Average Stress Criterion Research Articles

Coupling effect of roughness and normal pressure on mechanical characteristics and failure mode of deep-sea sediment interface

Revealing the failure mechanism at the soil–structure interface is an important prerequisite for foundations bearing capacity analysis. In deep-sea structures, due to the special sedimentary conditions and extremely low permeability coefficient of marine soil, obviously, the undrained behavior at the interface is becoming more complex and needs due attention. The objective of this study is to investigate the coupling effects of roughness and normal pressure on the interface mechanical characteristics and failure mode from the perspective of the pore pressure index through improved interface triaxial apparatus. The obtained results revealed that increased surface roughness makes it difficult for the soil particles in the grooves to climb along the structure surface and consequently the failure surface shifts from the interface to the soil. The confining pressure strongly influences the failure mode and critical roughness; Based on the response of two factors to the interface, a new piecewise Mohr-Coulomb failure criterion is established to reveal the real interface mechanical parameters, accordingly two new concepts are proposed and analyzed: transition roughness and critical normal stress; Furthermore, the relationship between the critical roughness and confining pressure is determined by the interface shear resistance equation considering the coupling effect; Finally, the interface failure mechanism and failure modes related to the confining pressure and surface roughness are revealed and distinguished. The dominant effect of effective interface friction parameters on the failure modes is evaluated for various surface roughness ranges.

Discrete element modeling and physical experiment research on the biomechanical properties of cotton stalk

The biomechanical properties of cotton stalk (CS) play an important role in the development of uprooting, crushing, and processing machinery, as well as in the comprehensive utilization of biomass resources. Establishing a discrete element model that can accurately reflect the biomechanical properties of CS can serve as a basis for the simulation of related operating machinery, including uprooting and processing machinery. In this study, a discrete element model of CS was developed using a combination of discrete element modeling and mechanical experimental tests. Eleven initial factors (including six contact and five bonding parameters) were screened using the Plackett–Burman design. The effects of these factors on the compression force–deformation (F–x) curve were investigated using the steepest climb test design. A second-order regression model of the compressive peak force (FC) and bonding parameters was established using the central composite design method, and the bonding parameters of the CS model were calibrated. Based on the results, the optimal combination of the bonding parameters was determined: a normal stiffness per unit area of 9.72 × 109 N·m−3, shear stiffness per unit area of 9.38 × 109 N·m−3, critical normal stress of 8.43 × 109 Pa, and bonded disc radius of 0.66 mm. Compared with the compressive experimental results, the deviation of FC was only 1.1 %, the F–x curve trend was consistent, and the error in the slope of the fitted equation for the elastic deformation stage was 0.7 %. Compared with the bending experimental results, the error in the peak bending force was 2.53 %. The results showed that the established discrete element model can be used to accurately simulate the compression of CS and that the method of calibrating the parameters for accuracy is reasonable. These results are important for studying the biomechanical properties of CS and optimizing the mechanisms whereby cotton stalks are uprooted and crushed.

Prediction of the transitional normal stress of rock joints under shear

The transitional normal stress of a rock joint subject to shear refers to the critical normal stress under which the normal dilation is completely suppressed. The transitional normal stress is involved in many shear strength/constitutive models of joints as a key material constant; however, this parameter is poorly constrained and its determination is mostly empirical. Here we propose a simple formulation to predict the transitional normal stress based on the systematic, well-calibrated PFC2D (two-dimensional particle flow code) simulation of the shear characteristics of both sawtooth and JRC -profiled rock joints. In the PFC2D modelling, rock joints are confined by low to high normal stresses approximating the uniaxial compressive strength (joint wall compressive strength of fresh, dry and closely matched joints) of the simulated rock. The formulation can satisfactorily quantify the transitional normal stress of regular and irregular joint surfaces as a function of the rock strength and the joint surface roughness , as validated by the laboratory data. Therefore, it can be readily introduced to the shear strength criteria and constitutive models of rock joints, which could significantly promote the accurate quantification of rock joint shear behaviour .

