Values Of Normal Stress Research Articles

A Static Friction Model for Elastic-plastic Contacting Surfaces Using Statistically Homogenized Technique

An improved static friction model is developed for elastic-plastic contacting surfaces. Two random variables of asperity height and curvature for Gaussian isotropic contacting surfaces are assumed to govern asperities distribution and they satisfy a joint probability distribution function. Based on the contact parametric representation of the KE model proposed by Kogut and Etsion, the expected macroscopic values of normal stress and shear stress are obtained by using statistically homogenized technique. Then the effect of roughness, the energy of adhesion and material properties on static friction coefficients are studied and the results show similar trend with literatures. A comparison of the present model with the SV friction model proposed by Sista and Vemaganti shows that the adhesion force has a more significant effect for smoother surface and SV model is more suitable for heavily loaded contacts. The static friction coefficient is related to the ratio of hardness to elastic modulus H/E. At the same roughness and normal load, lower H/E leads to lower critical interference ωc, higher plasticity index ψ, and smaller static friction coefficient.

Does In Situ State of Stress Affect Fracture Flow in Crystalline Settings?

AbstractWhether in situ state of stress affects fracture flow in approximately 1‐km‐deep crystalline rock setting is investigated by analyzing a unique fracture database, consisting of 193,698 discrete fractures out of which 6,223 were detected as flowing during steady state pumping. The database is based on fracture data acquired in deep surface‐based holes drilled during the site investigations for underground high‐level spent nuclear fuel repositories in Finland and Sweden. By computing effective normal and shear stress magnitudes on the planes of all identified fractures, we show that the probability for critically stressed fractures to be flowing is low and low normal stress values provide a slightly better indicator for fracture flow. Our results further show that at shallow settings, fracture criticality is driven by low normal stresses as the distributions of shear stress values for flowing and nonflowing fractures are very similar. The results also show that the effect of in situ stress on fracture flow is relatively low and in situ stress or its associated derivatives (shear and normal stresses) cannot be used directly in a deterministic manner for the prediction of fracture flow. As a consequence, if hydromechanical concepts are invoked for modeling fracture flow, a probabilistic approach is needed. In such an approach, we emphasize the role of low normal stresses over the concept of critically stressed fractures, as the use of normal stresses does not require the selection of a certain friction coefficient value, defining the boundary between critical and noncritical fractures.

Experimental study on long-term performance of steel-concrete composite bridge with an assembled concrete deck

Due to the new-old concrete age difference, the long-term behavior of steel-concrete composite bridge with an assembled concrete deck is different from that of a monolithic concrete deck. In order to address this issue, a 1/6 scale mock-up for the segmental beam of one practical cable-stayed composite bridge was precast and assembled. Long-term performance test of 390 days was performed on the assembled mock-up subjected to sustained loads. Moreover, shrinkage and creep tests were also conducted on concrete prisms for precast slabs and post-pouring joint of the assembled mock-up. Meanwhile, two types of finite element (FE) models were established by using Midas FEA. The first one adopted the proposed analysis strategy considering the influence of concrete age difference between the post-pouring joint and the precast slabs, and the other one adopted the traditional analysis strategy ignoring the influence of the new-old concrete age difference. The experimental results were compared with the numerical results calculated by these two FE models. The FEM results calculated by the proposed model agreed well with the experimental observations, while the traditional model severely underestimated the effect of creep and shrinkage of the post-pouring joint concrete. The effect of the long-term performance of the new-old concrete reduced the value of normal stress in the post-pouring joint, and tensile stress led by the shrinkage difference of the new-old concrete could appear the position adjacent to the post-pouring joint under the condition of insufficient axial force of the assembled concrete deck. Moreover, water curing of the whole concrete deck of the assembled composite beam after casting the concrete of post-pouring joint can reduce the tensile stress of the concrete deck. Much attention should be paid to the influence of concrete age difference between post-pouring joint and precast slabs, during the serviceability behavior analysis of the assembled steel-concrete composite bridge.

