Stability analysis of intermittently jointed rock slopes based on the stepped failure mode

In practical engineering,due to the noncontinuity characteristics of joints in rock slopes,in addition to plane failure,stepped sliding failure may occur for intermittently jointed rock slopes.Especially for intermittently bedding jointed rock slopes,the correlation and difference in strength parameters between joints and rock bridges,along with the various failure modes and intermittency of rock bridges,contribute to the complexity of stepped failure modes and the unpredictability of failure regions.Based on the upper-bound limit analysis method and multi-sliders step-path failure mode,considering the shear and tensile failure of rock bridges and the weakened relationship between the strength parameters of rock bridges and jointed surfaces,by introducing the modified M-C failure criterion and the formula for calculating the energy consumption of tensile failure of rock bridges,two failure mechanisms are constructed to obtain the safety factor(F_(s))of intermittently jointed rock slopes.The sequential quadratic programming method is used to obtain the optimal upper-bound solution for F_(s).The influence of multiple key parameters(slope height H,horizontal distance L,Slope angleβ,shear strength parameters of the rock bridgeφr and cr,Dimensionless parameter u,weakening coefficients of the internal friction angle and cohesion between the rock bridges and joint surfaces Kφand Kc)on the stability analysis of intermittently jointed rock slopes under the shear failure mode of rock bridges as well as under the tensile failure mode is also explored.The reliability of the failure mechanisms is verified by comparative analysis with theoretical results,numerical results,and landslide cases,and the variation rules of F_(s)with each key parameter are obtained.The results show that F_(s) varies linearly withφr and cr of the rock bridge and with K_(φ)and K_(c),whereas F_(s)changes nonlinearly with H and L.In particular,with the increase in Kφand Kc,Fs increases by approximately 52.78%and 171.02%on average,respectively.For rock bridge tensile failure,F_(s) shows a nonlinearly positive correlation withφr,cr,Kφand Kc.In particular,with the increase in Kφand Kc,Fs increases by approximately 13%and 61.69%on average,respectively.Fs decreases rapidly with increasing slope gradientβand decreasing dimensionless parameterμ.When Kφand Kc are both less than 1.0,the stepped sliding surface occurs more easily than the plane failure surface,especially in the case of tensile failure of the rock bridge.In addition,rock slopes with higher strength parameters,taller heights,and greater weakening coefficients are prone to rock bridge tension failure with lower Fs,and more attention should be given to the occurrence of such accidents in actual engineering.

Mechanical characteristic and failure mechanism of joint with composite sucker rod

Composite sucker rods are widely used in oil fields because of light weight,high strength,and corrosion resistance.Bonded technology becomes the primary connection method of composites.However,the joints with composite sucker rods are prone to debone and fracture.The connected characteristics are less considered,so the failure mechanism of the joint is still unclear.Based on the cohesive zone model(CZM)and the Johnson-Cook constitutive model,a novel full-scale numerical model of the joint with composite sucker rod was established,and verified by pull-out experiments.The mechanical properties and slip characteristics of the joint were studied,and the damaged procession of the joint was explored.The results showed that:a)the numerical model was in good agreement with the experimental results,and the error is within 5%;b)the von Mises stress,shear stress,and interface stress distributed symmetrically along the circumferential path increased gradually from the fixed end to the loading end;c)the first-bonded interface near the loading end was damaged at first,followed by debonding of the second-bonded interface,leading to the complete shear fracture of the epoxy,and resulted in the debonding of the joint with composite sucker rod,which can provide a theoretical basis for the structural design and optimization of the joint.

An Overview of Self-piercing Riveting Process with Focus on Joint Failures, Corrosion Issues and Optimisation Techniques

Self-piercing riveting(SPR)is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to predrill a hole.The application of SPR in the automotive sector has become increasingly popular mainly due to the growing use of lightweight materials in transportation applications.However,SPR joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation induced by the SPR process.In this paper,SPR joints of advanced materials and their corresponding failure mechanisms are discussed,aiming to provide the foundation for future improvement of SPR joint quality.This paper is divided into three major sections:1)joint failures focusing on joint defects originated from the SPR process and joint failure modes under different mechanical loading conditions,2)joint corrosion issues,and 3)joint optimisation via process parameters and advanced techniques.

