Uniform Stress State Research Articles

To justification of concrete strength criterion at biaxial compression

Introduction. Numerous experimental data from Russian and foreign researchers indicate that the classical plasticity hypotheses do not take into account the different resistance to uniaxial tension and compression, the influence of the spherical tensor. At the same time, experiments show that the ultimate resistance depends on the type of stress state, and hydrostatic pressure increases the strength and plasticity of solids. Aim. To establish the dependence of the influence of the second component of stresses during biaxial compression of concrete on the parameters of the complete diagrams of deformation of the material σbR and εbR it is necessary to describe these diagrams, and to construct a closed curve on the plane of the main stresses (concrete strength criterion). Materials and methods. Based on the experimental materials of foreign and domestic researchers, including the experiments of the authors of the article, methods of mechanics of deformed solids, limit curves and a closed curve on the plane of the main stresses in the form of a chain line forming the smallest area strength surface in the form of a catenoid are proposed. Results. The article provides an analysis of the known strength criteria from the point of view of their geometric interpretation in the stress space. It is shown that these studies relate mainly to metals and metal structures, and for the design of reinforced concrete and steel-concrete structures in a complex stress state, it is necessary to develop an appropriate criterion for the strength of concrete. Conclusions. As a result, the proposed limit curves and the surface (material strength criterion) on the plane of main stresses in the form of a catenoid accurately reflect the behavior of concrete under conditions of uniform and uneven flat stress state, and the equation of the surface in the form of a catenoid is a generalization of the equations of limit curves for each of the three types of flat stress state. At the same time, there is no overestimation of strength in the "compression – compression" area in this case.

Numerical Evaluation of Hydroformed Tubular Adhesive Joints under Tensile Loads

Adhesive joints are widespread in the aerospace, aeronautics, and automotive industries. When compared to conventional mechanical joints, adhesive joints involve a smaller number of components, reduce the final weight of the structure, enable joining dissimilar materials, and resist the applied loadings with a more uniform stress state distribution compared to conventional joining methods. Hydroformed tubular adhesive joints are a suitable solution to join tubes with identical cross-sections, i.e., tubes with the same dimensions, although this solution is seldom addressed in the literature regarding implementation feasibility. This work aims to numerically analyze, by cohesive zone modelling (CZM), hydroformed tubular adhesive joints between aluminum adherends subjected to tensile loads, considering the variation of material parameters (type of adhesive) and geometrical parameters. Initially, a validation of the proposed CZM approach is carried out against experimental data. Next, the aim is to numerically evaluate the tensile characteristics of the joints, measured by the maximum load (Pm) and energy of rupture (ER), considering the main geometrical parameters (outer tube diameter of the non-hydroformed adherend or dENHA, overlap length or LO, tube thickness or tAd, and joggle angle or q). CZM validation was successfully performed. The numerical study determined that the optimal geometry uses the adhesive Araldite® AV138, higher dENHA and LO highly benefit the joint behavior, tAd has a moderate effect, and q has negligible influence on the results.

Application of Deep Learning on Structure Displacement Measurement and Accuracy Analysis

Displacement serves as a crucial indicator in structural health monitoring within civil engineering, with computer vision being a commonly employed tool due to its cost-effectiveness and convenience. This paper introduces a methodology for tracking feature points computed via computer vision techniques SuperPoint and SuperGlue. Two test scenarios are conducted: a bridge loading test characterized by a low uniform stress state, and a tensile strength test marked by a high stress state. However, a notable drawback of this method is the potential misalignment between the survey point (the point under measurement) and the feature point, leading the errors. To address this issue, a fundamental principle for selecting feature points is presented. In the bridge loading test, the displacement of the survey point is recorded, and the influence of misalignment between survey point and feature point on accuracy is analyzed under low-stress uniform conditions. The findings demonstrate the feasibility of this method for field measurements in civil engineering. Subsequently, a tensile strength test is conducted to assess the influence of different feature point selection criteria on accuracy under high-stress conditions. Concurrently, the DIC software Ncorr is applied to analyze the displacement field of feature points near the survey point. The results highlight the significant influence of selecting feature points within regions of high stress concentration on measurement accuracy.