Study of the influence of stochastic inhomogeneity of a pipe on stress-strain behavior

The paper is devoted to stress-strain behavior of an elastic heavy-wall pipe from incompressible material, which is under the influence of compression hydrostatic pressure. We have obtained the analyticity condition for the solution of the problem in small parameters close to zero. These parameters are independent random variables. They characterize the deviation of the cross-section outline from a circle, and the inhomogeneity of the pipe material. The solution has been found by perturbation method up to the second order of smallness. It has been concluded that in the case of experimental analysis of the pipe state under the load meeting the specified condition, the average values of stress and displacement will be close to the values obtained at axially symmetric state.

PENGARUH VARIASI JUMLAH LAPISAN FIBERGLASS DAN ANYAMAN BAMBU TERHADAP KEKUATAN BENDING KOMPOSIT DENGAN METODE VACUUM ASSISTED RESIN INFUSSION (VARI)

The objective of this research is to obtain the bending properties of composite material produced by VARI method, and to get analysis of polyester resin distribution time on the layer of fiberglass composite and bamboo woven strip.The process of making composites varies, but the process of making this research is the process of Vacuum Assisted Resin Infusion (VARI). In the VARI process, dry fiber is placed between the fixmold and plactic bag, then the resin is injected after the chamber inside the low- pressure plastic bag and the process continues until all parts of the fiber are wetted by the resin.From the results of research, testing and discussion of bending test results that have been done then can be drawn conclusions include: Variation of the number of reinforcement layer on VARI method will affect the value of bending strength of composite. By using the VARI method The highest average value of bending stress occurs in variation 4 (206,69 MPa), while the lowest occurs in variation 1 (42,83 MPa), and the time required for the distribution of resin varies in time variation 1 which takes 5 minutes 02 seconds, while for variation 4 takes 8 minutes 3 seconds.Keywords: composites, VARI method, glass fiber, bending test

Evaluation of residual stress in HVOF stellite-6 coatings using non-contact drilling

Because of high hardness of stellite coatings it is impossible to evaluate the residual stress through the thickness using standard methods such as hole drilling (ASTME837). Applying a drilling method with hardness independent nature is necessary to find the stress state from the surface toward coat-substrate interface. In this study the residual stress profile of stellite-6 overlaid on steel substrate has been studied using electro-discharge milling. To have an exact evaluation on residual stress, the elasticity modulus of coating was measured applying Knoop indentation. Scanning electron microscopy and XRD were used to study the microstructure, morphology phase composition of powder and coating. The maximum compressive and average value of residual stress from surface to the coat-substrate interface obtained of −198 MPa and −108 MPa respectively. The explicit and implicit finite element was used in order to develop a model for simulation of the peening and cooling stresses respectively. The implicit result for coating sprayed in 740 m s−1 indicates the presence stresses of +170 MPa and −270 MPa at the surface and interface respectively with average of −93 MPa thorough the thickness which show good agreement between the experimental and numerical results.

An uncoupled thermoelasticity problem for a semi-infinite layer with regard of its proper weight

The exact solution of the uncoupled thermoelasticity problem for a semi-infinite elastic layer with regard of its proper weight was constructed. The solving method is differ from the known previous and is based on reducing of Lame equations to two jointly and one separately solvable equation. It allows application of integral transformations directly to the transformed equations of equilibrium and makes it possible to reduce the initial problem to a one-dimensional vector boundary problem. A special technique is given to calculate multiple integrals containing oscillating functions that appear during the inversion of the transformations. The character of the temperature and proper weight influence on the value of normal stresses on the lateral face of the semi-infinite layer, the zone of tensile stresses depending on the shapes of the distributed load section and the temperature and Poisson's ratio is established. The parameters of dimensionless mechanical load and temperature, when the separation of the side wall of the semi-infinite layer can be eliminated, were established. A study of the influence of the layer’s proper weight on the stress emerging on the layer’s edge is conducted. The constructed exact solution can be used as a model for solving a similar class of problems by numerical methods.

