Search

To address the engineering problem of wall collapse and destabilization during the construction of the slurry trench wall, the stability of the slurry trench wall under the condition of three-dimensional (3D) unsaturated stable seepage is studied using the upper limit analysis method and in combination with the three-dimensional rotating failure mechanism. The expression of the factor of safety (FS) for the unsaturated soil slurry trench was deduced by the gravity increase method. Additionally, a genetic algorithm was used to search for the minimum FS and the critical slip surface, and the obtained results were compared and validated with existing literature results. The effects of relevant parameters on the stability of the slurry trench under the condition of unsaturated seepage were analyzed in detail. The results of this paper indicated that ignoring the 3D effect of the slurry trench will underestimate the stability of the slurry trench. The presence of suction in unsaturated soil is beneficial to the stability of the slurry trench, and the impact of suction is closely associated with seepage conditions and the characteristics of the soil. The increase in surcharge load can lead to a decrease in the FS, and its effect on slurry trench stability increases with the enhancement of the three-dimensional effect. The impact of different seepage conditions on the sliding surface of the channel wall of clay strata is more significant, while the effect on the trench wall of sand strata is negligible. [ABSTRACT FROM AUTHOR]

The modified suction caissons (MSCs) have a cylindrical external skirt surrounding the top of the traditional suction caisson to increase the horizontal bearing capacity. MSCs will be subjected to inclined lateral loadings for anchoring offshore floating facilities, and the maximum lateral bearing capacity will be reached when the loading direction passes through the rotation center of the MSCs to generate horizontal translation in the seabed. This paper proposed an analytical model consisting of a meniscus failure wedge near the mudline and a purely lateral flow‐around mechanism in deeper soil for MSCs moving laterally in clays. The upper‐bound analysis is conducted to obtain the ultimate bearing capacity of MSCs by optimizing only one parameter to approach the critical failure mechanism that offers the minimum energy dissipation. The influences of the external skirt length (L) and radius (R) and adhesion factor (λ) at the caisson‐soil interface on the bearing capacity of MSCs are investigated. The finite element limit analysis (FELA) results are used to verify the rationality of the presented failure mechanisms and upper‐bound predictions. It is proved that the presented upper‐bound analyses can well predict the lateral ultimate bearing capacity of the MSCs with a horizontal translation failure. [ABSTRACT FROM AUTHOR]

Suction caissons supporting offshore wind turbines are subjected to eccentric horizontal loadings, which may lead to the overturning failure. This paper presents a modified three-dimensional (3-D) failure mechanism to predict the anti-overturning bearing capacity of suction caissons in clay under undrained condition. The modified failure mechanism is composed of a meniscus-conical wedge having meniscus shape at the mudline and a spherical failure in deeper soil. The meniscus-conical failure wedge predicts the deformation of the mudline better than the traditional ring-conical wedge, and the spherical failure keeps a kinematically admissible velocity field, ensuring the accuracy of the upper-bound theorem. Accordingly, the anti-overturning bearing capacity of suction caissons are deduced based on the work-energy balance equation, following by optimizations of two parameters to approach the critical failure mechanism that offers the minimum energy dissipation. In addition, the reliability of the presented upper-bound analyses are verified against the results from finite element limit analysis (FELA) as well as existing experimental and theoretical methods. [ABSTRACT FROM AUTHOR]