Effects of prestress implementation on self-stress state in large-scale tensegrity structure

Tensegrity structures are spatial, reticulated, and lightweight structures that are composed of struts and cables. Stability is provided by a self-stress state between tensioned and compressed members obtained by prestress. Although the form finding of tensegrity structures has attracted significant attention, few studies have focused on the structural concept implementation in large-scale systems and prestress implementation in particular. This work investigates the effects of the prestress implementation on the self-stress state in a large-scale tensegrity structure with interconnected struts. The suspended, 6.2-m-tall structure consists of stainless steel cables and carbon-fiber composite tubes. Different uniform and non-uniform prestress and support condition scenarios were applied and the effects on the self-stress state were investigated experimentally by measuring strains in the carbon tubes, stresses in cables, and reaction forces. Numerical modeling was performed using the dynamic relaxation method, whose predicted stress values were compared to experimental data. The results revealed that the cable-by-cable prestress implementation led to lower effective prestress levels than nominally predicted due to a continuous stress redistribution. Non-uniformly applied prestress resulted in a uniform stress state due to the same continuous stress redistribution. Small joint restrictions in rotational angles and small joint friction hindered geometry and stress distribution from changing when the support conditions changed. The importance of extending the analysis of tensegrity structures beyond form finding and sizing is thus demonstrated; prestress implementation and joint design also need attention.

A principal stress cap model for stresses in square silos, with some examples of exponential stress

The principal stress cap approach has been used to model stresses in square hoppers, based upon an orthogonal grid upon a spherical surface sector, bounded by “great circles”. The model agrees with Rotter's correlation for lateral variation of wall normal stress for concave, active systems at uniform state of stress. However, systems of convex, passive stress do not conform to Rotter. The data of Jarrett{Geotechnique, 1995, 45, 95–104} and Brown{Chemical Engineering Science, 2000, 55, 4399–4413} have been modelled successfully, both qualitatively and quantitatively. Regions of the exponential stress regime have been found in the corner regions of the silo, alongside conventional Janssenian stress at the wall mid-points. Equivalent data of Jarrett and Brown are in reasonable agreement when plotted in dimensionless form. • Stresses in square hoppers have been modelled. • Principal stress cap approach is easy-to-use and visual model. • The model has been fitted to 2 sources of experimental data. • The data and the model show regions of exponential stress. • Horizontal variations in wall normal stress are seen.

Pre-buckling vibration and buckling analyses of composite skew plate: An analytical investigation

In this work, the analytical investigation for pre-buckling vibration and buckling analyses of a composite skew plate subjected to parabolically and linearly varying in-plane edge load are presented. When the composite skew plate is subjected to parabolic (non-uniform) in-plane edge loading, the pre-buckling stresses within the composite skew plate are not known a priori. To estimate the pre-buckling stresses within the composite skew plate for which the in-plane elasticity problem is solved by minimizing the membrane strain energy using the Ritz method. It is observed that the rate of diffusion of applied parabolic in-plane edge load within the skew plate to a state of uniform in-plane stress is faster in the case of isotropic material than composite material. Using estimated pre-buckling stresses, the total energy functional is derived from the total strain energy, potential energy, and kinetic energy. The total energy functional is reduced into sets of an ordinary differential equation and algebraic equation, respectively for pre-buckling vibration and buckling problems using the Ritz method in conjunction with BCOPs. The associated linear eigenvalue problems are solved to compute the pre-buckling vibration frequency and buckling load of the stressed skew plate. The outcome of the study may provide crucial inputs in the design of skewed bridge decks, ship structures, and aircraft wing design.