Failure behavior around a circular opening in a rock mass with non-persistent joints:A parallel-bond stress corrosion approach

The stability of underground excavations is influenced by discontinuities interspaced in surrounding rock masses as well as the stress condition. In this work, a numerical study was undertaken on the failure behavior around a circular opening in a rock mass having non-persistent open joints using PFC software package. A parallel-bond stress corrosion(PSC) approach was incorporated to drive the failure of rock mass around the circular opening, such that the whole progressive failure process after excavation was reproduced. Based on the determined micro parameters for intact material and joint segments, the failure process around the circular opening agrees very well with that obtained through laboratory experiment. A subsequent parametric study was then carried out to look into the influence of lateral pressure coefficient, joint dip angle and joint persistency on the failure pattern and crack evolution of the rock mass around the circular opening. Three failure patterns identified are step path failure, planar failure and rotation failure depending on the lateral pressure coefficient. Moreover, the increment of joint dip angle and joint persistency aggravates the rock mass failure around the opening. This study offers guideline on stability estimation of underground excavations.

Research on the creep damage and interfacial failure of dissimilar metal welded joint between 10Cr9Mo1VNbN and 12Cr1MoV steel

The mechanical properties, creep damage, creep rupture strength and features of interfacial failures of welded joints between martensite (SA213T91) and pearlite steel (12Cr1MoV) have been investigated by means of argon tungsten pulsed arc welding, high temperature accelerated simulation, creep rupture, mechanical property tests and scanning electronic microscope (SEM). The research results indicate that the mechanical properties of overmatched and medium matched joint deteriorate obviously, and they are susceptible to creep damage and failure after accelerated simulation operation 500 h, in the condition of preheat 250℃, and post welding heat treatment 750℃×1 h. However, the mechanical properties of undermatched joint are the best, the interfacial failure tendency of undermatched welded joint is less than those of medium and overmatched welded joint. Therefore, it is reasonable that low alloy material TR31 is used as the filler metal of weld between SA213T91and 12Cr1MoV steel.

Influence of Shim Layers on Progressive Failure of a Composite Componentin Composite-Aluminum Bolted Joint in Aerospace Structural Assembly

The influence of varying shim layers on the progressive damage/failure of a composite component in a bolted composite-aluminum aerospace structural assembly was investigated using a non-linear three-dimensional(3 D)structural solid elements assembled model of a carbon fiber-reinforced polymer(CFRP)-aluminum single-lap joint with a titanium(Ti-6 Al-4 V)fastener and a washer generated with the commercial finite element(FE)software package,ABAQUS/Standard.A progressive failure algorithm written in Fortran code with a set of appropriate degradation rules was incorporated as a user subroutine in ABAQUS to simulate the non-linear damage behavior of the composite component in the composite-aluminum bolted aerospace structure.The assembled 3 DFE model simulated,as well as the specimen for the experimental testing consisted of a carbon-epoxy IMS-977-2 substrate,aluminum alloy 7075-T651 substrate,liquid shim(Hysol EA 9394),solid peelable fiberglass shim,a titanium fastener,and a washer.In distinction to previous investigations,the influence of shim layers(liquid shim and solid peelable fiberglass shim)inserted in-between the faying surfaces(CFRP and aluminum alloy substrates)were investigated by both numerical simulations and experimental work.The simulated model and test specimens conformed to the standard test configurations for both civil and military standards.The numerical simulations correlated well with the experimental results and it has been found that:(1)The shimming procedure as agreed upon by the aerospace industry for the resolution of assembly gaps in bolted joints for composite materials is the same for a composite-aluminum structure;liquid shim series(0.3,0.5 and 0.7 mm thicknesses)prolonged the service life of the composite component whereas a solid peelable fiberglass shim most definitely had a better influence on the 0.9 assembly gap compared with the liquid shim;(2)The shim layers considerably influenced the structural strength of the composite component by delaying its ultimate failure thereby increasing its service life;and(3)Increasing the shim layer′s thickness led to a significant corresponding effect on the stiffness but with minimal effect on the ultimate load.

