Abstract in English:Abstract A research on the microstructure and mechanical properties of the dissimilar joint was carried out so as to understand the effects of post-weld heat treatment (PWHT) on microstructural evolution, microhardness, tensile and flexural properties of the dissimilar friction stir welded (FSWed) joint. The results showed that there is a sufficient and rather complicated material mixing in the nugget zone (NZ). After PWHT, the grains in the joint area except for the NZ demonstrated a grain refinement with more homogeneous and equiaxed grains. Fine and clustered SiC particles in the NZ are confirmed by scanning electron microscope (SEM). In addition, the microhardness values of nugget zones both in as-welded and PWHT condition exhibited a higher Vickers compared to that of AA6061-O. Furthermore, a mean tensile strength of 183.30 MPa that demonstrates a 52.22% increase in tensile strength is also observed in the transverse tensile tests subsequent to PWHT. Considering the three-point bending test results, it is explicit that an increase of 121.96% in the bending extension is obtained as there is no significant increase in maximum bending force after the PWHT.
Abstract in English:Abstract The basic aim of this work is to present a method to treat structural mechanics problems dealing with tridimensional contact and large elastic deformations. The formulation presented in this article offers a B-Spline based discretization for the contact surface, which solves the continuity problems presented by the classic Lagrangian contact element formulation. A B-Spline surface is utilized to discretize the contact surface, and the B-Spline basis functions are used to distribute the contact forces between the contact surfaces nodes. The finite element utilized is an 8-node hexahedron, and the formulation is continuum mechanics based, written in the current configuration, utilizing a neo-Hookean material model. The contact restrictions are enforced by the augmented Lagrangian method.
Abstract in English:Abstract The paper deals with the effect of stress state on the dynamic damage behavior of ductile materials. The rate- and temperature-dependent continuum damage model has been enhanced to take into account the influence of the stress triaxiality and the Lode parameter on damage condition and on rate equations of damage strains. Different branches of these criteria depending on the current stress state are considered based on different damage and failure processes on the micro-level. To get more insight in the dynamic damage and fracture processes micro-mechanical behavior of void-containing representative volume elements have been analyzed numerically. These three-dimensional numerical simulations based on different dynamic loading conditions take into account a wide range of stress states in tension, shear and compression domains. Based on the numerical results general trends of dynamic damage and failure behavior can be shown and stress-state-dependent equations for damage criteria and for the formation of damage strains can be proposed.
Abstract in English:Abstract The complex structural behavior of shallow arches can be remarkably affected by many parameters. In this paper, the structural responses of a half-sine pin-ended shallow arch under sinusoidal and step loadings are accurately calculated. Additionally, the effects of environmental temperature changes are considered. Three types of sinusoidal loadings are separately investigated. Displacements, load-bearing capacity, the magnitude of the axial force and the locus of critical points (including limit and bifurcation points) are directly obtained without tracing the corresponding equilibrium path. Furthermore, the boundaries identifying the number of critical points are investigated. All mentioned structural responses are formulized based on the rise of the arch and the environmental temperature change, which are introduced in a dimensionless form. The proposed formulation is also developed for generalized sinusoidal loadings. Additionally, the structural behavior of the shallow arch under two types of step loadings is investigated. Finally, the accuracy of the suggested approach is examined by a non-linear finite element method.
Abstract in English:Abstract Gyroscopic systems and their properties have been extensively studied as Angular Momentum Devices (AMD), Control Moment Gyros (CMG) or Gyroscopes for various applications such as structure control, stability or energy storage. However, most of the works that have been done are theoretical and do not present experimental implementation. In this work we performed an experimental study of a gyroscope beam system (gyroelastic beam) focused systems on the deflection of cantilever beams or the control of flexural stresses. We first used a simple two-degree of freedom model to better understand the terms governing the design, construction and experimental evaluation of gyroelastic beam systems. We then performed experimental tests at different velocities of the gyroscopic actuator and measured the deflection of the system. The results showed that it is possible to have control of the deflection and the bending forces for this type of configurations which can be exported to helicopter blades or wind turbine blades.
