Stress Release Rate Research Articles

Improvement of Low-Temperature Strength of Composite Bonded Joints Based on Residual Plastic Strain in Adhesive Layers

In this study, an improvement in the low-temperature strength of composite double-lap bonded joints was developed based on the residual plastic strain through preloading at room temperature. The finite element method was employed to calculate the adhesive layer stress and energy release rate between the adhesive layer and the inner adherent of the bonded joint at low temperatures. In the calculation, the residual plastic strain of the adhesive layer was considered. The numerical results indicated that the residual strain of the adhesive layer could increase the low-temperature strength of the composite bonded joints. The strength improvement of the bonded joints was experimentally evaluated at -150ºC. From the experimental results, the low-temperature strength of the bonded joint preloaded at room temperature increased by approximately 7%.

Analysis of aWater-Inrush Disaster Caused by Coal Seam Subsidence Karst Collapse Column under the Action of Multi-Field Coupling in Taoyuan Coal Mine

Minin-induced water inrush from a confined aquifer due to subsided floor karst collapse column (SKCC) is a type of serious disaster in the underground coal extraction. Karst collapse column (KCC) developed in a confined aquifer occurs widely throughout northern China. A water inrush disaster from SKCC occurred in Taoyuan coal mine on February 3, 2013. In order to analyze the effect of the KCC influence zone’s (KCCIZ) width and the entry driving distance of the water inrush through the fractured channels of the SKCC, the stress, seepage, and impact dynamics coupling equations were used to model the seepage rule, and a numerical FLAC3D model was created to determine the plastic zones, the vertical displacement development of the rock mass surrounding the entry driving working face (EDWF), and the seepage vector and water inflow development of the seepage field. The hysteretic mechanism of water inrush due to SKCC in Taoyuan coal mine was investigated. The results indicate that a water inrush disaster will occur when the width of the KCCIZ exceeds 16 m under a driving, which leads to the aquifer connecting with the fractured zones of the entry floor. Hysteretic water inrush disasters are related to the stress release rate of the surrounding rocks under the entry driving. When the entry driving exceeds about 10 m from the water inrush point, the stress release rate reaches about 100%, and a water inrush disaster occurs.

Microcrack propagation induced by dynamic infiltration of calcium-magnesium-alumino-silicate in columnar structures for thermal barrier coatings

ABSTRACT Columnar-structured thermal barrier coating systems (TBCs) possess a strain tolerant columnar microstructure wherein the gaps between the columns are filled with high-porosity oxides. A numerical model is proposed to estimate the stress fields of the microcrack induced by calcium-magnesium-alumina-silicate (CMAS) infiltration and microcrack propagation behavior in the yttria stabilized zirconia column induced by dynamic CMAS infiltration is studied using the extended finite element method. The results demonstrate that both the stress and energy release rate near microcracks were found to dramatically increase when CMAS reached a microcrack. The present analysis reveals that the vertical cracks easily initiate from the microstructure column and coalesce with adjacent horizontal and vertical pores, thereby resulting in premature failure of TBCs via delamination.

Stability analysis of the middle soil pillar for asymmetric parallel tunnels by using model testing and numerical simulations

The stability of the middle soil pillar (MSP) is considered to be a crucial factor in ensuring the safety during the construction of asymmetric parallel tunnels. In this paper, both geomechanical model tests and numerical analysis were carried out to investigate the mechanical characteristics of the MSP between asymmetrically closely-spaced parallel-tunnels. First, large-scale three-dimensional (3-D) geomechanical model tests were conducted to study the stress release laws and mechanical behaviors of the MSP during the excavation and overloading conditions. Then, a numerical model was implemented based on the physical model for verifying the MSP’s stress response obtained from the model tests. Failure degree index (FDI) was put forward to evaluate the degree of disturbance to the MSP and to analyze its damage extent during tunnels excavation. Furthermore, the effect of construction parameters (i.e., support strength and excavation sequence) on the damage behaviors of the MSP were discussed by extended numerical analysis. The model tests results show that the stress was mainly released from the upper and lower ends of the middle pillar to lateral direction in sand ground. Both the horizontal stress release and horizontal stress release rate of the MSP increased along with the external applied loads. Meanwhile, the overloading process only increased the stress of surrounding soil, while the load-bearing pattern of the MSP was basically unchanged. In addition, the numerical results indicate that due to the differences in the cross-section dimensions and construction parameters (e.g., support strength and excavation sequence) of the closely-spaced parallel-tunnels, the final damage pattern of the MSP was asymmetrical.