Damage Failure Analysis of Z-Pins Reinforced Composite Adhesively Bonded Single-Lap Joint

In order to study themechanical properties of Z-pins reinforced laminated composite single-lap adhesively bonded joint under un-directional static tensile load,damage failure analysis of the joint was carried out bymeans of test and numerical simulation.The failure mode and mechanism of the joint were analyzed by tensile failure experiments.According to the experimental results,the joint exhibits mixed failure,and the ultimate failure is Z-pins pulling out of the adherend.In order to study the failure mechanism of the joint,the finite element method is used to predict the failure strength.The numerical results are in good agreement with the experimental results,and the error is 6.0%,which proves the validity of the numerical model.Through progressive damage failure analysis,it is found that matrix tensile failure of laminate at the edge of Z-pins occurs first,then adhesive layer failure-proceeds at the edge of Z-pins,and finally matrix-fiber shear failure of the laminate takes place.With the increase of load,the matrix-fiber shear failure expands gradually in the X direction,and at the same time,the matrix tensile failure at the hole edge gradually extends in different directions,which is consistent with the experimental results.

Strength and failure characteristics of brittle jointed rock-like specimens under uniaxial compression:Digital speckle technology and a particle mechanics approach

The strength and failure characteristics of most natural rock mass are influenced by discontinues such as fissures, joints, and weak surfaces. In the present study, the strength and failure behavior of ubiquitousjoint rock-like specimens under uniaxial loading have been investigated by DIC(digital image correlation) and discrete element numerical method(PFC2 D). The results are obtained. Firstly, the UCSJof specimens with c = 15° or 30° shows similar tendency while a goes from 0° to 75°. With c = 45° or 60°, the UCSJof specimens increases when a goes from 0° to 30° and decreases after a goes beyond 30°. With c = 75°, the peak UCSJvalue is reached when a = 0°. The UCSJvalue shows an increasing trend when a goes from 60° to 75°. Secondly, the ubiquitous-joint specimens present different failure modes for various levels of a and c(b-a). Based on the experimental results, the failure mode of ubiquitous-joint specimens can be classified into three categories: stepped path failure, failure through parallel plane, and failure through cross plane.

Bayesian Joint Modelling of Survival Time and Longitudinal CD4 Cell Counts Using Accelerated Failure Time and Generalized Error Distributions

Survival of HIV/AIDS patients is crucially dependent on comprehensive and targeted medical interventions such as supply of antiretroviral therapy and monitoring disease progression with CD4 T-cell counts. Statistical modelling approaches are helpful towards this goal. This study aims at developing Bayesian joint models with assumed generalized error distribution (GED) for the longitudinal CD4 data and two accelerated failure time distributions, Lognormal and loglogistic, for the survival time of HIV/AIDS patients. Data are obtained from patients under antiretroviral therapy follow-up at Shashemene referral hospital during January 2006-January 2012 and at Bale Robe general hospital during January 2008-March 2015. The Bayesian joint models are defined through latent variables and association parameters and with specified non-informative prior distributions for the model parameters. Simulations are conducted using Gibbs sampler algorithm implemented in the WinBUGS software. The results of the analyses of the two different data sets show that distributions of measurement errors of the longitudinal CD4 variable follow the generalized error distribution with fatter tails than the normal distribution. The Bayesian joint GED loglogistic models fit better to the data sets compared to the lognormal cases. Findings reveal that patients’ health can be improved over time. Compared to the males, female patients gain more CD4 counts. Survival time of a patient is negatively affected by TB infection. Moreover, increase in number of opportunistic infection implies decline of CD4 counts. Patients’ age negatively affects the disease marker with no effects on survival time. Improving weight may improve survival time of patients. Bayesian joint models with GED and AFT distributions are found to be useful in modelling the longitudinal and survival processes. Thus we recommend the generalized error distributions for measurement errors of the longitudinal data under the Bayesian joint modelling. Further studies may investigate the models with various types of shared random effects and more covariates with predictions.