Analytical determination of the soil strength parameters by the number of impacts of the dynamic instrument falling weight

The analytical connection of the number of impacts with the energy of gravity forces of the falling weight and instrument has been determined. The energy realized when striking the rod has two components: energy of the free falling weight when moving relative to the casing which occurs before the moment of the impact with the collar, and energy of the gravity forces of the weight and casing, which arises additionally while vertical lowering to the rod penetration depth in the transition process. The soil strength is numerically determined as per the normal stress value at the end of the rod being penetrated into the soil. The numerical determination of the normal stresses has been performed by means of the theorem on the mechanical system motion quantity change and theorem on the kinetic energy change. The identity of the numerical figures of the normal stress value at the end of the rod and specific energy of the impact penetration of the cylindrical rod into the test soil has been determined. The analytical connection of the impact energy and soil strength parameters with the dynamic instrument parameters has been obtained – with the falling weight mass, instrument casing mass, weight falling height, rod penetration depth, instrument geometric parameter. For the first time in relation to the impact instrument the loss of the kinetic energy and efficiency of the falling weight impact process in case of the direct inelastic impact of two bodies has been determined according to Carnot theorem. The values of the soil deformation specific energy and of normal stresses under the flat end of the rod for soil classes in the function of the number of the dynamic instrument impacts have been determined.

Cracking of thick-walled fiber composites during bending tests

When formulating fracture criteria for fiber composites, it is essential to know the characteristics of the material’s microstructure. The present research is carried out on specimens made of composite laminate with thermoplastic matrix PA6 reinforced by continuous glass fibers. Specimens are subjected to static three- and four – point bending tests at different spans between supports, resulting in different values of the ratio between maximal normal and shear stresses. Both distribution of normal stress coming from bending and distribution of shear stress, according to transverse shear stress in beams equation, coming from shear are considered. The results of the analysis are as follows: variation in the cross-section of shear stresses according to the transverse shear stress in beams equation, and distribution of bending stresses, according to the elastic model (value of normal stress increases linearly with increasing distance from the neutral plane). Depending on the combination of shear and normal stresses, cracking is initiated by either shear or normal stress.

Characterization of peak shear strength of rough rock joints using limited displacement multi-stage direct shear (LDMDS) tests

Current standard direct shear test methods for rock joints do not account for damage to the specimens' asperity profiles; tests require shearing of a single specimen to large displacements under successive normal stresses (the multistage test), or the use of similar specimens in multiple tests. Due to the inherently unique nature of rock joints and corresponding difficulty in obtaining specimens with identical or even similar geometries, multistage tests are more common. A major issue with the multistage test is that successive shearing of the specimen damages the surface asperities and changes its overall roughness profile, reducing the peak shear stress and consequently resulting in underestimation of the friction angle and overestimation of the joint shear intercept (cohesion). The limited displacement multistage direct shear (LDMDS) test method minimizes these testing imperfections by allowing shearing of a single specimen without extensive asperity damage, accomplished by immediately pausing shear displacement once peak shear stress has been reached, then proceeding to shear the specimen under the following normal stress value, and shearing into the post-peak region only after identifying multiple values of peak shear strength. The authors have validated the LDMDS procedure using cement replicates of rock joints, demonstrating that it yields more accurate strength parameters than the standard multistage direct shear test.

The influence of polygonal cavity on fracture behaviour of concrete

In this work, the influence of polygonal cavity on fracture behaviour of cement composite (concrete) is investigated. Specimens of the nominal dimensions 40 × 40 × 160 mm with polygonal cavity of 8 × 8 × 40 mm were provided with an initial central edge notch with a depth 12 mm, which was made by diamond blade saw. To determine the influence of polygonal cavity on fracture behaviour, fracture tests were conducted via three-point bending. The aim of this work is to analyse the behaviour of such specimen by means of finite element method (FEM) principles in Ansys, Inc. software. For this reason, a simplified 2D model was created for plane strain conditions and based on the fracture test configuration. The crack propagation assessment was based on generalized fracture mechanics approaches using a criterion of an average value of tangential stress determined in dependence on the polar angle θ. Results of numerical analysis indicates that the actual crack depth must be greater than it supposed. In other words, diamond blade saw damage the specimen more than expected.

