A creep model for ultra-deep salt rock considering thermal-mechanical damage under triaxial stress conditions

To investigate the specific creep behavior of ultra-deep buried salt during oil and gas exploitation,a set of triaxial creep experiments was conducted at elevated temperatures with constant axial pressure and unloading confining pressure conditions.Experimental results show that the salt sample deforms more significantly with the increase of applied temperature and deviatoric loading.The accelerated creep phase is not occurring until the applied temperature reaches 130℃,and higher temperature is beneficial to the occurrence of accelerated creep.To describe the specific creep behavior,a novel three-dimensional(3D)creep constitutive model is developed that incorporates the thermal and mechanical variables into mechanical elements.Subsequently,the standard particle swarm optimization(SPSO)method is adopted to fit the experimental data,and the sensibility of key model parameters is analyzed to further illustrate the model function.As a result,the model can accurately predict the creep behavior of salt under the coupled thermo-mechanical effect in deep-buried condition.Based on the research results,the creep mechanical behavior of wellbore shrinkage is predicted in deep drilling projects crossing salt layer,which has practical implications for deep rock mechanics problems.

Constitutive model of viscoelastic dynamic damage for the material of gas obturator in modular-charge howitzer

In order to investigate the mechanical response behavior of the gas obturator of the breech mechanism,made of polychloroprene rubber(PCR), uniaxial compression experiments were carried out by using a universal testing machine and a split Hopkinson pressure bar(SHPB), obtaining stress-strain responses at different temperatures and strain rates. The results revealed that, in comparison to other polymers, the gas obturator material exhibited inconspicuous strain softening and hardening effects;meanwhile, the mechanical response was more affected by the strain rate than by temperature. Subsequently, a succinct viscoelastic damage constitutive model was developed based on the ZWT model, including ten undetermined parameters, formulated with incorporating three parallel components to capture the viscoelastic response at high strain rate and further enhanced by integrating a three-parameter Weibull function to describe the damage. Compared to the ZWT model, the modified model could effectively describe the mechanical response behavior of the gas obturator material at high strain rates. This research laid a theoretical foundation for further investigation into the influence of chamber sealing issues on artillery firing.

Effect of neutral polymeric bonding agent on tensile mechanical properties and damage evolution of NEPE propellant

Introducing Neutral Polymeric bonding agents(NPBA) into the Nitrate Ester Plasticized Polyether(NEPE)propellant could improve the adhesion between filler/matrix interface, thereby contributing to the development of new generations of the NEPE propellant with better mechanical properties. Therefore,understanding the effects of NPBA on the deformation and damage evolution of the NEPE propellant is fundamental to material design and applications. This paper studies the uniaxial tensile and stress relaxation responses of the NEPE propellant with different amounts of NPBA. The damage evolution in terms of interface debonding is further investigated using a cohesive-zone model(CZM). Experimental results show that the initial modulus and strength of the NEPE propellant increase with the increasing amount of NPBA while the elongation decreases. Meanwhile, the relaxation rate slows down and a higher long-term equilibrium modulus is reached. Experimental and numerical analyses indicate that interface debonding and crack propagation along filler-matrix interface are the dominant damage mechanism for the samples with a low amount of NPBA, while damage localization and crack advancement through the matrix are predominant for the ones with a high amount of NPBA. Finally, crosslinking density tests and simulation results also show that the effect of the bonding agent is interfacial rather than due to the overall crosslinking density change of the binder.

Mechanical properties and damage constitutive model of sandstone after acid corrosion and high temperature treatments

Aiming at the problem of temperature-mechanics-chemical(T-M-C)action encountered by rocks in underground engineering,sandstone was selected as the sample for acid corrosion treatment at pH 1,3,5 and 7,the acid corrosion treated samples were then subjected to high-temperature experiments at 25,300,600,and 900℃,and triaxial compression experiments were conducted in the laboratory.The experimental results show that the superposition of chemical damage and thermal damage has a significant impact on the quality,wave velocity,porosity and compression failure characteristics of the rock.Based on the Lemaitre strain equivalent hypothesis theory,the damage degree of rock material was described by introducing damage variables,and the spatial mobilized plane(SMP)criterion was adopted.The damage constitutive model can well reflect the stress-strain characteristics of the rock triaxial compression process,which verified the rationality and reliability of the model parameters.The experiment and constitutive model analyzed the change law of mechanical properties of rock after chemical corrosion and high temperature thermal damage,which had certain practical significance for rock engineering construction.