Numerical simulations of failure behavior around a circular opening in a non-persistently jointed rock mass under biaxial compression

Pre-existing discontinuities change the mechanical properties of rock masses,and further influence failure behavior around an underground opening.In present study,the failure behavior in both Inner and Outer zones around a circular opening in a non-persistently jointed rock mass under biaxial compression was investigated through numerical simulations.First,the micro parameters of the PFC^(3D) model were carefully calibrated using the macro mechanical properties determined in physical experiments implemented on jointed rock models.Then,a parametrical study was undertaken of the effect of stress condition,joint dip angle and joint persistency.Under low initial stress,the confining stress improves the mechanical behavior of the surrounding rock masses;while under high initial stress,the surrounding rock mass failed immediately following excavation.At small dip angles the cracks around the circular opening developed generally outwards in a step-path failure pattern;whereas,at high dip angles the surrounding rock mass failed in an instantaneous intact rock failure pattern.Moreover,the stability of the rock mass around the circular opening deteriorated significantly with increasing joint persistency.

Simulation of failure process of jointed rock

A modified discontinuous deformation analysis (DDA) algorithm was proposed to simulate the failure behavior of jointed rock. In the proposed algorithm, by using the Monte-Carlo technique, random joint network was generated in the domain of interest. Based on the joint network, the triangular DDA block system was automatically generated by adopting the advanced front method. In the process of generating blocks, numerous artificial joints came into being, and once the stress states at some artificial joints satisfy the failure criterion given beforehand, artificial joints will turn into real joints. In this way, the whole fragmentation process of rock mass can be replicated. The algorithm logic was described in detail, and several numerical examples were carried out to obtain some insight into the failure behavior of rock mass containing random joints. From the numerical results, it can be found that the crack initiates from the crack tip, the growth direction of the crack depends upon the loading and constraint conditions, and the proposed method can reproduce some complicated phenomena in the whole process of rock failure.

Elasto-plastic Analysis of Circular Tunnels in Jointed Rock Masses Satisfy the Hoek-Brown Failure Criterion

The relationship between the Hoek-Brown parameters and the mechanical response of circular tunnels is il-lustrated. Closed-form and approximate solutions are given for the extent of the plastic zone and the stress and dis-placement fields under axisymmetrical and asymmetric stress conditions. For the same rock masses and under axisym-metrical stress conditions,the radius of the plastic zone in terms of Hoek-Brown criterion is generally an approximation of the radius in terms of the Mohr-Coulomb criterion. The radius in terms of the Hoek-Brown criterion is larger under low stress conditions. For poor quality rock masses (GSI<25),measures (such as grouting,setting rock bolts,etc.) that improve the GSI of rock masses are effective in improving the stability of tunnels. It is not advisable to improve the sta-bility of the tunnels by providing a small support resistance p through shotcrete,except for very poor quality jointed rock masses. Without reference to the quality of the rock mass,the disturbance factor D should not less than 0.5. Meas-ures which disturb rock masses during tunnel construction should be taken carefully when the tunnel depth increases.

Research status of corrosion environment failure and safety assessment of pipeline welded joint

Pipeline transportation is an economical, safe, and efficient transportation method for transporting oil, natural gas, mineral slurry, and other fluids.Welding is the most critical construction process in pipeline engineering and is crucial in the safe operation and service of an entire pipeline system.Theoretically, the girth welded joint is the weakest link in a pipeline system.The unevenness of the structure and performance of the joint caused by welding frequently results in the failure of the welded joint before the failure of the base material of the pipe body, causing the pipeline to leak or even break.For steel pipes used in an acidic corrosive medium environment, the integration of the corrosive medium and mechanical load will accelerate the failure of the welded joint.This article reviews the failure modes of pipeline welded joints in acidic corrosive media, including stress corrosion cracking, hydrogen-induced cracking, and corrosion fracture, and corrosion fatigue considering the diffusion and accumulation of H;at the crack tip.It also reviews service pipelines in acidic corrosive media.The general processing technology of pipe joint engineering critical assessment(ECA) is investigated to provide a reference for the future development of technology in this field.