Discrete element modelling of the yam root–soil complex and its verification

This paper constructs a discrete element simulation model of yam based on a bimodal model and uniform grid coordinates and performs parameter calibration on the simulation model based on three mechanical properties. The bonding parameters are: normal stiffness per unit area 8.0684 × 107 N·m−3, shear stiffness per unit area 2.0314 × 107 N·m−3, critical normal stress 1.2876 × 107 Pa and critical shear stress 8.9623 × 107 Pa. Using the root–soil complex shear test, based on the grey correlation analysis method, a root morphology index with the greatest correlation with the maximum shear force of the yam root–soil complex was obtained. The factor that has the greatest influence on the maximum shear force of the root–soil complex is the total surface area of the root, the correlation degree is 0.77. The root–soil complex bond parameters are as follows: normal stiffness per unit area 5.203 3 × 105 N·m−3, shear stiffness per unit area 2.493 8 × 106 N·m−3, the critical normal stress 1 × 105 Pa and the critical shear stress 5.319 0 × 105 Pa. The accuracy of the simulation model of the yam root–soil complex was verified by comparing the simulation test with the physical test. The relative error of the axial compression rupture pressure value of the yam root–soil complex simulation test and the physical test is 6.63%, the relative error of the displacement value is 8.83%, the relative error of the radial compression rupture pressure value is 6.83%, and the relative error of the displacement value is 6.38%.

Numerical Simulation of Rock Breaking by High-Temperature and High-Pressure Water under Thermal Driving

To solve the difficult problems in the field of unconventional oil and gas extraction and hard rock excavation in urban underground spaces, this paper proposes a rock-breaking technique with high-temperature and high-pressure water under thermally driven conditions and establishes a coupled thermal-fluid-solid model with COMSOL Multiphysics (COMSOL Co., Ltd. Shanghai, China). Different simulation groups are established by controlling variables to explore the effects of the surrounding rock load, heat source power, and Biot coefficient on the damage evolution during thermally driven rock breaking. To make the results relevant to practical engineering, the damage evolution results under the maximum normal stress criterion, maximum normal strain criterion, and Coulomb-Navier damage criterion are considered, and a comparative analysis is performed. The results of this study show that an increase in unilateral load and heat source power accelerates the damage evolution rate, while an increase in bilateral load and Biot coefficient has the opposite effect. The damage evolution rate controlled by the maximum normal stress criterion is the fastest under general conditions. Finally, the advantages in rock breaking provided by the established method are verified by a comparison of results from the proposed model and a conventional hydraulic fracturing model.

Unloading effect of the shear resistance of rock joints

To investigate the stress path dependent of rock joints, a series of tangential loading tests and normal unloading tests were conducted using 32 cement mortar replicas of artificially split rock joints with 4 different joint roughness coefficients (JRC). The critical normal stresses of the unloading tests were recorded, and a comparative study was conducted by referencing the results under different loading paths and joints conditions. The test results indicate that the shear resistance has a distinct unloading effect, which is mainly influenced by the morphologic characteristics, normal stress levels and stress paths. By analyzing the results, it can be concluded that the variation trend of shear/normal stress ratio against the normal stress and JRC of the two test conditions were identical. However, under low normal stress condition, the stress ratio of the joints under normal unloading stress path is the higher one; while under higher normal stress, the relationship becomes converse. Compared to that of the tangential loading condition, shear/normal stress ratio of the unloading stress path reduces rapidly as the increasing of normal stress, and the influence of the morphology is masked under lower normal stress.

Local stability of the web of a prestressed steel beam transport construction

A study of the local stability of the wall of a prestressed prefabricated beam of a steel bridge was carried out, which is not reflected in the technical literature. The critical normal stresses in a prestressed plate are determined by the energy method of S.P. Timoshenko. The stability of the plate in the zone of maximum shear forces is estimated under the assumption that the principal stresses on the web at the stage of fabrication of the structure and the principal stresses caused by the external load are oppositely directed and decrease when summed. The shear forces and shear stresses on the web are determined based on the theory of D.I. Zhuravsky. An experimental study was conducted on standard and prestressed beams of equal cross section to verify the theoretical conclusions. The graphs of web displacements from the plane of beam bending in the zone of maximum normal and shear stresses are presented. It was concluded that the prestressing of elements of steel beam has a positive effect on their local stability. Recommendations on the structural solution of prestressed beams and reduction of their mass were presented.