Damage statistical mechanics model of top coal in steep top caving coal

Damage statistical mechanics model of horizontal section height in the top caving was constructed in the paper. The influence factors including supporting pressure, dip angle and characteristic of coal on horizontal section height were analyzed as well. By terms of the practice project analysis, the horizontal section height increases with the increase of dip angle β and thickness of coal seam M. Dip angle of coal seam β has tremendous impact on horizontal section height, while thickness of coal seam M has slight impact. When thickness of coal seam is below 10m, horizontal section height increases sharply. While thickness exceeds 15m, it is not major factor influencing on horizontal section height any long.

Micro Model of Carbon Fiber/Cyanate Ester Composites and Analysis of Machining Damage Mechanism

Machining damage occurs on the surface of carbon fiber reinforced polymer (CFRP) composites during processing. In the current simulation model of CFRP, the initial defects on the carbon fiber and the periodic random distribution of the reinforcement phase in the matrix are not considered in detail, which makes the characteristics of the cutting model significantly different from the actual processing conditions. In this paper, a novel three-phase model of carbon fiber/cyanate ester composites is proposed to simulate the machining damage of the composites. The periodic random distribution of the carbon fiber reinforced phase in the matrix was realized using a double perturbation algorithm. To achieve the stochastic distribution of the strength of a single carbon fiber, a novel method that combines the Weibull intensity distribution theory with the Monte Carlo method is presented. The mechanical properties of the cyanate matrix were characterized by fitting the stress-strain curves, and the cohesive zone model was employed to simulate the interface. Based on the model, the machining damage mechanism of the composites was revealed using finite element simulations and by conducting a theoretical analysis. Furthermore, the milling surfaces of the composites were observed using a scanning electron microscope, to verify the accuracy of the simulation results. In this study, the simulations and theoretical analysis of the carbon fiber/cyanate ester composite processing were carried out based on a novel three-phase model, which revealed the material failure and machining damage mechanism more accurately.

A 3-D DDA damage analysis of brick masonry buildings under the impact of boulders in mountainous areas

In mountainous areas, geological disasters carrying large boulders can cause severe damage to the widely used masonry buildings due to the high impact forces. To better understand the damage of brick masonry buildings under the impact of boulders, a "block-joint" model is developed using threedimensional discontinuous deformation analysis(3-D DDA) to simulate the behaviour of the "brick-mortar" structure. The "block-joint" model is used to capture not only the large displacement and deformation of individual bricks but also the large-scale sliding and opening along the mortar between the bricks. The linear elastic constitutive model is applied to account for the non-plastic deformation behaviour of brick materials. Furthermore, the mechanical characteristics of the mortar are represented using the Mohr-Coulomb and Drucker-Prager criteria. To propose safe structural design schemes and effective reinforcement for brick masonry buildings, seven construction techniques are considered, includingdifferent grades of brick and mortar, effective shear areas and reinforced members. The proposed 3-D DDA model is used to analyse the velocity distribution and the key point displacements of the brick masonry building under the impact of boulders. The results show that upgrading the brick and mortar, increasing the wall thickness, making full use of the wall thickness, and adding a circular beam and structural column are very effective approaches for improving the impact resistance of brick masonry buildings.