This study introduces the Energy Method Upper-bound (abbreviated as EMU), originally proposed by Donald and Chen (1997), for calculating bearing capacities with a focus on earthquake loadings affecting layered geological strata. The theoretical components of this study consist of (1) an extension of Prandtl's solution for bearing capacity analysis to inclined surface loads, (2) a mathematical demonstration of the theoretical congruence between EMU and the Prandtl-Reissner solution, and (3) a validation of the numerical outcomes through four illustrative bearing capacity examples with known closed-form solutions. This innovative approach eliminates the semi-empirical coefficients typically required in conventional bearing capacity evaluations, thus enhancing its relevance to seismic analysis for stratified geological formation that can hardly be evaluated with empirical coefficients accurately. The study also discusses various technical aspects such as seismic load determination, the use of undrained shear strength, and specifications for allowable safety factors under seismic conditions. With these methodologies, the paper assesses the seismic bearing capacities of two different buildings, with different scales and footing types. Additionally, a computer program, BEARING-IWHR, featuring an Excel interface and open-source coding, is available online. This method provides a theoretically sound and practically feasible framework for addressing seismic bearing capacity on stratified foundations. • Presentation of an upper-bound numerical method capable of evaluating seismic bearing capacity on layered geological strata, eliminating the reliance on semi-empirical coefficients. • The theoretical foundation of this method has been extensively validated through analytical formulations and comparisons with established closed-form solutions, derived from the slip-line field theory. • The practical applicability of this method has been appraised by two case studies involving multilayer stratum by the spread sheet software BEARING-IWHR that is downloadable at the web, facilitating widespread accessibility and utilization. [ABSTRACT FROM AUTHOR]

When studying the stability of a slope, the first issue that needs to be clarified is the slip surface, which determines the minimum safety factor. The slopes investigated here are homogenous with three distinct gradients (1:1.5; 1:1; 2:1), two defined heights (H-3 m; H-8 m), and four different soil characteristics (S1—clayey silt, S2—sandy clayey silt, S3—sandy silty clay, S4—clay). The purpose of this paper is to develop a new methodology capable of estimating the safety factor and the shape and centre of the critical slip surface, delivering an improved estimate of slope probability of failure, which can represent a significant component in a more precise risk assessment. This paper compares distinct methods used in the slope stability analysis, examining their hypotheses and effects on the estimated safety factor and the centre and shape of the critical slip surface. The study compares the limit equilibrium results with those determined by the shear strength reduction method using an approach based on the upper-bound limit analysis to compare the predictions extracted from these methods with those from the finite element method (FEM) analysis. The finite element method discretizes the soil mass into finite elements. Hence, it establishes a kinematically admissible velocity field searching for the failure mechanism of the slope. Results for FEM show the influence of the slope geometry and the mesh size and density on the safety factor. In the study, plots of the regression curves of five different critical slip surface shapes, including a circular slip surface (benchmark), show that the shape of the failure surface depends on the shape and material of the slope. Furthermore, they show that the critical slip surface layout can approach a logarithmic spiral, damped sinusoid, parabola, etc.; the slip surface is not always circular. The analysis reveals that none of the approaches can consider all uncertainties concerning the factor of safety and the interpretations of critical slip surfaces. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Central South University is the property of Springer Nature and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract The exact calculations of the stress and strain distributions based on the controlling equations for a forming process with large deformation are often difficult. To circumvent such difficulties, some analytical methods such upper-bound analysis and slip-line field theory have been established by making a number of simplifying assumptions regarding the material properties and deformation modes. In this work an analytical model based on the upper-bound theory was successfully developed to predict material flow pattern and maximum process loads for an Equal Channel Angular Pressing (ECAP) die with circular cross-section and an intersecting channel angle of 120°. Based on the model, the power dissipated on all frictional and velocity discontinuity surfaces were determined and optimized in order predict the maximum process force as function of the channel geometry and the material plastic behavior. To validate the developed model, the ECAP die were produced and used to determine experimental load-displacement curves of AA6061-T6 specimens with different lengths. A good correlation between theoretical and experimental results was observed. In addition, the constant friction factor demonstrated to have a strong effect on the relative extrusion pressure.

