Journal of Engineering Mechanics

The electrical contact resistance between contacting rough surfaces was studied under various compressive stresses. The samples considered here were isotropically roughened aluminium disks with upper and lower surfaces modified through polishing and sand blasting using different sized glass beads. Fractal geometry and roughness descriptors, including root mean square values of roughness and slope, were used to describe the topography of sample surfaces, based on the digitized profiles obtained from interferometry-based profilometry. The electrical contact resistances at the interfaces were obtained by applying a controlled current and measuring the resulting voltage, through the following scenarios: (1) over time for various applied testing currents, the resistance relaxation curves were measured at constant loads; (2) through voltage-current characteristics by means of a logarithmic sweeping current, the influence of the testing current on the electrical response of contacting rough surfaces was evaluated; and (3) for a given testing current, the electrical resistance through interfaces of different surface structures was measured under increasing compressive stresses. The experimental results show that the measured resistance depends closely on the measurement time, testing current, surface topology, and mechanical loading. At stresses from 0.03 to 1.18 MPa, the electrical resistance as a function of applied normal stress is found to follow a power law relation, the exponent of which is closely linked to the surface topology.

The sudden collapse of sinkholes in the Dead Sea area represents a serious threat to infrastructure in the area. The formation of these sinkholes has been shown to be directly correlated with the drop in the Dead Sea water level which is accompanied by a corresponding lowering of the groundwater level and permits the penetration of low-salinity groundwater into coastal areas. This water causes dissolution of the salt layers which results in the formation of subsurface voids that develop into collapse sinkholes. Various tools and measurement methods have been investigated in order to attempt to detect the formation of sinkholes, but to date, there is no method capable of providing early warning of possible collapse. This paper investigates the use of fiber-optic Brillouin optical time-domain reflectometry (BOTDR) or Brillouin optical time-domain analysis (BOTDA) for such detection. Brillouin optical time-domain reflectometry or analysis (BOTDR/A) is an optical measurement technique that provides distributed measurements of strain along tens of kilometers of conventional optical fibers, based on the Brillouin frequency shift of backscattered light. The rationale for this approach is that the formation of an underground cavity causes strains in the soil which can be detected using a fiber-optic cable buried at a shallow depth. A closed-form solution for the expected surface sinkhole-induced strain profile attributable to spherical voids in elastic-plastic soil is developed, validated, and evaluated against more realistic conditions. The model is then used to develop a procedure that can differentiate between signals induced by a sinkhole and signals caused by disturbances. The suggested procedure uses wavelet decomposition to filter out the disturbances and extract the sinkhole contributions. This procedure is evaluated with signals measured in the field during a 50-day period on which were superimposed theoretical strains based both on the closed-form model and finite-difference analysis of more realistic cases. This analysis showed that in the depth range investigated (up to 50 m), detection can be achieved while the subsurface cavities are still stable and early enough to enable the implementation of countermeasures.

A superelement-based virtual distortion method (VDM) was proposed to improve the efficiency of the finite element model (FEM) updating of large-scaled bridges by using static information. On one side, VDM was deemed as a new type of metamodel for static analysis of structures, i.e., a surrogate model of FEM for bridges. In this way, the efficiency of FEM updating could be improved dramatically because massive iterative calculations need to be implemented during the process of FEM updating. On the other side, the superelement was applied to reduce the size of the influence matrix which was the kernel of building VDM; therefore, it is effective to enhance the efficiency of generating the metamodel (VDM) of large-scale bridges. Finally, with the measured static deformation data, the effectiveness of the proposed method was demonstrated by updating the FEM of a practical bridge.

The objective of this research is to use grain-scale numerical simulations to analyze the evolution of stress anisotropy exhibited in wetted granular matter. Multiphysical particulate simulations of unsaturated granular materials were conducted to analyze how the interactions of contact force chains and liquid bridges affect macroscopic responses under various suction pressure and loading histories. To study how the formation and rupture of liquid bridges affect the mechanical responses of wetted granular materials, a series of suction-controlled triaxial tests were conducted with two grain assemblies, one composed of large particles of similar sizes, the other composed of a mixture of large particles with significant amount of fines. The results indicate that capillary stresses are anisotropic in both sets of specimens, and that stress anisotropy is more significant in granular assemblies filled with fine particles. A generalized tensorial Bishop’s coefficient is introduced to analyze the connections between microstructural attributes and macroscopic responses. Numerical simulations presented in this paper indicate that the principal values and directions of this Bishop’s coefficient tensor are path dependent.

