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The study of constitutive relationship and damage degradation is crucial in solving the stability challenges faced in the rock engineering. In this work, the stress-strain relationships of different type of rocks subjected to uniaxial loading processes are investigated in details. Experimental results demonstrate measurements, such as uniaxial compressive strength (UCS), tangent deformation modulus, peak strain, and Poisson's ratio (v). A novel piecewise constitutive model is proposed that utilizes both a constitutive model during compaction and a conventional damage model using the strain equivalence assumption and logistic growth theory to represent the characteristics of stress-deformation curves during both compaction and post-compaction stages. The performance of the proposed constitutive models in capturing deformation characteristics of damaged rocks is demonstrated to be more outstanding in comparison to existing models. In all experimental cases discussed in this study, the proposed model outperforms existing reference models in terms of the coefficients of determination ( R 2 ), with the former having coefficients of determination greater than 0.95. Furthermore, physical meanings of relevant model parameters are found to be closely associated with properties of experimental stress-strain curves. [ABSTRACT FROM AUTHOR]

This work explores the dynamic fracture characteristics of recycled tyre polymer (RTP) fibre reinforced concrete after exposure to 20, 105, 250, 400 and 600 °C. Split Hopkinson pressure bar tests were performed to characterize the dynamic compressive properties under strain rates of 40–120 s−1, in terms of the failure mode, strain-stress response, dynamic strength, dynamic increase factor (DIF), peak strain and energy absorption capacity. Results indicate that the failure pattern varies from splitting failure to core failure, and finally to pulverization failure due to strain rate effect, regardless of exposure temperatures. The dynamic strength and DIF are enhanced by strain rate effect but drop by 56.1% and 23.9% at 600 °C, respectively, while no explosive spalling occurs because of the thermal expansion of RTP fibres. The temperature-dependent dynamic fracture mechanism is closely related to the physicochemical reactions in the concrete matrix, bridging or rupture of RTP fibres and breaking of coarse aggregates. [ABSTRACT FROM AUTHOR]

3D braided composites consolidated by heat-resistant resin are expected to become essential structural material of new generation aerospace vehicles. In this work, the thermo-mechanical behavior of 3D braided composites subjected to general off-axis tensile loadings at room and elevated temperatures is evaluated based on the meso-scale finite element (FE) model. A user-material subroutine UMAT is adopted to implement the thermal stress analysis, temperature-dependent failure initiation criteria and material properties degradation scheme of both fiber yarns and matrix. The meso-scale damage evolutions under thermal-mechanical coupling loads are simulated based on ABAQUS/Standard and the corresponding detailed failure mechanisms are revealed. The off-axial elastic moduli and strengths of 3D braided composites under elevated temperatures are also predicted. The numerical results indicate that high temperature affects the material properties mainly by weakening the matrix properties while off-axis loading affects the damage mechanisms by changing the load distribution of fiber yarn in each direction. The present work provides routine support for the numerical study of structural properties and damage behavior of other textile composites in service temperature environment. [ABSTRACT FROM AUTHOR]

This work aims to investigate the dynamic stress amplitude on the mechanical responses of rock‐backfill composite structure material under alternative low‐cycle fatigue loading and creep loading tests. Results show that deformation, damage propagation, and failure pattern are all impacted by the stress amplitude, and volumetric deformation is the largest for a sample subjected to strong stress disturbance. The equivalent lifetime decreases with the increase of disturbed amplitude. A notable positive interaction between the fatigue damage and time‐dependent damage is found, and they promote each other. Additionally, a novel damage evolution model is proposed by the irreversible radial strain, and the model matches well with the testing data. Moreover, post‐test computed tomography imaging reveals that cracks at rock‐backfill interfaces are relatively easy to be stimulated under low stress disturbance, and it is suggested that tension‐splitting failure is more likely to occur within the surrounding rock, and shear failure seems to occur within the cemented tailings backfill for all the tested samples. Highlights: Alternative LCF‐CL tests were performed on RBCS samples.A high disturbed stress amplitude is likely to induce greater volumetric deformation.The cyclic damage and time‐dependent damage promote each other.Splitting‐tension failure occurs within rock and shear failure is found within CTB. [ABSTRACT FROM AUTHOR]

