesh generation can be performed using well‐known procedures typically used in molecular dynamics. Although adjacent elements overlap, a double counting of the element contributions (as a result of the assembly process) cannot occur a priori. As a consequence, the assembly process can be performed efficiently line by line. The presented formulation can easily be implemented in standard finite element codes; thus, already existing features (e.g. equation solver, visualization of the numerical results) can be employed. The formulation is applied to various interatomic potentials that are frequently used to describe the mechanical behavior of carbon nanotubes. The effectiveness and robustness of this method are demonstrated by means of several numerical examples. Copyright © 2008 John Wiley & Sons, Ltd.     

Finite element implementation of large deformation micropolar plasticity exhibiting isotropic and kinematic hardening effects

Micropolar theories offer a possibility to model size effects in the constitutive behaviour of materials. Typical feature of such models is that they deal with a microrotation, which is supposed to represent an independent state variable, and its space gradient. As a consequence, the stress tensor is no longer symmetric and couple stresses enter the theory. Accordingly, a micropolar plasticity law exhibiting kinematic hardening effects should account for both, a back‐stress tensor and a back‐couple stress tensor. This has been considered in the micropolar plasticity model developed by Grammenoudis and Tsakmakis. The purpose of the current paper is to specify some constitutive functions in this model, to elucidate the finite element implementation as well as to demonstrate its capabilities in describing size effects. Copyright © 2005 John Wiley & Sons, Ltd.     

On the rectilinear shear of compressible and incompressible elastic slabs

We review some pseudo-planar deformations for the equations of incompressible isotropic nonlinear elasticity first introduced in 1985 by Rajagopal and Wineman. We extend this class of deformations to compressible isotropic and transverse isotropic materials, and also consider the influence of gravity. We consider some new approximate solutions and we discuss the possible relevance of such solutions to the understanding of the complex structure of the fields equations of nonlinear elasticity, using weakly nonlinear theories.     

Three Dimensional Creep Model for Wood Under Variable Humidity-Numerical Analyses at Different Material Scales

This paper presents a qualitative explanation of the creep phenomenon based on the physical and chemical mechanisms that occur at micro and ultra-structural levels of wood during moisture diffusion. This part is then completed by the formulation of a 3-dimensional non linear hydro-visco-elastic model, combined with hygro-expansion effects, and able to describe creep and recovery phenomena under variable humidity conditions. The constitutive relation is based on a generalised Maxwell model whose relaxation time functions depend on the moisture content rate, the history of accumulated moisture variations and the stress level. The model was implemented in a Finite Element (FE) program. Several applications based on external experimental tests and with creep periods ranging from 1 to 2735 days were carried out in order to prove the relevance of the approach at different structure scales. A local strain energy density criterion, associated with a flow law, allows the representation of the rupture phase initiated by the tertiary creep and is incorporated into the model in order to open up new horizons for the service life estimation of timber structures.     

Micromechanical deformations in PP/lignocellulosic filler composites: Effect of matrix properties

Polypropylene composites were prepared from three different PP matrices, a homopolymer, a random and a heterophase copolymer, and corn cob to study the effect of matrix characteristics on deformation and failure. The components were homogenized in an internal mixer and compression molded to 1 mm thick plates. Mechanical properties were characterized by tensile testing, while micromechanical deformations by acoustic emission measurements and fractography. The results proved that the dominating micromechanical deformation process may change with matrix properties. Yield stress determined from the stress vs. strain traces may cover widely differing processes. Debonding is the dominating process when the adhesion of the components is poor, while matrix yielding and/or filler fracture dominate when adhesion is improved by the introduction of a functionalized polymer. The dominating deformation mechanism is determined by component properties and adhesion. Interfacial adhesion, matrix yield stress and the inherent strength of the reinforcement can be limiting factors in the improvement of composite strength. The properties of polymer composites reinforced with lignocellulosic fillers are determined by micromechanical deformation processes, but they are independent of the mechanism of these processes.     