A strain rate dependent cohesive zone element for mode I modeling of the fracture behavior of adhesives

Adhesives are widely used in the automotive industry as they can be used to manufacture lightweight multimaterial structures with improved strength-to-weight ratio, contributing to lower energy consumption and emissions. In this industry, it is also crucial to ensure passengers' safety and, thus, mechanical behavior of the complete automotive structure should be tested for impact conditions, including the adhesive joints within it. This work presents the development of a finite element, which models the mechanical behavior of adhesives and takes into account the strain rate dependent property variation, in mode I. The trends for the property variation were set based on an experimental study of two adhesives under three different loading conditions: quasi-static, intermediate speed, and impact. The strength was determined using bulk tensile tests and toughness using double cantilever beam, respectively. Both adhesives studied have shown an increase in ultimate stress and critical energy release rate with increasing strain rate. The property variation was then implemented on a finite element, which revealed a good agreement with the experimental results.

Modeling snow slab avalanches caused by weak-layer failure – Part 1: Slabs on compliant and collapsible weak layers

Abstract. Dry-snow slab avalanche release is preceded by a fracture process within the snowpack. Recognizing weak-layer collapse as an integral part of the fracture process is crucial and explains phenomena such as whumpf sounds and remote triggering of avalanches from low-angle terrain. In this two-part work we propose a novel closed-form analytical model for a snowpack under skier loading and a mixed-mode failure criterion for the nucleation of weak-layer failure. In the first part of this two-part series we introduce a closed-form analytical model of a snowpack accounting for the deformable layer. Despite the importance of persistent weak layers for slab avalanche release, no simple analytical model considering weak-layer deformations is available. The proposed model provides deformations of the snow slab, weak-layer stresses and energy release rates of cracks within the weak layer. It generally applies to skier-loaded slopes as well as stability tests such as the propagation saw test. A validation with a numerical reference model shows very good agreement of the stress and energy release rate results in several parametric studies including analyses of the bridging effect and slope angle dependence. The proposed model is used to analyze 93 propagation saw tests. Computed weak-layer fracture toughness values are physically meaningful and in excellent agreement with finite element analyses. In the second part of the series (Rosendahl and Weißgraeber, 2020) we make use of the present mechanical model to establish a novel failure criterion crack nucleation in weak layers. The code used for the analyses in both parts is publicly available under https://github.com/2phi/weac (last access: 6 January 2020) (2phi, 2020).

Research and Application of Radial Borehole Fracturing Based on Numerical Simulation

The radial borehole fracturing technology has been applied in a certain number of oilfields with good results being achieved. However, the morphology and variation of fracture still require further study. In this paper, the reservoir model based on formation fluid-solid coupling equation is established with the extended finite element method (XFEM) in ABAQUS, and the fracture morphologies in the single-radial borehole, vertical multiradial borehole, and horizontal multiradial borehole are simulated and analyzed with criteria of maximum principal stress and maximum energy release rate as the damage mechanism. Moreover, the accuracy of numerical simulation results is verified with the large-scale true 3D physical simulation experiment. The results show that the induced stress field along the radial borehole during fracturing is the root cause of fracture directional propagation along the radial borehole whose effective guidance distance reaches 40 m. The vertical multiradial borehole can effectively enhance fracture directional propagation and is capable of reducing fracture initiation pressure. In the horizontal multiradial borehole, the major fracture propagating along each radial borehole is formed in the remote-borehole area, and the secondary fracture connecting the neighboring radial boreholes is formed in the near-borehole zone. Coordination of major and secondary fractures can effectively increase the drainage area and reduce the flow resistance in the near-borehole zone. Based on the research on fracture morphology of multiradial borehole fracturing, the scheme of radial borehole arrangement is optimized and verified through numerical simulation of deliverability. The final optimum borehole arrangement scheme is the intersectional angle of 45° between four orthogonal radial boreholes and horizontal maximum principal stress.