МАТЕМАТИЧЕСКОЕ МОДЕЛИРОВАНИЕ НАПРЯЖЕННО-ДЕФОРМИРОВАННОГО СОСТОЯНИЯ В ПОВЕРХНОСТНО УПРОЧНЕННЫХ ВТУЛКАХ С УЧЕТОМ ОСТАТОЧНЫХ КАСАТЕЛЬНЫХ НАПРЯЖЕНИЙ

We suggest the phenomenological mathematical model of the stress-strain state reconstruction in the surface hardened thin-walled tube with an inner diameter 45 mm and outer diameter 51.5 mm made of steel EI961 and treated by diamond smoothing of the outer surface. It is shown that if all stress components depend on radius only, then the components are in the cylindrical coordinate system. The experimental research is made for the samples which were softened under two load modes (radial force) of the diamond ball attachment of 200 and 300 N value. The experimental values of residual stresses , , and in the surface layer are obtained by the ring and strip method using the layer-by-layer electrochemical pickling of the hardened layer. The experimentally measured values of the strip beam deflection, split ring angular opening and axial displacement of cut edges relative to each other are used for this purpose. The hardening anisotropy parameter which relates the axial and circumferential components of plastic strain is included in the mathematical model. To solve the formulated problems we use the hypotheses of plastic incompressibility of the material, the absence of secondary plastic deformations of the material in the surface layer compression area and the hypotheses of flat sections and straight radii. We present the method for solving the stress-strain state reconstruction boundary value problems, which allows obtaining the missing component and all residual plastic strain components. The validation of the computational data obtained by mathematical modelling for adequacy to the experimental data for the two modes of hardening is made. There is a close agreement between the computational and experimental data. The numerical values for the hardening anisotropy parameter are given. By using this parameter we are able to theoretically describe the observable experimental layering of axial and circumferential stresses in depth of the hardened layer. It is theoretically and experimentally established that the absolute values of maximum shear stresses is an order of magnitude smaller than the absolute values of maximum normal stresses. We also discuss the questions of the effect of shear stresses on high-cycle fatigue and creep of the hardened thin-walled tubes. The main results of the research are illustrated by the tabular data and corresponding diagrams of the residual stresses distribution in depth of the hardened layer.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ ВЫСОКОТЕМПЕРАТУРНОЙ ПОЛЗУЧЕСТИ ЭЛЕМЕНТОВ КОНСТРУКЦИЙ ИЗ ЖАРОПРОЧНЫХ СПЛАВОВ С УЧЕТОМ ЭФФЕКТОВ НЕЙТРОННОГО ОБЛУЧЕНИЯ

The technique of numerical research on the basis of FEM processes of deformation and damage accumulation in the structural elements of heat-resistant alloys under conditions of high-temperature creep taking into account the influence of neutron irradiation is developed. The description of the mechanical behavior of the material is carried out within the framework of the previously developed general model of the damaged material and the creep model for non-irradiated heat-resistant alloys, supplemented by taking into account the effect of irradiation on the creep rate and the appearance of brittle fracture in a given range of temperature variation and irradiation intensity. The defining relations of the creep model of the irradiated material were obtained by modifying the creep model of the non-irradiated material: a material function was introduced, taking into account the effect of the flux of neutrons on the rate of thermal creep deformation; a material function was introduced that takes into account the effect of the neutron flux on the creep surface radius; A material function was introduced, which takes into account the effect of the neutron flux on the ultimate value of the dissipation energy at full power. To simulate the processes of brittle fracture during creep under neutron irradiation conditions, it is assumed that the destructive values of effective normal stresses are a function of temperature, flux of neutrons and the current value of accumulated creep. The material functions of the model were obtained from the results of basic experiments conducted at the Research Institute of Mechanics for the heat-resistant alloy without irradiation under consideration and the available experimental data on the study of the creep of this alloy during its irradiation. Based on the proposed model, a numerical method for solving problems of high-temperature creep of structures made of heat-resistant alloys under neutron irradiation was developed and implemented within the UPAKS computing complex. To verify and illustrate the capabilities of the developed methodological and software tools, a number of problems of modeling the processes of high-temperature creep and destruction of structural elements made of the high-temperature alloy under consideration are solved.