Multifunctional MEN-Doped Adhesives: Strengthening, Bond Quality Evaluation, and Variations in Magnetic Signal with Environmental Exposure

Adhesive bonding of polymer matrix composites offers various advantages over traditional fasteners, such as a uniform stress state, reduced weight, and delay of composite delamination. However, adhesive bonding has limited implementation due to challenges in the prediction of durability. This work introduces a new method to monitor an adhesively bonded composite joint by dispersing magneto-electric nanoparticles (MENs) into the polymer precursor and monitoring changes in their surface charge density by evaluating the output magnetic signal under an applied magnetic field. Real-time monitoring of the curing process of a polymer adhesive was performed and corroborated via thermal analysis and mechanical testing. Lap shear and end notch flexure testing showed that adding 1 vol% MENs led to a ~23% increase in shear strength and a ~12% increase in mode II critical energy release rates compared to the undoped adhesive. Adding 5 vol% MENs also increased the adhesive’s peak tensile stress by ~8%. Strengthening mechanisms of the doped adhesive were monitored using in situ electron microscopy. A correlation between water ingression and a change in the magnetic moment was observed. Results show the MENs’ potential as a structural health-monitoring tool for a wide range of materials and applications.

Evolution Mechanism of Mesocrack and Macrocrack Propagation in Carbonaceous Mudstone under the Action of Dry-Wet Cycles

The crack propagation evolution of carbonaceous mudstone under the action of dry-wet cycles is an important cause of the unstable failure of this type of slope. This paper attempts to reveal the evolution mechanism of mesocrack and macrocrack propagation in carbonaceous mudstone under the action of dry-wet cycles from chemical, physical, and mechanical perspectives. Firstly, the soaking solution of carbonaceous mudstone during the dry-wet cycles was extracted for an ion concentration test to analyze the chemical reactions of carbonaceous mudstone. Then, CT scans were performed on the carbonaceous mudstone samples to study the changing pattern of mesostructure of carbonaceous mudstone during the dry-wet cycles. In the end, the mechanical properties and failure characteristics of carbonaceous mudstone after dry-wet cycles were studied by triaxial compression tests. The results showed that chemical reactions such as calcite dissolution, potassium feldspar hydrolysis, and sodium feldspar hydrolysis occurred during the dry-wet cycle of carbonaceous mudstone. Affected by the dry-wet cycles, the mesostructure of the carbonaceous mudstone gradually changed from face-face contact and edge-face contact to edge-corner contact and corner-corner contact, and the interlayer flake structure was opened and was locally curled and fractured. With the increase in the number of dry-wet cycles, the failure characteristic of carbonaceous mudstone transformed from tensile failure to shear failure, the failure surface of carbonaceous mudstone was deflected from 90° to 60°, and the crack propagation path of carbonaceous mudstone became more complicated. The chemical reaction of carbonaceous mudstone minerals during the dry-wet cycle is an important reason for the initiation and development of pores. The dry-wet cycle aggregates the propagation of mesocracks and structural disorder, transforming the uniform stress state of the rock mesostructure to the concentrated stress state, which is the important reason for the macrocrack propagation evolution of carbonaceous mudstone.

Sequential reactivation of differently oriented weak planes of the basement under uniform state of stress: A possible mechanism of evolution of Tatapani–Ramkola basin, central India

Sequential reactivation of differently oriented weak planes of the basement under uniform state of stress: A possible mechanism of evolution of Tatapani–Ramkola basin, central India

Removing stress singularities in standard tests of composite materials

In many standard tests involving composite material, specially those needing tabs at the ends of the sample, configurations of multimaterial corners, with abrupt changes in geometry and material properties, typically appear giving rise to stress fields which achieve very high stresses and theoretically are unbounded. These stress intensifications may promote premature failures, or simply, alterations of the uniform stress state which is expected to occur in the specimen, and the test strength, calculated as a certain load divided by a certain failure area, may not be representative of the real strength of the material under analysis. In the present work three examples are presented, in which, by means of an addecuate selection of the geometrical parameters defining the test configuration which do not affect the mechanical property to be measured, the stress singularities at these multimaterial corners are removed (or at least reduced to a minimum). The three cases are: a) tests in tension and shear (Iosipescu) in bimaterial coupons, b) the off-axis test for the intralaminar shear strength in composite materials and c) a compression test of thick laminates in composite materials. In two of the three cases experimental results are available to corroborate that the removal of the stress singularities gives rise to higher strength values than those obtained in the original test configurations.