Investigation into rock damage is of great significance for evaluating and predicting the stability of underground rock applications, such as deep mining or tunneling structures. Considering the energy dissipation properties during rock deformation, this paper proposes a novel theoretical characterization of the damage induced by compressive-shear stress and its evolution in intact rocks. The linear energy dissipation (LED) law is derived from shear stress and deformation data of rocks resulting from the preset angle shear experiment. Based on the LED law, two damage variables are separately constructed from the theoretical and experimental aspects. Several sets of experimental data are subsequently utilized to validate the two constructed damage variables. Results show that both damage variables grow first slowly and then rapidly with shear displacement or shear stress in nonlinear relations. By comparison, however, it is found that the theoretical damage variable outperforms the experimental damage variable, which can accurately reflect the stress and deformation data during progressive rock damage with favorable continuity. This study contributes to a novel theoretical approach to quantifying the pre-peak damage in intact rocks subject to compressive-shear stress. [ABSTRACT FROM AUTHOR]

To deeply reveal the differences in geomechanical responses of rock during mining exposed to complicated stress disturbance of excavation unloading and cyclic loading conditions, cyclic triaxial fatigue loads with four cyclic stress amplitude (CSA) (60, 100, 130, and 160 MPa) were performed on deeply buried rock. Results show that rock deformation, energy evolution, and failure modes are all influenced by the CSA. Additionally, it is found that all the strain energy increases with increasing CSA during rock failure. Especially, the dissipated energy increases greatly at the confining stress unloading stage (CSUS) than the fatigue loading stage (FLS), indicating the influence of unloading on rock damage accumulation. Moreover, a damage evolution model reflecting a first slow and then quick increase pattern was established using the dissipated energy. Finally, a transition failure mode from the dominant tensile‐shear to pure shear is found. It is suggested that much more energy needs to be consumed to induce shear cracks than tensile cracks. Highlights: Complicated disturbed stress paths were applied to deeply buried rock.The influences of cyclic stress amplitude on damage and energy dissipation were revealed.Relatively large energy dissipation occurs at the CSUS compared to the FLS.A transition failure mode from the dominant tensile‐shear to pure shear is found. [ABSTRACT FROM AUTHOR]

Granite, as a typical anisotropic hard and brittle rock, is generally in a uniaxial, biaxial, or triaxial state of stress in deep underground engineering and is frequently influenced by extreme dynamic loads including earthquakes, occasional burst, and excavation disturbance. A series of dynamic compression tests were conducted on granite using a true triaxial split Hopkinson pressure bar (SHPB) system on initially anisotropic granite. Using X-ray three-dimensional (3-d) computed tomography (CT) scanning technology and ultrasonic testing, the mechanical properties and fracture behaviors of anisotropic granite under combined dynamic and static loading conditions were studied. Results show that the failure mode of granite changes from damage, to splitting failure, and finally to crushing failure with the rising strain rate under dynamic uniaxial compression. In addition, the axial prestress increases the proportion of transmitted energy in the rock and significantly influences failure strain of the rock. Under biaxial static prestresses, the granite is split along the direction of its free faces and the failure mode is affected by the value of the anisotropy index in the direction of free faces. Under the triaxial static prestresses, the rock is very unlikely to be damaged and the damage effect occurs only after multiple impacts. In the same prestress state, the strain rate of granite increases with the increasing impact velocity and the prestress constraint reduces the dynamic strain rate in the granite. The initial anisotropy of the longitudinal wave velocity of granite is gradually enhanced as the cyclic impact continues. Increasing the strain rate will induce more significant evolution of initial anisotropy of granite, while the prestress constraint restricts the development of initial anisotropy. The results improve understanding of the evolution of impact damage and the dynamic failure characteristics of anisotropic rocks under high geostress. Highlights: Conducted a series of dynamic fracture tests of granite under unprestressed, uniaxial, biaxial and triaxial prestressed conditions. Revealed the dynamic impact damage evolution mechanism of granite under multi-axial stress constraints from a microscopic perspective. Investigated the confining pressure effect and strain rate effect on dynamic damage anisotropy of granite under multi-axial stress constraint. [ABSTRACT FROM AUTHOR]

