Journal of Engineering Mechanics

A thermoelastic model for analyzing laminated composite plates under both mechanical and thermal loadings is constructed by the variational asymptotic method. The original three-dimensional nonlinear thermoelasticity problem is formulated based on a set of intrinsic variables defined on the reference plane and for arbitrary deformation of the normal line. Then the variational asymptotic method is used to rigorously split the three-dimensional problem into two problems: A nonlinear, two-dimensional, plate analysis over the reference plane to obtain the global deformation and a linear analysis through the thickness to provide the two-dimensional generalized constitutive law and the recovering relations to approximate the original three-dimensional results. The nonuniqueness of asymptotic theory correct up to a certain order is used to cast the obtained asymptotically correct second-order free energy into a Reissner–Mindlin type model to account for transverse shear deformation. The present theory is implemented into the computer program, variational asymptotic plate and shell analysis (VAPAS). Results from VAPAS for several cases have been compared with the exact thermoelasticity solutions, classical lamination theory, and first-order shear-deformation theory to demonstrate the accuracy and power of the proposed theory.

A new model is developed for non-Gaussian processes. The model is based on the spectral representation theorem for weakly stationary processes, can match the second moment properties and several higher order moments of any non-Gaussian process, and consists of a superposition of harmonics with uncorrelated but dependent random amplitudes. The calibration of the model to a target non-Gaussian process may require iterations. The Ito⁁ formula is used to calculate higher-order moments of the proposed model. These moments can be used to tune the model such that it matches some of the higher-order moments of a target non-Gaussian process in addition to its second-moment properties. The proposed model is useful for both Monte Carlo simulation and analytical studies on the response of linear and nonlinear systems to non-Gaussian noise. Examples are presented to illustrate the use of the proposed model.

Many materials exhibit elasto–visco–plastic behavior when subjected to loadings with certain strain rate. Examples include natural materials such as metals, clays, and soils and manmade materials such as some biomimic materials. Some voids may exist or be introduced in these materials. The effects of the voids on the material response are important in predicting the strength, reliability, and service life of structural systems containing these materials. This paper presents the results of applying a statistical micromechanical approach to describe the macroscopic behavior of elasto–visco–plastic materials containing many randomly dispersed spherical voids. Most existing micromechanics based models are only applicable to monotonic proportional loadings. The limitation is removed by integrating the material model into the framework of continuum plasticity. With the discrete integration algorithm and local return mapping algorithm, the proposed computation method is applicable to any loading and unloading histories and is ready for implementing into finite element analysis.

The water saturation degree of soils can strongly affect the propagation of blast wave. However, quantitative study of this problem has not been possible due to lacking an appropriate analytical model that is capable of describing the drastically changing behavior of the soil in close-in regions during the blast loading. In this paper, a newly developed three-phase soil model for shock loading is used to study the propagation of blast wave in soils, with particular focus on the influence of the water saturation degree on the stress wave parameters. The multiphase soil model enables the direct inclusion of the energy source in the simulation. Four kinds of fine-grained sand loam with different water saturation degree ranging from 37.5 to 100% are investigated. The numerically predicted camouflet and distribution of damage surrounding the charge show favorable agreement with general experimental observations. The predicted and measured peak wave parameters and their attenuation laws also show a reasonable agreement. The results indicate that a small amount of air in soils could affect significantly the blast wave parameters. For example, with 4% volume of air, the peak pressure in the soil could reduce by 1–2 orders of magnitude as compared to water-saturated soils within a scaled range of 0.5–4.0 (m kg^−1/3), while the peak particle velocity reduces by 2–6 times. The predicted attenuation relations show a noticeable nonlinear trend for soils with relatively high air content (e.g., 15%), and this phenomenon can be attributed to the high nonlinearity of the soil skeleton.

A baseline model is essential for long-term structural performance monitoring and evaluation. This study represents the first effort in applying a neural network-based system identification technique to establish and update a baseline finite element model of an instrumented highway bridge based on the measurement of its traffic-induced vibrations. The neural network approach is particularly effective in dealing with measurement of a large-scale structure by a limited number of sensors. In this study, sensor systems were installed on two highway bridges and extensive vibration data were collected, based on which modal parameters including natural frequencies and mode shapes of the bridges were extracted using the frequency domain decomposition method as well as the conventional peak picking method. Then an innovative neural network is designed with the input being the modal parameters and the output being the structural parameters of a three-dimensional finite element model of the bridge such as the mass and stiffness elements. After extensively training and testing through finite element analysis, the neural network became capable to identify, with a high level of accuracy, the structural parameter values based on the measured modal parameters, and thus the finite element model of the bridge was successfully updated to a baseline. The neural network developed in this study can be used for future baseline updates as the bridge being monitored periodically over its lifetime.

