Numerical Simulation of Impact Rockburst of Elliptical Caverns with Different Axial Ratios

Herein, a finite discrete element method was used to simulate the rockburst phenomenon of elliptical caverns with different axis ratios. Two situations were employed, namely when the disturbance direction is perpendicular and parallel to the ellipse. Based on the peak stress, maximum velocity, stress nephogram, and image fractal characteristics, the influence of axis ratio and direction of the disturbance on rockburst were analyzed. The results show that the samples with different axis ratios experienced the same process of quiet period, slab cracking period, and rockburst. The rockburst pit had V shape, and the failure modes of rockburst primarily included shear cracks, horizontal tension cracks, and vertical tension cracks. With the rise in axis ratio, the peak stress and maximum speed increased. Furthermore, the pressure area on the left and right sides of the sample cavern decreased when the disturbance direction was parallel to the short axis of the ellipse, while it increased for the sample with a disturbance direction perpendicular to the short axis. The fractal dimension value of the crack was gradually amplified with disturbance. The fractal dimension value of the sample whose disturbance direction was perpendicular to the minor axis of the ellipse was lower, and it was more difficult to damage.

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An Extension Strain Type Mohr–Coulomb Criterion

Rock Mechanics and Rock Engineering ◽

10.1007/s00603-021-02608-7 ◽

2021 ◽

Author(s):

Manfred Staat

Keyword(s):

Failure Modes ◽

Peak Stress ◽

Fracture Initiation ◽

Failure Surface ◽

Confining Pressure ◽

Biaxial Compression ◽

Simple Extension ◽

Strain Criterion ◽

Shear Component ◽

Extension Strain

AbstractExtension fractures are typical for the deformation under low or no confining pressure. They can be explained by a phenomenological extension strain failure criterion. In the past, a simple empirical criterion for fracture initiation in brittle rock has been developed. In this article, it is shown that the simple extension strain criterion makes unrealistic strength predictions in biaxial compression and tension. To overcome this major limitation, a new extension strain criterion is proposed by adding a weighted principal shear component to the simple criterion. The shear weight is chosen, such that the enriched extension strain criterion represents the same failure surface as the Mohr–Coulomb (MC) criterion. Thus, the MC criterion has been derived as an extension strain criterion predicting extension failure modes, which are unexpected in the classical understanding of the failure of cohesive-frictional materials. In progressive damage of rock, the most likely fracture direction is orthogonal to the maximum extension strain leading to dilatancy. The enriched extension strain criterion is proposed as a threshold surface for crack initiation CI and crack damage CD and as a failure surface at peak stress CP. Different from compressive loading, tensile loading requires only a limited number of critical cracks to cause failure. Therefore, for tensile stresses, the failure criteria must be modified somehow, possibly by a cut-off corresponding to the CI stress. Examples show that the enriched extension strain criterion predicts much lower volumes of damaged rock mass compared to the simple extension strain criterion.

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Dynamic characteristics and fractal representations of crack propagation of rock with different fissures under multiple impact loadings

Scientific Reports ◽

10.1038/s41598-021-92277-x ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Bing Sun ◽

Shun Liu ◽

Sheng Zeng ◽

Shanyong Wang ◽

Shaoping Wang

Keyword(s):

Fractal Dimension ◽

Crack Propagation ◽

Crack Growth ◽

Impact Test ◽

Fractal Theory ◽

Dynamic Change ◽

Fractal Dimensions ◽

Peak Stress ◽

Dynamic Mechanical Properties ◽

Gravitational Potential Energy

AbstractTo investigate the influence of the fissure morphology on the dynamic mechanical properties of the rock and the crack propagation, a drop hammer impact test device was used to conduct impact failure tests on sandstones with different fissure numbers and fissure dips, simultaneously recorded the crack growth after each impact. The box fractal dimension is used to quantitatively analyze the dynamic change in the sandstone cracks and a fractal model of crack growth over time is established based on fractal theory. The results demonstrate that under impact test conditions of the same mass and different heights, the energy absorbed by sandstone accounts for about 26.7% of the gravitational potential energy. But at the same height and different mass, the energy absorbed by the sandstone accounts for about 68.6% of the total energy. As the fissure dip increases and the number of fissures increases, the dynamic peak stress and dynamic elastic modulus of the fractured sandstone gradually decrease. The fractal dimensions of crack evolution tend to increase with time as a whole and assume as a parabolic. Except for one fissure, 60° and 90° specimens, with the extension of time, the increase rate of fractal dimension is decreasing correspondingly.