The stability and safety of rock mass in underground constructions are influenced by the damage processes of cracks evolution caused by the excavation under coupled hydro-mechanical loading conditions. In this paper, to understand the crack development and damage evolution of sandstone under different seepage conditions, a series of triaxial compression tests with acoustic emission under different water pressures (5, 10, 15 MPa) are carried out. The stress-strain behavior, crack stress thresholds, failure mode, AE characteristics and micromechanical mechanisms are analyzed and discussed. The experimental results demonstrate that the crack stress thresholds and the mechanical property of studied sandstone decrease with the increase of water pressure. Moreover, the water pressure has an important influence on and deformation behavior of sandstone, for instance, the elastic modulus and deformation modulus decreased when the water pressure increased. Based on the result of accumulated ring counts, the damage evolution of sandstone gradually increased with increasing water pressure. Finally, the micro-structural evolution under the coupling effects of water pressure and applied stress is discussed. The obtained results can be used to identify the initiation and propagation of studied sandstone in the engineering applications. [ABSTRACT FROM AUTHOR]

The utilization of deep mineral resources is severely restricted by dynamic disasters, such as rock burst, caused by mining disturbances. This study is aimed at investigating the instability failure mechanism and precursory response of coal specimen, with the aid of acoustic emission (AE) monitoring, under triaxial loading and unloading disturbance. The AE time-varying characteristics were analyzed, the crack propagation mode was explored, and the statistical b value and βt value (damage parameters) of AE events were introduced to evaluate the damage state of the loaded coal under disturbance. The results show that: the dynamic disturbance loads will accelerate the deformation process of coal specimen under high static confining stress, resulting in a positive relation between the AE characteristic parameters (AE counts and energy) and the increasing disturbance amplitude. The proportion of low-frequency and high-amplitude AE signals increases with the disturbance amplitude, indicating the disturbance with large amplitude will promote the formation of large-magnitude cracking events. Those stress-induced cracks, judged from the RA-AF value of AE signals, are mainly tensile mode (i.e., mode-I) under the applied disturbance load, but shear cracks are the critical factor affecting the instability and failure of the loaded coal specimen. Both the b value and βt value can reflect the time-varying evolution process of internal damage for the loaded coal specimen. The sudden decreasing of the b value together with the exponential increasing of the βt value can be regarded as a short-impending precursor indicator for the coal instability and failure. The results of this study will provide some experiment basic and theoretical guidance for dynamic disaster monitoring and early-warning during the deep coal mining. Highlights: The instability failure mechanism and the precursory response of coal specimens under triaxial loading and unloading disturbance were investigated. By analyzing the AE characteristic parameters, the internal damage evolution processes, crack propagation mode and the mechanisms of disturbance were studied. The damage accumulation process and precursor warning information of coal instability failure are extracted by the b and βt values. [ABSTRACT FROM AUTHOR]

To maintain safety and reliability in power plants, creep-life prediction models have received much attention over the years. This article was designed to focus on the conditions when a material structure is exposed to extremely high temperatures and pressures with the help of finite element analysis. A direct comparison of the feasibility of different models' fitness and suitability in predicting creep damage was presented in this article by simulating the damage evolution of a uniaxial SS-304 specimen under a pre-defined load, using established constitutive creep models. Comparative assessments of minimum creep strain rate, creep deformation, and stress rupture were demonstrated using the Norton–Bailey (NB), Kachanov–Rabotnov (KR), Theta projection (TP), and sine-hyperbolic (SH) models while standardizing them with the Omega model. The FE results of a dog-bone specimen, while implementing the models, were compared with the actual creep experiment results to check for the models' reliability and validation. Subsequently, sensitivity studies of the established creep models were conducted using the statistical tools RSM and ANOVA, with an analysis of how the parameters for operation, design, and material dependency came into effect. Thus, quantitative and qualitative correlation analyses of the FE creep response for these five established models were conducted together, resulting in finalizing the selection of the most suitable model, the sine-hyperbolic model, for the SS-304 material under the defined boundary conditions. The 0.84 R2 value of the sine-hyperbolic model proved the model's selection for predicting the creep response of stainless steel 304. The method can be applied to select a suitable creep damage model as per the feasibility of the operating conditions. [ABSTRACT FROM AUTHOR]

