Stress-displacement Relationships Research Articles

Exploring the relationship between embedment behaviour in cross-laminated timber and its layers: An experimental study

Cross-laminated timber (CLT) has become popular as a construction material and the design of CLT connections is typically based on empirical equations with an average embedment strength of the wood-based product. The experimental study presented herein aims at enhancing the understanding of the relationship between the CLT embedment behaviour and the single layers it is composed of. Non-linear embedment stress–displacement relationships were measured in 244 full-hole and half-hole embedment tests on structural timber boards as well as on CLT produced of parts of the same boards. Three different fasteners, namely a steel dowel with a diameter of 12 mm and the threaded part of screws with a nominal diameter of 6.5 mm and 10 mm, were studied. The design of the experiments allowed to validate a simple mechanical model with parallel springs for the CLT, which showed good agreement with experiments. In addition to a validation of the non-linear spring model, this unique experimental data-set is further exploited in a comparison with empirical equations and for the derivation of a non-linear embedment ratio, between parallel and perpendicular to the grain embedment stresses, over the displacement. Knowledge of the relationship between the non-linear embedment behaviour of the single layers and the CLT element can be further exploited in the engineering design and numerical modelling of CLT connections with dowel-type fasteners.

Large-scale field test on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking: A case study

With the rapid advancement of urbanization, research on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking has a high engineering application value. In this study, a large-scale field direct shear test apparatus and the corresponding test method were designed and proposed to investigate the shear properties of the concrete-silty soil interfaces in pipe jacking. A series of in-situ direct shear tests were conducted at five normal stress levels under four interface contact conditions. The results show that the shear stress-displacement relationships of the interfaces conform to the typical elastic-plastic constitutive relations, the shear strength-normal stress relationships of the interfaces can be well fitted by the Mohr-Coulomb failure criterion, and the interface shear properties are influenced by soil properties, normal stress, lubrication conditions and stagnation time. Without lubrication, the concrete-silty soil interfaces have a shear stress-displacement curve shape similar to that of the test results obtained from indoor direct shear tests, but their ductility and shear strengths are lower. Under lubrication conditions, the paraffin wax and thixotropic slurry can both effectively diminish the interface friction angle, thereby reducing the concrete-silty clay interface friction coefficient by 62.8% and 72.2% and reducing the concrete-silty sand interface friction coefficient by 60.8% and 59.2%, respectively. Based on the macroscopic and microscopic images observed in the tests, the lubrication mechanisms of the lubricants were analysed. The research results can provide a reference for the prediction of jacking force and construction disturbance in adjacent pipe jacking projects.

Integration of rock joint roughness into the Mohr-Coulomb shear behaviour model – application to dam safety analysis

Based on a previous experimental study that we conducted on granite joint replicas including thirty direct shear tests, this article proposes to integrate the joint roughness in the Mohr-Coulomb shear behaviour model (MC). The model integrates joint roughness into constitutive stress-displacement relationships describing mechanical behaviour of rock joint. The shear strength of rock joint is assessed by the MC shear criterion that takes into account the apparent cohesion. This MC model integrating rock joint roughness component is validated against other experimental results from the literature. It is able to accurately predict the peak shear strength of an unbounded rock joint with an average relative error of 7.9%. It is finally used to assess the role of joint roughness on the shear behaviour of a gravity dam foundation.

Experimental study on the shear behavior of frozen cemented sand-structure interface

The shear behavior of the interface between frozen cemented soil and structure is important for analyses of soil-structure interaction and foundation design involving cement grouting and ground freezing. This paper investigated the shear behavior of frozen cemented sand-structure interface. Direct shear experiments were conducted on the frozen sand- and frozen cemented sand-structure interface specimens at various sub-freezing temperatures and normal stresses. Experimental results, including shear stress-displacement curves, the adfreeze and residual strengths, and their differences between the frozen sand- and frozen cemented sand-structure interfaces, were presented and analyzed. The shear stress-displacement relationships of both interfaces display a wave pattern, and the adfreeze and residual strengths of the frozen cemented sand-structure interface were found to decrease significantly when compared with the one without cementation. On average, the adfreeze strength of the frozen cemented sand-structure interface decreases by 47% and the residual strength by 19% compared to the case without cement. As the temperature drops from −2 °C to −14 °C, the peak interface cohesion of the frozen sand case increases by 393%, while that of the frozen cemented sand case rises by 115%. Empirical formulas based on the Mohr-Coulomb model between the strengths and testing parameters, including temperature and normal stress, were proposed for both interfaces. The findings from this study help understand the impact of cement grouting on the shear behavior of frozen sand-structure interface and are significant for practices such as ground construction and foundation engineering when cement grouting is involved in ground freezing.