Investigation of force transmission, critical breakage force and relationship between micro-macroscopic behaviors of agricultural granular material in a uniaxial compaction test using discrete element method

In this paper, numerical DEM simulations are presented that were used to study the response of agricultural granular materials, such as dry soybeans, in a uniaxial compaction test. To this aim, the microscopic parameters of soybeans (i.e., input parameters for DEM simulations) were taken from our previous published work through a calibration between experiments and DEM simulations. The response of the granular medium to the uniaxial compaction stresses, ranging from 0 to 200 kPa with a step of 50 kPa, was investigated locally using various variables, such as displacement field, force chain network, number of particles in contact, and distribution of force acting on particles. The macroscopic responses, such as porosity and axial shortening, were also investigated as a function of applied normal stress. Such investigations allowed for exploring the arrangement of the material microstructure as well as its mechanical response under applied loading. A comparison between DEM simulations and the experiment of particle breakage showed that 200 kPa should be considered as critical normal stress for soybeans. Finally, parametric studies were conducted to highlight the variation in the local and macroscopic properties of the particulate system as a function of inter-microscopic parameters of soybeans.

Calibration of discrete element microparameters of coal based on the response surface method

Materials properties can be obtained using the discrete element method through appropriate parameter settings. The accuracy of setting parameters affects the reliability of simulation results. To accurately simulate the mechanical properties of coal, the discrete element models of coal were established. The effects of the parameters of the bonded-particle model on the macroperformance of coal were studied based on the response surface method. The simulation results showed that when the normal contact stiffness per unit area, the shear contact stiffness per unit area, critical normal stress, and critical shear stress are 3.066 × 1012 N/m3, 5 × 1011 N/m3, 2.747 × 107 Pa, and 1.424 × 107 Pa, respectively, the bonded parameters reliably simulated the crushing characteristics of long-flame coal. The increase in stiffness causes the model to fracture earlier but has less impact on the peak value; the strength has a greater impact on the mechanical properties of the model. In this study, the reliability of the comparison between the simulation and test was improved, and a basis was provided for the discrete element parameter calibration of the coal and the setting of simulation parameters for shearer cutting.

Determination and interpretation of bonded-particle model parameters for simulation of maize kernels

Investigating the relationship between parameters of the bonded-particle model (BPM) and the macroscopic properties of maize kernel may provide a useful insight into the better control of maize processing. This study provides a first step towards the simulation of maize kernel damage behaviour by determining BPM parameters for uniaxial-compressed maize kernels within the discrete element method. Reverse engineering technology was used to obtain a highly accurate representation of maize kernel shape. Rupture force and stiffness were determined by a reliable algorithm. The effects of BPM parameters on mechanical response were examined by employing Plackett–Burman experimental design in conjunction with response surface method to perform significance analysis for the effects of factor combinations. The results showed that shear modulus, coefficient of restitution between particles, and critical normal and shear stress have strong influences on rupture force and stiffness in respect to particle-based parameters, contact parameters, and parallel–bond parameters. Shear modulus and critical normal and shear stress have a significant interactions on stiffness.

Research on size effect of fracture toughness of sandstone using the center-cracked circular disc samples

In order to investigate the size effect and reason of rock fracture toughness, the split loading test was carried out in the GCTS rock mechanics testing machine using the center-cracked circular disc (CCCD) samples with diameters of 50 mm, 100 mm, 150 mm and 205 mm, respectively. The lengths of fracture process zone (lFPZ) were determined by the maximum normal stress criterion. The size effect of the lFPZ , fracture toughness test value (KIc), and fracture energy (Gf) were analyzed. The influence of the lFPZ on the size effect of the KIc was revealed. The crack growth paths and failure characteristics of the samples were discussed. The results show that the lFPZ and the KIc increase linearly with the sample size, and there is obvious size effect within the range of the test. The KIc and the lFPZ do not always increase linearly with the size in a larger size range, but gradually stabilize by extrapolation analysis. The lFPZ increases with the size, which means the release of stored energy increases with the size corresponding to the size effect of the Gf, and in turn leads to the size effect of the KIc. It is helpful to deeply understand the cause of the size effect reason of rock fracture toughness.

Fracture of quasi-brittle geomaterials with a circular hole under compression

The new nonlocal fracture criteria are proposed, which are the extension of the average stress criterion and the point stress criterion, and which contain a complex parameter that characterizes the size of the fracture process zone and accounts not only for the material microstructure, but also ductile properties of the material, geometry of the specimen, and its loading conditions. The formulas are obtained for the critical pressure in the problem of tensile fracture initiation in the specimens of geomaterials containing a circular hole and subjected to uniformly or nonuniformly distributed compression. The results of the calculations are in good agreement with the experimental data on failure of gypsum plates with a hole.