Analysis of the damage mechanism of strainbursts by a global-local modeling approach

Strainburst is the most common type of rockbursts.The research of strainburst damage mechanisms is helpful to improve and optimize the rock support design in the burst-prone ground.In this study,an improved global-local modeling approach was first adopted to study strainburst damage mechanisms.The extracted stresses induced by multiple excavations from a three-dimensional(3D)global model established by fast Lagrangian analysis of continua in 3 dimensions(FLAC3D)are used as boundary conditions for a two-dimensional(2D)local model of a deep roadway built by universal distinct element code(UDEC)to simulate realistic stress loading paths and conduct a detailed analysis of rockburst damage from both micro and macro perspectives.The results suggest that the deformation and damage level of the roadway gradually increase with the growth of surrounding rock stress caused by the superposition of mining-or excavation-induced stresses of the panel and nearby roadways.The significant increase of surrounding rock stresses will result in more accumulated strain energy in two sidewalls,providing a necessary condition for the strainburst occurrence in the dynamic stage.The strainburst damage mechanism for the study site combines three types of damage:rock ejection,rock bulking,and rockfall.During the strainburst,initiation,propagation,and development of tensile cracks play a crucial role in controlling macroscopic failure of surrounding rock masses,although the shear crack always accounts for the main proportion of damage levels.The deformation and damage level of the roadway during a strainburst positively correlate with the increasing peak particle velocities(PPVs).The yielding steel arch might not dissipate kinetic energy and mitigate strainburst damage effectively due to the limited energy absorption capacity.The principles to control and mitigate strainburst damage are proposed in this paper.This study presents a systematic framework to investigate strainburst damage mechanisms using the global-local modeling approach.

Thermal-hydro-mechanical coupling damage model of brittle rock

Based on fluid mechanics, thermodynamics and damage mechanics, thermal-hydro-mechanical （THM） coupling damage model of brittle rock is established by analyzing THM coupling mechanism, where THM coupling damage variable DTHM is dominated by TH coupling damage variable DTH, TM coupling damage variable DTM and HM coupling damage variable DHM, and DTH is firstly expressed in term of dimensionless total thermal conductivity of the water Nu. Permeability test, uni-axial compression test and THM coupling test are conducted to measure the permeability, elastic modulus and THM coupling stress-strain curves of brittle rock. The tested values of THM coupling elastic modulus E＇HM are in good agreement with the predicted values of THM coupling elastic modulus ETHM, which can verify the newly established THM coupling damage model.

Real-time assessment of tunnelling-induced damage to structures within the building information modelling framework

During the initial design phases of complex multi-disciplinary systems such as urban tunnelling,the appraisal of different design alternatives can ensure optimal designs in terms of costs,construction time,and safety.To enable the evaluation of a large number of design scenarios and to find an optimal solution that minimises impact of tunnelling on existing structures,the design and assessment process must be efficient,yet provide a holistic view of soil-structure interaction effects.This paper proposes an integrated tunnel design tool for the initial design phases to predict the ground settlements induced by tunnelling and building damage using empirical and analytical solutions as well as simulation-based meta models.Furthermore,visualisation of ground settlements and building damage risk is enabled by integrating empirical and analytical models within our Building Information Modelling(BIM)framework for tunnelling.This approach allows for near real-time assessment of structural damage induced by settlements with consideration of soil-structure interaction and non-linear material behaviour.Furthermore,because this approach is implemented on a BIM platform for tunnelling,first,the design can be optimised directly in the design environment,thus eliminating errors in data exchange between designers and computational analysts.Secondly,the effect of tunnelling on existing structures can be effectively visualised within the BIM by producing risk-maps and visualising the scaled deformation field,which allows for a more intuitive understanding of design actions and for collaborative design.Having a fully parametric design model and real-time predictions therefore enables the assessment and visualisation of tunneling-induced damage for large tunnel sections and multiple structures in an effective and computationally efficient way.