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Assimilation of Dynamic Combined Finite Discrete Element Methods Using the Ensemble Kalman Filter

Applied Sciences ◽

10.3390/app11072898 ◽

2021 ◽

Vol 11 (7) ◽

pp. 2898

Author(s):

Humberto C. Godinez ◽

Esteban Rougier

Keyword(s):

Kalman Filter ◽

Ensemble Kalman Filter ◽

Failure Modes ◽

Split Hopkinson Pressure Bar ◽

Peak Stress ◽

Discrete Element ◽

Material Behavior ◽

Model Parameters ◽

Hopkinson Pressure Bar ◽

Parameter Values

Simulation of fracture initiation, propagation, and arrest is a problem of interest for many applications in the scientific community. There are a number of numerical methods used for this purpose, and among the most widely accepted is the combined finite-discrete element method (FDEM). To model fracture with FDEM, material behavior is described by specifying a combination of elastic properties, strengths (in the normal and tangential directions), and energy dissipated in failure modes I and II, which are modeled by incorporating a parameterized softening curve defining a post-peak stress-displacement relationship unique to each material. In this work, we implement a data assimilation method to estimate key model parameter values with the objective of improving the calibration processes for FDEM fracture simulations. Specifically, we implement the ensemble Kalman filter assimilation method to the Hybrid Optimization Software Suite (HOSS), a FDEM-based code which was developed for the simulation of fracture and fragmentation behavior. We present a set of assimilation experiments to match the numerical results obtained for a Split Hopkinson Pressure Bar (SHPB) model with experimental observations for granite. We achieved this by calibrating a subset of model parameters. The results show a steady convergence of the assimilated parameter values towards observed time/stress curves from the SHPB observations. In particular, both tensile and shear strengths seem to be converging faster than the other parameters considered.

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Hysteretic Behavior of Geopolymer Concrete with Active Confinement Subjected to Monotonic and Cyclic Axial Compression: An Experimental Study

Materials ◽

10.3390/ma13183997 ◽

2020 ◽

Vol 13 (18) ◽

pp. 3997 ◽

Cited By ~ 3

Author(s):

Huailiang Wang ◽

Yuhui Wu ◽

Min Wei ◽

Lang Wang ◽

Baoquan Cheng

Keyword(s):

Failure Modes ◽

Peak Stress ◽

Hysteretic Behavior ◽

Stress And Strain ◽

Geopolymer Concrete ◽

Cement Concrete ◽

Cyclic Compression ◽

Stress States ◽

Loading Modes ◽

Good Agreement

This paper investigated the performance of actively confined geopolymer concrete (GPC) through experiments. The mechanical properties of GPC under triaxial stress states were analyzed and discussed from the prospects of failure modes, axial peak stress and strain, monotonic and cyclic constitutive relationships. The experimental results demonstrated that the loading modes (monotonic loading and cyclic loading) had little effect on the failure mode and axial peak stress and strain. The improvement of the strength and ductility of GPC with the increase in confinement level was consistent with that of the conventional cement concrete while the strain enhancement of confined GPC was lower than that of confined conventional cement concrete at the same confinement level. The curves of the monotonic stress–strain and the envelop of cyclic compression were predicted through Mander’s model with good accuracy. The unloading/reloading models proposed by Lokuge were modified and the predicted cyclic hysteresis curves for actively confined GPC were in good agreement with the cyclic compression results. Findings from this study provide references for the application of geopolymer concrete.

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Failure Characteristics and Mechanism of Multiface Slopes under Earthquake Load Based on PFC Method

Shock and Vibration ◽

10.1155/2021/9329734 ◽

2021 ◽

Vol 2021 ◽

pp. 1-11

Author(s):

Tao Yang ◽

Yunkang Rao ◽

Huailin Chen ◽

Bing Yang ◽

Jiangrong Hou ◽

...

Keyword(s):

Moisture Content ◽

Failure Mechanism ◽

Failure Modes ◽

Shear Failure ◽

Shaking Table ◽

Tensile Failure ◽

Particle Flow Code ◽

Shear Cracks ◽

Local Shear ◽

Shear Slip

Understanding the failure mechanism and failure modes of multiface slopes in the Wenchuan earthquake can provide a scientific guideline for the slope seismic design. In this paper, the two-dimensional particle flow code (PFC2D) and shaking table tests are used to study the failure mechanism of multiface slopes. The results show that the failure modes of slopes with different moisture content are different under seismic loads. The failure modes of slopes with the moisture content of 5%, 8%, and 12% are shattering-shallow slip, tension-shear slip, and shattering-collapse slip, respectively. The failure mechanism of slopes with different water content is different. In the initial stage of vibration, the slope with 5% moisture content produces tensile cracks on the upper surface of the slope; local shear slip occurs at the foot of the slope and develops rapidly; however, a tensile failure finally occurs. In the slope with 8% moisture content, local shear cracks first develop and then are connected into the slip plane, leading to the formation of the unstable slope. A fracture network first forms in the slope with 12% moisture content under the shear action; uneven dislocation then occurs in the slope during vibration; the whole instability failure finally occurs. In the case of low moisture content, the tensile crack plays a leading role in the failure of the slope. But the influence of shear failure becomes greater with the increase of the moisture content.