Ultimate pressure determination in hydraulic fracturing of boreholes for different rock failure criteria

The problem on elastoplastic deformation and rock failure around boreholes under the action of internal pressure is considered. The stress state in the intact rock mass is assumed to be hydrostatic. The hydraulic fracturing pressure is conventionally determined by the maximum normal stress criterion. However, the experimental studies described in scientific articles show that this criterion is not consistent with the results obtained in the course of laboratory tests on failure of cylindrical and spherical cavities made in rock samples. It has been known that test results on complex loading for solid samples of various materials are not confirmed by the theory of maximum normal stress both in terms of ultimate load value and direction of failure surface propagation. In this case, for the complex stress state of rocks, it is proposed to determine the ultimate pressure by the experimentally substantiated fracture criteria which are in good agreement with the results of laboratory tests on hydraulic fracturing of boreholes.

Compressed stiffened composite panels design with skin postbuckling behavior

The aim of this paper is development of stiffened composite panels design method taking thin skin postbuckling behavior and stringers elastic behavior in case of longitudinal compression into account. Statement of the problem takes geometrically non-linear correlations into account. Variant of rigid boundary conditions is considered for both stiffened panels and skin. Analytic solution of non-linear problem using Ritz method which is reduced to solution of non-linear system of two equations in skin and stringer deflection amplitudes is shown. Obtained analytical solution is used for development of load-bearing panels design method based on postbuckling state. A design method based on thin skin postbuckling state not taking stringers influence into account is shown as an example. In this case following variants of strength criteria are considered: reaching longitudinal normal or tangential transverse critical stress. Besides stiffened composite panels design method taking skin postbuckling behavior in case of stringers elastic behavior into account is shown. With known stringer parameters design task is reduced to solution of non-linear system of two equations in skin thickness and stringer deflection amplitude. In this case stipulation of reaching critical normal stress in the skin is taken as strength criterion.

Modeling of progressive debonding of interphase- matrix interface cracks in particle reinforced composites using VCFEM

The Voronoi cell finite element model (VCFEM) containing three phases of inclusion, interphase and matrix in an element has been proven to be very accurate and efficient in the analysis of inclusion reinforced composites. In this paper, we extended our previous study to include the matrix-interphase interface debonding problems. The traction-free at the debonded matrix-interphase interface and the traction reciprocity at the bonded matrix-interphase interface were introduced into the functional by displacement as Lagrange multiplier. A modified complementary energy functional was constructed. When the matrix-interphase interface was debonded according to critical normal stress law, additional nodes are added as node pairs with existing nodes whose stress exceeds the critical value. This process was interated until the stress of all nodes did not exceed the critical value. Thus, the mesh re-division technology was realized for the interface debonding progressively. The proposed VCFEM is validated by results from MARC. The results show that the proposed VCFEM was more efficient. Based on the developed model, the influence of interphase properties on stress-strain cure was studied during the debonding process. This showed that the interphase modulus and thickness had great influence on the overall mechanical behavior of composites.

A Comprehensive Study on Burst Pressure Performance of Aluminum Liner for Hydrogen Storage Vessels

Abstract This paper presents a comparative study on the burst pressure performance of aluminum (Al) liner for type-III composite overwrapped pressure vessels (COPVs). In the analysis, the vessels were loaded with increasing internal pressure up to the burst pressure level. In the analytical part of the study, the burst pressure of the cylindrical part was predicted based on the modified von Mises, Tresca, and average shear stress criterion (ASSC). In the numerical analysis, a finite element (FE) model was established in order to predict the behavior of the vessel as a function of increasing internal pressure and determine the final burst. The Al pressure vessels made of Al-6061-T6 alloy with a capacity of 5 L were designed. The manufacturing of the metallic vessels was purchased from a metal forming company. The experimental study was conducted by pressurizing the Al vessels until the burst failure occurred. The radial and axial strain behaviors were monitored at various locations on the vessels during loading. The results obtained through analytical, numerical, and experimental work were compared. The average experimental burst pressure of the vessels was found to be 279 bar. The experimental strain data were compared with the results of the FE analysis. The results indicated that the FE analysis and ASSC-based elastoplastic analytical approaches yielded the best predictions which are within 2.2% of the experimental burst failure values. It was also found that the elastic analysis underestimated the burst failure results; however, it was effective for determining the critical regions over the vessel structure. The strain behavior of the vessels obtained through experimental investigations was well correlated with those predicted through FE analysis.

