Levels Of Pre-compression Research Articles

Unreinforced masonry walls under two-way out-of-plane bending: Comparison of FE model predictions with analytical models

Four existing design guidelines based on the virtual work method have been compared to predict the out-of-plane (OOP) bending capacities of unreinforced masonry (URM) walls to one-way and two-way actions. The two-way OOP bending design equation as per the Australian masonry standard (AS 3700) and the three subsequent models developed by other researchers are based on specific wall geometries, support conditions, dimensions and strengths of masonry units and mortar. Due to limited experimental data available on URM walls under two-way bending, it is difficult to carry out a detailed investigation of the performance of these two-way design models against all the design parameters. This paper applies the finite element (FE) modelling technique to generate data and extensively investigate the influence of support conditions, pre-compression and aspect ratio on the two-way OOP bending capacity of URM walls. An explicit FE model previously developed by the authors where masonry is macroscopically modelled as an anisotropic composite material, is used for this purpose. Performances of three key parameters (i.e., moment capacity under horizontal bending and diagonal bending and torsional shear strength of masonry) that differentiate the four design models are investigated against varying levels of vertical pre-compression and flexural tensile strength of masonry. The OOP bending capacity predicted by the FE model is compared with the predictions of the design models. Results show that three out of four design models generally overpredict the two-way OOP bending capacities of square and taller walls. Further refinement to the AS 3700 design equation has been proposed in a modified model. The outcomes of this research will help to improve the design of masonry walls under OOP bending.

ТРІЩИНОСТІЙКІСТЬ І МІЦНІСТЬ БЕТОНІВ ПРИ РОЗТЯЗІ ПІСЛЯ ПОПЕРЕДНЬОГО ОБТИСНЕННЯ НА ОСНОВІ ЙОГО СТРУКТУРНОЇ ТЕОРІЇ

The results of theoretical researches on the basis of the structural theory of crack resistance and tensile strength of concrete after preliminary long compression of various intensity are stated. For the structural scheme in the form of a checkerboard arrangement of the aggregate in the cement matrix, the stress-strain state of concrete during unloading or subsequent stretching to stresses when the cement stone has exhausted its bearing capacity (cracks are formed) and destruction of concrete as a whole is considered.Analytical dependences of definition of the specified parameters for various levels of precompression are resulted.

Experimental Research and Numerical Analysis of CFRP Retrofitted Masonry Triplets under Shear Loading.

This paper presents an experimental and numerical study into the shear response of brick masonry triplet prisms under different levels of precompression, as well as samples reinforced with carbon fiber-reinforced polymer (CFRP) strips. Masonry triplets were constructed with two different mortar mix ratios (1:1:3 and 1:1:5). In this study, finite element models for the analysis of shear triplets are developed using detailed micro-modelling (DMM) approach and validated with the experimental data. The failure mechanisms observed in the masonry triplets were simulated using a coupled XFEM-cohesive behaviour approach in ABAQUS finite element software. The nonlinear behaviour of mortar and brick was simulated using the concrete damaged plasticity (CDP) constitutive laws. The cohesive element with zero thicknesses was employed to simulate the behaviour of the unit–mortar interfaces. The extended finite element method (XFEM) was employed to simulate the crack propagation in the mortar layer without an initial definition of crack location. CFRP strips were simulated by 3D shell elements and connected to masonry elements by an interface model. The changes in failure mechanism and shear strength are calculated for varying types of mortar and fiber orientation of CFRP composite. Based on this study, it was concluded that the ultimate shear strength of masonry triplets is increased due to the external bonding of CFRP strips. The performance of masonry specimens strengthened with CFRP strips is assessed in terms of gain in shear strength and post-peak behaviour for all configurations and types of mortar considered. The comparison of FE and experimental results proved that the models have the potential to be used in practice to accurately predict the shear strength and reflect damage progression in unreinforced and CFRP-reinforced masonry triplets under in-plane loading, including the debonding of the CFRP reinforcement. Additionally, XFEM was found to be a powerful technique to be used for the location of crack initiation and crack propagation in the mortar layer.

Prestrain-induced bandgap tuning in 3D-printed tensegrity-inspired lattice structures

In this letter, we provide experimental evidence of bandgap tunability with global prestrain in additively-manufactured tensegrity-inspired lattice structures. These lattices are extremely lightweight and designed to exhibit a nonlinear compressive response that mimics that of a tensegrity structure. We fabricate them out of a stiff polymer but, owing to their peculiar design, they are compliant and remain elastic up to high levels of precompression. In turn, unlike tunable metamaterials made of soft polymers, the response of our lattices is not dominated by damping. We perform experiments on a one-dimensional lattice subject to longitudinal wave excitation and varying levels of static longitudinal precompression, and observe continuous tuning of both the wave speed and the location and width of the lowest-frequency bandgap.