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Applicability of Design Rules for Openings in Shells, ASME B&PV Code, Section VIII, Division 2 - A Case Study

10.1115/pvp2021-62020 ◽

2021 ◽

Author(s):

Gurumurthy Kagita ◽

Krishnakant V. Pudipeddi ◽

Subramanyam V. R. Sripada

Keyword(s):

Stress Analysis ◽

Failure Modes ◽

Element Analysis ◽

Design Rules ◽

Area Method ◽

Replacement Method ◽

Local Stresses ◽

Pressure Area ◽

Section Viii ◽

Division 2

Abstract The Pressure-Area method is recently introduced in the ASME Boiler and Pressure Vessel (B&PV) Code, Section VIII, Division 2 to reduce the excessive conservatism of the traditional area-replacement method. The Pressure-Area method is based on ensuring that the resistive internal force provided by the material is greater than or equal to the reactive load from the applied internal pressure. A comparative study is undertaken to study the applicability of design rules for certain nozzles in shells using finite element analysis (FEA). From the results of linear elastic FEA, it is found that in some cases the local stresses at the nozzle to shell junctions exceed the allowable stress limits even though the code requirements of Pressure-Area method are met. It is also found that there is reduction in local stresses when the requirement of nozzle to shell thickness ratio is maintained as per EN 13445 Part 3. The study also suggests that the reinforcement of nozzles satisfy the requirements of elastic-plastic stress analysis procedures even though it fails to satisfy the requirements of elastic stress analysis procedures. However, the reinforcement should be chosen judiciously to reduce the local stresses at the nozzle to shell junction and to satisfy other governing failure modes such as fatigue.

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The meso-fracturing mechanism of marble under unloading confining pressure paths and constant axial stress

Scientific Reports ◽

10.1038/s41598-021-97359-4 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Linna Sun ◽

Liming Zhang ◽

Yu Cong ◽

Yaduo Song ◽

Keqiang He

Keyword(s):

Shear Failure ◽

Axial Stress ◽

Confining Pressure ◽

Particle Flow Code ◽

Shear Cracks ◽

Confining Pressures ◽

Tension Cracks ◽

Trial And Error Method ◽

Unloading Confining Pressure ◽

Fracturing Mechanism

AbstractFailure tests on marble during unloading confining-pressure under constant axial stress and simulations with the particle flow code were performed. The influence mechanism of the unloading rate of the confining pressure, initial unloading stress, and confining pressure on the failure characteristics of, and crack propagation in, marble was studied. By using the trial-and-error method, the conversion relationship between the unloading rates of confining pressures in laboratory tests and numerical simulations was ascertained. Micro-cracks formed in the unloading process of confining pressure are dominated by tension cracks, accompanied by shear cracks. The propagation of shear cracks lags that of tension cracks. As the confining pressure is increased, more cracks occur upon failure of the samples. The proportion of shear cracks increases while that of tension cracks decreases. The failure mode of samples undergoes a transition from shear-dominated failure to conjugated shear failure.

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Permeability Characteristic and Failure Behavior of Filled Cracked Rock in the Triaxial Seepage Experiment

Advances in Civil Engineering ◽

10.1155/2019/3591629 ◽

2019 ◽

Vol 2019 ◽

pp. 1-12

Author(s):

Xinyu Liu ◽

Zhende Zhu ◽

Aihua Liu

Keyword(s):

Growth Rate ◽

Rock Mass ◽

Failure Modes ◽

Peak Stress ◽

Confining Pressure ◽

Strength Characteristics ◽

Failure Behavior ◽

Compression Tests ◽

Secondary Cracks ◽

Safety And Stability

Filling is commonly found in natural cracked rock mass. As the weakest part of the rock, the filling properties directly affect the rock deformation and strength, permeability, and so on and affect the safety and stability of the rock mass engineering. In this study, a single slit has been preset in sandstones and filled with different physical properties materials. Based on the laboratory triaxial seepage test, the permeability and strength characteristics of filled cracked sandstones are analyzed, and the failure modes are obtained. The main findings of this study are as follows: (1) The permeability coefficient peak value of the filled cracked rock appears before the stress peak. (2) At the same confining pressure growth rate, the peak stress growth rate of the filled cracked rock is generally higher than that of the intact rock and the strength growth rate of the cracked rock increases with the length of the fracture. The strength characteristics of the filling in the uniaxial compression tests and triaxial seepage tests are significantly affected by the hydraulic properties. (3) The strength and permeability coefficients of cracked rock filled with cement mortar are more sensitive to the change of confining pressure, while under the same condition, the ones of cracked rock filled with gypsum mortar are stable. (4) According to the failure mechanism, under the seepage stress, the secondary cracks can be divided into 3 types and the failure modes can be divided into 2 types.