Dynamic Mechanical Characteristics and Damage Evolution Model of Granite

By using the technique of the split Hopkinson pressure bar（ SHPB）,impact tests at different stress wavelengths（ 0. 8-2. 0 m） and strain rates（ 20-120 s^（-1）） were conducted to study the dynamic mechanical properties and damage accumulation evolution lawof granite. Test results showthat the dynamic compressive strength and strain rate of granite have a significantly exponential correlation;the relationship between peak strain and strain rate is approximately linear,and the increase of wavelengths generally makes the level of peak strain uplift. The multiple-impacts test at a lowstrain rate indicates that at the same wavelength,the cumulative damage of granite shows an exponential increasing form with the increase of strain rate; when keeping the increase of strain rate constant and increasing the stress wavelength,the damage accumulation effect of granite is intensified and still shows an exponential increasing form; under the effect of multiple impacts,the damage development trend of granite is similar overall,but the increase rate is accelerating. Therefore the damage evolution model was established on the basis of the exponential function while the physical meaning of parameters in the model was determined. The model can reflect the effect of the wave parameters and multiple impacts. The validity of the model and the physical meaning of the parameters were verified by the test,which further offer a reference for correlational research and engineering application for the granite.

Damage evolution model of strain hardening cementitious composites under the uniaxial stress state

The deformation and damage behaviors of strain hardening cementitious composites （SHCC） under the uniaxial stress state were investigated in this paper. Two ductile failure-based constitutive models were introduced to describe the uniaxial tension and compression properties of SHCC only using a few parameters. The computation method of model parameters was developed to ease the simulation procedures. Damage evolution of the SHCC was simulated by the formulation of continuum damage mechanics subsequently. The results show that the proposed models fit the stress-strain curves reasonably well, and the damage variables show different growth rules under uniaxial tension and compression. It is concluded that the proposed method can not only simply simulate the constitutive behavior of SHCC with the reasonable accuracy but also capture the characteristic of material degradation.

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.

Mechanical responses and acoustic emission behaviors of coal under compressive differential cyclic loading(DCL):a numerical study via 3D heterogeneous particle model

The stability of coal walls(pillars)can be seriously undermined by diverse in-situ dynamic disturbances.Based on a 3D par-ticle model,this work strives to numerically replicate the major mechanical responses and acoustic emission(AE)behaviors of coal samples under multi-stage compressive cyclic loading with different loading and unloading rates,which is termed differential cyclic loading(DCL).A Weibull-distribution-based model with heterogeneous bond strengths is constructed by both considering the stress-strain relations and AE parameters.Six previously loaded samples were respectively grouped to indicate two DCL regimes,the damage mechanisms for the two groups are explicitly characterized via the time-stress-dependent variation of bond size multiplier,and it is found the two regimes correlate with distinct damage patterns,which involves the competition between stiffness hardening and softening.The numerical b-value is calculated based on the mag-nitudes of AE energy,the results show that both stress level and bond radius multiplier can impact the numerical b-value.The proposed numerical model succeeds in replicating the stress-strain relations of lab data as well as the elastic-after effect in DCL tests.The effect of damping on energy dissipation and phase shift in numerical model is summarized.

A fatigue damage model for asphalt mixtures under controlled-stress and controlled-strain modes

A fatigue damage model based on thermodynamics was deduced for asphalt mixtures under controlled-stress and controlled-strain modes. By employing modulus of resilience as the damage hardening variable, a damage variable related with dynamic modulus was extracted as the evaluation index. Then, the damage evolution law under two control modes was proposed, and it has a similar form to the Chaboche fatigue model with a nonnegative material parameter m related to its loading level. Experimental data of four loading levels were employed to calibrate the model and identify the parameter in both control modes. It is found that the parameter m shows an exponential relationship with its loading level. Besides, the difference of damage evolution under two control modes was explained by the law. The damage evolves from fast to slow under a controlled-strain mode. However, under a controlled-stress mode, the evolution rate is just the opposite. By using the damage equivalence principle to calculate the equivalent cycle numbers, the deduced model also interprets the difference of damage evolution under two control modes on the condition of multilevel loading. Under a controlled-strain mode, a loading sequence from a low level to a high level accelerates damage evolution. An inverse order under the controlled-stress mode can prolong fatigue life.

