International Journal of Structural Stability and Dynamics

In this paper, we investigate the natural frequencies and buckling loads of functionally graded material (FGM) plates and shells, using a quasi-conforming shell element that accounts for the transverse shear strains and rotary inertia. The eigenvalues of the FGM plates and shells are calculated by varying the volume fraction of the ceramic and metallic constituents using a sigmoid function, but the Poisson ratios of the FGM plates and shells are assumed to be constant. The expressions for the membrane, bending and shear stiffness of FGM shell elements are more a complicated combination of material properties than a homogeneous element. In order to validate the finite element numerical solutions, the Navier solutions for rectangular plates based on the first order shear deformation theory are also presented. The present numerical solutions for composite and sigmoid FGM (S-FGM) plates and shells are verified by the Navier solutions and various examples of composite and FGM structures. The present results are in good agreement with the Navier theoretical solutions.

A statistical method using frequencies of structures under control is proposed for detecting damage. In the study, feedback control based on independent modal space control is first used to assign the pole of the system under detection intentionally. Then the prescribed characteristic frequencies of the structure under control, which may be more sensitive to damage, are obtained and further employed to constitute a sensitivity-enhanced damage indicator (SEDI). The principle of sensitivity-enhancing feedback control for damage detection of multi-degree-of-freedom systems is elaborated. To overcome the effect of measurement noise on modal frequencies, a hypothesis test involving the t-test that utilizes the SEDI is employed to estimate the occurrence of damage, while a statistical pattern recognition method that uses the feature vectors including the SEDI is employed to locate damage. Based on the perturbation theory, the feature vectors are normalized in order to eliminate the effect of damage extent on damage localization. The proposed method is verified by examples including a three-span continuous beam with a single damaged element and the IASC-ASCE benchmark structure with a single damaged brace. Simulation results show that, by using the frequencies of the structures under control, the proposed damage indicators are more sensitive to damage and are capable of detecting and locating small damage of structures.

A change in the representation of discrete motion equations for nonlinear structural dynamics of two-dimensional bodies is developed. The objective is to write the motion equation in a less nonlinear form. This leads to a significant increase in the range of stability of the time integration process and a reduction in the number of Newton iterations required in the time integration step.

This paper presents a statistical model updating technique for damage detection of underwater pipeline systems via vibration measurements. To verify the reliability of the method, laboratory tests of a scaled pipeline model were carried out in a towing tank. The model includes a plastic pipe and some removable springs which are designed and fabricated to link the pipe and the steel base to simulate the bedding conditions. Different damage scenarios, in terms of location and severity of scouring under the pipe, were simulated by removing one or several springs. The natural frequencies, damping ratios and mode shapes of the pipeline system were extracted from the measured vibrations using a stochastic subspace identification technique. Both the numerical and the experimental results show that the method is effective and reliable in identifying the underwater pipeline bedding conditions and the damage in the pipe structure.

This paper is concerned with the free vibration analysis of Timoshenko piles partially embedded in elastic soil, semi-rigidly connected at the upper end, and subjected to an axial force. The pile is divided into three regions: the pile portion above the soil constitutes the first region, while the second and third regions are the pile portion that is embedded in two different layers of the soil type. The pile material is assumed to be linearly elastic and the axial force is constant along the pile length. The soil is idealized by the Winkler model and the semi-rigid connection of the pile is modeled by a rotational spring. The natural frequencies of the piles are calculated from the transfer matrix for different axial forces, rotational spring constants, subgrade reaction moduli and embedded lengths of the pile. The results indicate that the natural frequency of the pile decreases as the axial force increases. Further, the increase in the stiffness of the rotational spring at the upper end of the pile causes only a small increase in the natural frequency. Finally, both the pile length and the subgrade reaction of the soil influence significantly the natural frequency of the pile.

The HHT-α method previously developed shows favorable numerical dissipation, in addition to unconditional stability, in the solution of linear elastic systems. However, its performance in the solution of nonlinear systems has not been studied yet. In this paper, the numerical properties of a subfamily of this integration method with -⅓ ≤ α ≤ 0, β = ¼(1 - α)² and γ = ½-α for the solution of nonlinear systems are analytically explored using a linearized analysis, where the stiffness term in total form is used to determine the restoring force. A theoretical proof of unconditional stability for nonlinear systems is presented. The subfamily of the integration method is verified to possess favorable numerical dissipation for both linear and nonlinear systems. Period distortion affected by the step degree of either nonlinearity or convergence is studied. Although the analysis is conducted for a single-degree-of-freedom nonlinear system, the application to a multiple-degree-of-freedom nonlinear system is also illustrated. It is confirmed that the performance of the HHT-α method for nonlinear systems is generally the same as that for linear elastic systems, except for the high dependence on the step degree of nonlinearity.

This paper deals with the experimental results obtained by in situ and model testing of a segment of the pipeline system of a thermal power plant. The field testing has been performed by using the forced and ambient vibration method. The model testing has been performed by means of a shaking table. The model was designed and constructed to the scale of 1/3 and tested on the seismic shaking table in the IZIIS' laboratory. The adopted modeling concept was an adequate model with artificial mass simulation, using the same material as that of the prototype. The spring hangings, as well as the special rolling support, have also been simulated. The model was subjected to random, harmonic and earthquake motion in horizontal, vertical and biaxial directions. The results show that the support springs can accept displacements in both the horizontal and the vertical direction in the elastic range of deformation, while the stop point base support is sensitive to the intensity of earthquake motion and is required to be limited to the horizontal and vertical directions.

This technical note is concerned with the buckling of single-walled carbon nanotubes with one atomic vacancy. An elastic beam theory is developed to predict the buckling strain of defective CNTs, and the strain prediction via the continuum mechanics model is verified from comparison studies by molecular dynamics simulations. The results demonstrate the effectiveness of the continuum mechanics theory for longer CNTs. In addition, a local kink is revealed in the morphology of the buckling of shorter defective CNTs via molecular dynamics.