Abstract in English:Abstract In this study, the discrete-time Volterra series are used to update parameters in a nonlinear finite element model. The main idea of the Volterra series is to describe the discrete-time output of a nonlinear system using multidimensional convolutions between the Volterra kernels represented in a Kautz orthogonal basis and the excitations. A metric based on the residue between the experimental and the numerical Volterra kernels is used to identify the parameters of the numerical model. First, the identification of the linear parameters is performed using a metric based only on the first order Volterra kernels. Then the nonlinear parameters are identified through a metric based on the higher-order kernels. The originality of this nonlinear updating method stems from the decoupling of linear and nonlinear parameters and the use of global nonlinear model. In order to put in light the applicability of this technique, this work focus on the identification of the parameters in a nonlinear finite element model of a beam that was preloaded by compression mechanism. This work shows that the updated numerical model was able to represent the behaviour observed in the experimental measurements.
Abstract in English:Abstract Reinforced concrete (RC) beam-column connections especially those without transverse reinforcement in joint region can exhibit brittle behavior when intensive damage is concentrated in the joint region during an earthquake event. Brittle behavior in the joint region can compromise the ductile design philosophy and the expected overall performance of structure when subjected to seismic loading. Considering the importance of joint shear failure influences on strength, ductility and stability of RC moment resisting frames, a finite element modeling which focuses on joint shear behavior is presented in this article. Nonlinear finite element analysis (FEA) of RC beam-column connections is performed in order to investigate the joint shear failure mode in terms of joint shear capacity, deformations and cracking pattern. A 3D finite element model capable of appropriately modeling the concrete stress-strain behavior, tensile cracking and compressive damage of concrete and indirect modeling of steel-concrete bond is used. In order to define nonlinear behavior of concrete material, the concrete damage plasticity is applied to the numerical model as a distributed plasticity over the whole geometry. Finite element model is then verified against experimental results of two non-ductile beam-column connections (one exterior and one interior) which are vulnerable to joint shear failure. The comparison between experimental and numerical results indicates that the FE model is able to simulate the performance of the beam-column connections and is able to capture the joint shear failure in RC beam-column connections.
Abstract in English:Abstract Due to their robustness in handling the inherent singularity difficulties associated with crack analysis, mesh-reduction methods present an avalanche of formulations in the literature which, sometimes, entails modifications to their conventional/standard forms for better results. Although such formulations provide a pool of alternative choices to the analyst, increase in their number requires some relative assessment between them in order to guarantee optimum choice of analysis tool. The present study assesses the applicability and relative performance of three such mesh-reduction methods, namely the radial basis function (RBF) method, the boundary element method (BEM), and the method of fundamental solution (MFS) for mode III crack analysis. In order to have a common ground for performance comparison, these methods are, first, tested in their most basic forms and simplest conventional formulations possible. Failure of some of them to provide reliable results calls for some enrichments. Yet, unless where necessary, efforts are made to ensure that unnecessary computationally expensive formulations are avoided. Consequently, the BEM formulation is not altered in any way, and modifications to both the RBF and MFS are limited to enrichment by the addition of, at most, one singular term and/or the domain-decomposition technique. Verification is achieved using the literature results and/or those obtained by FEM in this study. Summary of the relative advantages and limitations of the methods for mode III crack analysis is given to serve as a yard-stick based on which the choice of one over the others may be influenced.
Abstract in English:Abstract Dynamic compression behaviors of density-homogeneous and density-graded irregular honeycombs are investigated using cell-based finite element models under a constant-velocity impact scenario. A method based on the cross-sectional engineering stress is developed to obtain the one-dimensional stress distribution along the loading direction in a cellular specimen. The cross-sectional engineering stress is contributed by two parts: the node-transitive stress and the contact-induced stress, which are caused by the nodal force and the contact of cell walls, respectively. It is found that the contact-induced stress is dominant for the significantly enhanced stress behind the shock front. The stress enhancement and the compaction wave propagation can be observed through the stress distributions in honeycombs under high-velocity compression. The single and double compaction wave modes are observed directly from the stress distributions. Theoretical analysis of the compaction wave propagation in the density-graded honeycombs based on the R-PH (rigid-plastic hardening) idealization is carried out and verified by the numerical simulations. It is found that stress distribution in cellular materials and the compaction wave propagation characteristics under dynamic compression can be approximately predicted by the R-PH shock model.
Abstract in English:Abstract This paper studies the stiffening effects of the material strain rate sensitivity and strain hardening on the saturated impulse of elastic, perfectly plastic plates. Finite element (FE) code ABAQUS is employed to simulate the elastoplastic response of square plates under rectangular pressure pulse. Rigid-plastic analyses for saturated impulse, which consider strain rate sensitivity and strain hardening, are conducted. Satisfactory agreement between the finite element models (FEM) and predictions of the rigid-plastic analysis is obtained, which verifies that the proposed rigid-plastic methods are effective to solve the problem including strain rate sensitivity and strain hardening. The quantitative results for the scale effect of the strain rate sensitivity are given. The results for the stiffening effects suggest that two general stiffening factors n 1 and n 2, which characterizes the strain rate sensitivity and strain hardening effect, respectively can be defined. The saturated displacement is inversely proportional to the stiffening factors (i.e. n 1 and n 2) and saturated impulse is inversely proportional to the square roots of the stiffening factors (i.e. n1 and n2). Formulae for displacement and saturated impulse are proposed based on the empirical analysis.