Improvement of DDA with a New Unified Tensile Fracture Model for Rock Fragmentation and its Application on Dynamic Seismic Landslides

Discontinuous deformation analysis (DDA) method is a discrete element method, presenting a great advantage in modelling deformation and rigid body movements, and it is also an alternative approach for problems involving the fracturing process from continuity to discontinuity if the failure mechanism in DDA is well constituted. This paper presents a new united tensile fracture model (UTFM) for the two-dimensional DDA method to simulate the fracture behaviors of various brittle materials (e.g., rock, soil, and concrete). The new fracture model unifies four classical failure modes, including the maximum normal stress criterion, Tresca criterion, Mohr–Coulomb criterion, and the von Mises criterion, for tensile fracture. By incorporating UTFM into the original DDA frame, the improved DDA (I-DDA) can predict the crack initiation and propagation paths in Brazil disc and simulate rock fracture of various brittle materials. Numerical examples of the direct tensile test and the Brazil disc split tests are investigated to verify the accuracy and validity of the I-DDA method. The simulated results agree well with those obtained from physical tests and other numerical analyses, suggesting that the I-DDA has obvious advantage in simulating the fracture behaviors of the Brazil disc split test. Further, the I-DDA is applied to analyze the failure process of a practical earthquake-induced landslide with consideration of the tensile strength of the rock mass. The results indicate that the I-DDA is more feasible to analyze the slope failure, which can consider both the tensile and shear characteristics simultaneously compared with the original DDA.

Research on ballast breakage under tamping operation based on DEM–MBD coupling approach

Tamping can effectively improve the service quality of ballast bed, but could cause ballast breakage. To mitigate the ballast damage caused by tamping, an application programming interface was innovatively developed to establish a ballast model that could actually simulate ballast breaking. This model was subsequently verified by a laboratory test. On this basis, a tamper-sleeper-ballast bed coupling model was constructed by the coupling of discrete element method and multibody dynamics. Finally, the coupling model was utilized to analyze the breaking behavior of ballasts in different sections and the influence of ballast properties on particle breakage. The results show that ballasts in the crib are the most probable to be broken in a tamping operation, whose breakage degree is 1.65%. Ballast properties can greatly influence particle breakage in a tamping operation. The recommended critical normal and tangential stresses of ballasts are 45 MPa and 15 MPa, respectively. Elongation and flakiness indexes are recommended to be limited in 20%. Wide gradation together with large size is recommended for ballast gradation. This study can provide critical insights for improving the service life of ballast bed.

An approach to calibration of BPM bonding parameters for iron ore

To accurately simulate the crushing process of the ore in the crusher based on the discrete element method (DEM), the modelling of large-sized iron ore particles based on the bonded-particle model needs to be accurately performed. Through the uniaxial compression test, in this paper, the influence of the normal stiffness per unit area, shear stiffness per unit area, critical normal stress, critical shear stress, and the radius of the filling sphere on the bonded-particle model performances, including the compressive strength and the average gradient of stress-strain, are first explored based on EDEM software. On this basis, a calibration method for the bonding parameters of the bonded-particle model is proposed. The results show that the compressive strength of the bonded-particle model is jointly determined by the critical normal stress and the critical tangential stress that they are small, and it is affected by the large radius of the filling sphere. When the critical normal stress and the filling sphere radius are in small, the variation of the average gradient of stress-strain is significant. Further, taking three compressive strength values as the target values, the reliability and validity of the calibration method are preliminarily verified by the DEM simulation.

Microanalysis of smooth Geomembrane–Sand interface using FDM–DEM coupling simulation

Very limited attention was paid on the micro–response of sand as it interacts with geomembrane and the effect of surface hardness on the interaction at the microscopic level. In this study, a coupled finite difference–discrete model was adopted with which to analyze the shear behavior of sand–smooth geomembrane interface. The model was validated using published experimental data. The numerical results show that the effects of normal stress and surface hardness on the interface strength depend on the shear mechanism (sliding or plowing) at the interface. There exists a critical normal stress at which the mechanism transforms from predominant sliding to predominant plowing. There is a high level of coupling effect between normal stress and surface hardness on the interface strength. Micro–topographical analysis of geomembrane provides clear insights into the shear mechanism at the interface, supporting the results obtained from interface shear tests. No shear band is formed during shearing for a sand–smooth geomembrane interface. The shear resistance of sand–smooth geomembrane interface relies on interface indentations and characteristics rather than on the formation of shear band.