Study on shear behavior and damage constitutive model of tendon-grout interface

Ground anchors are responsible for maintaining the stability and integrity as well as controlling the deformation, displacement, and crack development in the surrounding ground. Tendon-grout (T-G) interface, which is the weak part of anchorage structures, affects the anchorage performance of the ground anchor during load-transfer processes. To investigate the shear mechanical properties of the tendon-grout interface, a series of pull-out tests were conducted on anchorage element under constant radial stiffness boundary conditions. Moreover, the equivalent calculation for the constant radial stiffness boundary condition was analyzed. The mechanical performance of cement grouts cured for different periods was investigated, including the compressive strength and shear strength. Additionally, the failure modes of tendon-grout systems and effects of different tendon quantity, spacing and offset, were assessed. The failure evolution mechanism of the interface during the pull-out process was clarified. In addition, the microscopic structural characteristics of the tendon-grout interface were investigated. Two interfacial shear stress-displacement relationships were developed based on the interface failure modes, i.e., stress attenuation type and ideal elasto-plastic type. According to the test results, the peak strength of the interface increased with the tendon spacing, while the residual strength decreased. The effect of the tendon offset on the shear stiffness and strength of the interface is significant. A damage constitutive model was proposed to model the shear behavior of the tendon-grout interface. The proposed model verified by comparing with the test results and further adopted in pull-out test simulation. The conclusions will provide a better understanding on the shear behaviors of tendon-grout interface, and facilitate the development of a general evaluation method for ground anchor systems.

Calibration of discrete element parameters of crop residues and their interfaces with soil

The existence of crop residues in fields affects the performance of agricultural equipment. To develop high-efficiency agricultural equipment, it is necessary to identify the mechanical properties of crop residues and understand how they interact with soils. Through tests and simulations, this study focused on understanding the macro and microscopic shear properties of five different crop residues (canola, corn, flax, oats, and wheat) and their interaction with soil. Ring shear test results showed that internal friction angles varied from 16.4 to 26° for the crop residues, while direct shear test results showed that soil-residue interface friction angles varied from 41.3 to 47.4°. These measured test data were then used to calibrate microparameters: friction coefficient (µ), and the particle normal (kn) and shear (ks) stiffnesses. The calibrated crop residue kn and ks ranged from 5e3 to 5e4 N m−1, and µ ranged from 0.025 to 0.425. The soil-residue interface microparameters were calculated using the surface property inheritance approach. After calculations, the resulting soil-residue interface kn and ks ranged from 3.4e3 to 9.7e3 N m−1, and µ ranged from 0.025 to 0.425. Using these calibrated values, the discrete element models of shear tests simulated well the measured stress-displacement relationships and friction angles with relative errors less than 2%. The microparameter values obtained from this research will be beneficial to improve the reliability of discrete element models that simulate soil and crop residue interactions in tillage.

A shear damage model of the interface between soil and sulfate-corroded concrete

In order to investigate the mechanical properties of the interface between sulfate-corroded concrete piles and soil, a corrosion shear damage model and a method to determine the model parameters were proposed based on statistical damage theory. Assuming that the corrosion damage of the interface obeys Weibull random distribution, a damage model is established, which contains only six parameters. By fitting curves, the model parameters are obtained from the interface direct shear test. The shear damage model can not only predict the shear deformation behaviors, but also consider the effects of normal stress and corrosion time on the shear properties of the interface after corrosion. The results show that the shear stress-displacement relationships are significantly affected by normal stress and corrosion time. The peak shear stress increases with normal stress, and the corresponding peak shear displacement also increases. The longer the corrosion time is, the less obvious the shear softening phenomenon of soil-concrete interface is, and as the corrosion time reaches a threshold level, the softening phenomenon of shear stress-displacement relationship changes into weak softening. Finally, the rationality and feasibility of the proposed model are verified by comparing the theoretical curves with the experimental curves.