Composites of piezoelectric materials are widely used in practical applications such as medical devices, nondestructive testing devices, and intelligent or smart structures. The study of mechanics and effective properties of piezocomposites is crucial to the design and development of this class of materials. In this paper, a micromechanics model is developed to determine the effective properties of piezoelectric fiber-reinforced composite materials with imperfect fiber-matrix interface bonding conditions. The micromechanics analysis is based on the periodic microfield micromechanics theory and the boundary element method (BEM). The imperfect bonding between the piezoelectric fibers and matrix is taken into account and represented by a spring-factor parameter. Selected numerical results are presented to show the influence of fiber-volume fraction, material properties of fiber and matrix, and interface bonding conditions on the effective properties of piezoelectric fiber-reinforced composites.

The successful development of structural damage prognosis (SDP) capability requires the further development and integration of many technology areas, such as hardware, software, modeling capabilities, and so forth. By combining the damage-sensitive feature extraction, the higher statistical moments of structural responses and the fuzzy c-means (FCM) clustering algorithm with the traditional time-series analysis-based SDP method, an integrated method is proposed for the SDP of a truss bridge model with end connector bolts under environmental and operational variability simulated in laboratory. The theoretical formulation of the integrated method is developed first. A six-bay truss bridge model is then designed and fabricated in laboratory for assessing the performance of the proposed method. Various damage cases are simulated by partially or completely loosening some end connecter bolts of the truss bridge under the environmental variability. The performance of the integrated SDP method is assessed based on the time-series acceleration response data measured at 16 ball nodes of the truss bridge model in each reference state and test state. The illustrated results show that the proposed integrated SDP method can effectively identify the damages of a truss bridge with end connector bolts. The introduction of higher statistical moments can provide a beneficial supplement to the traditional damage-sensitive indexes.

Different types of sensors in a structural health monitoring (SHM) system installed in a structure enable various types of structural responses to be measured. However, their distinct properties and limitations considerably complicate multisensing structural condition assessment. As a result, the information from these sensors is often used separately, and the potential advantage of multisensing information has not been used effectively. This paper first proposes a covariance-based multisensing (CBMS) damage detection method in the time domain in terms of a CBMS vector as a new damage index and a sensitivity study for damage detection. The proposed method has the merit of assimilating heterogeneous data and reducing the adverse effect of measurement noise. The CBMS damage detection method is then used in two stages for detecting damage location and severity consecutively. Numerical studies are finally performed to investigate the feasibility and accuracy of the proposed framework using an overhanging beam with two damage scenarios. The results show that the two-stage CBMS damage detection method improves the accuracy of damage detection and that the proposed method can be effectively used to combine multisensing information for better damage detection.

A theoretical investigation is presented for the dynamic interaction of a rigid circular disc and its surrounding space, which is a trimaterial transversely isotropic full-space, and the disc is undergoing a prescribed horizontal harmonic vibration. Because the equations of motion form a system of coupled partial differential equations in a cylindrical coordinate system, a system of two scalar potential functions are used to decouple the equations of motion. This approach results in a set of two separated partial differential equations, which may be transformed to some ordinary differential equations by using Fourier series along the angular coordinate and the Hankel integral transformations with respect to the radial coordinate in a cylindrical coordinate system. After solving the ordinary differential equations, the unknown functions are determined by satisfying relaxed boundary conditions, which themselves are transformed to a set of four coupled integral equations. These coupled integral equations are reduced to two coupled Fredholm-Volterra integral equations of the second kind, which are numerically solved in this paper. The proposed solutions are applied for a transversely isotropic half-space, and the results showed the conformation of the existing solutions.