Fast self-navigated wall shear stress measurements in the murine aortic arch using radial 4D-phase contrast cardiovascular magnetic resonance at 17.6\u2009T

Purpose4D flow cardiovascular magnetic resonance (CMR) and the assessment of wall shear stress (WSS) are non-invasive tools to study cardiovascular risks in vivo. Major limitations of conventional triggered methods are the long measurement times needed for high-resolution data sets and the necessity of stable electrocardiographic (ECG) triggering. In this work an ECG-free retrospectively synchronized method is presented that enables accelerated high-resolution measurements of 4D flow and WSS in the aortic arch of mice.Methods4D flow and WSS were measured in the aortic arch of 12-week-old wildtype C57BL/6 J mice (n = 7) with a radial 4D-phase-contrast (PC)-CMR sequence, which was validated in a flow phantom. Cardiac and respiratory motion signals were extracted from the radial CMR signal and were used for the reconstruction of 4D-flow data. Rigid motion correction and a first order B0 correction was used to improve the robustness of magnitude and velocity data.The aortic lumen was segmented semi-automatically. Temporally averaged and time-resolved WSS and oscillatory shear index (OSI) were calculated from the spatial velocity gradients at the lumen surface at 14 locations along the aortic arch. Reproducibility was tested in 3 animals and the influence of subsampling was investigated.ResultsVolume flow, cross-sectional areas, WSS and the OSI were determined in a measurement time of only 32 min. Longitudinal and circumferential WSS and radial stress were assessed at 14 analysis planes along the aortic arch. The average longitudinal, circumferential and radial stress values were 1.52 ± 0.29 N/m2, 0.28 ± 0.24 N/m2 and − 0.21 ± 0.19 N/m2, respectively. Good reproducibility of WSS values was observed.ConclusionThis work presents a robust measurement of 4D flow and WSS in mice without the need of ECG trigger signals. The retrospective approach provides fast flow quantification within 35 min and a flexible reconstruction framework.

Generation of Optimized Voronoi Based Interior Structures for Improved Mechanical Properties

General design approaches are related with and inspired by natural examples. Since evolution itself pushes beings to their full potentials in the most efficient way, it is important for us to understand, observe and mimic the nature itself. In this study, naturally inspired internal structures are constructed by using hollow Voronoi cells and models are optimized to meet the design requirements. For this purpose, design volume firstly undergoes topology optimization with specific load and support conditions, and then the volume is divided into several density regions. Cell generation algorithm advances according to the average stress values in these corresponding regions, the number of seeds in structurally critical regions is higher than others. Thus, the results of the optimization are reflected on the models. Since the Voronoi based interior has high porosity, the selected test models are manufactured via Fused Filament Fabrication process. The proposed approach is tested on these cases and the performance of the method is compared with respect to the cases having random Voronoi site distributions. According to the test results, strength-to-weight ratios of the tested artifacts are enhanced.

Numerical and experimental tests of steel-concrete composite beam with the connector made of top-hat profile

The paper presents solution for a steel-concrete composite beam structure. The connectors were executed from sections of a non-weldable top-hat profile 80 × 85 × 3 made of S235 steel. The connectors were fastened to the beam with four shot nails.In the first part of the work the push-out tests of connectors were carried out. The tests allowed to optimize the length of the connector. The connectors with a length of 60 and 100 mm were considered. The results were verified in a numerical analysis.In the second part of the work, for the best variant of the connector, two models of the composite ceiling beam were made: a numerical one and in real scale. The ceiling model was constructed from an IPE I-beam, galvanized trapezoidal sheet, concrete, reinforcing mesh and top-hat connectors. The values of normal stresses and displacements were determined based on the numerical analysis. These results were compared with the displacements obtained in experimental test.The numerical analysis was carried out using the ADINA System program based on the Finite Element Method. The stresses and displacements in the steel and concrete parts as well as cracking in concrete were assessed.

Necessary height of the vertical stiffeners in steel silos on discrete supports

The steel silos are interesting complex facilities. In order to ensure unloading of whole amount of stored product by gravity, the steel silos are often placed on supporting frame structure. Values of stresses in the joints between the thin walled shell and supporting frame elements are very high. It can causes local loss of stability in the shell. To prevent its local buckling, many designers put stiffening elements above the supports. Here the question is how high should be the stiffening elements? The right solution is that they should reach that level till which the values of the meridional normal stresses above the supports and in the middle between them are equalized. Under this level the cylindrical shell will be considered as a ring beam, stiffened by elements above the supports. Above it, the cylinder can be calculated as continuously supported shell. But where is this level? A lot of researchers worked on values and way of distribution of normal meridional stresses above the supports of the cylindrical shells. As a result of their efforts are determined critical height Hcr of the shell and the ideal position HI of intermediate stiffening ring. But these heights are considerably different between each other. To which of them our vertical stiffening elements should achieve?