The investigation on the machining damage evolution and material removal mechanism of anisotropic brittle material is of vital interest for controlling surface and subsurface damage. In this work, with the combination of stress field analysis and scratching experiments, the evolution of various cracks and the mechanism of material removal in the scratching process of anisotropic brittle material were decoupled to determine the inducing conditions and coupling modes of different cracks. Firstly, an improved elastic stress field model was established considering anisotropy characteristics, tool shape, and material elastic recovery. Furthermore, the scratching experiments of single crystal silicon with varying crystal orientations were carried out. Finally, the characteristics of subsurface damage were analyzed by transmission electron microscopy, and the interaction between dislocation and crack was investigated. The results show that the lateral crack first initiates and propagates on the material subsurface under the tensile stress behind the tool. With the influence of the cleavage surface, radial cracks gradually form on the material surface. Median cracks induced by the tensile stress initiated at the front of the tool. The geometric overlap of the opening radial crack and lateral crack resulting in brittle removal in the form of massive brittle stripping. The theoretical results agreed well with the experimental results, the average error of prediction of radial crack propagation direction is less than 9.4%. There is a strong crystal orientation dependence in the damage evolution process. The scratching of crystal orientation [100] is more conducive to the ductile machining due to its lower elastic stress and higher dislocation density. This research contributes to a deeper comprehension of the damage evolution and the crystal orientation dependence of anisotropic brittle material. [Display omitted] • An elastic stress field model is established considering anisotropy characteristics, tool shape, and elastic recovery. • Damage evolution and material removal mechanism of anisotropic brittle material induced by scratching are clarified. • Geometric overlap of deep radial crack and lateral crack induces brittle chunk removal. • Damage characteristics show obviously crystal dependent, orientation [100] is more conducive to ductile domain machining. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) is often accompanied by the propagation of internal microcracks in loaded rock samples, and it essentially reflects microinstability phenomena driven by energy redistribution under stress. In this paper, loading and unloading tests were carried out to investigate the internal nonlinear damage evolution characteristics of diorite samples under different unloading confining-pressure rates. The nonlinear mechanical characteristics of the strain energy sequence of diorite were studied by applying nonlinear dynamics and basic chaos theory and MATLAB software. Moreover, the evolution characteristics of AE counts and AE energy of rock samples were investigated, and their microcrack-propagation modes were analyzed based on the RA–AF scatter distribution of AE and a two-dimensional Gaussian mixture model. Finally, according to the evolution characteristics of energy and AE, the nonlinear damage evolution mechanism of diorite under loading and unloading conditions was revealed. The results show that, before the loading and unloading peak strength, when the strain-energy-promotion coefficient, r, is equal to 1 or changes in the ranges of 1–3, 3–3.57, and ≥3.57, the strain-energy evolution of diorite presents the characteristics of supercritical stability, nonlinear stability, period-doubling stability, and chaos, respectively. Meanwhile, the greater the rate of the unloading confining pressure, the earlier the period-doubling bifurcation and chaotic mechanical behavior will occur. After loading and unloading peak strength, the sudden decrease of high-density AE counts and AE energy or the sudden transition of the strain-energy-promotion coefficient from >0 to <0 can be used as an important criterion for the complete failure of rock samples. [ABSTRACT FROM AUTHOR]

Under thermal cyclic fatigue loading, temperature and cyclic-dependent damage mechanisms affect thermal fatigue damage evolution in fiber-reinforced ceramic-matrix composites (CMCs). Stress-strain hysteresis loops occur with the changing of temperature and correspond to internal damage evolution in CMCs. In this work, fatigue hysteresis-based damage features, including hysteresis-based dissipated energy, hysteresis modulus, hysteresis width, and peak strain, were developed to monitor thermal cyclic fatigue damage evolution in CMCs considering synergistic impacts of thermal and mechanical loading. Experimental thermal fatigue damage evolution of the 2.5D woven C/SiC composite subjected to various thermal fatigue loads were predicted with the newly developed fatigue hysteresis-related damage parameters. The effects of the composite's material features, peak stress, damage state, and cycle number on thermal fatigue damage evolution in the C/SiC composite were analyzed. [ABSTRACT FROM AUTHOR]