This paper develops the 2D and 3D kinematically admissible mechanisms for analyzing the passive face stability during shield tunneling using upper-bound analysis. The mechanisms consider trapezoidal distribution of support pressure along tunnel face and partial failure originated at tunnel face above invert. For cohesionless soils, the support pressure is a function of soil effective frictional angle φ′ which determines the inclination of failure block and the normalized soil cover depth C/D (soil cover depth/tunnel diameter) which affects the origination of the passive failure. For cohesive soils, the support pressure is a function of φ′, C/D, and the effective cohesion c′. The cohesion c′ has a relatively smaller impact on the support pressure than φ′ and C/D have. The mechanisms are verified by comparing the current solutions with a previous upper-bound solution. The comparison shows that the current solutions are a general solution which is capable of predicting the passive face failure originated at any depth along tunnel face and the previous solution is a particular solution with the assumption that the face failure originated at tunnel invert. The mechanisms are validated through application to a practical project of shallowly buried, large diameter underwater tunnel. The validation shows that the mechanisms are capable of assessing the tunnel face passive instability rationally. [ABSTRACT FROM AUTHOR]

Soilbags have good reinforcement effect on soil slopes. In this paper, traditional soilbag is modified by adding a tail to it. Model tests have been used to study the influence of soilbag tail length on the reinforcement effect in soilbags reinforced slopes. Moreover, a permissive failure pattern and a corresponding velocity field have been established based on the experimental results. Then, the ultimate failure heights of the reinforced slopes are obtained by using the upper-bound solution theory and compared with the experimental results. Both the experimental and analytical results show that within a certain limit, the reinforcement effect improves with the increase of tail length. However, when the tail length is over a certain value, the increase of the tail length does not improve the reinforcement effect any more. This provides theoretical basis to the optimized design of soilbags reinforced slopes. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a new 3D collapse mechanism to investigate the face stability of a shallow tunnel embedded in the saturated and multi-layered soils with short-term condition. The new collapse mechanism is composed of five rigid truncated elliptical cylinders on which a distributed force acts. The limit support pressure is determined by an upper-bound analysis. A simple method given by the fitting formula is presented to estimate the limit support pressure. Analytical solutions obtained by the new collapse mechanism are validated by numerical simulations. It has shown that the proposed collapse mechanism has good accuracy and efficiency. [ABSTRACT FROM AUTHOR]

Highlights • A theoretical analysis based on upper-bound analysis for wavy surfaces with assumptions supported by previous slip-line plasticity analyses and observations from surface deformation experiments. • A perturbation method that yields the average fluid pressure accounting for the effect of the squeezed fluid film with leakage at groove ends. • Solutions of normal approach, average surface rise, cavity volume, and surface roughness of the deformed surface versus fractional contact area, with direct applications to metal forming processes. • Confirmation of relations and assumption of uniform surface rise at noncontact regions due to contact deformation. • A new definition of the real contact area that resolves some paradoxical reasoning for multiscale rough surfaces. Abstract An upper-bound plasticity analysis based on volume conservation and presumed uniform surface rise of noncontact regions was used to develop analytical models of a rigid-perfectly plastic half-space with a flat surface indented by a sinusoidal rigid surface and a rigid-perfectly plastic half-space with a sinusoidal surface compressed by a rigid flat surface. The hydrodynamic effect of lubricant fluid pressurized in the surface grooves of the deformed half-space on the deformation process was analyzed by solving the Reynolds equation of a squeezed film using a perturbation method. Analytical solutions of the fractional contact area, normal approach, average surface rise, cavity volume, and surface roughness were obtained after plastic contact deformation. Numerical results of a finite element analysis of relatively compliant and stiff elastic-nearly perfectly plastic materials are shown to be in good agreement with analytical results. The hypothesis of uniform surface rise of noncontact regions is shown to be a reasonable assumption, leading to a good approximation of global deformation quantities and providing an accurate description of changes in the surface profile for relatively large contact areas. The results of the present study provide a basis for explaining the full plastic merger of asperity contacts in multi-scale rough (fractal) surfaces and elucidate the role of surface roughness in metal-working processes. [ABSTRACT FROM AUTHOR]