Molecular dynamics simulation of enhanced interfacial cohesive behavior of Ni-coated MWCNT/Mg composites

ABSTRACT Debonding processes of different Ni-coated MWCNT/Mg interface models were studied by molecular dynamics simulations, and the stress–displacement relationships of different interface models were obtained by fitting a modified exponential cohesive zone law for interface. In addition, the debonding behaviors of Ni-coated and uncoated MWCNT/Mg interface were compared. The results show that CNT wall numbers and inner diameters as well as interface orientations have significant effects on the Ni-coated and uncoated MWCNT/Mg interface cohesive-zone model parameters. Compared with the above two kinds of uncoated MWCNT/Mg interface models, the average values of the peak stress and the work of separation for the Ni-coated MWCNT/Mg interface models are both increased significantly, showing that the addition of a Ni-modified coating on the surface of MWCNTs can effectively enhance the adhesion capacity of the MWCNT/Mg interface.

Cyclic shear behavior of GMB/GCL composite liner

The composite liner system consisting of geomembrane (GMB) and geosynthetic clay liner (GCL) has been widely used in landfills. Although there have been a lot of studies on the monotonic shear behavior of GMB/GCL composite liner, the dynamic test data are still very limited and consequently, the dynamic shear mechanism is not clear. A series of displacement-controlled cyclic shear tests were conducted to study the shear behavior of GMB/GCL composite liner, including the shear stress versus horizontal displacement relationships, backbone curves, and shear strengths. Hysteretic loops in the shape of parallelogram were obtained and equivalent linear analyses revealed that the secant shear stiffness decreased and the damping ratio increased with the rise in loading cycles. According to the test results, it is generally acceptable to predict the dynamic peak strength of a GMB/GCL composite liner with its static strength envelope. Furthermore, the dynamic softening mechanism and rate-dependent shear stiffnesses were well described by the proposed equations, which also facilitate the accurate modeling of the cyclic shear behavior.

Study on the mechanical performance and interface damage of CRTS II slab track with debonding repairment

In this work, the effect of the carrying-out debonding repairment on the mechanical performance and interface damage of China Railway Track System (CRTS) II slab track was studied. The bilinear cohesive zone model (CZM) based on the stress-displacement relationships for describing the interface behaviors of the debonding repaired CRTS II slab track was obtained through the experimental tests. A finite element model (FEM) of the CRTS II slab track incorporating the debonding repairment between the track-slab and the mortar layer was established, which was applied in the simulations under the vertical temperature gradient load. The results show that the temperature deformation of the track-slab could be well restored and no structural damage occurred under the vertical temperature gradient load. The damage only initiates and develops at the edge of the interface between the track-slab and the mortar layer (T-M interface) under the vertical positive temperature gradient. However, the evolutions of the maximum damage variable and the damage area indicate that the influence of the interface damage emerging at the T-M interface is relatively limited.

The effect of expansive concrete on the side resistance of sockets in weak rock

In situ load tests and theory are used to investigate the effectiveness of expansive concrete on the improvement of side resistance of sockets in weak rock. Two in situ load test databases are used where rock sockets are constructed with normal and expansive concrete. Both databases include the side shear stress-displacement relationships, the rock mass properties and the socket geometry. Using the side shear stress-displacement relationships, the initial shear stiffness, the peak side shear stresses (and the corresponding displacements) and the post-peak reduction in the side shear stresses at a post-peak displacement of 15 mm are determined. Assessment of the load test data, show that variability in side resistance behavior in sockets constructed with expansive concrete is generally lower than those constructed using regular concrete. In particular, assessment of data show sockets with expansive concrete constitute an upper-bound to the peak side shear stress data. The use of expansive concrete, however, does not result in significant improvements in the post-peak behavior of sockets and initial shear stiffness. Lastly, sockets with expansive concrete generally reached peak shear stress at smaller shear displacements.

