Stress-deformation Curves Research Articles

Mechanical impairment characteristics and a novel constitutive model for rocks subjected to uniaxial loading process

The study of constitutive relationship and damage degradation is crucial in solving the stability challenges faced in the rock engineering. In this work, the stress-strain relationships of different type of rocks subjected to uniaxial loading processes are investigated in details. Experimental results demonstrate measurements, such as uniaxial compressive strength (UCS), tangent deformation modulus, peak strain, and Poisson’s ratio ([Formula: see text]). A novel piecewise constitutive model is proposed that utilizes both a constitutive model during compaction and a conventional damage model using the strain equivalence assumption and logistic growth theory to represent the characteristics of stress-deformation curves during both compaction and post-compaction stages. The performance of the proposed constitutive models in capturing deformation characteristics of damaged rocks is demonstrated to be more outstanding in comparison to existing models. In all experimental cases discussed in this study, the proposed model outperforms existing reference models in terms of the coefficients of determination ([Formula: see text]), with the former having coefficients of determination greater than 0.95. Furthermore, physical meanings of relevant model parameters are found to be closely associated with properties of experimental stress-strain curves.

Coarse-Grained Artificial Intelligence for Design of Brush Networks.

The ability to synthesize elastomeric materials with programmable mechanical properties is vital for advanced soft matter applications. Due to the inherent complexity of hierarchical structure-property correlations in brush-like polymer networks, the application of conventional theory-based, so-called Human Intelligence (HI) approaches becomes increasingly difficult. Herein we developed a design strategy based on synergistic combination of HI and AI tools which allows precise encoding of mechanical properties with three architectural parameters: degrees of polymerization (DP) of network strands, nx, side chains, nsc, backbone spacers between side chains, ng. Implementing a multilayer feedforward artificial neural network (ANN), we took advantage of model-predicted structure-property cross-correlations between coarse-grained system code including chemistry specific characteristics S = [l, v, b] defined by monomer projection length l and excluded volume v, Kuhn length b of bare backbone and side chains, and architecture A = [nsc, ng, nx] of polymer networks and their equilibrium mechanical properties P = [G, β] including the structural shear modulus G and firmness parameter β. The ANN was trained by minimizing the mean-square error with Bayesian regularization to avoid overfitting using a data set of experimental stress-deformation curves of networks with brush-like strands of poly(n-butyl acrylate), poly(isobutylene), and poly(dimethylsiloxane) having structural modulus G < 50 kPa and 0.01 ≤ β ≤ 0.3. The trained ANN predicts network mechanical properties with 95% confidence. The developed ANN was implemented for synthesis of model networks with identical mechanical properties but different chemistries of network strands.

Continuous softening up to the onset of failure: A hyperelastic modelling approach with intrinsic softening for isotropic incompressible soft solids

This paper seeks to put forward an alternative notion for modelling the continuous softening in the finite deformation of incompressible isotropic rubber-like materials, up to the onset of failure. Based on the proposition that ‘there is no fundamental reason as to why the basic constitutive parameters of the (hyper)elastic state of a material cannot determine the natural evolution of softening and failure in that material’, the concept of hyperelasticity with intrinsic softening is posited. In contrast to the currently available theories of continuum damage mechanics and energy limiters, which superpose external parameters onto the basic hyperelastic model, the alternative approach presented here postulates that if a hyperelastic strain energy function accommodates a comprehensive set of constitutive parameters and has an appropriate functional form, it will intrinsically capture the observed continuous softening in the stress–deformation curves of soft solids, up to the onset of failure. Examples of this notion are presented using a specific hyperelastic model previously proposed by the author, applied here to extant datasets of a wide variety of isotropic incompressible soft solids, ranging from soft tissues to protein gels and 3D printed biomaterials, that include the softening behaviour. The favourable modelling results obtained via this approach are demonstrated. It is conferred that these results portend a versatile modelling tool for application to the whole-range deformation of soft solids, up to the onset of failure, and make the case for devising a unified theory for modelling both continuous and discontinuous softening in the finite deformation of rubber-like materials.

Engineering properties of fine-grained red mud

ABSTRACT Basic engineering parameters of fine-grained red mud tailings were evaluated via laboratory tests; the coupled effect of mineralogy and microstructure was investigated by appraising the stress-deformation characteristics; and the static and seismic liquefaction susceptibilities were evaluated via critical state soil mechanics and empirical equations, respectively. Results indicated the presence of cementitious minerals without swollen clay minerals. The textural and compression properties were similar to those of clays, whereas the strength parameters were similar to those of sand. The stress-deformation curves show overall strain softening with a temporary strain-hardening curve at the beginning, which can be qualitatively explained by the coupled effect of the microstructure and mineral components. Hydroxysodalite precipitated at the contact points among the particles afforded microstructural cementation bonds, allowing the aggregates to resist external loads at the initial loading stage. Red mud presented strong resistance to static liquefaction but was potentially seismically liquefiable.