In this paper, an upper-bound analysis is developed to model the rod extrusion process of the strain hardening materials. A velocity field, including radial and angular velocity components, is assumed in the deformation zone. This field permits to predict the possible formation of a central burst. The strain rate levels were calculated in each region of billet to apply strain hardening effect in the presented central bursting criterion. The presented criterion is compared with FEM simulation data and the experimental data of the published papers. The results show that the central bursting defects are affected primarily by the strain hardening exponent. So that by increasing the strain hardening exponent, the possibility of the central bursting defects and the relative extrusion pressure is being decreased. Based on the obtained criterion, the diagram is presented for the safe zones in a wide range of process variables. [ABSTRACT FROM AUTHOR]

• The face failure mode of shield or TBM launching with a concrete box is revealed. • A failure model based on the LA method for launching in semi-infinite body is built. • The soil arching effect of the slope overburden is considered using the LE method. • Application effects of proposed models for two project cases are analyzed. An emerging construction method using a concrete box can effectively reduce the risks such as face collapse and extrusion at the launch stage of shield or tunnel boring machine (TBM) tunnelling. It occupies a small construction plant and can be applied to complex urban environments, especially in mountainous cities. However, the failure mechanisms of the tunnel face at the launch stage of the novel method are still not properly understood. Based on the upper-bound theorem of the limit analysis (LA), two multi-block failure models are proposed in this work for tunnelling launch in semi-infinite body and in areas with slope overburden. The limit equilibrium (LE) method is used to obtain the vertical earth pressure of upper finite soil mass considering the soil arching effect. The comparisons of the support pressure, failure zone and dip angle of the slip surface obtained from the proposed models with the results of numerical simulations and previous studies indicate that the proposed approach provides realistic and satisfactory predictions. The effects of soil cohesion, internal friction angle, slope dip angle, reduction coefficient and the ratio of tunnel diameter to overburden width are further assessed. The proposed models were applied to two practical engineering cases to determine reinforcement measures using grouting and pipe-roof methods required during the tunnelling launch, and were demonstrated to be able to provide useful guidance for the practical operations. [ABSTRACT FROM AUTHOR]

T-shaped equal channel angular pressing (T-shaped ECAP) is a novel process of severe plastic deformation (SPD). The maximum effective strain per pass in the T-shaped ECAP is larger than that in the ECAP. Deformation models of the T-shaped ECAP have been proposed by a designed grids deformation experiment considering the deformation dead zone (DDZ) formation or without it. Based on the deformation models, an upper bound approach has been used to analyse the T-shaped ECAP process. The effects of the plastic deformation zone (PDZ) angles and the friction factor on the relative pressing pressure and the DDZ are analysed. It is found that the relative pressing pressure increases with an increase of the angle and the friction factor m, however, the DDZ decreases. The upper bound solution for the T-shaped ECAP with the DDZ is more close to the experimental load than that without the DDZ. The results show that there is a good agreement between the theoretical value and the measured maximum load required for the T-shaped ECAP conducting.