Fracture Modeling of Concrete in Plain and Reinforced Concrete Members

To simulate response of concrete members and structures, comprehensive description of concrete behavior is required. This paper presents two groups of models that could be used to simulate behavior of concrete in tension for finite element modeling. The first group of models is based on stress–strain relationships and the second is based on stress–displacement relationships. Six relationships available in literature are investigated to model the response of plain concrete struts of various shapes and the behavior of reinforced concrete frame. Three of these relationships define tension in terms of stress–strain, whereas the other three describe tension in terms of stress–displacement. Comparing model estimates with results from experiments on plain and reinforced concrete members, it is concluded that tensile behavior of concrete can be represented well using stress–strain relationship. Modeling tensile behavior of concrete based on stress–displacement relationships can simulate response of these members, too, but not as well. The stress–strain relationship given by Tsai is the best option to simulate tension in concrete compared to the other stress–strain relationships and stress–displacement relationships considered in this study.

Interaction of Polymer Flexible Joint with Brittle Materials in Four-point Bending Tests

Paper presents the possibility of application of Polymer Flexible Joint (PFJ) as a connection for brittle materials (bricks, limestones) where the PFJ is capable to carry loads. The aim of this paper is to describe the influence of application of such joints on repaired elements.In the article the idea and examples of using the Polymer Flexible Joints in civil engineering are depicted. Mechanical properties of polymer are described. Experimental investigation was conducted to determine the behaviour of selected brittle materials in four-point bending test of original specimens and after that of specimens repaired with PFJ. Based on the results of experimental research, the characteristic stress-displacement relationships are presented. The differences between various joint thickness and velocity of applied displacement are shown. The influence of flexible joint on connected elements is described, particularly taking into consideration the stress redistribution around the joint.The application of flexible joint has a positive influence on load capacity of the connection of tested elements. Furthermore, it is demonstrated that repaired elements can carry larger load due to stress redistribution. The direction of further research is indicated for connecting selected brittle materials with the described Polymer Flexible Joint.

Cohesive zone model properties for evaluating delamination of spray-applied fire-resistive materials from steel structures

This paper presents cohesive zone model (CZM) fracture properties of spray-applied fire-resistive material (SFRM) for modeling delamination of fire insulation from steel structures. For characterizing cohesive zone properties, namely cohesive strength, cohesive fracture energy and cohesive displacement ductility, a set of experiments are conducted on three types of commercially available SFRM namely medium density Portland cement-based, medium density gypsum-based and mineral fiber-based. Data from experiments is utilized to develop a cohesive stress–displacement relationships in both mode-I and mode-II delamination. The recorded stress–displacement relationship indicates noticeable strain-softening zone verifying that SFRM is not a completely brittle material, rather, it is quasi-brittle.

Dynamic delamination of fire insulation applied on steel structures under impact loading

This paper presents an experimental-numerical approach for evaluating dynamic fracture and delamination of fire insulation from steel structures during impact loading. The experiments encompass drop mass impact tests on steel beams insulated with three types of sprayed applied fire resistive material (SFRM), namely Portland cement-based, gypsum-based and mineral fiber-based, commonly utilized in steel construction. The impact tests are conducted at two kinetic energy levels to evaluate the strain rate-dependency of fracture energy and extent of delamination at steel-SFRM interface. Results from experiments show that the cracking and delamination of SFRM is mainly localized on the bottom flange with slight extension into lower part of web of beam at the mid span. Further, Portland cement-based SFRM can withstand the applied impact energy and no delamination or substantial cracking in SFRM occurs, whereas two other types of SFRM experienced significant fracture and delamination on the bottom flange. A fracture mechanics-based numerical approach is subsequently employed to simulate the conducted experiments using LS-DYNA finite element code. In the explicit numerical model, cohesive zone approach is adopted to model fracture process zone at the interface of steel and SFRM. By quantifying and calibrating the extent of delamination on the bottom flange, the dynamic increase factor of fracture energy and stress–displacement relationships, determined through previous static fracture tests, is estimated. According to numerical simulations, extent of delamination in mineral fiber-based SFRM is not dependent on strain rate, whereas in the case of gypsum-based and Portland cement-based SFRM extent of delamination is a function of strain rate.

