Journal of Engineering Mechanics

Metamaterials, which draw their origin from a special class of structured (periodic) materials characterized by a dynamic filtering effect, have recently emerged as a prospective means for structural seismic protection. This paper explores such a periodic arrangement in the form of local adaptive resonators buried in the soil, serving as a seismic protection barrier. As a starting point, a simplistic representation is chosen herein that comprises chains of mass-in-mass unit cells. A robust-to-uncertainties optimization of such a chain, addressing uncertainties at the level of the excitation, the system properties and the model structure itself, is conducted. The optimization problem is formulated within the context of reliability assessment, where the objective function is the failure probability of the structure to be protected against seismic input. The problem is solved through adoption of the subset optimization algorithm enhanced through the simultaneous implementation of a stochastic approximation algorithm. It is demonstrated that not all parameters of the chain model require optimization, because the failure probability proves to be a monotonic function of a subset of the parameters. A primary objective herein lies in optimizing the internal unit-cell stiffness properties. It is further demonstrated that the effectiveness of the protection offered by the metamaterial is improved for spatially varying unit-cell properties. The optimization procedure is carried out in the frequency domain, with an example application confirming that a time domain optimization is expected to yield similar optimal configurations. A parametric study using a nonlinear model is also presented, offering a starting point for more refined future investigations.

A meshfree particle approach, namely moving particle semi-implicit (MPS) method is used to investigate the sloshing behavior in a partially filled liquid container. The validity of the numerical model is checked by comparing the sloshing motion in tanks undergoing sinusoidal and earthquake excitations with the published results. A comprehensive study of seismically excited violent sloshing motion and wave impact using a particle method, not reported in open literature, were taken up in the present study. Three ground acceleration time histories of varying characteristics were selected as the external excitations to the liquid tank system. The influence of the frequency content of ground acceleration on the violent sloshing characteristics was studied by scaling the peak ground acceleration to a fixed value for all the earthquake events. The assessment of dynamic responses, such as base shear, overturning moment, pressure load exerted on tank walls and ceiling, and slosh displacement arising out of vicious sloshing phenomenon, was competently carried out subjected to the selected ground excitations. A higher magnitude of dynamic responses, such as impact pressure load, base shear, and overturning moment, was evaluated for low frequency-content earthquake compared with that of a high frequency-content earthquake.

The heat evolved during the hydration process may not dissipate properly due to the large size and poor thermal conductivity of mass concrete. Embedded cooling pipes of circulating water are generally used for heat removal from the interior of the concrete mass. This study experimentally shows that forced cooling helps reduce the interior temperature significantly; however, it leads to a reverse thermal gradient around the cooling pipe. This paper conducts a three-dimensional finite-element simulation of the hydration heat in concrete with a forced cooling system, modeling the circulating water with three-dimensional elements with diffusion and dispersion properties which accurately predict the experimentally observed temperature profile. Finite-element analysis also indicates the presence of high thermal stresses in concrete near the cooling pipe, mainly due to the extreme temperature gradient and temperature fluctuations. The results suggest a careful monitoring of heat removal using forced cooling to avoid a large thermal gradient and the likelihood of cracking.

A self-powered active support system, which produces a control force using the energy regenerated by a transversely mounted linear electromagnetic (EM) damper, is applied for two-dimensional (2D) (streamwise/cross-flow) vortex-induced vibration (VIV) suppression of an oscillating circular cylinder in low Reynolds number flow ($\mathsf{R}=90$). In addition, a passive energy generating system is devised that achieves a proper trade-off between VIV suppression and energy harvesting actions with the EM damper acting in either the regeneration or the dissipation mode. A multifield cosimulation scheme, which links an active proportional-integral-derivative (PID) controller along with the switch-based energy harvesting circuit of the EM damper (implemented in MATLAB/Simulink) to the plant model (constructed via a user-defined function in a commercial finite volume solver), is designed and implemented. Numerical simulations compare the effects of the passive and active EM-generating systems on 2D VIV suppression of the elastic cylinder as well as on the time evolution of cylinder force coefficients. They demonstrate that the adopted self-powered (tuned) active energy harvesting system, which does not require an external power supply and makes intelligent use of the stored electrical energy for producing the required transverse control force, efficiently balances the regenerated and consumed energies by proper switching between three distinct (regeneration, drive, and brake) modes of EM device operation. It can particularly provide improved cylinder VIV suppression with lower power consumption via enhanced collaborative action between different modes of the EM device. Moreover, the selected system is also found to be effective for other Reynolds numbers in the lock-in region.