Double Plane Direct Shear Apparatus for Determination of Adhesion Factor in Clays

Abstract A modified direct shear assembly with two parallel shear planes was designed to enable the simultaneous and independent measurement of cohesive and adhesive resistance; it is named the “double-plane direct shear device.” The three-block specimen box of the device has top and middle blocks filled with soil specimen and the bottom block is made of solid steel. The middle box is laterally displaced while restraining the top and bottom box, leading to concurrent shearing along two parallel shear planes. Results from quick undrained testing of kaolinite at five different consistency indexes (Ic) and three different vertical stresses (σv) are presented. The stress–deformation curves show greater mobilization of adhesive stress than cohesive stress at low deformations at all Ic and σv. The deformation required for the complete mobilization of cohesive and adhesive stresses and their variation with Ic and σv is discussed. A parameter crossover deformation (dco) is defined to identify the region where adhesive stress is greater than cohesive stress, which would be useful in applications where soil stickiness is of concern. The results show that the presence of partial drainage increases the dco, which results in an increased deformation range over which adhesive stress is greater than cohesive stress, thereby increasing the stickiness susceptibility. Furthermore, results show that the adhesion factor (α, defined as the ratio of adhesion to undrained cohesion) always stayed below unity under undrained conditions, whereas under partial drainage conditions, values of α greater than unity were obtained. The study concludes that for the application where soil stickiness is of concern, the determination of α and its usage to analyze stickiness susceptibility should be performed under simulation of appropriate drainage boundary conditions.

A one dimensional ubiquitiform damage model for quasi-brittle materials

Considering the self-similar characteristic of irregular pore network of materials during the damage evolution process, a pore ubiquitiform model (PUM) is developed to characterize the multiscale pore or microcrack network of quasi-brittle materials, and then based on the PUM, the ubiquitiform damage model (UDM) is proposed to describe the damage evolution process of quasi-brittle materials under quasi-static uniaxial tensile loading. The values of porosity and pore specific surface area estimated from the PUM and the stress-deformation curves including the softening curve calculated from the UDM are in good agreement with previous experimental data, respectively. It can be found that, the PUM can be adopted to characterize the self-similar multiscale pore network and estimate the porosity and pore specific surface area, and the UDM can describe the damage evolution process of quasi-brittle materials under quasi-static uniaxial tensile loading. Meanwhile, the UDM can characterize the self-similar characteristic of pore network during the damage evolution process of quasi-brittle materials through the evolving ubiquitiform complexity.

Experimental Investigation on Seepage Characteristics of Clay-Structure Interface after Shear Deformation.

There exist shear and seepage behaviors on the interface between clay core and concrete slab in clay core dams. In order to investigate the seepage characteristics of the clay–structure interface after shear deformation, a shear-seepage test system is proposed, in which the seepage direction is perpendicular to the shear direction. The shear test and shear-seepage test are performed on clay–metal and clay–mortar interfaces under different normal stresses (100, 200, 400, 800, and 1600 kPa). The shear stress-deformation curves of two clay–structure interfaces exhibit softening behavior and residual friction behavior. The interface roughness can enhance peak and residual shear strength and increase peak displacement. The shear-seepage test results show that specimen permeability decreases first and then increases to a stable value as shear deformation increases under low normal stress, while it decreases continuously and then retains stability under high normal stress. The interface roughness enhances specimen permeability under low normal stress, whereas it has a weak effect on specimen permeability under high normal stress. Compared with initial permeability, shear deformation reduces specimen permeability rather than raise it. The ratio of stabilized permeability coefficient to initial value ranges from 0.6 to 0.8. The clay–structure interface still has a good resistance to seepage failure after shear deformation. The shear dilation features and interface pore decrease caused by shear behavior are the internal attributions of clay–structure specimen permeability evolution.