A solid rocket motor is in storage for most of its life cycle, and propellant creep occurs subjected to gravity load. To analyse the creep characteristics of HTPB propellant during long‐term storage, MSC Marc is used to calculate the maximum von Mises stress of the motor under horizontal and vertical storage conditions. Based on the numerical results, the short‐term reciprocating creep test and long‐term creep test are designed and carried out. The results illustrate that the process of propellant creep from deformation to final failure can be divided into four stages: instantaneous deformation stage, attenuation creep stage, steady‐state creep stage and accelerated creep failure stage. When the creep stress level is less than 0.1 MPa, the creep characteristics tend to be stable. When the creep strain exceeds a certain critical value, the propellant will enter the steady‐state creep stage. The damage caused by creep rises with the increase of creep time and stress. When the maximum creep strain is less than 10 %, the creep damage can be fully recovered. The splicing curve of short‐term creep test data is in good agreement with that of long‐term creep test data. A mechanical long‐term creep test method is provided, and the parameters of the modified Burgers model are fitted. The text methods and conclusions can provide a reference for the evaluation of structural integrity and storage life of solid motor. [ABSTRACT FROM AUTHOR]

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Thin-shell structures are widely used in various engineering applications. It is essential to investigate the impact resistance of thin-shell structures, to provide theoretical support for engineering applications. Numerous impact tests have been conducted on thin spherical shells using ballistic guns. The effects of the impact velocity and shell thickness on the deformation and fracture of thin spherical shells are summarized. Moreover, a novel damage model based on statistic damage mechanics is proposed to better predict dynamic responses of thin shells impacted by projectiles. Considering that fracture surfaces are formed by void evolution and are affected by the stress states, the damage level is defined as the ratio of the statistical cross-sectional area of the voids to the cross-sectional area of the representative elements. Utilizing statistical methods, the incorporation of continuous void nucleation, ellipsoidal void growth, and the acceleration of dynamic void evolution are introduced into the novel damage model. Subsequently, numerical investigations of the dynamic response of spherical shells under impact are conducted based on the proposed damage model. The numerical results are consistent with the experimental results in terms of the depression deformations and strain signals. The effects of shell thickness and double-layer structures on the dynamic response of spherical shells are investigated via numerical simulations considering the novel damage model in detail. The results demonstrate that the proposed model can accurately predict the dynamic response of spherical shells impacted by flat-nose projectiles, thus serving as a valuable reference for engineering design. • A new damage model was derived to predict the penetration of spherical shell. • The effect of stress states on damage evolution was considered in our model. • Dynamic response of spherical shell under impact is affected by its thickness. • Response characteristics of double-layer shells were predicted via our model. [ABSTRACT FROM AUTHOR]

Damage evolution of SiC f /SiC ceramic matrix mini-composites (CMMCs) was characterised by using a 4D in-situ micro-computed tomography (CT) tensile test and digital volume correlation (DVC) technique. Additionally, a CT damage data-driven shear lag model was developed to predict its tensile stress-strain response. A 4D in-situ X-ray CT tensile test of a unidirectional SiC f /SiC mini-composite was first carried out. Then two ad-hoc deep-learning image segmentation models were developed to automatically identify its microstructure and damages induced by tension, respectively. Damage evolution was quantitively characterised by visualising the initiation and propagation of matrix cracks in three-dimensions (3D). A two-step approach was employed to evaluate its 3D internal strain distributions at various loading levels, which further revealed strain concentrations and helped establishing the tensile stress-strain response of the CMMCs. It was observed that transverse cracking is the predominant damage mode, and the average crack opening displacement increases with loading. A high-fidelity X-ray CT data-driven shear lag model was developed, incorporating inputs of transverse matrix crack spacing calculated by the 4D in-situ CT test data. The predicted stress-strain response showed a good correlation with the experimental results. [Display omitted] [ABSTRACT FROM AUTHOR]