The Prevention and Control Mechanism of Rockburst Hazards and Its Application in the Construction of a Deeply Buried Tunnel

Rockburst hazards induced by high geostress are particularly prominent during the construction of underground engineering. Prevention and control of rockburst is still a global challenge in the field of geotechnical engineering, which is of great significance. Based on the tunnel group of the Jinping II hydropower station of China, this paper analyzed the mechanical principle of support in the process of construction, and discussed in detail the active release and passive support by numerical simulation and field application. The results show that as two active measures, stress relieve holes and advanced stress relief blasting can release the energy of the microseismic source and transfer the high stress to the deeper surrounding rock, make the surface rock wall with a relatively low stress act as a protective barrier. Their stress release rate is about 12% and 33% in this project, respectively. In term of passive measure, the combined rapid support, which is mainly composed of water swelling anchor and nano-admixture shotcrete, is also an effective way to prevent and control the rockburst under high geostress.

Sensitivity analysis of ground settlement induced by pipe-jacking construction in soft soil stratum

Taking the ground settlement while pipe-jacking construction in soft soil as the engineering background, the effect of pipe jacking were studied by numerical simulation. It focused on the factors that mainly influenced surface settlement of ground including in-situ stress release rate (or ground loss ratio), chamber pressure, elastic modulus of soil, buried depth and diameter of pipe. That would provide theoretical basis for taking measures to reduce surface subsidence while jacking. The main conclusions were as follows: (1) The in-situ stress release rate and pipe diameter were proportional to the surface subsidence. (2) Chamber pressure, elastic modulus and buried depth were inverse proportional to the surface subsidence. (3) From the point of view of the influence on surface settlement, the sensitivity of pipe diameter and elastic modulus of soil was the greatest, followed by burial depth and chamber pressure, and the stress release rate was the smallest.

Numerical Analysis of Pipelines Settlement Induced by Tunneling

Three‐dimensional finite element method analysis on the tunnel‐soil‐underground pipeline was carried out based on the ABAQUS program. PSI element was applied to simulate the interaction between the pipelines and soil. Parameters such as an elastic modulus of soil, stress release rate, at‐rest lateral pressure coefficients, an elastic modulus of pipelines, and buried depths of tunnels were analyzed. The effects of tunnel excavation on the displacement of existing pipelines were investigated, and the settlement relationships were obtained. The relationship between each parameter and surface settlement was determined by the grey relational analysis method to analyze each parameter’s sensitivity to the settlement of the pipeline, which can provide a reference for emphasis and methods of shield tunneling support. Finally, a formula of the settlement relationship between the maximum surface settlement and pipelines deformation was proposed for different pipe‐soil relative stiffness. The formula was applied in the practical case. Compared with the field monitoring results and FEM computer results, it has been found that the proposed normalized formula is consistent with the measured results and numerical simulation of the pipeline settlement.

Optimization of Supporting Structures and Parameters of Tunnels in Soft Phyllite

The study investigated the extra-long Shuangxiling tunnel in soft phyllite within Nanchang–Ningdu highway in Jiangxi province, China. The supporting scheme of reducing the size and density of anchor rods while strengthening the stiffness and strength of preliminary support was put forward aiming at the repeated occurrence of great deformation of surrounding rocks during tunneling. Moreover, two groups of comparative supporting schemes were introduced for comparison. Based on the numerical simulation, the study calculated the amount of displacement deformation, stress on anchor rods, development of plastic zones, stress on shotcrete, and stress on secondary lining structure under different supporting schemes. By conducting field monitoring test, the surrounding rock deformations and pressures under different supporting schemes were monitored and measured in real time. The research results showed that after strengthening the stiffness and strength of preliminary support, the surrounding rock pressure on the supporting structure increased. It limited the preliminary stress release rate of surrounding rocks, protected the integrity of surrounding rocks and controlled the sustained deformation of surrounding rocks. However, weakening the size and density of anchor rods had an insignificant effect on the whole supporting effect. The result of field test was basically consistent with that obtained through numerical simulation, which theoretically and practically verified the scientificity and feasibility of the supporting scheme of weakening the size and density of anchor rod while strengthening the stiffness and strength of preliminary support. The research result provides references for the support optimization of similar tunnels along the highway.