Effects of process parameters on tablet critical quality attributes in continuous direct compression: a case study of integrating data-driven statistical models and mechanistic compaction models

Continuous manufacturing of oral-dosage drug products is increasing the need for rigorous process understanding both from a process design and control perspective. The purpose of this study is to develop a methodology that analyzes the effects of upstream process parameters on continuous tablet compaction and then correlates associated upstream variables to the final tablet attributes (e.g. relative density and hardness). The impact of three process parameters (system throughput, blender speed, and compaction force) on tablet attributes is investigated using a full factorial experimental design. As expected, the compaction force was found to be the most significant process parameter. However, importantly, throughput was discovered to have a non-negligible impact which was previously unaccounted for. This impact is proposed to be related to differing levels of powder pre-compression. An empirical model for this relationship is regressed and incorporated into a flowsheet model. The flowsheet model is then used to develop an in silico design space which is compared favorably to that built from experiments. Moreover, in the future, the in silico design space based on the validated flowsheet model can provide better manufacturing flexibility and make control strategy development simpler.

Shear behavior of cement stabilized rammed earth assemblages

Adoption of rammed earth technology in building construction has witnessed a surge during the recent times and hence it is essential to understand the mechanical behavior of rammed earth under various loading conditions. In this study, in-plane shear behavior of cement stabilized rammed earth (CSRE) is assessed by conducting two types of tests namely (i) Direct shear test of CSRE causing shearing along two adjacent rammed earth layers similar to triplet shear test of masonry bed joints which is henceforth called as triplet test and (ii) Diagonal tension (shear) test of CSRE panels. Three types of bonding techniques between the rammed layers of CSRE are explored in this study, namely, (i) making conical dents which act as shear connectors between rammed earth layers, (ii) applying a coat of fresh cement slurry along the interfaces of rammed layers and (iii) combination of dents and fresh cement slurry and their influence in enhancing the interface shear strength of CSRE is assessed. The bonding techniques are adopted for both, triplet specimens and diagonal panels. Further, the effect of compressive stress normal to the rammed earth layers on the interface shear behavior of triplet specimens is examined. The different levels of normal pre-compression considered are 0.05 MPa, 0.3 MPa and 0.9 MPa. The triplet and diagonal tension (shear) tests on CSRE assemblages are conducted under both, dry and wet conditions. Shear behavior is assessed in terms of shear strength, shear modulus, strain at peak stress, post-peak behavior and failure patterns. Among the different bonding techniques assessed in this study, CSRE specimens with a coat of fresh cement slurry exhibited higher shear strength in comparison to specimens with other types of bonding techniques or no bonding technique. Further, CSRE triplet specimens exhibited steady increase in interface shear strength with increase in normal pre-compression, both in dry and wet conditions. Also, shear strength of CSRE was found to be lower in wet condition, in comparison to shear strength in dry condition, for all the bonding techniques. A Mohr-Coulomb failure envelope was developed for CSRE under wet and dry conditions based on the results of triplet tests conducted in this study. Overall, the study aims to understand the efficacy of the proposed bonding techniques in enhancing the shear strength of CSRE and the influence of normal pre-compression and moisture content on shear behavior of CSRE.

Salt Attack Effects on the Shear Behavior of Masonry: Preliminary Results of an Experimental Campaign

In this paper, preliminary results of an experimental campaign carried out on masonry triplets subjected to weathering cycles in salt solution are presented. Weathering cycles are constituted of a wetting phase by capillary rise of a sodium chloride solution and a drying phase in oven. At the end of the last conditioning cycle, triplets are shear tested in order to quantitatively assess the effects of salts on their mechanical behaviour, in particular on the Mohr-Coulomb relationship. To this aim, three levels of pre-compression are applied during the shear tests. For comparison purposes, unconditioned triplets are also tested under the same loading conditions.