The post-peak behaviour of rocks subjected to cyclic loading is very significant to appraise the long-term stability of underground excavations. However, an appropriate testing methodology is required to control the damage induced by the cyclic loading during the failure process. In this study, the post-failure behaviour of Gosford sandstone subjected to the systematic cyclic loading at different stress levels was investigated using the double-criteria damage-controlled testing methodology, and the complete stress–strain relations were captured successfully. The results showed that there exists a fatigue threshold stress in the range of 86–87.5% of the average monotonic strength in which when the cyclic loading stress is below this threshold, no failure occurred for a large number of cycles and in turn, the peak strength improved up to 8%. Also, the variation of the energy dissipation ratio, rock stiffness and acoustic emission hits for hardening tests showed that cyclic loading in the pre-peak regime creates no critical damage in the specimen, and a quasi-elastic behaviour dominates the damage evolution. The post-failure instability of such tests was similar to those obtained for monotonic tests. On the other hand, by exceeding the fatigue threshold stress, the brittleness of the specimens increased with an increase in the applied stress level, and class II behaviour prevailed over total post-peak behaviour. A loose-dense-loose behaviour with different extents was also observed in the post-peak regime of all fatigue cyclic loading tests. This was manifested then as a secondary inverted S-shaped damage behaviour by the variation of the cumulative irreversible axial and cumulative irreversible lateral strains with the post-peak cycle number. Furthermore, it was confirmed that the damage per cycle in the post-peak regime decreases exponentially with an increase in the applied stress level. [ABSTRACT FROM AUTHOR]

In general, the microdefects in rocks obey Weibull distribution. Based on the laws of statistics of microdefects in rocks, the relationship between the damage and the strain is established. Moreover, by considering characteristics of the viscoelastic–plastic and the damage evolution in rocks, a nonlinear creep model for rocks considering damage evolution is proposed to predict the creep behaviors of rocks based on the modified Nishihara model. In the proposed model, a dashpot element and a nonlinear viscoplastic body are added into the classical Nishihara model to simulate the accelerating creep stage of soft rocks. Moreover, the damage of rocks is considered in the process of creep. The effects of the shape parameter, the scale parameter, the peak stress, the peak strain, and the confining stress on the damage are investigated using the proposed method. The novel model is suitable to predict the creep behaviors of hard rocks and soft rocks simultaneously, and the difficulty to simulate the accelerating creep stage using the traditional creep model is overcome. To verify the robustness and precision of the proposed model, the creep behaviors of granite, coal, and red sandstone at different stresses are investigated. The predicted creep behaviors of soft and hard rocks using the proposed method are in good agreement with the experimental data, especially for the nonlinear accelerating creep stage. [ABSTRACT FROM AUTHOR]

Accurately predicting the time-dependent behavior of surrounding rock is crucial for supporting design, revealing the mechanism of lining cracking and understanding long-term stability in deep-buried tunnels engineering. Based on the component combination model method and continuous damage mechanics theory, a New Elasto-Visco-Plastic Damage (NEVPD) model was proposed considering the characteristics of viscoelastic-plastic and the damage evolution of rocks. Three groups of experimental data were used for validating the model, and the results indicate that the model can effectively describe the time-dependent deformation characteristics of rocks at different stress levels. The dynamic link computer application program of the NEVPD model was procured through the secondary development interface of the FLAC3D software, and its validity was ascertained through test simulations. By investigating the influence of stress levels on creep parameters, a creep simulation approach considering the impact of rock mass stress state on the creep parameters was proposed for used in the long-term stability analysis of underground rock engineering. Finally, the time-dependent behavior of deep tunnel was predicted using two simulation approaches (the traditional method and the proposed method) based on the Burger-Creep Viscoplastic (CVISC) model built-in FLAC3D and the NEVPD model. The findings suggest that the proposed tunnel creep simulation approach provides a more precise reflection of the local deformation features of the surrounding rock compared to traditional methods. The simulation outcomes based on the NEVPD model exhibit the highest agreement with the field monitoring data. [ABSTRACT FROM AUTHOR]

