Journal of Engineering Mechanics

A novel dynamic physical model for a magnetorheological (MR) damper is proposed. In this model, the hysteretic properties are considered to result from the difference between the working mechanism of MR fluids in their preyield and postyield stages. In the preyield stage, the MR damper’s force is caused by the chain deflection of MR fluids, which can be described by a particle-chain model. However, in the postyield stage, the MR damper’s force is caused by the flow gradient of MR fluids, which can be accurately predicted by the quasi-static model. Therefore, the proposed model is a combination of a particle-chain model and quasi-static model. The input-current dependence and loading-condition dependence of MR damper are then addressed. By establishing the relationship between model parameters and input current, the input-current dependence is solved. By analyzing the physical mechanism of the proposed model and its consistency with experimental data, the relevance with the loading amplitude and frequency is revealed. For the purposes of validation, comparative studies between the proposed physical model and phenomenological models such as the extended Bouc-Wen model under different loading conditions are carried out. Numerical results show that the proposed model produces a highly accurate description on MR damper’s hysteretic properties and maintains better robustness for the cases with various loading conditions. Also, the dilemma between accuracy, robustness, and complexity inherent in traditional phenomenological models can be elegantly solved by the proposed physical model.

In vegetated open channel flow, sediment transport is affected by both boundary shear and vegetation-induced turbulence. This study presents a theoretical derivation of the critical flow condition for incipient sediment motion, which is evaluated in the reach scale in the presence of emergent vegetation simulated with cylindrical rods. To take into account the combined effects of both mean flow and turbulence on sediment motion, a spatiotemporal average is applied to the force balance equation. The analysis shows that the critical condition can be formulated in terms of the dependence of the densimetric Froude number on the vegetation concentration for the condition of hydrodynamically rough turbulent flows. The derived theoretical formula is consistent with laboratory measurements reported in previous studies.

In this paper, the proposed yield function is the function of stress, hardening parameter, and strain rate. To fully describe the stress state and strain rate condition of material during dynamic loading, a new coordinate system, which is composed of a stress axis and strain rate axis, is employed to present the proposed rate-dependent yield surface. The strain rate is regarded as a variable in applying the consistency condition. The rate-dependent consistency is satisfied strictly during the whole dynamic loading. The nonassociated flow rule is employed to describe the dilatancy behavior of concrete. The loading–unloading criterion based on Ilyushin’s postulate is developed into the rate-dependent loading–unloading criterion that can consider reasonably the change of elastic domain caused by strain rate. Furthermore, a three-dimensional (3D) dynamic elastoplastic model of concrete is established within the framework of the rate-dependent consistency condition. Under the dynamic uniaxial compression loading condition, the model characteristics related to rate-dependent consistency condition are discussed and analyzed. Five sets of dynamic experiment results of concrete are employed to evaluate the performance of the model.

The problem involving mitigation of wave energy by trench-type structure, in particular, a pair of trenches imposed on the seabed in the absence and presence of a vertical wall, is examined for its solution. The problem under consideration leads to multiple series relations involving trigonometric functions. Instead of converting these relations into a system of integral equations, direct algebraic approaches are utilized for solving the reduced system of overdetermined algebraic equations and the corresponding solutions are obtained approximately. Here, the overdetermined system of algebraic equations are solved with the aid of the well-known least-squares (LS) and singular value decomposition (SVD) methods. Results involving the hydrodynamic quantities such as reflection and transmission coefficients related to the single trench problem are derived and are found to be in excellent agreement with the results available in the literature. The present algebraic methods appear to be very direct and quick. The energy balance relation for the given scattering problem is derived and used to check the accuracy of numerical results. Some important results such as the behavior of singularity in flow near each edge of the trenches, surface elevation profiles, and force experienced by the wall are investigated and analyzed through graphs to analyze the transformation of wave energy by a pair of trenches imposed on a seabed. It is observed that creation of a pair of trenches imposed on a seabed helps to reduce the wave load on the wall; consequently, the wall as well as the seashore (in absence of a wall) is protected.

Corrosion resistant alloy steel (MMFX) reinforcing bar is increasingly utilized in structural engineering applications due to their high yield strength and corrosion resistance. This paper aims to characterize the thermomechanical response of MMFX rebar by examining their elastic and inelastic properties at a wide range of temperatures and strain rates. To achieve that, a series of quasi-static tensile tests at rates of 0.0015, 0.015, and $0.15{\mathrm{s}}^{-1}$ were conducted at a range of temperatures between room temperature and 650°C. In addition, dynamic tests were conducted at room temperature using a drop mass bench at strain rates of up to $500{\mathrm{s}}^{-1}$. Results showed that at room temperature, the flow stress of the MMFX steel alloy observed slight dependence on the quasi-static strain rate. However, the strain rate-sensitivity became more effective as the temperature increases with very active regions of dynamic strain aging encountered at different levels of strains, strain rates, and temperatures. Moreover, the reduction in the elastic properties was less than 18% at temperatures up to 450°C, then followed by a sharp decrease at 650°C. The stress–strain responses were utilized to identify the material constants for constitutive modeling. The Voyiadjis–Abed constitutive model was utilized to describe the material flow stress and was implemented into a finite-element (FE) model that was developed using ABAQUS version 2017. The FE model is capable of reproducing the experimental results and simulating the dynamic hammer-tests at room temperatures.