Influence of pre-compression on the ductility of AA6xxx aluminium alloys

Reversed loading experiments were conducted to study the influence of pre-compression on the ductility of three aluminium alloys. Diabolo-shaped specimens were machined from extruded profiles along the transverse direction, and heat treated to peak strength (T6 temper). The specimens were subjected to five different levels of pre-compression (0, 10, 20, 30, 40%), i.e., the specimens were first compressed to a prescribed strain and then pulled to fracture in tension. Using a laser-based measuring system, the minimum diameter in the extrusion direction and thickness direction were continuously measured during the tests until fracture. The three aluminium alloys AA6060, AA6082.25 and AA6082.50 had different grain structure and texture. The AA6060 and AA6082.50 alloys had recrystallized grain structure with equi-axed grains and large elongated grains, respectively. The AA6082.25 alloy had a non-recrystallized, fibrous grain structure. It was found that pre-compression has a marked influence on the ductility of the aluminium alloys, which depends on the microstructure and strength of the alloy. Using the compressed configuration as the reference configuration, the relative failure strain could be calculated. For the AA6060 alloy, the relative failure strain increased for increasing pre-compression, and was approximately doubled for 40% pre-compression compared to pure tension. For the AA6082.25 alloy, a slight increase in the relative failure strain was observed for increasing pre-compression, while for the AA6082.50 alloy the relative failure strain was low and approximately constant for different levels of pre-compression.

The dependence of Weibull parameters on preloads and its implication on brittle fracture probability prediction using a local criterion

A linear applicable relationship is proposed to improve the cleavage failure probability prediction of preloaded specimens using the modified Beremin model. Maximum stress triaxility factor shows fracture load independency for enough high loads and a good sensitivity to crack tip stress changes due to residual stresses and preloads. Hence, it is selected as representative of crack tip stress state. Using recalibration for six experimental datasets of CT including different levels of pretension and precompression shows that only reference stress changes with changing the preload level and type. A linear relationship is derived to modify the reference stress for the preloaded cases as a function of maximum stress triaxiality factor which can be easily computed from the FE analysis. Using the proposed relationship simply leads to a considerable improvement for failure predictions in all cases with different geometries, materials and preloading conditions.

Diagonal compression tests on stone masonry panels: experimental results and analytical interpretations

The models available in literature and codes for the strength evaluation of masonry piers subjected to in-plane lateral forces may provide very different predictions depending on the degree of anisotropy of the examined type of masonry; furthermore, the experimental evaluation of the parameters required by them may pose some problems, especially in those masonries (as the historical ones) in which mortar joints are not regularly arranged. The paper presents the results of a set of diagonal compression tests on irregular stone masonry panels with different levels of lateral pre-compression. The test campaign allowed to provide experimental data on the mechanical behaviour of traditional masonry made by rubble stones and air lime mortar; we found very low values of diagonal compressive strength. Afterwards, we interpreted these results referring to two widely accepted literature models. This way, it was possible to obtain further mechanical parameters (diagonal tensile strength, cohesion and friction coefficient) of the masonry and to discuss the applicability and reliability of these latter models.

Diagonal compression tests on stone masonry panels: experimental results and analytical interpretations

The models available in literature and codes for the strength evaluation of masonry piers subjected to in-plane lateral forces may provide very different predictions depending on the degree of anisotropy of the examined type of masonry; furthermore, the experimental evaluation of the parameters required by them may pose some problems, especially in those masonries (as the historical ones) in which mortar joints are not regularly arranged. The paper presents the results of a set of diagonal compression tests on irregular stone masonry panels with different levels of lateral pre-compression. The test campaign allowed to provide experimental data on the mechanical behaviour of traditional masonry made by rubble stones and air lime mortar; we found very low values of diagonal compressive strength. Afterwards, we interpreted these results referring to two widely accepted literature models. This way, it was possible to obtain further mechanical parameters (diagonal tensile strength, cohesion and friction coefficient) of the masonry and to discuss the applicability and reliability of these latter models.

Masonry elements with multi-layer bed joints: Behaviour under monotonic and static-cyclic shear

The goal of the research project presented here is to investigate the effect of multi-layer bed joints on the shear behaviour of unreinforced masonry elements subjected to cyclic actions, and to assess the shear mechanical characteristics of multi-layer bed joints. Such joints consist of a core soft layer protected with two thin extruded elastomer membranes, which in turn are placed in a bed mortar joint. The extruded elastomer membranes are employed to prevent and/or limit the deterioration of the core soft layer during the cycling action, which has been observed in previous investigations. Five different core soft layer materials, namely rubber granulate, cork-rubber granulate, cork, bitumen and polyvinylchloride were employed. A total of 57 clay block masonry triplets were subjected to both monotonic and static-cyclic loading under varying levels of pre-compression.The shear behaviour of the specimens was greatly influenced by the applied pre-compression level and by the loading speed. Increasing the level of pre-compression leads to the higher values of shear strength. The same is true for increasing loading speed. Further, for the prediction of the failure shear force a Mohr–Coulomb failure criterion with zero cohesion can be applied. Finally, experimental findings showed that extruded elastomer layers largely fulfilled the intended protective role.The results indicate that multi-layer bed joints with adequate material properties have the potential to change the typical brittle shear response of unreinforced masonry walls to a more desirable quasi-ductile behaviour. Moreover, based on the observed hysteretic behaviour, considerable energy dissipation could be expected in masonry structures with multi-layer bed joints. Such behaviour would be desirable for enhanced seismic performance.