微視的な不均質構造によって生じる巨視的な不均一変形のモデル化に関する研究

Engineering materials typically have a heterogeneous microstructure, and it induces microscopic non-uniform deformation even under the macroscopically uniform stress state. With the decrease in the dimension of engineering structural components, such a microscopic non-uniform deformation affects the macroscopic deformation state. In this study, the development of the microscopic heterogeneity-induced non-uniform deformation is modeled using the rate-form nonlocal constitutive equation. First, the inelastic strain is decomposed into local and nonlocal inelastic strains. Then, the nonlocal strain is assumed to be developed by the distribution of plastic compliance, which is characterized by an intrinsic size of the material microstructure. Finally, the multiaxial rate-form constitutive equation, in which the stress on the material points is calculated by both the local and nonlocal strains, is formulated. The finite element simulations of the tensile tests of specimens with curved and parallel gauge sections are performed using the proposed model. The obtained results clarified that the mechanical response and the non-uniform deformation are characterized by the microscopic heterogeneity of the material.

Nonlinear characteristics and a new fractional constitutive model for warp-knitted NCF composites under normal loading conditions

This paper presents a new fractional constitutive model to reveal the nonlinear and viscoelastic properties of fabric composites under normal loading conditions. The uniaxial tensile tests for warp-knitted non-crimp fabric (NCF) composites under different strain rates were carried out, and their nonlinear characteristics and quasi-yield behaviors under normal loading conditions were investigated. A new nonlinear constitutive model was then proposed by employing the fractional calculus operator theory, and the proposed models were verified by the experimental results and implemented in the simulations based on UMAT subroutine in FEA code ABAQUS. Results show that the warp-knitted NCF composites' filling yarns would produce greater yarn geometric deformation than the warp yarns due to their yarn waviness. The variations of initial elastic modulus under increasing strain rate show rising tendencies, which uncovers the materials’ strain-rate effect. Owing to the expressiveness of deformation mechanisms offered by model parameters, the proposed model could effectively present the nonlinear and viscoelastic characteristics of the fabric materials under normal loading conditions. In addition, The UMAT subroutine shows a good applicability, and the FE model with the Equation Constraint could simulate the uniform stress states accurately.

DETERMINATION OF THE STRESS STATE AND THE FORCE OF DEFORMATION OF BALL-SHAPED BILLETS IN A CLOSED MATRIX

At the present stage of development of science and technology, the problem of improving the quality of metal products by integrating methods of severe plastic deformation into existing technological processes is urgent. Intense plastic deformation makes it possible to obtain nanostructured metallic materials with improved quality and an attractive set of properties due to the saturation of the metal with nanoscale defects. Spherical billets with two ends along the edges are a common form of metal products. For the manufacture of such metal products, the effect of mutual influence of all-round compression and ECAP pressing in a device for the implementation of equal-channel angular pressing in a closed matrix was used. The purpose of this study is to develop a theoretical approach to determining the stress state and deformation force of spherical blanks in a closed matrix. To achieve this goal, an integrated approach was used to determine the stress state and deformation force by the method of slip lines and computer simulation in the Deform-3D software package. Analysis of the results of the stress state of the workpieces obtained by the slip line method showed that a uniform stress state is formed with a predominance of the maximum compressive stresses. The combination of the latter with angular metal extrusion into the lateral channels of the matrix predicts the production of workpieces with sub-ultrafine-grained and/or nanostructure. By the method of joint solution of differential equations of equilibrium and plasticity conditions, as well as computer modeling, the deforming force of the blanks at the final stage of deformation, when the metal flows out into the lateral channels of the matrix, is determined. The analysis of the obtained results shows that the value of the deformation force obtained by the two methods is comparable with a difference of up to 2 %, which confirms the correctness of the obtained values.

Core-shell heterostructure-enabled stress engineering in vanadium dioxide nanobeams