Study on Failure Characteristics and Acoustic Emission Characteristics of Sandstone Under Variable Angle Shear

To study the shear damage and failure characteristics of red sandstone under different normal stress conditions, the failure process of sandstone under three different shear angles (50°, 55°, 60°) were studied by using variable angle shear test device. The shear stress-deformation curves and failure characteristics of sandstone were obtained, and the relationships between shear cracks and acoustic emission impact times, amplitude, peak frequency were established. With the increase of shear angle, the normal stress, shear stress and peak shear stress decrease gradually. The development of micro-cracks in the shear plane appear more earlier. The rock spalling mainly occur in the penetrating area of main and secondary cracks surrounding the two ends of specimen. The spalling degree was obviously weakened with the increase of shear angle. In acoustic emission monitoring,the high frequency signal decreases significantly, which may have a significant corresponding relationship with the rock friction and shear effect. The failure mode of rock changes from plasticity to brittleness. The amplitude changes are concave, and more acoustic emission energy is released at compaction stage and plastic(failure) stage. The results have important guiding value for judging and predicting the instability mechanism of rock engineering.

Experimental and Numerical Investigation on Repairing Effect of Polymer Grouting for Settlement of High-Speed Railway Unballasted Track

Uneven settlement of high-speed railway subgrade leads to the irregularity of high-speed railway line, which seriously affects high-speed train operation. The skylight point of high-speed railway operation is short and the maintenance time is limited. Therefore, how to quickly lift and repair the ballastless track slab in the subsidence section is an urgent problem to be solved in the maintenance of high-speed railways. The two-component non-aqueous reactive polymer material has the advantages of strong expansive force, fast reaction speed, and wide application range, which is extremely suitable for the repair of high-speed railway track slab subsidence and lifting. In this study, the expansion force characteristics of different density polymer materials and the stress-deformation curves at corresponding density are tested in laboratory to propose the mechanical parameters of polymer. Then, a three-dimensional finite element (FE) model of high-speed railway train ballastless-track subgrade is established based on ABAQUS. The mechanical characteristics of CRTS III ballastless track under different repair materials, different elevation, and different density of polymer grouting materials are analyzed. The results show that, under the dynamic load of the train, the stress value of polymer repairing material is less than that of cement slurry, presenting a compressive stress state, which is similar to that of the complete subgrade surface. In addition, within a certain thickness range, increasing the thickness of polymer is beneficial to reducing the difference of stress variation between polymer filling layer and complete pavement. Once beyond this range, the thickness of polymer has little effect on the force variation.

The sustainability of micropolar concrete plasticity model to the finite element size

Introduction. As it is known, deformation of concrete can be divided into several stages. The first stage is characterized by a linear dependence of deformations and stresses, elastic deformations and small loads that, as they increase, lead to the second stage. At the second stage, the dependence becomes curvilinear, while deformations are irreversible, since micro-cracks are formed. Further consolidation of the micro-cracks into meso- and main cracks refers to the third stage and is accompanied by a redistribution of energy to the area of the main crack mouth. However, reaching the ultimate strength is not accompanied by an instant loss of bearing capacity due to the effect of decompression. This phenomenon should be taken into account in the numerical simulation of concrete and reinforced concrete structures, because it significantly affects their strength characteristics. The introduction of such a refinement in the design models will allow reducing cross-sections of the construction components and accordingly getting rid of material overruns.&#x0D; Materials and methods. A digital sample is created for the study using the ANSYS software. A beam model is simulated as a single-span beam with longitudinal reinforcement in the bending zone. The load is applied as a 70 mm offset to the nodes in the line along the application point. Reinforcement is simulated as bilinear isotropic strengthening elements (LINK180). For uniform load distribution, load plates with linear elastic properties are specified at the points where boundary conditions and load are applied.&#x0D; Results. According to the obtained data, stress-deformation curves are constructed identically to the concrete deformation diagram. The values of loads when the first cracking occurs (end of the linear-elastic state), peak loads when the main crack is formed (maximum load for the unreinforced case and the beginning of the steel softening for the reinforced case) as well as ultimate loads and maximum deflections at the mid-span are compared.&#x0D; Conclusions. The results give insignificant (up to 5 %) discrepancies when changing the finite element size. Therefore, when working with calculation software, developers will be able to create correct models with any spacing of the finite element mesh depending on the available computational capabilities. Micropolar theory for simulating the concrete decompression can be considered sustainable to the size of the finite elements.

Experimental Study on the Complete Tensile Stress-Deformation Curve of Fully Graded Concrete

Concrete is a type of quasi-brittle material with high compressive strength and low tensile strength. To obtain the complete tensile stress-deformation curve of massive concrete specimens, a specially designed loading machine is proposed with a pair of hydraulic oil dampers parallel to the direction of the tensile loading. The dampers were designed with valves to modify the stiffness of the loading machine, enhancing the stiffness of the machine before the applied stress reaches 90% of the ultimate stress of the concrete. Several experiments were conducted with the proposed loading machine on prism-shaped specimens (size: 1350 mm× 450 mm × 450 mm), and the stress-deformation curves and mechanical parameters of the concrete were obtained. Finally, an empirical formula is presented and compared with the experimental results.

