Ozgur Demir

This study investigates the nonlinear forced vibration of three-layered curved sandwich beams with a viscoelastic core and elastic face sheets. By using Hamilton’s principle and First-order Shear Deformation Theory (FSDT), the governing equations are derived to account for large-deflection geometric nonlinearities. The core's frequency-dependent behaviour is modeled via fractional-order viscoelasticity. A novel, computationally efficient framework is developed by integrating the Harmonic Balance Method (HBM) with the Generalized Differential Quadrature Method (GDQM). The nonlinear equations are linearized using the Newton–Raphson method and discretized into algebraic form. To the best of the authors’ knowledge, this represents the first application of a combined GDQM–HBM framework to curved sandwich structures under fractional Zener modelling. Validation against existing literature confirms the model's accuracy and precision. The results show that viscoelastic core thickness primarily enhances damping, while increased beam curvature significantly intensifies membrane-induced axial forces and triggers a transition from hardening to softening behaviour. Furthermore, load positioning fundamentally alters the nonlinear response even under identical geometric conditions. These findings provide a practical benchmark for the precise design and stability analysis of viscoelastic curved structures in large-deflection regimes.

This study aims to determine the stiffness values of under sleeper pad (USP) and rail pad (RP) components to reduce the high-amplitude vibrations that occur in the transition zones of some specific structures such as viaducts in ballasted railways. The conventional method of simulating USPs and RPs as spring–dashpot elements in the Kelvin–Voigt model is inadequate due to the absence of frequency and temperature dependencies in the model. The study proposes a new analytical model that considers USPs and RPs as viscoelastic (VE) materials and integrates them into the ballasted railway superstructure model by adding unit masses avoiding mathematical singularity. The process includes material testing, field measurements, and validation of the proposed model with finite element model analysis. The effect of ambient temperature and material modelling on the superstructure’s dynamic response in the frequency domain is analysed in detail. To account for VE behaviours of the resilient elements, the generalised Maxwell model (GMM) is chosen via unit mass implementation compared to other VE models. The obtained results show that the dynamic response of the railway superstructure is 8–10 times sensitive to temperature variation. This demonstrates how important it is to include the temperature-dependent dynamics of the elastomer material in the model. According to the other results that were obtained, the use of USP in transition zones does not solve the vibration problem radically. Bridge dynamic responses are also sensitive to the mass of the bridge rather than its stiffness.

This paper focuses on the geometrically nonlinear dynamic analyses of a three-layered curved sandwich beam with isotropic face layers and a time-dependent viscoelastic core. The boundary conditions and equations of motion governing the forced vibration are derived by using Hamilton’s principle. The first-order shear deformation theory is used to obtain kinematic relations. The spatial discretization of the equations is achieved using the generalized differential quadrature method (GDQM), and the Newmark-Beta algorithm is used to solve the time variation of the equations. The Newton–Raphson method is used to transform nonlinear equations into linear equations. The validation of the proposed model and the GDQM solution’s reliability are provided via comparison with the results that already exist in the literature and finite element method (FEM) analyses using ANSYS. Then, a series of parametric studies are carried out for a curved sandwich beam with aluminum face layers and a time-dependent viscoelastic core. The resonance and cancellation phenomena for the nonlinear moving-load problem of curved sandwich beams with a time-dependent viscoelastic core are performed using the GDQM for the first time, to the best of the authors’ knowledge.

Purpose: The purpose of this paper is to parametrically investigate the vibration and damping characteristics of a functionally graded (FG) inhomogeneous and porous curved sandwich beam with a frequency-dependent viscoelastic core. Design/methodology/approach: The FG material properties in this study are assumed to vary through the beam thickness by power law distribution. Additionally, FG layers have porosities, which are analyzed individually in terms of even and uneven distributions. First, the equations of motion for the free vibration of the FG curved sandwich beam were derived by Hamilton's principle. Then, the generalized differential quadrature method (GDQM) was used to solve the resulting equations in the frequency domain. Validation of the proposed FG curved beam model and the reliability of the GDQ solution was provided via comparison with the results that already exist in the literature. Findings: A series of studies are carried out to understand the effects on the natural frequencies and modal loss factors of system parameters, i.e. beam thickness, porosity distribution, power law exponent and curvature on the vibration characteristics of an FG curved sandwich beam with a ten-parameter fractional derivative viscoelastic core material model. Originality/value: This paper focuses on the vibration and damping characteristics of FG inhomogeneous and porous curved sandwich beam with frequency dependent viscoelastic core by GDQM – for the first time, to the best of the authors' knowledge. Moreover, it serves as a reference for future studies, especially as it shows that the effect of porosity distribution on the modal loss factor needs further investigation. GDQM can be useful in dynamic analysis of sandwich structures used in aerospace, automobile, marine and civil engineering applications.