In strongly correlated materials (SCMs), especially for vanadium dioxide (VO2), manipulating physical properties through stress engineering is an important issue for the use of ultrafast metal-insulator transition (MIT) in device applications. Recent research efforts have mainly focused on modulation and related phenomena of physical properties by epitaxial and mechanical stresses in VO2 films or anisotropic nanocrystals. However, inhomogeneous stress in such planar and nanocrystal systems leads to complications induced by phase competitions or the creation of intermediate phases. Here, we demonstrate the core-shell heterostructures-enabled stress engineering on MIT, which provides accommodation of uniform axial stress and control of phase transition pathways in VO2 nanobeams. Specifically, the VO2 nanobeams with an amorphous Al2O3 shell undergo a simple and direct MIT at lower temperatures without intermediate phases, distinctly different from pristine nanobeams with coexisting phases. For the core-shell nanobeams, the VO2 core sustains a uniform compressive stress state along the nanobeam length caused by shell formation, which can be attributed to the different thermal behaviors coupled to the elastic modulus between the VO2 and shell. Our results can lead to better engineering of phase transitions in SCMs, providing the beneficial effect of suppressing inhomogeneities during the MIT process.

Transformation of concrete deforming diagrams in conditions of uniform and non-uniform stress states

The aim of the research is formulated, which is the theoretical substantiation, development and testing of the concrete state diagram, suitable for use to calculations of the strength of bended reinforced concrete elements based on the deformational model, as well as for more complete assessment of non-uniform stress-strain state of the cross-sections of elements at all stages of their loading up to the destruction directly on the values of the relative strains of concrete and the corresponding stresses.
 In this paper, an analytical method of converting the parameters of reference concrete deforming diagrams, which are determined experimentally during standard tests of concrete prismatic specimens by short-term axial compression and tension up to the destruction in «soft» mode of loading, into parameters of simulated transformed diagrams, which correspond to the «rigid» mode of loading of concrete, is developed. The way of simultaneous application of analytical expressions for the specified defoming diagrams, which describe the operation of concrete in conditions of uniform compression, for transition to the state diagram, which displays the functional relationship between the stresses and strains of the concrete compressed zone in bended concrete and reinforced concrete elements, cross-sections of which operate in conditions of non-uniform stress state, is proposed.
 The results of testing the obtained mathematical model of the concrete state diagram during the calculation by different methodics of strength of the normal cross-sections of bendable reinforced concrete elements are presented.
 It has been determined that a promising direction for further experimental and theoretical research is the study of the application of the developed methodics of graphical building and mathematical descripting of concrete deforming diagrams under conditions of uniform and non-uniform compression to the calculation of concrete and reinforced concrete elements that are operated under the action of long-term and short-term low-cycle loadings.

Effect of inherent microcrack populations on rock tensile fracture behaviour: numerical study based on embedded discontinuity finite elements

Inherent microcrack populations have a significant effect on the fracture behaviour of natural rocks. The present study addresses this topic in numerical simulations of uniaxial tension and three-point bending tests. For this end, a rock fracture model based on multiple intersecting embedded discontinuity finite elements is developed. The inherent (pre-existing) microcrack populations are represented by pre-embedded randomly oriented discontinuity populations. Crack shielding (through spurious locking) is prevented by allowing a new crack to be introduced, upon violation of the Rankine criterion, in an element with an initial crack unfavourably oriented to the loading direction. Rock heterogeneity is accounted for by random clusters of triangular finite elements representing different minerals of granitic numerical rock. Numerical simulations demonstrate the strength lowering effect of initial microcrack populations. This effect is substantially stronger under uniaxial tension, due to the uniform stress state, than in semicircular three-point bending having a non-uniform stress state with a clear local maximum of tensile stress.

Modeling of elastic-plastic deformation of the ring parts as applied to the analysis of the results of testing the material of the fuel-element cladding

An analytical formula for a smooth description of the tension diagram of EK-181 steel and a method for rearranging the diagram when changing the direction of deformation are proposed for the first time. The process of straightening a quarter of an annular sample and further stretching is numerically modeled. It is shown that the conditional yield strength of the material of the straightened sample is 7.5% less than the actual conditional yield strength of steel. It is shown that the test for pure bending of a cantilever sample in the form of a semicircle with the processing of the bending diagram (by analogy with GOST 3565–80 for torsion) provides an estimate of the conditional yield strength which is 32% higher than the actual yield strength. The possibility of numerical reconstruction of the tension diagram from the diagram of pure bending of a cantilevered semi-ring sample is proved. It is shown that this procedure really gives the value of the conditional yield strength of steel EK-181 with a tolerance for the residual deformation of 0.2%. The analysis of the test procedure for the rings of fuel element cladding and the proposed algorithm for determination of the conditional yield stress of the ring material is carried out. Attention is drawn to the arbitrariness of the choice of the designed load on the two-stage diagram of the diametrical tension of the ring and to the lack of scientific substantiation of the possibility of determining the yield stress on the second part of the diagram. It is shown that this method in the current form contradicts GOST for tensile testing due to the absence of a base with a uniform stress state on the ring. Therefore, the considered method is not recommended for determining the values of the conditional yield strength suitable for strength calculations.