• Rock properties and relations between AE and stress-strain properties were analyzed. • Difference of AE parameters response at different strain rates were studied. • Rock damage variable was derived based on accumulative AE count. • The damage evolution law of rock samples at different rates were described. To further understand rock damage mechanisms under loading, evaluate coal burst danger and ensure stopping safety during coal mining, uniaxial compression tests of rock samples at different mining-induced loading rates were carried out. A systematical analysis has been conducted on strain rate effect on rock mechanical properties, AE parameters response and damage evolution law. The results show that along with the increase of mining-induced loading rate, an increase of peak strength and elastic modulus of rock together with a decrease of peak strain and dynamic failure duration in post-peak period are shown. Several acoustic characteristics are found decreasing in power function when loading rate increases such as AE hits, counts and events, but an obvious increase of AE energy is shown. Peak frequencies of AE events are mostly between 95 ∼ 125 kHz and 270 ∼ 310 kHz. A well-defined correspondence between AE count curve and rock stress-strain curve is presented. Rock damage variable was derived based on accumulative AE count, and three stages of rock damage evolution at different loading rates can be divided: initial damage, steady evolution, and accelerated development. A constitutive damage equation has been established on the basis of equivalent strain hypothesis and AE count. Calculated results can basically reflect the characteristics of rock stress-strain curves at different strain rates. The conclusions of this paper can provide references for the issues of coal burst control and hazard assessment. [ABSTRACT FROM AUTHOR]

In this paper, a thermomechanical fatigue hysteresis-based damage evolution model for fiber-reinforced ceramic–matrix composites has been developed. Upon unloading and reloading, the fiber/matrix interface debonded length, interface counter-slip length, and interface new-slip length change with increasing or decreasing applied stress, which affects the stress–strain fatigue hysteresis loops and fatigue hysteresis-based damage parameters. The reloading/unloading stress–strain relationships when fiber/matrix interface partially or completely debonding are determined as a function of interface debonding/sliding, peak stress, applied cycle number, and thermal cycle temperature. The relationships between thermomechanical fatigue loading parameters (i.e. peak stress, applied cycle number, and thermal cyclic temperature), fiber/matrix interface debonding/sliding lengths, and fatigue hysteresis-based damage parameters (i.e. fatigue hysteresis dissipated energy, fatigue hysteresis modulus, and fatigue peak strain) have been established. The effects of fiber volume fraction, peak stress, matrix cracking space, and thermal cyclic temperature range on damage evolution under the out-of-phase thermomechanical cyclic loading have been discussed. The differences in damage evolution between in-phase/out-of-phase thermomechanical fatigue and isothermal fatigue loading at the same peak stress have been analyzed. The damage evolution of cross-ply SiC/magnesium aluminosilicate composite under the out-of-phase thermomechanical and isothermal fatigue loading has been predicted. [ABSTRACT FROM AUTHOR]

• Investigate the effects of hydrostatic pressure and anisotropic in-situ stress fields on blast-induced damage to rock. • The image processing technique is employed to quantitatively assess blast-induced damage to rock under different in-situ stress fields. • The mathematical relationship between the damage variable and in-situ stress fields is established. In-situ stress significantly affects rock blast damage but there is a paucity of quantitative assessments of damage evolution in rocks affected by confining pressure. The present paper analyses the effect of envelope pressure on blast-induced rock damage through theoretical analysis and numerical simulations. Damage clouds obtained from numerical simulations are processed using image processing techniques. The concept of the damage variable (η) is proposed to facilitate the presentation of the image processing results. The damage variable is found to be negatively correlated with the hydrostatic pressure (P x) at the same moment, in equiaxial in-situ stress fields. In contrast, in anisotropic in-situ stress fields, η is not negatively correlated with P x due to the presence of hoop tensile stresses in the rock. The mathematical relationship between η and P x in equiaxial and anisotropic stress fields are established. An anisotropic damage variable (η k) is introduced to describe the effect of the anisotropy ratio (K) on rock damage, which is found to increase with increasing values of K. The sharp increase in K equal to 4 and 5 is explained in terms of the state of the rock stress distribution under static loading. This study provides insights into the effect of in situ stress on rock blast damage and presents new approaches for analyzing and presenting the data. [ABSTRACT FROM AUTHOR]