Abstract in English:Abstract The influence of the frecuency content of seismic excitations on the behavior of an optimal tuned mass damper (TMD) is studied in the context of a system with explicit consideration of soil-structure interaction. A stochastic analysis is made in the time domain for two random processes, one considering a broad bandwidth process (BBP) and other considering a narrow bandwidth process (NBP). A structure built over three different types of soil (soft, medium and hard) is considered. For the optimization of the TMD, the minimization of the ratio between the standard deviation of the displacement of the main structure with TMD with respect to a structure without TMD, is used as the target function. It is found that for seismic excitations with high frecuency content, the ratio of the TMD frequencies compared to the fixed base frequency of the structure approaches to 1 as the soil becomes more rigid. It is also observed that the TMD become tuned with the flexible base frequency for all soil types, producing perfect tuning for small mass ratios and detuning gradually for higher mass ratios. On the other hand, the TMD optimal damping ratio increases as the TMD mass ratio is higher, independently of the soil type. The TMD is more efficient for higher values of the TMD mass ratios, especially on soft soil. In structures built over flexible base, that are subjected to low frequency content excitations, the optimal TMD is tuned with the flexible base, independently of the type of soil and the fixed base period of the main structure. The TMD optimal damping is not sensitive to the flexible period for small mass ratios, and reaches its minimum value when it matches with the predominant period of the seismic event. On the other hand, the TMD reaches its maximum efficiency when it is tuned with the flexible period of the soil-structure system, and coincides with the predominant period of the seismic exitation and is higher on soft soil. A deterministic analysis is made using two seismic records, an artificial earthquake compatible with the Chilean code NCh2745 characterized by high frequencies content and other similar to the event in 1985 in Mexico, characterized by low frequencies content. It is seen that the optimal TMD is efficient controlling the response of the structure in all types of soil analyzed.
Abstract in English:Abstract A spiral case with its steel spiral case (SSC) being embedded in reinforced concrete under a pressurized condition is called a preloaded filling spiral case structure (PFSCS) in a hydropower plant. As a steel-concrete composite structure, a PFSCS is designed to work reliably. The non-uniform gap and contact nonlinearity between the SSC and the surrounding mass concrete have a great effect on the bearing mechanism of the composite structure. However, the description of the gap and contact nonlinearity in a PFSCS is a tough work. With the aim of efficiently describing the evolution process of the non-uniform gap and contact nonlinearity, we performed an experimental investigation and proposed a novel numerical simulation technique for structural finite element analyses (FEA) of PFSCSs. In the technique, the gap and contact nonlinearity as well as the construction process and operation process of a PFSCS are taken into account. A friction-contact model is used to simulate the sliding of the SSC against the concrete. A plasticity damage model is employed to describe the concrete. The development of the gap, contact status between the steel liner and the surrounding concrete, stresses of the steel liner and the steel bars, as well as the concrete cracking time and crack pattern, are presented in this work. The FEA results agree well with the experimental results. The agreement provides evidence that the applicability and competence of the proposed technique are valid and satisfactory.
Abstract in English:Abstract Currently, visual inspections for damage identification of structures are broadly used. However, they have two main drawbacks; time limitation and qualified manpower accessibility. Therefore, more precise and quicker technique is required to monitor the condition of structures. To aid the aim, a data mining based damage identification approach can be utilized to solve these drawbacks. In this study, to predict the damage severity of single-point damage scenarios of I-beam structures a data mining based damage identification framework and a hybrid algorithm combining Artificial Neural Network (ANN) and Imperial Competitive Algorithm (ICA), called ICA-ANN method, is proposed. ICA is employed to determine the initial weights of ANN. The efficiency coefficient and mean square error (MSE) are used to evaluate the performance of the ICA-ANN model. Moreover, the proposed model is compared with a pre-developed ANN approach in order to verify the efficiency of the proposed methodology. Based on the obtained results, it is concluded that the ICA-ANN indicates a better performance in detection of damage severity over the ANN method used only.
Abstract in English:Abstract In this work, a finite element-based approach is presented to study the effective width variation in non-pre-stressed steel-concrete beams under the serviceability stage, including time dependent effects such as concrete creep, shrinkage and cracking. For this purpose, the viscoelasticity theory in conjunction with a nonlinear cracking monitoring algorithm is used to trace the nonlinear viscoelastic response of the structure along time. The present numerical model is fully three-dimensional and permits the inclusion of partial interaction at the slab-beam interface. A comprehensive study is carried out on the long-term response of a composite girder bridge previously studied by other researches. Then, previous results are revised and extended herein. Potential shortcomings of some standard codes related to the effective width evaluation are also investigated. It is demonstrated that the slab effective width varies sharply along the beam axis in the short-term, while it approaches to the actual slab width in the long-term. For the studied example, the common assumption of using only the middle layer of the reinforced concrete (RC) slab for the effective width calculation is revised with a through-thickness integration procedure. The influence of some creep and shrinkage models as well as the ultimate tensile concrete strain on the effective width response is also assessed. Finally, a simple formula is proposed to evaluate the short-term slab effective width for the studied example.
Abstract in English:Abstract A finite element model for structural analysis of media with embedded inclusions is presented. The “embedded element concept” is adopted to model the contact interaction of two medium components along the contact interface considering a mixed 3D-1D formulation. The Mohr-Coulomb interface model is employed to define the bond-stress and bond-slip relation and strains associated with bond-slip are assumed to remain infinitesimal along the interface. Nonlinear analysis is performed with a corotational kinematics description introduced in the context of embedded approach. The problem of load transfer in mooring anchor systems was investigated and reasonable results were obtained using the present model.