Dimensioning of Punctiform Metal-Composite Joints: A Section-Force Related Failure Criterion

Reliable line production processes and simulation tools play a central role for the structural integration of thermoplastic composites in advanced lightweight constructions. Provided that material-adapted joining technologies are available, they can be applied in heavy-duty multi-material designs (MMD). A load-adapted approach was implemented into the new fully automatic and fault-tolerant thermo mechanical flow drill joining (FDJ) concept. With this method it is possible to manufacture reproducible high strength FRP/metal-joints within short cycle times and without use of extra joining elements for the first time. The analysis of FDJ joints requires a simplified model of the joint to enable efficient numerical simulations. The present work introduces a strategy in modeling a finite-element based analogous-approach for FDJ-joints with glass fiber reinforced polypropylene and high-strength steel. Combined with a newly developed section-force related failure criterion, it is possible to predict the fundamental failure behavior in multi-axial stress states. The functionality of the holistic approach is illustrated by a demonstrator that represents a part of a car body-in-white structure. The comparison of simulated and experimentally determined failure loads proves the applicability for several combined load cases.

Deformation and failure mechanism of slope in three dimensions

Understanding three-dimensional (3D) slope deformation and failure mechanism and corresponding stability analyses are crucially important issues in geotechnical engineering. In this paper, the mecha-nisms of progressive failure with thrust-type and pull-type landslides are described in detail. It is considered that the post-failure stress state and the pre-peak stress state may occur at different regions of a landslide body with deformation development, and a critical stress state element (or the soil slice block) exists between the post-failure stress state and the pre-peak stress state regions. In this regard, two sorts of failure modes are suggested for the thrust-type and three sorts for pull-type landslides, based on the characteristics of shear stress and strain (or tensile stress and strain). Accordingly, a new joint constitutive model (JCM) is proposed based on the current stability analytical theories, and it can be used to describe the mechanical behaviors of geo-materials with softening properties. Five methods, i.e. CSRM (comprehensive sliding resistance method), MTM (main thrust method), CDM (comprehensive displacement method), SDM (surplus displacement method), and MPM (main pull method), for slope stability calculation are proposed. The S-shaped curve of monitored displacement vs. time is presented for different points on the sliding surface during progressive failure process of landslide, and the rela-tionship between the displacement of different points on the sliding surface and height of landslide body is regarded as the parabolic curve. The comparisons between the predicted and observed loadedis-placement and displacementetime relations of the points on the sliding surface are conducted. The classification of stable/unstable displacementetime curves is proposed. The definition of the main sliding direction of a landslide is also suggested in such a way that the failure body of landslide (simplified as“collapse body”) is only involved in the main sliding direction, and the strike and the dip are the same as the collapse body. The rake angle is taken as the direction of the sum of sliding forces or the sum of displacements in collapse body, in which the main slip direction is dependent on progressive defor-mation. The reason of non-convergence with finite element method (FEM) in calculating the stability of slope is also numerically analyzed, in which a new method considering the slip surface associated with the boundary condition is proposed. It is known that the boundary condition of sliding surface can be described by perfect elasto-plastic model (PEPM) and JCM, and that the stress and strain of a landslide can be described properly with the JCM.

Strength Reliability Analysis of Stiffened Cylindrical Shells Considering Failure Correlation

The stiffened cylindrical shell is commonly used for the pressure hull of submersibles and the legs of offshore platforms. There are various failure modes because of uncertainty with the structural size and material properties, uncertainty of the calculation model and machining errors. Correlations among failure modes must be considered with the structural reliability of stiffened cylindrical shells. However, the traditional method cannot consider the correlations effectively. The aim of this study is to present a method of reliability analysis for stiffened cylindrical shells which considers the correlations among failure modes. Firstly, the joint failure probability calculation formula of two related failure modes is derived through use of the 2D joint probability density function. Secondly, the full probability formula of the tandem structural system is given with consideration to the correlations among failure modes. At last, the accuracy of the system reliability calculation is verified through use of the Monte Carlo simulation. Result of the analysis shows the failure probability of stiffened cylindrical shells can be gained through adding the failure probability of each mode.