Abstract in English:Abstract Thin walled multi-cell columns are highly efficient energy absorbers under axial compressions. Multi-cell features are usually obtained by placing stiffeners or ribs to get the required geometry features in the columns. In this study, the curved stiffeners with varying numbers in different configurations are used to support the circular single and bi-tubular metallic tubes and their shape, position and number effects are numerically investigated under dynamic axial crushing loads. Number of configurations can be classified by the number of tube/tubes and curved stiffened shells. This study can be divided in three parts; single tubes with cross-curved stiffeners with equal number of cells, single tubes with curved stiffened wall supports with ascending number of stiffeners, and bi-tubular tubes with ascending cross-curved stiffeners. The mass in each configuration is maintained as same by adjusting the thicknesses of the stiffeners. Energy absorption for all of the configurations are compared with published experimental literature of standard single tube and single tube with cross-straight stiffeners. Deformation modes and energy absorption characteristics are evaluated and discussed for all of the proposed configurations. Results indicate that the proposed configurations with curvy stiffeners are superior in energy absorption with increased mean crushing force and enhanced energy absorption along with a high value of crush force efficiency in comparison with the refernce configurations. Bi-tubular configurations with curvy stiffeners show a more stabalized response with the lowest peak crushing force in all of the proposed configurations.
Abstract in English:Abstract Polyester pins are incorporated in polyurethane foam filled honeycomb core sandwich panel to increase the interfacial strength between the faces and core in order to improve the performance of sandwich structures. Foam filled Honeycomb Sandwich panel (FHS) and Pin incorporated Foam filled Honeycomb sandwich panels (PFHS) were developed. The developed sandwich panels are tested for flexural and vibration characteristics. The influence of strain rates on flexural behaviour of sandwich panels were also evaluated. The material used for face sheets and pins are same, ends of pin act as chemically cross linked polyester joint at the interfaces of faces and core in addition to the adhesive area. This modification had effectually increased the interfacial strength thereby increased the flexural and damping properties of panels significantly. Moreover, increasing the pin diameter has a larger effect, whereas, the strain rate had a moderate influence on the failure load of both types of sandwich panels. The investigation brings to light a novel pin incorporated foam filled honeycomb sandwich panels that can be used for various applications.
Abstract in English:Abstract The present paper addresses free vibration of multiple cracked Timoshenko beams made of Functionally Graded Material (FGM). Cracks are modeled by rotational spring of stiffness calculated from the crack depth and material properties vary according to the power law throughout the beam thickness. Governing equations for free vibration of the beam are formulated with taking into account actual position of the neutral plane. The obtained frequency equation and mode shapes are used for analysis of the beam mode shapes in dependence on the material and crack parameters. Numerical results validate usefulness of the proposed herein theory and show that mode shapes are good indication for detecting multiple cracks in Timoshenko FGM beams.
Abstract in English:Abstract The impact phenomenon is one of many subjects that is interesting and has become an inseparable part of naval architecture and ocean engineering fields. Limitless possibilities in cause and scenario make the demand to understand the physical behavior of ship structures due to impact phenomenon is increasing. In present work, a series of virtual experiment is performed by finite element method to solve several defined collision scenarios. Two involved ships are classified as the striking ship which penetrates the target and struck ship as the target. Hull arrangement of the struck ship is considered as main parameter which single and double hull configurations are proposed to be assessed. An observation of the damage extent on struck ship subjected to collision loads are presented. The results indicate that the internal arrangement of the struck ship provides significant effects to resistance capability and structural failure after collision process. Finally, an analysis regarding the extent of the damage is summarized with a statistical calculation to provide distribution of crashworthiness criteria of the defined scenarios.
Abstract in English:Abstract The present paper explores the stability and failure response of elastoplastic Ni/Al2O3 functionally graded plate under thermomechanical load using non-linear finite element formulation based on first-order shear deformation theory and von-Karman’s nonlinear kinematics. The temperature dependent thermoelastic material properties of FGM plate are varied in the thickness direction by controlling the volume fraction of the constituent materials (i.e., ceramic and metal) with a power law, and Mori-Tanaka homogenization scheme is applied to evaluate the properties at a particular thickness coordinate of FGM plate. The elastoplastic behavior of FGM plate is assumed to follow J2-plasticity with isotropic hardening, wherein the ceramic phase is considered to be elastic whereas the metal is assumed to be elastic-plastic in accordance with the Tamura-Tomota-Ozawa model. Numerical studies are conducted to examine the effects of material and geometrical parameters, viz. material in-homogeneity, slenderness and aspect ratios on the elastoplastic bucking and postbuckling behavior and the failure response of FGM plate. It is revealed that material gradation affects the stability and failure behavior of FGM plate considerably. Furthermore, it is also concluded that FGM plate with elastic material properties exhibits only stable equilibrium path, whereas the elastoplastic FGM plate shows destabilizing response after the ultimate failure point.