Limit load instabilities of anisotropic tubes under combined tension and torsion

Under tensile loadings, thin-walled structures such as sheet metal and tubes develop load maxima, or limit loads, beyond which deformation localizes leading to rupture. For uniform stress states, Considère-type estimates of limit loads can serve as forming limits in applications. The paper explores the effect of anisotropy on such estimates for thin-walled Al-alloy tubes under combined tension and torsion. Anisotropy is modeled using Yld04-3D with an exponent of 8. Material hardening originates from a simple shear test using this yield function, taking into account material axes rotation caused by the shearing. A Considère formulation is developed for the problem, which also incorporates the effect of material frame rotation. The analysis is used to establish limit loads for a set of circumferentially constrained tension–torsion experiments tested under radial nominal tension-shear stress paths. The predictions reproduce the strains measured at the limit loads for the range of biaxiality ratios considered. By contrast, corresponding results produced using the isotropic yield functions of von Mises and Hosford(8) increasingly deviate from the measured results as the shear stress increases. Considère-type formulation is also developed for the same tension–torsion loadings for a uniform thickness tube. The results exhibit a similar trend but the limit strains for shear dominant paths are significantly lower, demonstrating the stabilizing effect of the circumferential constraint used in the experiments.

Understanding the effect of anisotropy in composite materials on the performance of cruciform specimens

The anisotropic nature of fibre-reinforced composite materials is often not accounted for during the design process of composite cruciform specimens. Here, the response of cruciform specimens depends on both - the layup configuration and the variations in the biaxial loading conditions. This work studies the effect of anisotropy in composite materials on the uniformity of the developed multi-axial stress state in cruciform specimens. For that, a combination of 3D finite element and analytical models are used to analyze the behaviour of 12 different cruciform specimen geometries consisting of different layups and subjected to different biaxial loading conditions. In this study, it is shown how the shape of the gauge zone, the shape of the corners formed by the adjacent arms and the presence of slots in the arms can be used to design cruciform specimens with uniform stress states in the gauge zone, while accounting for the varying anisotropy.

Comparison of the shear behavior in graphite-epoxy composites evaluated by means of biaxial test and off-axis tension test

Abstract Characterization of shear behavior in composite materials remains a not fully solved problem. In the last fifty years, many different approaches have been proposed to solve this problem (rail shear, thin-walled tube torsion, off-axis tensile, ±45° tensile, Arcan, Iosipescu, asymmetric four-point bend, plate twist, v-notched rail shear, off-axis flexural, and shear frame), although none of these approaches have achieved an unquestionable solution. For this reason, proposals of alternative methods and comparison between different experimental techniques are of interest. In the present work, the use of cruciform samples with the fiber oriented at 45° with respect to the load directions, and subjected to tension-compression (creating a pure shear stress state at the central part of the samples), is studied. The experimental results of the cruciform samples have been compared with the off-axis tests (with the fiber at 10°) for the same material (AS4/8552), finding a good agreement between the shear experimental curves, especially at the initial part of the curve, where the shear modulus is calculated. Nevertheless, the shear strength value obtained by means of the cruciform specimen has shown to be significantly lower than that obtained using the off-axis test. A Finite Element numerical model of the cruciform specimen has been developed to analyze the stress field of the samples. Numerical results have shown that there is a central area of the cruciform specimens where a pure and uniform shear stress state is developed, which is suitable for the evaluation of the shear constitutive law of the material. It has been observed that there is a (σ 22) stress concentration in the transition between the straight and curved parts of the boundary geometry of the samples, which explain some premature failures of the samples. This premature failure could be avoided with tabs extended up to the beginning of the central part of the sample.