A comprehensive constitutive model has been formulated to capture the complex behaviors of gravelly soils under irregular cyclic loading. The application of this model aims to analyze the nonlinear deformation behavior of earth structures involving gravelly soils, such as rockfill dams and retaining walls subjected to earthquake loading. The proposed model is based on the basic framework of a two-mechanisms model structure (the first mechanism due to the deviatoric stress ratio increment ${\mathrm{d}r}_{\mathrm{ij}}$, the second to effective mean pressure increment ${\mathrm{d}p}^{\prime }$). The distinctive feature of this model is that the concept of scaled memory is extended and incorporated into the plastic modulus and dilatancy, which are defined in three-dimensional stress space by the relative configuration of past stress reversal memory surfaces. The sizes of these memory surfaces are assumed to expand with cyclic accumulated volumetric strain. Consequently, the influence of any loading and unloading history on stress-strain behaviors can be determined for complex loading paths, particularly for irregular cyclic loading. Subsequently, the key assumption of the plastic flow direction under the ${\mathrm{d}p}^{\prime }$ mechanism is modified to better reflect the shear characteristic under complex cyclic loadings. The model is formulated in a general three-dimensional stress space, and the modeling of cyclic hysteresis, cyclic hardening and densification, and cyclic residual deformation of gravelly soils is verified through comparisons of the predicted behaviors and the known experimental results with various one-way and two-way cyclic loading paths. Finally, the cyclic loading history is determined to have a considerable impact on the calculated strain that should not be ignored.

This paper investigates the stress concentration of an adhesive material containing a micro air void subjected to a harmonic dynamic surface load. The elastodynamic Green’s function was used to derive the elastic field and take into account the boundary effect. Using the equivalent inclusion method, the micro air void was replaced by the matrix but an eigenstrain and fictitious force were introduced to simulate the material mismatch. Coupling with the boundary element method, one can assemble a linear equation system from which the microvoid and boundary effect are fully taken into account and the eigenstrain, fictitious force, and unknown boundary responses can be calculated. Therefore, the elastodynamic field can be obtained from the equivalent inclusion problem with the prescribed boundary conditions for any bounded domain. The results have been verified and compared with the finite-element simulation for a static case, the classic Eshelby solution for a low-frequency case, and the traditional boundary element method for a full time-harmonic case. A parametric study of stress concentration factors in the adhesive for one void with various depths and frequencies was performed. This method can be extended to general cases of many particles embedded in a matrix with arbitrary boundary conditions.

This paper is to study the propagation of plane waves in a rotating thermoelastic solid with voids with temperature-dependent elastic moduli. The modulus of elasticity is taken as a linear function of reference temperature. The entire elastic medium is rotated with a uniform angular velocity. The formulation is applied under three theories of generalized thermoelasticity: Lord-Schulman (L-S), Green-Lindsay (G-L), and the coupled theory (CD). The normal mode analysis is used to obtain the exact expressions for the considered variables. Some particular cases are also discussed in the context of the problem. Numerical results for the considered variables are obtained and illustrated graphically. Comparisons are also made with the results predicted by different theories (CD, L-S, G-L) in the absence and presence of rotation and temperature dependent of properties, also with and without voids.

In this article a new testing method using a shake table and dynamic actuators at the same time is presented and optimized for performance. The original idea of the method is further developed in this article in extent and application. Here the input motion is divided between the shake table and actuators and is applied at the same time to the experimental model. It is shown that by using this method it is possible to minimize the total power required for the experiment compared with when each of the testing apparatuses are used separately. To optimize the test procedure, different techniques for dividing the ground motion between the two facilities are presented that include dividing the amplitude by a constant number, dividing the frequency content, and dividing the amplitude on a timely basis based on a minimization scheme, which is called the dynamic optimization method in this article. It is shown that the dynamic optimization method is the preferred technique requiring a minimum testing power.

Model reduction has been performed for several decades to allow for correlation of an analytical model to experimental data at a reduced number of points; however, traditional approaches have limitations. Static stiffness matrix reduction methods (e.g., Guyan) may not accurately capture the system dynamics, while techniques based on mode shapes [e.g., the system equivalent reduction expansion process (SEREP)] may experience rank deficiency issues. A new model reduction approach presented herein addresses these limitations by combining the accuracy of SEREP with the full-rank attributes of Guyan reduction. The advantages of the presented methodology over traditional reduction techniques are showcased via analytical studies on a cantilevered beam and general plate-type structure.

Blast-load effects on structures are notably dangerous, and the damage and destruction that they cause are considered a great threat to human lives. Nonlinear dynamic analysis (NDA) is recognized as an accurate method for analyzing structures under blast waves. However, because of the time-consuming nature of this analysis, a simple alternative method with a sufficient degree of accuracy is essential. This study aims to extend the concept of the well-known modal pushover analysis (MPA) to assess structures under blast loading. MPA is currently used to analyze structures under seismic excitation. In an earthquake analysis, forces are distributed over the height of the structure in proportion to the mass concentration. However, in the blast-loading scenario, the distribution of forces on the building depends on the magnitude and location of explosion, regardless of the mass distributions. Based on these differences, the fundamental formulations of the MPA are modified, and a step-by-step procedure is accordingly proposed. The effectiveness of the proposed procedure is assessed by examining an eight-story shear-wall structure. The results show that the proposed procedure is sufficiently accurate in comparison with the exact NDA.