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Impact of endoscopic ultrasound-guided fine needle aspiration of small lymph nodes

Journal of Digestive Endoscopy ◽

10.4103/0976-5042.189137 ◽

2016 ◽

Vol 07 (02) ◽

pp. 051-054

Author(s):

Narendra S. Choudhary ◽

Rinkesh Kumar Bansal ◽

Rajesh Puri ◽

Vijay Bodh ◽

Gaurav Kumar Patil ◽

...

Keyword(s):

Lymph Nodes ◽

Endoscopic Ultrasound ◽

Fine Needle Aspiration ◽

Tertiary Care ◽

Needle Aspiration ◽

Study Cohort ◽

Short Axis ◽

Axis Ratio ◽

Fine Needle ◽

Significant Difference

Abstract Background: There is very limited literature on results of fine needle aspiration (FNA) of small (defined as ≤1 cm at long and short axis) lymph nodes, particularly in the setting of pyrexia of unknown origin (PUO). Methods: The study was conducted from July 2014 to December 2015 at a tertiary care center. A total of 34 endoscopic ultrasound (EUS)-guided FNAs in 33 patients were done for lymph nodes ≤1 cm at long and short axis and these were included in the analysis. Results: The study cohort comprised 33 patients; 23 males and 10 females, mean age of 58 ± 12 years. Indication of FNA was to look for malignancy (n = 15), PUO (n = 16), unexplained weight loss (n = 1), and presence of lymphadenopathy in prospective liver donor (n = 1). The FNA was taken from mediastinal nodes (n = 20, 14 subcarinal) and abdominal (n = 14, 8 at porta). The mean size of lymph nodes was 87 ± 11 mm at large axis and 68 ± 17 mm at short axis. A total of 3 (8.8%) FNAs were nondiagnostic (inadequate material). The cytopathologic diagnosis was malignancy in 8 (23.5%), granulomatous change in 8 (23.5%), and reactive lymphadenopathy in 15 (44.1%). Thus, EUS-guided FNA of these small nodes changed the management decisions in 44% of cases (one patient had tubercular lymphadenopathy at two sites). The 22-gauge EUS FNA needle was used in majority of patients (n = 26). There was no significant difference between pathologic (malignant and granulomatous) and reactive lymph nodes regarding size at long or short axis, ratio of long and short axis, hypoechogenicity, and sharply defined borders. Conclusion: EUS-guided FNA of small lymph nodes showed pathological enlargement in 44% of cases.

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A NONLINEAR FINITE ELEMENT ANALYSIS OF PRECAST INDUSTRIALISED BUILDING SYSTEM BEAM UNDER FLEXURAL TEST

Jurnal Teknologi ◽

10.11113/jt.v81.12486 ◽

2019 ◽

Vol 81 (3) ◽

Author(s):

Chun-Chieh Yip ◽

Jing-Ying Wong ◽

Ka-Wai Hor

Keyword(s):

Finite Element Analysis ◽

Finite Element ◽

Reinforced Concrete ◽

Concrete Beam ◽

Failure Modes ◽

Reinforced Concrete Beam ◽

Shear Cracks ◽

Flexural Test ◽

Element Analysis ◽

Failure Behaviour

Software simulation enables design engineers to have a better picture of possible structural failure behaviour and determine the accuracy of a design before the actual structural component is fabricated. Finite element analysis is used to simulate the behaviour of the reinforced concrete beam under the flexural test. During the flexural test, results are recorded for both simulation and experimental tests. By comparing the results, beam displacement, crack patterns, and failure modes can be studied with better accuracy. The accuracy percentage for yield load and ultimate load between the two tests results were 94.12 % and 95.79 %, respectively, whereas the accuracy percentage for elastic gradient before the yielding stage was 81.08 %. The behaviour between simulation and laboratory models described is based on crack pattern and failure mode. The progression of von Mises (VM) stresses highlighted the critical areas of the reinforced concrete beam and correlation between the experimental specimen, in terms of flexural cracks, shear cracks, yielding of tension reinforcement, and the crushing of concrete due to compressive stress. This paper concludes that simulation can achieve a significant accuracy in terms of loads and failure behaviour compared to the experimental model.

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