Hyperbolic models for a 2-D backfill and reinforcement pullout

ABSTRACT: A series of direct shear tests were performed using an idealised two-dimensional test medium consisting of rhombically stacked steel rods to facilitate displacement computations for reinforced slopes using a force-equilibrium-based finite-displacement method. A series of pullout tests using extensible and stiff reinforcement with various embedment lengths (Lt) were also performed. Modelling for the non-linear behaviour of shear stress–displacement and pullout force–displacement relationships was performed using hyperbolic models. In modelling shear stress–displacement relationships for the steel rod backfill, a linear Mohr–Coulomb failure envelope was assumed for the investigated range of confining pressures. It was found that model parameters for the initial shear stiffness, namely K and n, are significantly different from those for ordinary soils because of the regular and dense packing of the steel rods, but are advantageous as a backfill material for model tests because of its consistency and repeatability. Results of hyperbolic curve fitting for the pullout force–displacement relationship indicate that the model parameters describing non-linear and pressure-dependent behaviour of reinforcement pullout, namely Kt, m, and Rt, are functions of Lt. The modelled hyperbolic shear stress–displacement relationships and the pullout force–displacement relationships are in good agreement with the experimental ones and are applicable for simulating the behaviour of model slopes consisting of steel rod assemblies.

Mechanical Behavior of Interface between Composite Geomembrane and Permeable Cushion Material

An accurate description of composite geomembrane-cushion interface behavior is of great importance for stress-deformation analysis and stability assessment of geomembrane surface barrier of rock-fill dam. A series of direct shear tests were conducted to investigate the friction behaviors of interfaces between composite geomembrane and two different permeable cushion materials (crushed stones and polyurethane mixed crushed stones). The shear stress-displacement relationships of the two interfaces show different characteristics and were described by the nonlinear-elastic model and nonlinear-elastic perfectly plastic model, respectively. Then the two models were implemented into the Fast Lagrangian Analysis of Continua in Three Dimensions (FLAC3D) procedure correctly. By verification of a numerical example, numerical calculation results showed a good agreement with the theoretical solutions and test results.

Ab initio investigation of tensile strengths of metal(1 1 1)/α-Al2O3(0 0 0 1) interfaces

Ab initio calculations using plane wave pseudopotential method within density funtional theory are applied to investigate mechanical and electronic properties of Al-terminated Me(1 1 1)/Al2O3(0 0 0 1) (Me = Al, Ag, Cu, Nb) interfaces. Stress–displacement relationships of separation perpendicular to the interface are calculated. It is shown that obtained results such as work of separation and tensile strength can be understood from electronic structure.

Developing a new slice method for slope displacement analyses

Conventional slice methods of slope stability use a constant value of safety factor along the entire failure surface, providing no information of displacements and possible variations about safety margins for the analyzed slope. Therefore, it is of practical significance to extend the force-based slice method into a displacement-based one to provide useful information about shear displacements and associated safety margins along the potential failure surface. This study extends Janbu's Generalized Procedure of Slices (GPS) by supplementing force and moment equilibrium and the Mohr–Coulomb failure criterion, which are employed in the Janbu's GPS, with additional criteria regarding slice displacement compatibility and non-linear stress–displacement relationships represented by a hyperbolic soil model. The extended displacement-based slice method constitutes a statically determinate system which is capable of generating local stress- and displacement-based safety factors along a potential failure surface. The principles and procedures proposed here can also be applied to other slice methods, regardless of their accuracy. Results of a case study show that the observed slope displacement induced by a ground table rise during a rainstorm event can be well simulated using the proposed method. More case studies into various mechanisms of slope displacements will be performed in the future to validate the proposed method for practical engineering applications.

繰り返しせん断力を受ける接着系あと施工アンカーの力学モデル

Recently, it has become increasingly important to instigate more widespread seismic retrofitting for seismically weak structures in Japan, which is frequently subject to massive earthquakes. Generally, post-installed anchors are used to connect the seismic retrofitting members to existing members. However, no computational models of post-installed anchors have yet been proposed in previous studies to evaluate stress-displacement relationships. Accordingly, we developed a new mechanical model for post-installed anchors. In this model, the kinking behavior of anchor bolts, bearing behavior of concrete, and tensile behavior of anchor bolts are considered. The proposed model can reasonably simulate the mechanical behaviors of the previous test results.