Journal of Engineering Mechanics

This study investigates the theoretical aspects of the localization of plastic buckling patterns, which results in the strength deterioration of steel structures under cyclic loading. Because the localization phenomena is caused by the plastic bifurcation from the decreasing equilibrium path in the post-peak range, we first presented a method based on the finite-element method (FEM) to analyze the plastic bifurcation. Then, using the proposed method, we analyzed a multiply supported column and simply supported rectangular compression plates subject to repeated loading. As a result of numerical anlysis, it was confirmed theoretically that the localization of buckling patterns has a large influence on the strength deterioration under cyclic loading. Furthermore, it was observed from the numerical results that the occurrence of localization is accelerated by the accumulation of residual bending deformations as well as the Bauschinger effect of materials.

The paper analytically examines the axisymmetric interaction between a rigid, circular, flat indentor and a poroelastic half-space that is saturated with a compressible fluid. The contact between the indentor and the poroelastic half-space is assumed to be smooth. The drainage conditions at the surface of the poroelastic half-space are considered as either completely drained, or partially drained, or completely undrained. By using the integral transform techniques, the paper develops the governing coupled integral equations. These governing integral equations are further reduced to systems of standard Fredholm integral equations of the second kind in the Laplace transform domain. Efficient computational algorithms are proposed to evaluate the time-dependent behavior of the rigid, circular indentor. The numerical results presented in the paper illustrate the manner in which the three variations in pore-pressure boundary conditions and the undrained compressibility of pore-fluid influence the consolidation response of the indentor.

This paper presents damage-mechanics-based models for predicting the nonlinear static response of unreinforced concrete dams. An anisotropic formulation of the damage because of cracking is proposed. It leads to a reasonable finite-element mesh size. Correct convergence of energy dissipation is ensured by using the energy equivalence concept. The proposed expression for the evolution of damage is a function of the basic properties of concrete strength, ${\mathit{f}}_{\mathit{c}}^{\prime }$ and ${\mathit{f}}_{\mathit{t}}^{\prime }$, and the fracture energy G _f . Three examples are presented to illustrate the performance and accuracy of the proposed formulation. The first example compares the numerical results with experimental measurements obtained from a reduced model of a concrete dam. The second example uses Koyna dam as a test case to compare the damage mechanics theory with other theories on cases of reservoir overflow. The last example investigates the influence of initial damage on the seismic response of a gravity dam analyzed using the equivalent static loads.

Rigid, closed-cell, polyurethane foams possess excellent energy-absorbing properties and are used as impact limiters for various packaging applications. They can absorb large quantities of energy as they transmit a stress equal to their crushing strength. Recently, their mechanical properties have come under increased scrutiny due to applications in nuclear-waste shipment packages. In such an application, the material is subjected to multiaxial state of stress. Strain-rate effects due to the dynamic nature of the loading in an hypothetical accident situation is also likely to be encountered. This paper describes a number of tests performed on polyurethane foams of various densities along several loading paths. These include uniaxial, hydrostatic and triaxial compression. Strain-rate effects were also studied under compressive loading. Since the material is significantly anisotropic, the experimental data are used to infer elasticity parameters and the shape of the anisotropic yield surface. Some strain-rate effects are also examined.

The present paper extends the results of discontinuous bifurcation analysis of elastoplastic solids to materials that exhibit elastic degradation. As a starting point, the concepts of elastic degradation are formulated in terms of a secant relationship, a threshold function, and an elastic stiffness degradation rule. Differentiation of the secant law renders the governing tangent operator for both stress- or strain-based formulations of elastic degradation. Scalar- and tensor-valued proposals in continuum damage mechanics are particular cases of this unified formulation of elastic degradation, when a reduced set of damage variables is considered. The traditional (1 −D) approach of scalar damage describes the stiffness degradation by a single variable, whereby all components of the secant stiffness are affected in a self-similar manner. The paper presents general analytic results for distributed and localized failure of elastic-degrading materials, further specialized for the traditional (1 −D) scalar damage model, when it is subjected to specific loading scenarios. For illustration, a geometric localization criterion is introduced to highlight the features of localized failure in the Mohr coordinates.

A new neural-network-based control algorithm has been developed and tested in the computer simulation of active control of a three-story frame structure subjected to ground excitations. First, an emulator neural network has been trained to forecast the future response of the structure from the immediate history of the system's response, which consists of the structure plus an actuator. The trained emulator has been used in predicting the future responses and in evaluating the sensitivities of the control signal with respect to those responses. At each time step of the simulation, the control signal has been adjusted to induce the required control force in the actuator based in a control criterion. A controller neural network has been trained to learn the relation between the immediate history of response of the structure and actuator, and the adjusted control signals. The trained neurocontroller has been used in controlling the structure for different dynamic loading conditions. Results of this initial study indicate that the neural-network-based control algorithms have the promise of evolving into powerful adaptive controllers after further research.

The physical meaning, geometric interpretation, and applications of the polar decomposition theory and Jaumann strains and stresses in nonlinear modeling and analysis of solids and structures are considered. Jaumann strains and stresses prove to be objective geometric measures defined with respect to the deformed system configuration, and constant material stiffnesses obtained from experiments using engineering stresses and strains can be directly used in the constitutive equation. Moreover, the relations of Jaumann strains and stresses to Green-Lagrange strains and Piola-Kirchhoff stresses are derived, and it is shown that Jaumann strains can be easily derived by using a new concept of local displacements.