Analysis on the Bonding Failure Mechanism of High Modulus Carbon Fiber Composites

In order to explore the bonding failure mechanism of high modulus carbon fiber composite materials,the tensile experiment and finite element numerical simulation for single-lap and bevel-lap joints of unidirectional laminates are carried out,and the stress distributions,the failure modes,and the damage contours are analyzed. The analysis shows that the main reason for the failure of the single-lap joint is that the stress concentration of the ply adjacent to the adhesive layer is serious owing to the modulus difference,and the stress cannot be effectively transmitted along the thickness direction of the laminate. When the tensile stress of the ply exceeds its ultimate strength in the loading process,the surface fiber will fail. Compared with the single-lap joint,the bevel-lap joint optimizes the stress transfer path along the thickness direction,allows each layer of the laminate to share the load,avoids the stress concentration of the surface layer,and improves the bearing capacity of the bevel-lap joint. The improved bearing capacity of the bevellap joint is twice as much as that of the single-lap joint. The research in this paper provides a new idea for the subsequent study of mechanical properties of adhesively bonded composite materials.

Assessment of cyclic deformation and critical stress amplitude of jointed rocks via cyclic triaxial testing

Jointed rock specimens with a natural replicated joint surface oriented at a mean dip angle of 60were prepared,and a series of cyclic triaxial tests was performed at different confining pressures and cyclic deviatoric stress amplitudes.The samples were subjected to 10,000 loading-unloading cycles with a frequency of 8 Hz.At each level of confining pressure,the applied cyclic deviatoric stress amplitude was increased incrementally until excessive deformation of the jointed rock specimen was observed.Analysis of the test results indicated that there existed a critical cyclic deviatoric stress amplitude(i.e.critical dynamic deviatoric stress)beyond which the jointed rock specimens yielded.The measured critical dynamic deviatoric stress was less than the corresponding static deviatoric stress.At cyclic deviatoric stress amplitudes less than the critical dynamic deviatoric stress,minor cumulative residual axial strains were observed,resulting in hysteretic damping.However,for cyclic deviatoric stresses beyond the critical dynamic deviatoric stress,the plastic strains increased promptly,and the resilient moduli degraded rapidly during the initial loading cycles.Cyclic triaxial test results showed that at higher confining pressures,the ultimate residual axial strain attained by the jointed rock specimen decreased,the steadystate dissipated energy density and steady-state damping ratio per load cycle decreased,while steadystate resilient moduli increased.

Shear behaviour of a rock bridge sandwiched between incipient joints under the influence of hydraulic pressures

The rock bridges sandwiched in incipiently jointed rock mass were considered as barriers that block the fluid seepage,and provide certain shear strength reservation.For better revealing the influence of hydraulic pressure on the failure behaviour of rock bridges,direct shear tests were carried out through a newly proposed method on rock samples that contain two parallel incipient joints.By developing the gypsum-silicone pad coupling samples,a conventional triaxial test system was qualified to implement direct shear tests with satisfied sealing capability.The results showed that the rock bridges could be failed through the tensile failure,shear failure and mixed failure mechanism.The hydraulic pressure would facilitate the tensile failure mechanism and induce rougher fracture surfaces;while the normal stress would facilitate the shear failure mechanism and induce less rough fracture.The hydraulic pressure reduced the global shear strength of the rock block through reducing the efficient normal stress applied on the rock bridge area,which was highly dependent on the joint persistence,k.Moreover,because of the iterating occurrence of the hydraulic pressure lag with the fracture propagation,the rock bridge failure stage in the shear stress-shear displacement curves displayed a fluctuation trend.