FAILURE-MODE TRANSITION SPEED IN THREE-DIMENSIONAL TRANSIENT DEFORMATIONS OF A MICROPOROUS HEAT-CONDUCTING THERMOELASTOVISCOPLASTIC PRENOTCHED PLATE

ABSTRACT

We analyze three-dimensional finite coupled thermomechanical deformations of a rectangular plate with two parallel notches placed symmetrically about the horizontal centroidal plane of the plate. The edge surface of the plate between the two notches is struck by a cylindrical projectile of diameter equal to the distance between the notches and made of the same material as the plate. The plate material is modeled as heat-conducting, microporous, elastoviscoplastic, and isotropic. Both the brittle and the ductile failures initiate at points adjoining the notch-tip surface that are on the midplane of the plate and propagate toward the outer surfaces. Even for a relatively thin plate, the difference in the times of initiation of failures on the mid and front surfaces is significant. Also the two failure modes on the mid surface initiate much later than that predicted by the plane strain analysis. Thus an experimentalist observing fracture on the front or the back face of the plate will see it initiate much later than the times given by the plane strain analysis of the problem. For a steel plate, it is found that the failure mode transitions from brittle to ductile at an impact speed of about 21.8 m/s.     

数字图像相关法测量金属薄板焊接的全场变形

提出了一种基于数字图像相关法和双目视觉技术的全场三维变形测量方法来测量金属薄板焊接过程中的高温变形.首先,提出一种基于种子点的高精度图像匹配算法求解相关匹配非线性优化初值.然后,介绍了三维坐标重建以及三维位移、三维应变的求解算法.最后,借助于VC+ +6.0开发环境,研制了用于薄板焊接全场变形测量的实验系统.为验证本文方法在材料力学性能实验方面的可行性,利用标准材料试验机和自主研制的图像采集装置设计了钢试件的标准拉伸实验,并采用Q235板材件进行了焊接变形测量实验.实验表明:本文方法的应变测量精度为0.5％,与引伸计的测量结果基本相当;与传统的测量方法相比,提出的方法可以更全面、更直观地测量金属薄板在整个焊接过程中的三维移和应变场,并且测得的3个方向的位移变化曲线过渡自然、数据合理,是研究焊接变形规律的有效手段.     

Stabilization of mixed tetrahedral elements at large deformations

This paper presents stabilized mixed finite element formulations for tetrahedral elements at large deformations using volume and area bubble functions. To this end, the corresponding weak formulations are derived for the standard two‐field method, the method of incompatible modes and the enhanced strain method. Then, the weak formulations will be linearized. Furthermore, the matrix formulations for the weak formulations and its linearizations are summarized. The numerical results for incompressible rubber‐like materials using a Neo‐Hookean material law show the locking‐free performance and the drastic damping of the stresses for the new stabilized tetrahedral elements in finite deformation problems. This paper is an extension of the works published by the authors regarding small deformation problems for linear elasticity and plasticity. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis, structure and properties of polyhydroxyalkanoates: biological polyesters

High molecular weight polyhydroxyalkanoates (PHA) are synthesized and stored in the cell cytoplasm as water-insoluble inclusions by various microorganisms. This intriguing biological polyester initially attracted the attention of microbiologists and managed to keep many polymer scientists occupied over the second half of the last century. Concerted multidisciplinary scientific approaches have been directed to the elucidation of various aspects of PHA. Of significant interest are the findings that PHA can consist of various hydroxyalkanoate monomers, and the cloning of its biosynthesis genes. This has resulted in the production of PHA with various physical properties by genetically engineered microorganisms. In fact, it is now possible that large-scale production of PHA by transgenic plants can be achieved in the near future. The physical properties of PHA homopolymers as well as co- and heteropolymers have been the subject of study in various laboratories all over the world. By controlling the monomer composition of PHA, polymer scientists have shown that the polymer's physical properties can be regulated to a great extent. Furthermore, it is also clear that the rate of degradation of PHA in various environments can be controlled by judiciously altering its monomer compositions. This review attempts to bring together the biochemical and physicochemical aspects of PHA along with new perspectives on its potential therapeutic applications.