Measured force-displacement sensing data are analyzed to create nonlinear hysteretic models for elastic sliding bearings and steel dampers. These data were collected during full-scale experimental tests of a base-isolated structure at Japan’s National Research Institute for Earth Science and Disaster Resilience (NIED) “E-Defense” Hyogo Earthquake Engineering Research Center in 2013 that employed historical and synthetic earthquake inputs. Several models are explored for each isolation-layer device, including Coulomb friction models for the elastic sliding bearings and Bouc-Wen hysteresis models for both devices. Parameters for each device model are estimated via constrained nonlinear optimization to minimize model prediction error using testing data. The resulting models are applied to data from other seismic tests to evaluate the models’ ability to provide accurate force predictions. The model for the steel dampers demonstrates the capacity to be generalized across seismic excitations and devices, whereas the model for the elastic sliding bearing reproduces well its behavior during other seismic excitations but is limited to the device to which it was calibrated. The residual error between the model predictions and measurements does not contribute a significant degree of variance in the model parameter estimates.

Standardized shrinkage models for concrete incorporate a function which depends on the ratio of volume to sample surface. These empirically calibrated functions are rationalized based on nonuniform moisture distributions during drying. Nonuniform moisture distributions also lead to crack development during early exposure to drying conditions. The role of early crack patterns on the cement drying shrinkage size effect is studied by companion three-dimensional X-ray microscope scans and shrinkage studies of cement paste samples. The scanned internal surface area allows for the incorporation of crack surface area to provide a more accurate representation of the drying surface. It is found that cracks generated during drying shrinkage will accelerate the moisture diffusion and cause additional shrinkage strain, and both effects follow a size effect that becomes more prominent for larger sections. Discussion of the experimentally observed impact of cracking on the shrinkage size effect behavior separates the contributions from diffusion and strain using a finite difference model. Based on these results, insights for concrete construction practice of aggregate arrangement to minimize differential shrinkage strains and cracking in nonuniform sections, such as bridge girders, are also discussed.

This study presents a novel modal parameter identification method enabling approximation of the mode shapes of linear systems using white-noise or earthquake inputs. The majority of well-established existing system identification methods perform successfully when the system is excited by broadband white-noise excitation. However, they encounter serious limitations when analyzing the vibrations triggered by nonstationary earthquake inputs. Thus, the presented technique extends the applicability of system identification and modal-based structural health monitoring methods. The method operates in modal space and is based on mode superposition in short windows. The mode shapes are identified using an optimization algorithm minimizing the weighted sum of cross-correlation of frequency response spectra. The technique is validated analytically using simulation results of a simple three-dimensional (3D) structure representing a simplified model of a real bridge pier structure, which enables exact comparison with known properties. The results show the method provides relatively good identification accuracy of modal parameters of systems excited by white-noise and earthquake inputs. The identified modal frequencies showed $<1\%$ error, where the mode-shape coefficients were identified within 5% error. The method performs robustly even for high levels of simulated sensor noise and can be readily applied to more complex multiple degree of freedom (MDOF) systems.

The transport of spheres as a bed-load through a sparse network of immobile, bed-mounted submerged cylinders was experimentally studied to quantify how the mean velocity of the spheres in the streamwise direction is altered, with respect to transport in a bare bed. Two regimes of transport were observed: (1) a tracer-type motion regime in which the spheres are able to circumvent the cylinders with few collisions between them and (2) a collisional regime in which impacts play a key role in predicting the experimental results. It was quantified as the critical value of a dimensionless parameter, below which the first regime takes place, whereas the second one triggers above this value. This parameter plays a central role in a one-dimensional (1D) model, which includes a dissipative force based on the average contact work between moving and motionless particles. At a critical flow velocity, all the spheres stopped, remaining motionless within the network, suggesting the appearance of a threshold for the onset of the spheres motion due to the network.

Rapid filling of a horizontal pipe with an entrapped air pocket has attracted significant attention as overpressurization induced by air compression may cause safety concerns. This study developed a linearized model for a rapidly filling horizontal pipe, by using a Jacobian linearization around the equilibrium point of the nonlinear system. The analytical expressions explicitly show the relationship between the characteristics of pressure oscillation and parameters including the driving head, the length of the water column, the length of the entrapped air pocket, the location of the equilibrium point, and the polytropic exponent. The oscillation frequency increases with the driving head whereas it decreases with the length of the air pocket. The obtained analytical solution compares well with the rigid-column model for a limited range, and it is improved by means of introducing power-law fitting for general cases. The linearization method also can be used in other cases, such as rapid filling in a horizontal pipe with the entrapped air pocket above an initial water layer or in slope/vertical pipes.