Study on Mechanical Properties and Damage Evolution Law of Secondary Destruction Induced by Simultaneous Unloading after the Peak of the Curve of Sandstone

The research adopts the French ROCK600‐50 three‐axis experiment instrument and SH‐II system to evaluate the acoustic emission (AE) peak of the rear axle of yellow sandstone by carrying out the confining pressure synchronous unloading experiment, and evolution of mechanical properties and characteristics of energy transformation and damage of a yellow sandstone, which is a two‐damage process, the results of which have shown the following: (1) there are two damages appearing when the confining pressure is 5 MPa, 10 MPa, and 20 MPa. In contrast, these two damages do not appear when the confining pressure is 30 MPa and 40 MPa. The fracture degree and crack number of the two fractures are larger than those without two damages. (2) A failure stress release rate showed an obvious “√” trend. With the increase of confining pressure, the stress release rate of the two failures is increasing. (3) By keeping the strain of the body constant, it is determined to be the first time to destroy the warning. The two failures of rock are predicted by the inflection point of the axial strain slope. No two failures occurred, and the strain unloading process showed two characteristics. (4) Meanwhile, with the increase of confining pressure, the energy released from the peak decreases first and then increases; the law of releasing energy from one failure and two destructions is the opposite. No damage occurred two times. As the confining pressure increases, there is a continuous decrease of the energy released from the peak. As the confining pressure increases, the release rate of the primary energy decreases at first and then increases, and the rate of energy released two times is increasing. The plasticity coefficient shows the trend of increasing first and then decreasing. (5) The damage degree of the second rupture is greater than the first rupture.

Acoustic emission characteristics and stress release rate of coal samples in different dynamic destruction time

Considering the importance of the prediction of rock burst disasters, and in order to grasp the law of acoustic emission (AE) of coal samples in different dynamic destruction time, the SH-II AE monitoring system was adopted to monitor the failure process of coal samples. The study of the change rule of the AE numbers, energy, ‘b’ value and spectrum in the micro crack propagation process of the coal samples shows that as dynamic damage time went by, AE presented high-energy counts and the accumulated counts increased during the compression phase. The AE energy and cumulative counts increased during the elastic stage. The AE blank area increased gradually and the blank lines were more and more obvious in the molding stage. The AE counts and energy showed a trend of decrease in the residual damage phase. AE ‘b’ values gradually became sparse, and the large scale cracks percentage compared with micro cracks decreased and the degree of damage decreased. The AE frequency spectrum peak went from the residual damage phase to the molding phase, and finally it was nearly stable, besides the bandwidth of the main frequency is gradually narrowed. Also, the frequency peak changed from single peak frequency to bi-peak frequency and to the single peak frequency. Uniaxial compressive strength is more sensitive than the elastic modulus to dynamic damage time.

Relaxation times in deformed polymer glasses: A comparison between molecular simulations and two theories

Relaxation times in polymer glasses are computed with molecular dynamics simulations of a coarse-grained polymer model during creep and constant strain rate deformation. The dynamics is governed by a competition between physical aging that increases relaxation times and applied load or strain rate which accelerates dynamics. We compare the simulation results quantitatively to two recently developed theories of polymer deformation, which treat aging and rejuvenation in an additive manner. Through stress release and strain rate reversal simulations, we then show that the quantity governing mechanical rejuvenation is the rate of irreversible work performed on the polymer.

Parameter estimation of an anisotropic damage model for concrete using genetic algorithms

This article presents an anisotropic damage model for concrete that couples between elasticity and continuum damage mechanics. The formulation of constitutive model is based on the elastic strain energy in the framework of irreversible thermodynamics. The thermodynamic free energy is represented as a scalar function of elastic strain and damage tensors and used to derive the constitutive law and thermodynamic conjugate force of damage that is used to derive the dissipation potential. The damage evolution law is governed by the normality rule. The formulation of elastic strain energy of damaged material is capable of modeling the concrete anisotropic behavior under different loadings without decoupling the stress or damage release rate. A series of unknown parameters in the model formulation was used to control the constitutive behavior and damage surface. A Genetic algorithm FORTRAN subroutine is used to estimate these parameters based on the coupling between the constitutive and damage evolution equations. The performance of the damage model is verified with the experimental data from the literature. The model has shown a good agreement with the experimental results. It describes the anisotropy induced by the crack development within the concrete.

A clamped punch-loaded blister test for adhesion: a lumped parameter model and the bending-to-stretching behavior

This paper presents a clamped punch-loaded blister test and the corresponding lumped parameter model for the adhesion between thin film and rigid substrate. In the test, circular thin film is adhered to rigid substrate and an external load is applied with a clamped punch to cause the deflection of thin film. The relations among load, deflection, residual stress and strain energy release rate are thoroughly investigated. Analytical solutions in linear and nonlinear behaviors are obtained and the bending and stretching effects are associated through a lumped parameter model. The result of lumped-parameter model shows that the blister deflection is directly proportional to the applied load in the linear bending dominant region and the blister deflection is in direct proportion to the cubic root of the applied load in the nonlinear stretching dominant region. This has been proved by the result of nonlinear finite element analysis and the experimental result as well.

