Strengthening of three-leaf stone masonry walls: an experimental research

The paper summarizes the results of an experimental research carried out on three-leaf masonry walls of typical granite stone constructions from the North of Portugal. The research aimed at studying the behaviour under compression of this wall typology, as well as the improvements introduced by common strengthening techniques applied for the structural rehabilitation of masonry heritage buildings. Ten masonry specimens were tested, plain or strengthened by transversal tying of the external leaves, with GFRP bars, or/and by injection of the inner leaf, with a lime-based grout. The results obtained showed that these strengthening techniques were successful in increasing the compressive strength of the walls and in improving their behaviour under compressive loads.     

Fluid particle group reaction model and experimental verification

Based on a shrinking core model (SCM) that takes into account the particle size distribution (PSD) and product characteristics, a model is established to simulate the reaction process of the fluid particle group. In this model, a total reaction rate coefficient k_m is defined to describe the reaction behavior. Simulation results show that the particle group conversion is determined by k_m, the PSD pattern, and reaction condition. The dissolution characteristics of five kinds of limestone from different places in China are studied by using the pH-stat method in a stirred tank at different temperatures (30–60 °C), pH values (4.6–6.2) and PSD. By choosing proper k_m, the model can well describe the limestone dissolution process in acid with different reaction conditions. For different reaction conditions, the average total reaction coefficient can be obtained via iteration. Moreover, k_m can be deduced numerically at a given reaction time, which we propose a new way of obtaining the reaction parameter from experiment.     

多孔材料性能模型研究2:实验验证

在三维网状多孔材料"八面体结构模型"及其系列基本物理、力学性能相关数理模型和表征方式基础上,本文对传导和拉伸等若干性能指标的数理关系验证进行了综述。重点讨论了数理关系的实践性、修正系数的合理性、对计算结果的影响、对应致密体的许用应力取值和塑性指数取值等问题。按照这种数理关系,通过多孔产品孔率等基本参量即可计算其电阻率等性能指标,实验结果证明了其可行性。本方法可以优越于有限元等复杂计算。     

Solute-homogenization model and its experimental verification in Mg-Gd-based alloys

Composition homogenization in solid solution is important for industrial alloys. In the present work, a solute homogenization model is proposed based on the chemical short-range-order tendency in Mg-Gd-based alloys. After a calculation using the cluster-plus-glue-atom model, the stable Mg-Gd structural unit is derived, [Gd-Mg₁₂]Mg₆, where one solute Gd is nearest-neighbored with twelve Mg atoms to form the characteristic hcp cluster [Gd-Mg₁₂] and this cluster is matched with six Mg glue atoms. Such a local unit is then mixed with [Mg-Mg₁₂]Mg₃, the stable unit for pure Mg. Assuming that the Gd-containing units are arranged in fcc- or bcc-like lattice points and the Mg units in their octahedral interstices, three proportions between the two units are obtained, 1:1, 2:3, and 1:3, which constitute three solute homogenization modes. The prevailing Mg-Gd-based alloys are consequently classified into three groups, respectively exemplified by GW103 K (Mg-10Gd-3Y-0.4Zr, wt%), GW83 K (Mg-8Gd-3Y-0.4Zr), and GW63 K (Mg-6Gd-3Y-0.4Zr). Mg-Gd-Y-Zr alloys were designed following the model (where Y and Zr were also added in substitution for Gd) and prepared by permanent-mould casting. According to their mechanical properties, the 1:3 alloy (Mg-5.9Gd-1.6Y-0.4Zr) shows the best comprehensive properties (ultimate tensile strength 305 MPa, yield strength 186 MPa, elongation 9.0%) in solution plus ageing state.     

A new model of shear creep and its experimental verification

Mechanics of Time-Dependent Materials - Creep exerts a significant role in rock engineering safety. In engineering practice, it is helpful to develop a mathematical model representing rock creep...     