Bi-variable damage model for fatigue life prediction of metal components

Based on the theory of continuum damage mechanics,a bi-variable damage mechanics model is developed,which,according to thermodynamics,is accessible to derivation of damage driving force,damage evolution equation and damage evolution criteria. Furthermore,damage evolution equations of time rate are established by the generalized Drucker＇s postulate. The damage evolution equation of cycle rate is obtained by integrating the time damage evolution equations,and the fatigue life prediction method for smooth specimens under repeated loading with constant strain amplitude is constructed. Likewise,for notched specimens under the repeated loading with constant strain amplitude,the fatigue life prediction method is obtained on the ground of the theory of conservative integral in damage mechanics. Thus,the material parameters in the damage evolution equation can be obtained by reference to the fatigue test results of standard specimens with stress concentration factor equal to 1,2 and 3.

Deformation and damage mechanism of aluminum alloy under different stress states

The deformation and damage mechanism of aluminum alloy (6063) were investigated by 0°, 30°, 45°, 60°and 90°tensile tests and tensile-unload tests with the modified Arcan fixture on the butterfly specimens. The results show: the curves of engineering stress-engineering strain under different stress states are obviously different. There were microvoids in the specimen when 0°direction loading was preformed. The microcracks were produced in the root of notch as the result of the microvoids shearing fracture and then they led to specimen fracture with microcracks being coalesced. With tensile angle increasing, the shear stress in the center of butterfly specimen increases gradually, while the deformation bands become more and more concentrative. In these concentrative deformation bands, the microcracks are produced and then microcracks propagation and coalescence result in specimen fracture. When 90°direction loading is preformed, the shear bands are obviously formed. The G-T-N damage model and the Johnson-cook model were used to simulate 0°tensile test and 90°tensile test respectively. The simulated engineering stress-engineering strain curves fit the measured ones very well.

基于24model的建筑施工机械伤害事故脆弱性分析

为预防建筑施工机械伤害事故,本研究采用事故致因2-4模型(24model)和复杂网络理论(CN)对施工机械事故进行分析整理。首先采用24model梳理86起典型机械事故报告中各层次事故的致因,对伤害事故中的安全管理体系、习惯性行为、不安全动作和不安全物态进行定性分析;随后根据复杂网络理论构建风险要素网络模型,通过网络攻击的手段找到导致事故的关键要素。研究结果表明:未经允许冒险作业、无相关施工资质、作业环境恶劣是事故关键致因。通过对关键节点的防控,可有效提升建筑施工安全水平。

INTERFACE DAMAGE ANALYSIS OF FIBER REINFORCED COMPOSITES WITH DUCTILE MATRIX

A cohesive zone model is employed to simulate the fiber/matrix interface damage of composites with ductile matrix. The study is carried out to investigate the dependence of the interface damage and the composite tensile strength on the micro parameters of the composite. These parameters contain fiber packing pattern, fiber volume fraction, and the modulus ratio of the fiber to the matrix. The investigation reveals that though the high fiber vo lume fraction, the high fiber′s modulus and the square fiber packing can supply strong reinforcement to the composite, the interface damage is susceptible in these cases. The tensile strength of the composite is dominated by the interface strength when the interface debonding occurs.

Seismic fragility assessment of RC frame structure designed according to modern Chinese code for seismic design of buildings

Following several damaging earthquakes in China, research has been devoted to find the causes of the collapse of reinforced concrete （RC） building sand studying the vulnerability of existing buildings. The Chinese Code for Seismic Design of Buildings （CCSDB） has evolved over time, however, there is still reported earthquake induced damage of newly designed RC buildings. Thus, to investigate modern Chinese seismic design code, three low-, mid- and high-rise RC frames were designed according to the 2010 CCSDB and the corresponding vulnerability curves were derived by computing a probabilistic seismic demand model （PSDM）.The PSDM was computed by carrying out nonlinear time history analysis using thirty ground motions obtained from the Pacific Earthquake Engineering Research Center. Finally, the PSDM was used to generate fragility curves for immediate occupancy, significant damage, and collapse prevention damage levels. Results of the vulnerability assessment indicate that the seismic demands on the three different frames designed according to the 2010 CCSDB meet the seismic requirements and are almost in the same safety level.