An accurate prediction for the response of tall buildings subject to strong wind gusts or earthquakes requires the information of in situ dynamic properties of the building, including natural frequencies and damping ratios. This paper presents a method of identifying natural frequencies and damping ratios of in situ tall buildings using ambient wind vibration data. Our approach is based on the empirical mode decomposition (EMD) method, the random decrement technique (RDT), and the Hilbert–Huang transform. Our method requires only one acceleration sensor. The noisy measurement of the building acceleration is first processed through the EMD method to determine the response of each mode. Then, RDT is used to obtain the free vibration modal response. Finally, the Hilbert transform is applied to each free vibration modal response to identify natural frequencies and damping ratios of in situ tall buildings. The application of the proposed methodology is demonstrated in detail using simulated response data of a 76-story benchmark building polluted by noise. Both the along-wind and across-wind vibration measurements have been illustrated. Simulation results demonstrate that the accuracy of the proposed method in identifying natural frequencies and damping ratios is remarkable. The methodology proposed herein provides a new and effective tool for the parametric identification of in situ tall buildings.

This paper addresses the development of a nonconforming axisymmetric quadrilateral variable-node element which is designated as NAQV. The NAQV element is an improved version of the axisymmetric quadrilateral variable-node (AQV) element by adding nonconforming modes selectively. Both NAQV and AQV elements are effectively used in overcoming some problems in connecting the different layer patterns in the adaptive h refinement on a quadrilateral mesh. The use of elements with variable nodes eliminates the necessity of imposing displacement constraints on irregular (hanging) nodes in order to enforce the interelement compatibility. Therefore, the NAQV and AQV elements can be used effectively in the local mesh refinement for the axisymmetric structures which have stress concentrations. As verified through some numerical tests, the NAQV element shows excellent performance and can be used in the analysis of practical problems.

An energy parameter based on the concept of the M integral is proposed for describing the fracture behavior of a two-dimensional multidefect rubbery mechanical system under the action of large deformation. To this end, we first define a problem-invariant parameter $M_c$ by suitably choosing a closed contour and taking the integration with respect to the geometric center of all the enclosed defects. Attention is then addressed to discussion of the physical meaning of $M_c\text{,}$ which is shown to be related to the surface energy corresponding to formation of the defects. Also, the integration appears to be path independent so that the complicated singular stress field in the near-tip areas need not be involved in the calculation. Based on the above characteristics, it is suggested that $M_c$ may be possibly used as a fracture parameter for describing the degradation of material and/or structural integrity caused by irreversible evolution of defects in rubbery media.

Drained true triaxial tests on dense Santa Monica Beach sand deposited with a cross-anisotropic fabric have been performed to study the failure condition in the principal stress space. The failure surface was assumed to be symmetric around the vertical axis (on the octahedral plane of the principal stress space), but varying as a function of the Lode angle. Data from previously performed consolidated-undrained true triaxial tests on San Francisco Bay Mud and data from triaxial compression, triaxial extension, and plane strain tests on Toyoura sand showed similar behavior in terms of effective stresses. A three-dimensional failure criterion is proposed for characterization of failure in cross-anisotropic soils, under commonly occurring conditions when loading and depositional directions coincide and no significant rotation of principal stresses occur. This cross-anisotropic criterion is developed using a coordinate rotation of the principal stress space and utilization of an existing isotropic failure formulation. Derivation of the three required parameters is explained and illustrated. The proposed criterion is compared with various experimental results; and it is demonstrated that the failure criterion for cross-anisotropic soils captures the experimental behavior with good accuracy.

A three-dimensional finite element modeling of high damping rubber bearings is studied. At first, the constitutive model of high damping rubber materials proposed by the writers is formulated in order to derive the constitutive tensor, which is required in the application of the finite element method. Second, a mixed finite-element method consistent with the proposed constitutive model is described. In this method, slightly compressible materials with rate form constitutive models are applied. Then, using the constitutive model and the finite-element method, a three-dimensional finite element model of high damping rubber bearings is constructed. The simulations by the model are found to be in good agreement with the experimental results of the bearing. Finally, complex deformation such as torsional or rotational deformation of the bearing are simulated by the finite-element model, and the design equations for these deformation are proposed on the basis of the simulations or experimental results.