Journal of Engineering Mechanics

Instability of granular material may lead to catastrophic events such as the gross collapse of earth structures, and thus it is an important topic in geotechnical engineering. In this paper, we adopt the micromechanics approach for constitutive modeling, in which the soil is considered an assembly of particles, and the stress-strain relationship for the assembly is determined by integrating the behavior of the interparticle contacts in all orientations. Although analyses regarding material instability have been extensively studied for a soil element at the constitutive level, it has not been considered at the interparticle contact level. Through an eigenvalue analysis, two modes of instability are identified at the local contact level: the singularity of tangential stiffness matrix and the loss of positiveness of second-order work. The constitutive model is applied to simulate drained and undrained triaxial tests on Toyoura sand of various densities under various confining pressures. The predictions are compared with experimentally measured instability at the assembly level. The modes of stability at the interparticle contact level and their relations to the overall instability of the assembly are also analyzed.

The split Hopkinson pressure bar (SHPB) is one of the most widely used experimental techniques to study the behavior of materials at medium to high strain rates. This paper deals with the use of the SHPB to investigate the effect of the specimen diameter on the mechanical properties of materials. Specimens made of aluminum alloy 6061-T6, with length 5 mm and varying diameters of 5, 6, 8, 10, and 12 mm, are tested. For same strain rate, higher stresses are observed for the smaller-diameter specimens. The smaller-diameter specimens give rise to higher and more uniform strain rates but oscillating stress-strain curves. They result in a low transmission coefficient that affects the output signals; therefore, smaller diameters should be selected carefully with the transmission coefficient of 0.5 or better. With some compromise on oscillating stress-strain behavior, the specimens with smaller diameters can be used successfully to achieve high strain rates.

Vast governmental budgets are spent annually to face corrosion problems of steel reinforcement in concrete bridges attributable to the extensive use of deicing salts. Corrosion controls the lifetime of a bridge, which has two distinct periods. During the first period, chlorides diffuse through the cover. When sufficient chlorides are formed at the rebars, corrosion initiates. This marks the start of the second period, during which rust with higher volume to bare steel is produced. The rust puts pressure on the cover, which finally leads to cover spalling. In this paper, a model is developed to determine the time span of the second period. The model includes a volume compatibility condition that allows for the proper introduction of compaction of all materials that contribute to cover spalling, including the rust. A new condition for marking failure of the cover is also established, based on fracture mechanics and strain energies. Finally, a new formula is proposed for the rate of rust production, which allows for the constant rust production at early and nonlinear diffusion dependant rates at latter stages of corrosion.

Finite-element (FE) analysis makes it possible to investigate different parameters and their effect on the carrying capacity or failure behavior of a component in an easy and cost-effective way. But to do this, the numerical model needs to reproduce the material behavior as close to reality as possible. This paper presents a numerical model developed to simulate the complex failure behavior of dowel connections in wood loaded perpendicular to grain. This includes both the ductile behavior, such as the embedding or bearing failure under the dowel or supports, and the brittle failure, such as cracking of the wood near the dowels. Using contact elements, the crack growing under tension perpendicular to grain and/or shear stresses can be modeled explicitly. The linear elastic-plastic stress-strain behavior of the wood allows one to simulate the plastic deformations of the ductile behavior. The comparison of the numerical results with the load capacity of different experimental test series confirms that the numerical model is accurate and useful to extend existing experimental test series.

In this research, an enhanced flexibility- (force-) based formulation is developed for a shear deformable beam-column element by using force interpolation functions. The development is derived from Reissner’s exact stress resultant theory and its finite strain field for a Timoshenko frame element. Here, the state-space approach is applied, and the differential-algebraic equation system, consisting of a conservation global system and local constitutive equations, is formed and solved simultaneously. To improve the element’s accuracy, a polynomial-based displacement field approximation is implemented to evaluate the element flexibility matrix. The proposed technique is verified by nonlinear examples, including material and geometric nonlinearities, to examine its precision. It is shown that shear-locking is alleviated by the element; therefore, shear effects can easily be considered. The proposed method is more reliable and accurate than general displacement-based methods and existing force-based formulations.

The writers recently proposed a new type of mass damper system to integrate structural and environmental control systems for buildings. External shading fins are used as mass dampers such that they can (1) control building energy consumption by adjusting the fins and, thus, the amount of sunlight entering the building; and (2) control structural movements by dissipating energy with the dampers during strong motions. Because shading fins are placed along the height of the building, the mass dampers are distributed along the building height instead of concentrated in one or a few locations like traditional tuned mass dampers (TMDs). The distributed mass damper (DMD) system is formulated and simulated for earthquake motions. Optimization is performed on damper parameters (i.e., masses, stiffness, and damping coefficients) of the passive DMD system to minimize structural responses. A near-optimal DMD system outperforms a single TMD system. The movable shading fins are also briefly discussed; they show a substantial savings in building energy consumption.

Previously a Bayesian theory for modal identification using the fast Fourier transform (FFT) of ambient data was formulated. That method provides a rigorous way for obtaining modal properties as well as their uncertainties by operating in the frequency domain. This allows a natural partition of information according to frequencies so that well-separated modes can be identified independently. Determining the posterior most probable modal parameters and their covariance matrix, however, requires solving a numerical optimization problem. The dimension of this problem grows with the number of measured channels; and its objective function involves the inverse of an ill-conditioned matrix, which makes the approach impractical for realistic applications. This paper analyzes the mathematical structure of the problem and develops efficient methods for computations, focusing on well-separated modes. A method is developed that allows fast computation of the posterior most probable values and covariance matrix. The analysis reveals a scientific definition of signal-to-noise ratio that governs the behavior of the solution in a characteristic manner. Asymptotic behavior of the modal identification problem is investigated for high signal-to-noise ratios. The proposed method is applied to modal identification of two field buildings. Using the proposed algorithm, Bayesian modal identification can now be performed in a few seconds even for a moderate to large number of measurement channels.