A threshold fatigue crack closure model: Part II – experimental verification

Predictions from an analytical model that considers contributions and interactions between plasticity, roughness, and oxide induced crack closure are presented and compared with experimental data. The analytical model is shown to correctly predict the combined influences of crack roughness, oxide debris, and plasticity in the near‐threshold regime. Furthermore, analytical results indicate closure mechanisms interact in a non‐linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism.     

Numerical and experimental verification of a damping model used in DEM

This paper presents a study on the damping ratio \(({\upbeta })\) used in discrete element simulations. Physical experiments are performed by dropping particles from a predetermined height. Two kinds of granular particles, aluminum and steel spheres, are used. The size of these particles are the same. The process of particle depositing under gravity is simulated using the discrete element method. The experimental observation is compared with the numerical result to identify the appropriate \({\upbeta }\). The result indicates that the appropriate damping ratio used in discrete element simulations is between 0.2 and 0.3 %. Various \({\upbeta }\) are then used in the numerical simulations to study the effect of \({\upbeta }\) on the dropping process. The final height of the sample relates to \({\upbeta }\) and the drop height. The effect of \({\upbeta }\) is more profound for small drop height. For greater drop height, the effect of \({\upbeta }\) is negligible.     

Homogenized limit analysis of FRP-reinforced masonry walls out-of-plane loaded

A three-dimensional (3D) homogenized limit analysis model for the determination of collapse loads of out-of-plane loaded FRP reinforced masonry walls is presented. Homogenization is performed on unreinforced masonry, whereas strips are applied at a structural level on the already homogenized material. Unreinforced masonry strength domain is obtained by means of a compatible approach in which bricks are supposed infinitely resistant and joints are reduced to interfaces with frictional-cohesive behavior and associated flow rule. A sub-class of elementary deformation modes is a-priori chosen in the representative volume element (RVE), mimicking typical failures due to joints cracking and crushing. Masonry strength domains are obtained equating power dissipated in the heterogeneous model with power dissipated in a fictitious homogeneous macroscopic plate. Afterwards, an upper bound FE limit analysis code is implemented to study entire unreinforced and FRP reinforced walls out-of-plane loaded. For unreinforced masonry, rigid infinitely resistant wedge-shaped 3D elements are used. The utilization of 3D elements is necessary to simulate the flexural strength increase induced by the introduction of FRP strips with negligible thickness, which are modeled by means of triangular rigid elements. FRP strips contribution is taken into account assuming that masonry and FRP layers interact by means of interfacial tangential actions. Internal power dissipation is possible at the interfaces between wedge adjoining elements (masonry failure), at the interfaces between triangular FRP and wedge masonry elements (delamination) and between triangular FRP adjoining elements (FRP failure). Two different structural examples are presented to validate the numerical model, namely a FRP reinforced masonry wall in cylindrical flexion and a set of masonry walls with openings in two-way bending. Results obtained with the model proposed fit well both experimental and numerical data available for all the cases analyzed, meaning that the procedure proposed can be used in building practice.     

High-power CMUTs: design and experimental verification

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with -28 dBc second harmonic at the surface of the array.     

Compressive response of Kevlar-epoxy composites: experimental verification

The initial misalignment of Kevlar fibres in Kevlar-epoxy composites is quantitatively investigated. This misalignment has been found to be one of the most important factors for determining the compressive response of these composites. A theoretical model, which considers initial fibre misalignment and assumes that the compressive response of Kevlar-epoxy composites is dominated by kink band failure, is in good agreement with experimental results. In addition, photomicrographs of the failure surfaces suggest that kink band formation is the predominant failure mode in this composite system.     

Fringing-field effect in liquid-crystal beam-steering devices: an approximate analytical model

An approximate analytical model was developed that links the fringing-field broadening of the phase profile of a liquid-crystal (LC) beam-steering device, and the resulting diffraction efficiency, to the physical parameters of the device including the cell thickness as well as the dielectric, optical, and geometrical constants of the device. The analysis includes a full solution of the Laplace equation for the LC device in which the broadening of the initial voltage profile into an effective voltage-drop profile, due to the fringing-field effect, is derived. It is shown that within the linear approximation used, the broadening of the phase profile is identical to the broadening of the effective voltage profile in the presence of the fringing field. On the basis of this model, the resulting broadening kernel of the phase profile is found to be proportional to the LC cell thickness. These results are found to be in an excellent agreement with high-precision computer simulations performed on the LC beam-steering structure, thereby validating this approximate linear model.     