EFFECTS OF CULVERT SHAPES ON POTENTIAL RISK OF HYDRAULIC FRACTURING ADJACENT TO CULVERTS IN EMBANKMENT DAMS

Hydraulic fracturing is generally considered as one of the most probable causes of dam failures and concentrated leakage occurring adjacent to the outlet conduits in embankment dams. It is thought that hydraulic fracturing will occur in a fill dam when the stress in the dam is reduced to levels that are lower than the water pressure, causing the subsequent propagation of cracks in the dam body. This phenomenon might be closely related to the arching action which occurs around the culverts due to the effects of the culvert configurations. The aim of this study is to investigate the effects of culvert shapes on the potential for hydraulic fracturing close to culverts in fill dams using the finite element method. Many numerical analyses are taken here to determine the stress distributions around culverts with various shapes. The possibility of hydraulic fracturing is then predicted by comparing the values of normal stress and water pressure. The results reveal that there is a probable risk of hydraulic fracturing occurring adjacent to box-shaped culverts in embankments. In addition, box-shaped culverts with inclining chamfers or arc-chamfers on the culvert top also have a similar potential for hydraulic fracturing because the chamfers have a negligible effect on the stress on the sides of the culverts. However, in culverts with slanted walls, that have a gradient for the slanted walls equal to 0.4 or greater, the risk of hydraulic fracturing might be reduced.

Alternative Analytical Solution of Consolidation at Constant Rate of Deformation

In this paper, we present an alternative solution of consolidation at constant rate of deformation (CRD). Governing equation of CRD consolidation is formulated considering excess pore water pressure as primary variable. Formulation of the governing equation is done for soil as linear as well as nonlinear materials. An analytical solution to the governing equation is derived which consists of transient state and steady state components. The solution derived herein yields profile of excess pore water pressure distribution with depth at any time during the test. The present paper analyzes the excess pore water pressure distribution profile during CRD consolidation and factors affecting it in detail. Analysis of the obtained solution indicates the existence of moving boundary condition within the test specimen at early stage of the test where transient state condition prevails. It also indicates that the pore water pressure ratio affects significantly the excess pore water distribution profile, in turn, average value of pore water pressure and average effective stress in the specimen. An expression for consolidation parameters for steady state condition are derived using the present solution. Further, evolution of excess pore water pressure at the base of CRD consolidation test specimen during the test is predicted for normally consolidated as well as for over consolidated soils at different rates of deformation.

Evaluation of indoor heat stress on workers of bakeries at Assiut City, Egypt

This study investigates indoor heat stress among employees engaged in bakeries in Assiut city. Heat stress is one of the occupational hazards especially in arid climates that impose negative impacts on workers such as heat rashes, heat exhaustion, heat cramps, heat fainting or heat stroke. In this research, 100 subjects were chosen randomly from 20 bakery stations to testify heat stress impacts. Heat stress levels were measured using wet-bulb globe temperature index (WBGT) to evaluate the impacts of environmental factors on the employees. The response of worker’s body to this heat stress is expressed as physiological strain index (PSI) that can be estimated by measuring heart rate and core temperature. Results obtained from WBGT index measurements indicated that the average value of heat stress in bakeries reached 31.6 °C that is exceeding than threshold limit value (28 °C) and action limit (25 °C) recommended by American Conference of Government Industrial Hygienists. In addition, WBGT values were compared with the Egyptian Environmental Law 4/1994 and Egyptian Labor Low 12/2003. PSI values fall in the range of 200 < M (metabolic rate) < 260 indicating that workload in bakeries environment lies in the moderate class. According to the questionnaire data, several workers faced heat disorders including heat exhaustion (23 cases), heat cramps (17 cases), and heat stroke (5 cases). Application of control measures such as engineering control, administrative control, and personal protective equipment should be applied.