Journal of Engineering Mechanics

Stress-strain response of optical fibers in direct tension is introduced in this article. The research involved direct tension tests of optical fibers and development of theoretical relationships describing the tensile behavior of the fibers. Two types of optical fibers, namely ribbon and standard single mode fiber (SMF-28) were included in the experimental and theoretical investigations. The impetus for the study was the need for the elastic properties of the optical fibers, such as the modulus of elasticity and the elastic limit for accurate interpretation of strains measured by optical fiber sensors. Ribbon fibers have proven to be robust for sensing applications in civil structures which prompted the research described herein. By employing the experimental data and by using the generalized Ramberg-Osgood law, it was possible to establish the theoretical stress-strain responses of the optical fibers. The experimental program allowed for distributed measurement of optical fiber strain by a Brillouin Optical Time Domain Analysis system (BOTDA). These results were compared with the direct measurement of the applied displacements at the fiber ends.

Interest in self-healing techniques that can enhance the performance of cementitious materials has been ever increasing over the past two decades. Alongside the experimental developments, a great deal of progress has been made on the development of numerical models for simulating the self-healing behavior. In spite of this, many models do not consider the coupled physical processes that govern the healing response. In addition, few are developed in a 3D setting that is necessary for many self-healing systems. This study aims to address this through the development of a new 3D coupled model for simulating self-healing cementitious materials. Key features of the model are a new embedded strong discontinuity hexahedral element that employs a damage-healing cohesive zone model to describe the mechanical behavior, a new approach for describing the dependence of the mechanical regain on healing agent transport based on a local crack filling function, and a generalized healing front model that is applicable to different healing agents. The performance of the model is demonstrated with a healing front study and experimental tests on self-healing cementitious specimens. The examples consider a vascular self-healing cementitious specimen that uses a sodium silicate solution as the healing agent and the autogenous healing of a cementitious specimen with and without crystalline admixtures. The results of the validations show that the model is able to reproduce the experimentally observed behavior with good accuracy.

In order to investigate the compaction behavior of tight shale and obtain the apparent preconsolidation stress and critical stress of brittle-ductile transition, uniaxial strain tests were conducted at various temperatures. This method utilizes a cold-stretched 45# carbon steel sleeve to restrain the lateral deformation of tight shale, which successfully simulates the rigid constraints of rock stratum and overcomes the challenging issues of measuring the apparent preconsolidation stress and brittle-ductile transition based on existing tests. The results show that (1) the test curve can be divided into three stages: the pore compaction stage, the elastic compaction stage, and the cataclastic flow stage; (2) there is no obvious relationship between the front in situ stress coefficient and temperature, whereas the middle and later in situ stress coefficients increase at elevated temperatures; and (3) the apparent preconsolidation stress and critical stress of brittle-ductile transition decrease as the temperature rises, as does the elliptical yield surface of tight shale. In addition, the thermal stress distribution law of rock units was thermodynamically analyzed. The average increase in axial thermal stress is nearly equivalent to the decrease in apparent preconsolidation stress at various temperatures, and thermal stress has a compensating effect on the axial load. A new method is provided for studying the compaction behavior of tight rock under various coupling conditions.

This paper studied the thermomechanical nonlinear stability analysis of simply supported geometrically imperfect porous functionally graded nanoplates (FG-nPs) resting on an elastic medium. The inverse trigonometric shear deformation theory was used in conjunction with the nonlocal strain gradient theory, which accounts for small-scale effects. The non-linear stability equations using the von Karman sense of the strain–displacement relation and generic imperfection function were derived for FG-nPs under thermomechanical loading conditions. The FG-nP was subjected to mechanical and thermal loading. An expression for the critical buckling load and temperature of a geometrically imperfect porous FG-nP was obtained. The impact of geometric imperfection, porosity inclusion, and geometric and boundary conditions on the nonlinear stability characteristics of FG-nPs was addressed thoroughly after validation of the superior accuracy of the derived expression.

Perforated unreinforced masonry (URM) walls are used in most existing masonry buildings as structural and nonstructural elements. Depending on the size and position, openings may detrimentally affect the stiffness and seismic capacity of URM walls. This research investigates the structural behavior of perforated URM walls with different opening sizes and proportions subjected to lateral loading using the discrete element method (DEM). In the applied modeling strategy, masonry walls are composed of rigid blocks, where their mechanical interactions are simulated via point-contact hypotheses. Once the numerical approach is validated, parametric analyses are performed to better understand the effect of different opening sizes and their aspect ratios on the failure mechanism and shear capacity of perforated URM walls. The results quantify the lateral load–carrying capacity and demonstrate its inverse relationship with the opening size. Furthermore, a slight influence of contact stiffness (varied from 10 to $120\mathrm{GPa}/\mathrm{m}$) on the ultimate lateral load in DEM-based simulations is noted. Finally, the obtained shear force capacities are compared against the strength prediction equations provided in current US standards. In most cases, the predictions of the US standard provide conservative values relative to the DEM results.

Performance-based engineering for natural hazards facilitates the design and appraisal of structures with rigorous evaluation of their uncertain structural behavior under potentially extreme stochastic loads expressed in terms of failure probabilities against stated criteria. As a result, efficient stochastic simulation schemes are central to computational frameworks that aim to estimate failure probabilities associated with multiple limit states using limited sample sets. In this work, a generalized stratified sampling scheme is proposed in which two phases of sampling are involved: the first is devoted to the generation of strata-wise samples and the estimation of strata probabilities, whereas the second phase aims at the estimation of strata-wise failure probabilities. Phase-I sampling enables the selection of a generalized stratification variable (i.e., not necessarily belonging to the input set of random variables) for which the probability distribution is not known a priori. To improve the efficiency, Markov Chain Monte Carlo Phase-I sampling is proposed when Monte Carlo simulation is deemed infeasible, and optimal Phase-II sampling is implemented based on user-specified target coefficients of variation for the limit states of interest. The expressions for these coefficients are derived with due regard to the sample correlations induced by the Markov chains and the uncertainty in the estimated strata probabilities. The proposed stochastic simulation scheme reaps the benefits of near-optimal stratified sampling for a broader choice of stratification variables in high-dimensional reliability problems with a mechanism to approximately control the accuracy of the estimators of multiple failure probabilities. The practicality and efficiency of the scheme are demonstrated using two examples involving the estimation of failure probabilities associated with highly nonlinear responses induced by wind and seismic excitations.

Reasonable simulation of stochastic fields is a key prerequisite for addressing stochastic mechanics problems with Monte Carlo simulation solver. This paper presents a novel method to simulate m-variate and n-dimensional ($m\mathrm{V}\text{-}n\mathrm{D}$) homogenous non-Gaussian fields according to the prescribed power spectral density matrix (PSDM) and first four moments. In the proposed method, the unified Hermite polynomial model (UHPM) is first extended to $m\mathrm{V}\text{-}n\mathrm{D}$ homogenous non-Gaussian fields. Then, based on the extended UHPM, a complete transformation model from the normalized non-Gaussian correlation function matrix (CFM) into the underlying Gaussian CFM with its applicable range is derived. In addition, two types of potential incompatibility of the transformation arising from the prescribed PSDM and first four moments are defined and neatly solved. Finally, a unified simulation framework for $m\mathrm{V}\text{-}n\mathrm{D}$ homogenous non-Gaussian fields is presented, where the fast Fourier transform technique can be embedded in both spectral representation method and Wiener-Khintchine transformation to speed up the simulation progress. Two numerical examples including the simulations of non-Gaussian wind fields and non-Gaussian fields of material properties are presented to demonstrate the effectiveness of the proposed method.