Co-extrusion of strips of different strength and ductility is a process in the production of bimetal rods of desirable physical and mechanical properties. Equal channel angular extrusion (ECAE) is an innovative process for applying severe plastic deformation to materials. The present study is concerned with the ECAE of different bi-layer strips. Developing a kinematically admissible velocity field using a Bezier-type streamline is the aim of present research. In this analysis, using the mentioned velocity field and the upper bound theorem, the extrusion force as well as the strain distribution in the deformation zone of the ECAE process are predicted. The solution takes into account the die profile, friction conditions, strength, arrangement and thickness ratio of the layer materials. Experimental results of ECAE of aluminum/copper and aluminum/mild steel bi-layer strips showed a good agreement with the predicted results. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Geotechnical Journal is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Fastener holes used in the mechanical joints are vulnerable to failure due to development of stress concentration at their edges. Inducing compressive residual stresses by different techniques has been the most common method to reinforce the holes to date. In this work, a new reinforcement technique called internal torsion, which can be classified as a localized severe plastic deformation process, is proposed as an alternative to the cold expansion pre-stressing. A special specimen is designed to represent the behavior of a typical fastener hole during the internal torsion process. The deformation of the specimen in the vicinity of its hole surface is studied by introducing a parametric kinematically admissible velocity field (PKAVF) within the deformation affected zone (DAZ). Calibration of the parameters in relation to the deformation of the material during the process is done by an elastic-plastic finite element solution that was performed in ABAQUS for a specimen made of interstitial free (IF) steel. Numerical analysis of the deformation is carried out to understand the process and to estimate the optimum process parameters. Subsequently, the calibrated model is used in an upper-bound solution of the problem to estimate the torque-twist response of the specimen during internal torsion. Finally, the results of upper-bound solution are compared with those of finite element analysis. There is a good agreement between the upper-bound solution and finite element results, which verifies validity of the calibrated velocity field model and the upper-bound solution based on the model for the internal torsion problem. [ABSTRACT FROM AUTHOR]

Abstract: In this research in order to predict the strain distribution and extrusion load in equal channel angular extrusion (ECAE) process of circular cross section a new three dimensional kinematically admissible velocity field has been developed based on Bezier formulation. The strain distribution in deformation zone and the effects of various terms on extrusion load were determined. The results were compared both with the experimental and numerically predicted results reported in the literature. It was found that extrusion pressure decreases with increasing both the die angle and the outer curved corner angle and increases with increasing the friction coefficient. Also, it was exhibited that increasing the angle of outer curved corner increases the inhomogeniety of strain field of deformation zone. The predictions were in a good agreement with the FEM and experimental results reported by previous researchers for different die angles and frictional conditions. [Copyright &y& Elsevier]

Much research has been carried out in the manufacturing of helical gears by cold forging or by extrusion. Although cold forging is applied to some bevel, spur, and helical gears, problems in connection with the reduction forming load and tool life still make it difficult for these methods to be commercialized. In this study, focusing on reducing a load in forming helical gears, the extrusion of helical gears by two-step process is proposed. This process is composed primarily of extruding a billet to a spur gear and then twisting the previous spur gear extruded to a helical gear. Cylindrical billets of Cr-Mo steel(SCM 415) and aluminium alloy(Al60 series) were used as specimen materials for the experiments. The maximum loads obtained by upper-bound analysis and FEM are compared with the results of experiments. The loads of the analysis have good agreements with those of the experiment. The newly proposed method can be used as an advanced technique that remarkably reduces the forming load and replaces the conventional forming. [ABSTRACT FROM AUTHOR]

This paper provides an in-depth study of the upper-bound slope stability analysis method proposed by Donald and Chen using inclined slices to simulate the sliding mass (Energy Method Upper-bound, abbreviated to EMU). After a brief review of the theoretical background and numerical method of the EMU, the authors present a weightless slope subjected to a vertical ultimate load to demonstrate its theoretical background. It has been proven that the analytical formulation of the EMU is identical to the closed-form solution given by the slip line method (SLM) for this selected example. Conversely, using its computer program, EMU has produced several closed-form solutions selected from Sokolovski. Thus, the potential for applications in practical engineering problems is demonstrated. With its theoretical background and numerical feasibility, EMU is extended to the area of bearing capacity of strip footings whose theoretical background is based on the slip line theory by Prandtl but involves various semi-empirical factors accounting for the effect of the soil weight, inclined and eccentric loading, and multi-layer foundations. This paper presents a preliminary investigation of semi-empirical approaches to the factors accounting for soil weight. Alternatively, the authors present a computer program Bearing-IWHR at the web, which is an Excel/VBA coded spreadsheet to perform numerical bearing analysis without the need for the semi-empirical factors. [ABSTRACT FROM AUTHOR]