Attenuation of short stress pulses in strongly nonlinear dissipative metamaterial

Attenuation of short stress pulses under different levels of precompression was investigated in a one-dimensional strongly nonlinear discrete metamaterial assembled using alternating steel disks and toroidal Nitrile O-rings. The results were compared with the numerical modeling. A double power-law is used to describe the nonlinear interaction between the disks due to the compression of rubber O-rings. The dispersion behavior caused by the periodic arrangement of elements is contributing to the attenuation of pulse, but could not explain the experimental observations. It was explained by taking into account the nonlinear viscous behavior of O-rings. The numerical simulations were able to predict the dependence of the signal speed on the precompression force, a significant decrease of the pulse width with the precompression and the attenuation of the leading positive pulse, the latter of major significance in the protection against impact. This strongly nonlinear dissipative metamaterial has a potential for attenuation of dynamic loading and allows an enhanced tunability of signal speed and degree of attenuation.

Masonry wallettes with a soft layer bed joint: Behaviour under static-cyclic loading

The main goal of the research project presented herein is to investigate the effect of damp proof courses and soft layer wall bearings on the seismic shear behaviour of unreinforced masonry walls subjected to cyclic actions, in particular how the lateral load resisting mechanism of the wall is influenced thereby. Unreinforced clay block masonry wallettes with soft layers of three different thicknesses (3, 5 and 10mm), and two different materials (rubber granulate and extruded elastomer) were subjected to static-cyclic shear under varying levels of pre-compression. The soft layer was placed either in the first bed joint of the wallette or in the interface joint between the wallette and the underlying concrete slab.Soft layers were beneficial when the tested wallettes failed in sliding. Such sliding failure was observed for wallettes with rubber granulate soft layers placed in the first bed joint. This resulted in quasi-ductile behaviour of the wallettes, with a displacement and energy dissipation capacities significantly larger than those of the conventional unreinforced masonry control specimens. Furthermore, wallettes failing by sliding along the joint containing the soft-layer showed considerably less damage compared to wallettes that exhibited other failure modes at the same displacements levels. The progressive deterioration of the rubber granulate layers lead to a decreasing shear strength at larger horizontal displacements. Wallettes with extruded elastomer layers and wallettes with layers placed in the interface joint did not fail in sliding. Apart from slightly increased energy dissipation, no beneficial effect on the shear force–deformation behaviour of the wallettes could be observed in these tests.The results indicate that soft-layers with adequate material properties placed in the first bed joint have the potential to change the typical brittle shear response of unreinforced masonry walls to a more desirable quasi-ductile behaviour.

Effect of twinning and detwinning on the spring-back and shift of neutral layer in AZ31 magnesium alloy sheets during V-bend

Abstract In order to investigate the effect of twinning and detwinning on springback and shift of neutral layer, pre-stretch (PRS) of 3% and 5%, and pre-compression (PRC) of 1%, 3% and 5% along rolling direction was conducted in an AZ31 magnesium alloy sheets at room temperature. 90° V-bending tests were performed at 423 K with various pre-strained samples. The results show that springback increases after both PRS and PRC deformation. The coefficient of neutral layer ( k-value ) decreases with the increase of the levels of PRS and PRC. After PRS, twinning in an inner compressive region during bending is restrained and tension–compression asymmetry decreased. After PRC, detwinning occurs which implying that it dominates the deformation in the outer tensile region, while in inner compressive region twinning still governs the deformation. Twinning and detwinning leads to a decrease of asymmetry mechanism between the outer and inner layers. The decrease of asymmetry mechanism after PRT and PRC results in a drop of the neutral layer offset.

Propagation of short stress pulses in discrete strongly nonlinear tunable metamaterials.

The propagation of short pulses with wavelength comparable to the size of a unit cell has been studied in a one-dimensional discrete metamaterial composed of steel discs alternating with toroidal nitrile O-rings under different levels of precompression using experiments, numerical simulations and theoretical analysis. This strongly nonlinear metamaterial is more tunable than granular chains composed of linear elastic spherical particles and has better potential for attenuation of dynamic loads. A double power-law relationship for compressed O-rings was found to describe adequately their quasi-static and dynamic behaviour with significantly different elastic moduli. It is demonstrated that the double power-law metamaterial investigated allows a dramatic increase in sound speed and acoustic impedance of three to four times using a moderate force.