Tensile properties of concrete at very early ages

Proper knowledge of tensile properties of concrete from very early ages is essential for effective control of not only early-age cracking but also of residual stress due to restrained early-age deformation as well as for demolding and handling of young concrete members in the precast industry. Despite significant past research, such knowledge is currently inadequate and based on experimental data with questionable reliability, due mainly to the considerable challenges in testing early-age concrete.This paper first highlights the challenge and importance of collecting reliable test data on full tensile stress-deformation curves for very early-age concrete. Through identifying and effectively addressing critical drawbacks in previous test setups, an improved direct tensile testing system that can reliably capture simultaneously stress and deformation of concrete from the age of several hours after mixing has been successfully developed. Key features of the improved setup, including the air-bearing box for friction minimisation and digital image correlation for non-contact full-field deformation capturing, are then reported in the paper. Based on the newly collected data, fundamental tensile properties of concrete at very early ages are re-assessed and presented. Such properties include tensile strength, Young’s modulus, strain at peak stress, fracture energy, performance under cyclic loading and tensile relaxation.

A transverse isotropic viscoelastic constitutive model for aortic valve tissue.

A new anisotropic viscoelastic model is developed for application to the aortic valve (AV). The directional dependency in the mechanical properties of the valve, arising from the predominantly circumferential alignment of collagen fibres, is accounted for in the form of transverse isotropy. The rate dependency of the valve's mechanical behaviour is considered to stem from the viscous (η) dissipative effects of the AV matrix, and is incorporated as an explicit function of the deformation rate (). Model (material) parameters were determined from uniaxial tensile deformation tests of porcine AV specimens at various deformation rates, by fitting the model to each experimental dataset. It is shown that the model provides an excellent fit to the experimental data across all different rates and satisfies the condition of strict local convexity. Based on the fitting results, a nonlinear relationship between η and is established, highlighting a ‘shear-thinning’ behaviour for the AV with increase in the deformation rate. Using the model and these outcomes, the stress–deformation curves of the AV tissue under physiological deformation rates in both the circumferential and radial directions are predicted and presented. To verify the predictive capabilities of the model, the stress–deformation curves of AV specimens at an intermediate deformation rate were estimated and validated against the experimental data at that rate, showing an excellent agreement. While the model is primarily developed for application to the AV, it may be applied without the loss of generality to other collagenous soft tissues possessing a similar structure, with a single preferred direction of embedded collagen fibres.

Investigations and Non-destructive Testing in New Building Design

Mechanical rebar couplers are preferable in the advanced building construction and structural design of antiseismic elements. The paper presents destructive inspection techniques used to investigate stress fields (tensile and compressive) and deformation curves for mechanical rebar splicing. The properties of mechanical rebar splicing are investigated by the non-destructive testing digital radiography. The behavior of real connections (column-to- column, beam-to-column) is studied under static and dynamic loads. Investigation results allow the elaboration of recommendations on their application in the universal prefabricated antiseismic structural system developed at Tomsk State University of Architecture and Building, Tomsk, Russia.

Tensile Fracture Characteristics of Compacted Soils under Uniaxial Tension

AbstractThe tensile fracture characteristics of compacted soils are very important for the stability of earth structures. In this work, a horizontal displacement-controlled, uniaxial, direct-tension test apparatus was developed to capture the complete tensile stress-deformation response of cohesive soils. A series of uniaxial tensile tests were performed on two compacted soils, and the cracking process was observed. The tensile stress-deformation curves showed a clear postpeak softening behavior along with the development of tensile cracks. The influences of water content and dry density on the tensile strength and softening behavior of compacted soils were analyzed. On the basis of the fracture process zone concept, a method was proposed to extract the tensile stress-strain curve for the fracture process zone from the test data and a constitutive model was established to describe the complete stress-strain relationship of compacted soils under uniaxial tension. The model fits the experimental results well.

Extrusion of ceramic emulsions: Plastic behavior

This paper presents the plastic characterization of several extruded ceramic emulsions prepared by emulsification of different ceramic suspensions: i) red clay, ii) kaolin and iii) alumina, which were performed in order to obtain some insights to upscale the ceramic emulsion extrusion process. Stress-deformation curves obtained by compression of the pre-extruded bodies were used to obtain the plasticity characterization of the emulsions with different water contents. Deformation amplitude, yield stress and rupture stress revealed the most sensitive parameters, thus giving valuable information about the plastic behavior of the ceramic emulsions, as well as important guidelines for extrusion upscale. The microstructural features of the sintered bodies were kept constants even after the extrusion process, making this process be feasible for large-scale production of cellular ceramic bodies.