Fracture of thermal barrier coating with multiple surface cracks and delaminations: Interlayer effect

Multiple surface cracks and interfacial delamination are the major failure mechanisms in film/substrate systems. The effect of interlayer upon the failure mechanisms of interfacial delamination concomitant to surface crack was explored. Finite element model was developed to obtain the stress and energy release rate (ERR), which governs the propagation of interface cracks. The dependences of delamination upon the geometry and constitutive properties of interlayer were examined. The results indicate that the effect of elastic modulus of interlayer on the steady state ERR is insignificant. In cases of different geometrical parameters, however, the steady ERR decreases with the increase of the interlayer thickness. These findings lead to the conclusion that the interlayer constraint has significant effect on the ERR and thus coating life, which can be adopted to modify the ceramic top coat.

Comparing Effects of Biobinder with Other Asphalt Modifiers on Low-Temperature Characteristics of Asphalt

AbstractThis paper examines effects of a new biobased modifier, “biobinder,” on low-temperature properties of asphalt binder and compares the results with those of conventional modifiers: crumb rubber, Gilsonite, styrene-butadiene-styrene (SBS), and polyphosphoric acid (PPA). Low-temperature characteristics of modified and nonmodified asphalts were studied using experimental and modeling approaches. A three-point bending beam rheometer was used to measure the stress-strain response of each specimen. A theoretical approach was used to determine stiffness and stress release rate; in addition, the Burgers model was implemented to predict the stored and dissipated energy ratio and quantity of derivation of creep compliance in each of modified and nonmodified asphalt specimens. Effects of various modifications were compared based on both the calculated stored and dissipated energy ratio and quantity of derivation of creep compliance. The proposed biobinder is produced from the thermochemical conversion of biom...

Numerical Simulation Study on the Delay of Rockburst Based on Rock Mass Stress Release Rate

Rockburst is a phenomenon of geological hazard due to excavation in brittle rockmass of high in-situ stress which endanger to the engineers and construction equipments with unexpectedly damaged. At present, researchers and engineers mainly concentrate to the requirement of rockburst and whether to arise. Although the delay characteristic of rockburst (DCR) has been realized and recorded, but the knowledge of the mechanism of this feature is insufficient. In the paper, the delay characteristic was researched from the stress release rate (SRR) of the excavation rock mass using the numerical simulation software. Firstly, using the core replacement technique, the relation of the SRR and core modulus reduction (CMR) was determined. Secondly, the mechanism of the DCR was analyzed from the elastic strain energy distribution and the plastic strain energy distribution in the different SRR. Finally, the plastic field shape and range under different SRR was contrasted and analyzed. Conclusions can be drawn as follows: with the increase of CMR value, the SRR shows increase trend in the form of exponent. In the process of excavation, the rockmass elastically deform under the low SRR value. When the SRR value equals certain degree, the portion of rockmass will be plastic field and behind the plastic region there will be arise a elastic strain energy centralized phenomenon. Under the different the SRR value, the field style change to the tension-shear field from the shear field in the rock mass and the rockburst rank obviously different.

Characteristics of the unloading process of rocks under high initial stress

The unloading process of rocks under high initial stress is complex, and verifying the mechanism of the unloading process in the field or in a laboratory is not straightforward. In this study, the unloading process of rocks under high initial stress was characterised by a mathematical physics model, which was then implemented in the finite element program LS-DYNA for analysis. In particular, the implicit and explicit methods were performed in sequence in the finite element simulation of rocks with initial stresses. In the numerical simulation, the characteristics of the dynamic unloading process of rocks were investigated for various peak initial stresses, initial stress release paths and initial stress release rates (ISRRs). The numerical results indicated that the rock failure could be induced by the release of the initial stress; furthermore, there is a relationship between the magnitude of the unloading failure and the peak initial stresses, the initial stress release paths and the ISRRs. When the initial stresses were at the same level, the equivalent initial stress release rate (EISRR) was introduced to quantitatively describe the characteristics of the unloading process. Using the numerical results, the unloading failure process was characterised, and a method for the static stress initialisation-dynamic unloading of rock was developed.