The purpose of this paper is to study the axisymmetric vibrations of circular three-layered sandwich plates with a frequency-dependent fractional viscoelastic core and elastic face sheets. First, the equations of motion and related boundary conditions are derived using the Hamilton’s principle for the free vibrations. Then, the governing equations obtained for various boundary conditions are solved and parametric studies are carried out to examine the vibration behavior of circular sandwich plates with a viscoelastic core. The differential transform method (DTM), a well-known semi-analytical–numerical solution technique, is utilized for the eigenvalue analysis. In addition, the finite element (FE) solution obtained with the commercial code ANSYS is added to this comparison. The effect of face and core layer thicknesses and the location of the core layer and core material on the dynamic and damping characteristics of circular sandwich plates with a fractional derivative viscoelastic core is studied in detail.

Underwater explosions are tremendously hazardous for floating structures and human life inside. When compared to an air blast, an underwater blast is more effective due to the effect of the fluid domain. Therefore, the design of marine structures with high strength to underwater explosions is a difficult subject as well as important. With the rapid progression of material technology, composite materials are widely preferred over metals in fields of marine and defense industries due to low density, high elasticity and high strength in their nature. In this study, a sandwich panel with a soft-core was modelled using the finite element method, and the dynamic behavior of the panel subjected to an underwater blast load is investigated. In the validation study of the model and method, firstly, the dynamic responses of the carbon epoxy composite plate structure subjected to underwater explosion load were confirmed with the results available in the literature. In this part, ALE (Arbitrary Lagrangian-Eulerian) and Load_SSA (Sub Sea Analysis) methods were used to simulate the dynamic response of the plate and a very good agreement was observed by using the ALE method. Then, the composite sandwich panels with a soft-core (foam) were investigated by the finite element-based ALE method. Finally, the maximum deflection results of the plate structure subjected to underwater explosion loads were examined with parametric studies. Increasing core layer thickness caused a decrease in plate deflections and an increase in post-explosion oscillation frequency under the same blast conditions.

Purpose: In this study, a railway superstructure is modeled with a new approach called locally continuous supporting, and its behavior under the effect of moving load is analyzed by using analytical and numerical techniques. The purpose of the study is to demonstrate the success of the new modeling technique. Design/methodology/approach: In the railway superstructure, the support zones are not modeled with discrete spring-damping elements. Instead of this, it is considered to be a continuous viscoelastic structure in the local areas. To model this approach, the governing partial differential equations are derived by Hamilton’s principle and spatially discretized by the Galerkin’s method, and the time integration of the resulting ordinary differential equation system is carried out by the Newmark–Beta method. Findings: Both the proposed model and the solution technique are verified against conventional one-dimensional and three-dimensional finite element models for a specific case, and a very good agreement between the results is observed. The effects of geometric, structural, and loading parameters such as rail-pad length, rail-pad stiffness, rail-pad damping ratio, the gap between rail pads and vehicle speed on the dynamic response of railway superstructure are investigated in detail. Originality/value: There are mainly two approaches to the modeling of rail pads. The first approach considers them as a single spring-damper connected in parallel located at the centroid of the rail pad. The second one divides the rail pad into several parts, with each of part represented by an equivalent spring-damper system. To obtain realistic results with minimum CPU time for the dynamic response of railway superstructure, the rail pads are modeled as continuous linearly viscoelastic local supports. The mechanical model of viscoelastic material is considered as a spring and damper connected in parallel.

This paper focuses on the vibration analysis of three-layered curved sandwich beams with elastic face layers and viscoelastic core. First, the equations of motion that govern the free vibrations of the curved beams together with the boundary conditions are derived by using the principle of virtual work, in the most general form. Then, these equations are solved by using the generalized differential quadrature method in the frequency domain, for the first time to the best of the authors’ knowledge. Verification of the proposed beam model and the generalized differential quadrature solution is carried out via comparison with the results that already exist in literature and the ANSYS finite element solution combined with the modal strain energy method. The effect of system parameters, i.e. layer thicknesses, the lamination angle of layers and the curvature on the vibration and damping characteristics of a curved sandwich beam with laminated composite face layers and a frequency dependent viscoelastic core is investigated in detail.

Underwater explosion induced shock loads are capable of causing considerable structural damage. Investigations of the underwater explosion (UNDEX) effects on structures have seen continuous developments because of security risks. Most of the earlier experimental investigations were performed by military since the World War I. Subsequently; Cole [1] established mathematical relations for modeling underwater explosion shock loading, which were the outcome of many experimental investigations This study predicts and establishes the transient responses of a panel structure to underwater explosion shock loads using non-linear finite element code Ls-Dyna. Accordingly, in this study a new MATLAB code has been developed for predicting shock loading profile for different weight of explosive and different shock factors. Numerical analysis was performed for various test conditions and results are compared with Ramajeyathilagam's experimental study [8].