Large deformation behavior and effective network chain density of swollen poly(N-isopropylacrylamide)–Laponite nanocomposite hydrogels

Hydrogels were often used in aqueous and/or physiological saline media, thus, the mechanical properties of the swollen gels were of particular importance. Until now, these data were lacking due to the most swollen hydrogels being too weak and brittle to undergo the deformation test at all. The mechanical behavior of swollen poly(N-isopropylacrylamide) (PNIPAm)–Laponite nanocomposite hydrogels (NC gels) were studied with compression and elongation under large strains causing deformation. The swollen NC gels were intact after compression and sustained elongation of up to 500%. Strain hardening was observed in the swollen NC gels for the first time, which was considered to be the extensional limitation of the polymer network chains due to the orientation of the Laponite platelets. Therefore, the strain hardening was enhanced by increasing the Laponite content in the NC gels. When the deformation was low, the compression and elongation stress-deformation curves of the as-prepared and swollen NC gels were described quantitatively with the Mooney–Rivlin model. Whilst for high deformation, Creton's model considering the finite extensibility enabled the prediction of these curves almost quantitatively. Stress-deformation hysteresis was found in the as-prepared and swollen NC gels, which was the result of the deorientation of the clay platelets and relaxation of the polymer chains. The orientation process of the clay platelets dissipated energy during deformation, resulting in a high toughness of the NC gels even in the swollen state. The effective network chain density in the swollen NC gels manifested that the swelling in water only expanded the gel volume and did not damage the cross-linking points. The number of effective network chains attached to one Laponite XLS platelet was almost independent of the clay content, so the addition of clay platelets in the NC gel formed new cross-linking junctions.

Characterizing rock mass deformation mechanisms during plate load tests at the Bakhtiary dam project

The results of 26 plate load tests carried out at the Bakhtiary dam site were analyzed to define and characterise the mechanisms governing the rock mass deformation during the tests. Tests were categorized into groups based on the relative direction of loading to the orientation of major discontinuity sets running through the rock mass. Five mechanisms were distinguished as the main modes controlling deformations of the rock mass during the tests. These mechanisms were based on analysis of the shape of stress-deformation curves, total and plastic deformation, anisotropy and deformation hysteresis. This study shows that part of the scatter commonly observed in the moduli calculated from plate load tests can be attributed to the different deformation mechanisms that are mobilized during the tests. Incompatibility of the theoretical assumptions, made for the interpretation of the test results, with actual deformation mechanisms is shown to be another factor causing scatter in the calculated moduli.

Simulation of Dilatometer Tests by Neural Networks

Rock rigidity may be experienced in a wide range depending on several factors, and different methods can be used to consider their load-deformation behaviors. In this context, dilatometer tests (DTs) can be applied to obtain the modulus of elasticity of rock masses; therefore, it is possible to evaluate in-situ stress-strain behavior of rock masses realistically. Nevertheless, the application of this test is expensive as well as time-consuming, and necessitates mobilization of the equipment to construction site by trucks. The aim of this study is to simulate the load-deformation curve obtained of DT by neural networks (NNs). Therefore, the dilatometer test can be modeled as well as synthetic simulation of the test enables analyzers to characterize the material behavior. In order to investigate this, 50 different stress-deformation curves are obtained from DTs conducted on limestone formation underlying a dam (Dim dam) construction site in the Southern part of Turkey. The developed database by the curves was used for training and testing of the NN models. The results revealed that NN technique is quite successful for modeling the stress-deformation behavior of specific rocks based on DT results. It is therefore possible with the help of this alternative tool developed for the simulation of DT i) to model DT numerically, ii) to simulate the stress-strain behavior successfully, iii) to calculate the modulus of deformation efficiently, iv) to generate additional DT data synthetically, v) to develop material model alternatively, and vi) to make assumptions on the characterization of the rock mass behavior using previous information gathered by DTs.

Behaviour of different industrial ceramic pastes in extrusion process

It is well known that the extrudability of a ceramic paste is strongly dependent on materials composition and formulation, basically because it controls the resulting plasticity. Accordingly, the characterisation of the plastic behaviour is essential to adjust the fluxing properties of the material. In the current work, three industrially prepared ceramic pastes (porcelain, earthenware and terracotta) were tested, using stress–deformation curves obtained by compression of cylindrical probe tests. To achieve good extrusion performance, the design and operating conditions also need to be considered in detail.The ram extrusion of adjusted pastes was followed by the Benbow–Bridgwater model. Differences in the flow behaviour were related with stress–deformation curves and with the surface quality of the extrudate (by using the static friction coefficient).