Abstract: Technological metal forming processes of extrusion, forging and rolling with imposed cyclic torsion or shear deformation are of engineering interest in view of their advantages with respect to monotonic loading processes. The present work is aimed at the analysis of an axisymmetric extrusion process assisted by cyclic torsion, which can be induced by a cyclically rotating die with a specified rotation amplitude and frequency. The material is assumed to be rigid-perfectly plastic and the upper-bound analysis for a kinematically admissible flow field is presented. The field parameters are specified from the condition of minimum plastic dissipation. The evolution of the extrusion force and torsional moment is studied and the corresponding plastic flow field is specified, with process control parameters such as the ratio of extrusion and rotation rates, and the amplitude of die rotation. For increasing rotation frequency, it is shown that localization of plastic flow occurs in the bottom zone close to the rotating die. The analysis allows for specification of process control parameters and their relation to extrusion force, torsional moment, and the associated plastic dissipation for varying reduction ratios. [Copyright &y& Elsevier]

Abstract: An upper-bound analysis of the pressure-assisted injection forging (PAIF) of thick-walled tubular components with hollow flanges was conducted to predict the maximum forming-pressure requirements. The polymeric pressurizing material and the pure aluminium work material employed were assumed to obey the von-Mises’ yield criterion, the two materials being divided into three and six zones, respectively, and described using kinematically admissible velocity fields. An upper-bound solution was derived based on this model. Experiments were conducted to validate the solution. The results suggest that the upper-bound analysis has sufficient accuracy for the prediction of the forming-pressure requirements. Subsequently, a parametric analysis was conducted to study the effects of variation of the principal parameters of the process and the geometry, such as tube thicknesses, flange dimensions and friction, on the forming-force requirements, which showed other useful applications of the upper-bound solutions. [Copyright &y& Elsevier]

Abstract: The kinematically admissible velocity fields were determined by the two kinds of metal flow models, and then the general formulae for calculating the punch pressures in back extrusion of various kinds of box-shaped products (such as rectangle, square, regular, polygon, ellipse or circle) were derived through the computation of total power compute and the optimization of height of deformed zone. The value results for the technological design of extrusion process were obtained by the analysis of upper-bound solution. The research shows that the upper-bound solutions calculated by presented in this paper velocity fields are very close to the experimental values. [Copyright &y& Elsevier]

Abstract: Deformation of the material during a 90° equal-channel angular extrusion (ECAE) process is analyzed using upper-bound theorem. The model suggested includes the effect of friction between the sample and the die walls, radius of inner corner of the die and the dead metal zone on the deformation patterns during ECAE. The parameters of the model is explored in relation to the deformation of the material during the process. Further directions for progress in deformation analysis in severe plastic deformation processes are outlined. [Copyright &y& Elsevier]

In this investigation a simple theoretical analysis of the cylindrical compression test has been developed so that the constant friction factor can be estimated quantitatively by using cold/hot compression test. The compression tests have been carried out on the Ti-IF-steel without lubricant, with mica sheets and with glass powder lubricants the commercial aluminum with oil lubricant to confirm this analysis. This new method for obtaining the constant friction factor may be called “The Barrel Compression Test”. The advantages of the proposed method are: (i) very simple method, (ii) high sensitivity of geometrical shape of cylinder to friction condition, (iii) does not need force measurement, and (iv) the constant friction factor can be determined for forming processes corresponding to actual processing condition. [Copyright &y& Elsevier]

In this paper the pressures needed for non-friction equal-channel angular pressing (ECAP) of perfectly plastic or strain-hardening materials are analysed using analytical approximations (upper-bound type) and numerical (finite-elements) methods. The approximate solutions agree very well with the FEM results for different ECAP die angles or materials. The convenience of using back-pressure for improving the strain pattern homogeneity of deformed strain-hardening materials is highlighted. [Copyright &y& Elsevier]