Oscillator phase noise: systematic construction of an analytical model encompassing nonlinearity

This paper offers a derivation of phase noise in oscillators resulting in a closed-form analytic formula that is both general and convenient to use. This model provides a transparent connection between oscillator phase noise and the fundamental device physics and noise processes. The derivation accommodates noise and nonlinearity in both the resonator and feedback circuit, and includes the effects of environmental disturbances. The analysis clearly shows the mechanism by which both resonator noise and electronics noise manifest as phase noise, and directly links the manifestation of phase noise to specific sources of noise, nonlinearity, and external disturbances. This model sets a new precedent, in that detailed knowledge of component-level performance can be used to predict oscillator phase noise without the use of empirical fitting parameters.     

Theory and experimental verification of a model for high gradient magnetic separation.

A mathematical model of high gradient magnetic separation (HGMS) is presented, along with data on a characterized and sized feed material. The data are fitted to the model which uses elliptical co-ordinates to approximate the ribbon-like nature of the fibers. Magnetic force terms are developed for both paramagnetic and ferromagnetic particles in the vicinity of idealized matrix fibers which can either be magnetically saturated or unsaturated. The fluid flow is simulated by superimposing a boundary layer upon the solution for potential flow thus extending the range of validity to low Reynolds Numbers. Single particle trajectories are calculated in a piecewise linear manner by considering the force balance of magnetic, hydrodynamic, gravitational and inertial forces over each increment of the trajectory. By taking orientation of the fiber with respect to the field and flow direction into account, loading can be allowed for by assuming elliptical deposits. Experimental data were generated using high grade hematite prepared in ten separate size fractions. Correlation with the model is generally fairly good except for large particles where mechanical entrapment dominates the process. Considerable discussion of the results is included by analysing the physical concepts upon which the model is based. The validity of various assumptions pertinent to HGMS modeling is tested.     

Heterogeneous upper-bound finite element limit analysis of masonry walls out-of-plane loaded

A heterogeneous approach for FE upper bound limit analyses of out-of-plane loaded masonry panels is presented. Under the assumption of associated plasticity for the constituent materials, mortar joints are reduced to interfaces with a Mohr–Coulomb failure criterion with tension cut-off and cap in compression, whereas for bricks both limited and unlimited strength are taken into account. At each interface, plastic dissipation can occur as a combination of out-of-plane shear, bending and torsion. In order to test the reliability of the model proposed, several examples of dry-joint panels out-of-plane loaded tested at the University of Calabria (Italy) are discussed. Numerical results are compared with experimental data for three different series of walls at different values of the in-plane compressive vertical loads applied. The comparisons show that reliable predictions of both collapse loads and failure mechanisms can be obtained by means of the numerical procedure employed.     

Non-smooth mode I fracture of fibre-reinforced composites: an experimental, numerical and analytical study

We present a novel approach to analyse the fracture of fibre-reinforced composites. Experimental results on mode I fracture of glass fibre and carbon fibre unidirectional laminates presented here and published by others in the open literature formed the basis for the analytical and numerical results presented. When details of the external loading rate are explicitly accounted for, a new picture of fracture emerges, which encompasses the possibility for non-smooth crack growth and the necessity to relax the use of a critical strain energy release rate as a criterion for crack advancement. Results predicted by adopting the analytical model presented here are seen to capture a wide variety of fracture responses that have been observed previously.     

Design for prestressing tendons layout in stiffening walls via an experimental technique

The present paper describes the implementation of an experimental technique that allows one to optimise the efficiency of layout design for ropes, tendons, etc in prestressed structures. An application with a certain grade of complexity has been developed, i.e. a stiffening partition wall in hollow sections, with a central opening. This structural typology is normally used in bridges.