Path dependent high strain, strain-rate deformation of polymer toroidal elements

The dynamic behavior of toroidal elements (o-rings) is investigated at the range of global engineering strains up to 0.7 and strain rates about 100 s−1. It was observed that the corresponding average dynamic stiffness of rubber toroidal elements increases up to 3 times in comparison with their quasistatic compression. The viscoelastic dynamic model using linear strain-rate dependence and Hertz damped model did not satisfactory agree with experimental data in investigated range of strains and strain-rates. In order to reflect experimental results, a modified viscoelastic model with power-law strain-rate dependence was proposed. Path dependent deformation of o-rings with different levels of pre-compression was investigated under dynamic loading conditions. It was found that dynamic response of pre-compressed o-rings at the initial strain range of 0.04–0.25 is similar to the behavior of uncompressed o-rings, but further increasing pre-compression to 0.4 and 0.5 results in different force-strain curves demonstrating memory effect. This phenomenon is explained using a model incorporating dependence of dynamic force on initial pre-compression introducing critical level of dynamic strain, after which memory of initial pre-compression fades. This model predicts that force history of weakly compressed o-rings (initial strain 4%) on the stage of loading represents an envelope for all other data in agreement with experiments. In all cases, the dynamic behavior was characterized by stiffer force-displacement curves in comparison with quasistatic compression of o-rings.

In-plane shear tests on masonry panels with plaster: correlation of structural damage and damage on artistic assets

In the paper, possibilities of correlation of structural damage and damage of attached artistic assets on multi-leaf stone masonry walls by means of destructive and non-destructive testing (NDT) methods are investigated. Results from two testing campaigns carried out at the University of Genoa and University of Ljubljana are briefly presented. In particular, diagonal tests with different levels of pre-compression and shear tests under different boundary conditions were carried out in Genoa and Ljubljana, respectively. During the tests, different damage limit states (DL) of both masonry walls (SE) and attached plasters (AA) were investigated by means of NDT’s. Two different types of rubble stone masonry were considered (uncoursed random rubble vs. coursed squared rubble). For AA the results are presented in relative form in dependence from the DL of SE. Obtained results are influenced both by the type of test and tested masonry. Results of NDT revealed strong potential and a need for data fusing of both investigated methods for the evaluation of the state of the degradation behind the plaster.

Cold pre-compression of membrane electrode assembly for PEM fuel cells

Direct compression from the land structure of bipolar plate in a PEM fuel cell is considered as an important factor for the higher performance under the land than under the channel areas. Therefore the objective of this study is to determine if a cold pre-compression treatment on the whole membrane electrode assembly (MEA) area may have a significant positive effect on the overall performance of the cell. Five different levels of cold pre-compression have been applied and the experimental results show that the overall performance of the cell first increases with the level of compression to a maximum, and then decreases. These results clearly show that cold pre-compression of the MEA can significantly enhance the performance of the entire cell and there exists an optimal level of compression. Results of electrochemical impedance spectroscopy (EIS) show that the cold pre-compression results in a significant reduction in charge transfer resistance, especially in the high current density region. Further study by the cyclic voltammetry (CV) shows that the electro-chemical area (ECA) is changed with the different cold pre-compressed MEAs and there exists an optimal compression that results in the maximum ECA.

Robustness as a criterion for use of hollow clay masonry units in seismic zones: an attempt to propose the measure

In order to provide adequate seismic behavior of masonry walls, local brittle failure of masonry units in the most stressed zones of structural walls should be prevented. Although robust behavior is required by the code, no specifications are given regarding the criteria to fulfill this requirement. To propose such criteria, a series of 28 masonry walls, built with six different types of hollow clay masonry units, currently available on the market, have been tested by subjecting them to cyclic lateral load at two levels of constant precompression. Besides, the strength characteristics of the units, like compressive strength orthogonal and parallel to the bed joints and tensile and shear strength of the units have been determined by standardized and specifically designed testing procedures. By correlating the parameters of seismic resistance of the walls and strength characteristics of the units, no specific indicator for robustness could have been determined on the basis of the mechanical characteristics of the tested units. It has been found that in all cases the level of precompression, i.e. the ratio between the compressive stresses in the walls and the compressive strength of masonry, represents the governing parameter.