Experiment and finite element analysis were performed in this study to estimate the deformation characteristics of small punch creep test. Considering the disadvantage of the traditional model owing to numerous unknown parameters, ductility exhaustion model was introduced and used to estimate the damage and multiaxial creep evolution process. The results show that the evolution trends of creep deflection and strain with three stages are similar. During deformation, apparent damage localization is observed. The maximum strain and damage also occurred in the necking region due to the stress concentration. Thus, a critical damage value is proposed for service-exposed Cr5Mo in this study. In the primary creep stage, a tensile deformation is observed at the lower surface and a compression deformation is observed at the upper surface. Subsequently, the stress triaxiality tends to be constant in the second creep stage. Thus, steady creep parameters of small punch test can be used to replace traditional creep results in the creep properties analysis of in-service components. Finally, the multiaxial creep and damage evolution characteristics of the small punch creep specimen are obtained. [ABSTRACT FROM AUTHOR]

Studying the thermal damage constitutive model of rock using statistical theory can better reflect the damage evolution process and the stress–strain relationship of rock under temperature and loading, which is one of the key problems especially in deep rock mechanics. The thermal-mechanical coupling damage constitutive model of rock is established using the Hoek–Brown strength criterion, based on the Weibull distribution and the continuous damage theory. The rationality of the model is also verified by experiments. The main conclusions are as follows. The stress–strain curves of rock can be divided into four stages according to the damage evolution characteristics, including the non-damage of loading, damage stability expansion, damage intensification expansion, and damage stability expansion to saturation, and the method of determining the demarcation points of each stage is given clearly. The initial damage point of the rock is about 25% of the peak stress, the damage value is about 0.3 when the rock reaches the peak stress and about 0.6 when reaches the residual stress. Both the damage value and the strain energy release rate of the rock corresponding to the peak stress show exponential growth with the increase in confining pressure. The maximum damage evolution rate of the rock shows exponential decay as the confining pressure rises, indicating that the confining pressure can delay the development of cumulative damage. The modified damage constitutive model considering compaction coefficient is in good agreement with the test curves in the stage of compaction, linear elasticity, yield, and pre-peak strength. It is hoped that through the research of this paper, it can provide references for studying the macroscopic mechanical response from the damage propagation characteristics of the rock in the future. [ABSTRACT FROM AUTHOR]

Abstract: Damage evolution in carbon fibre reinforced plastic subjected to fatigue loading (R = 0.5, −1, 2) has been studied using digital image correlation to obtain full‐field surface strains. Damage initiation being a local phenomenon, its effect on global parameters is not significant. The local transverse strain is a better indicator of delamination which affects transverse strain more than the longitudinal strain. Variation of normalized local transverse strain (ratio of local transverse strain to applied stress) near the initiated delamination indicates that the damage evolution occurs over 2 to 3 stages. Each stage has a stable damage growth with a drastic increase between the stages. The stages correspond to different damage mechanisms (matrix cracking, fibre‐matrix debonding, delamination, and fibre breakage) dominating at different periods during the fatigue life. Scatter in normalized local transverse strain plots due to large relative displacements was eliminated using different reference images for DIC. Waviness due to shift in the time at which the images are captured during the loading cycles was avoided using a sine curve fit to obtain maximum transverse strain in a cycle. Normalized local transverse strain plots were found to qualitatively reflect the physical extent of damage, thereby providing confidence in the methodology. Fatigue life curves were generated and run‐out lives were determined. [ABSTRACT FROM AUTHOR]

In this paper, the damage monitor and life prediction of carbon fiber-reinforced ceramic-matrix composites (C/SiC CMCs) have been investigated using the hysteresis dissipated energy-based damage parameter. The evolution of the interface shear stress, hysteresis dissipated energy, hysteresis dissipated energy-based damage parameter and the broken fibers fraction vs. cycle number, the fatigue life S‒N curves of unidirectional, cross-ply and 2.5D C/SiC composites at room temperature and 800 °C in air atmosphere have been analyzed. For unidirectional C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter first increase and then decrease with cycle number, and the fatigue limit stress decreases from 88% tensile strength at room temperature to 20% of the tensile strength at 800 °C in air atmosphere; for cross-ply C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter decrease with increasing applied cycles, and the fatigue limit stress decreases from 85% tensile strength at room temperature to 22% tensile strength at 800 °C in air; and for 2.5D C/SiC, the hysteresis dissipated energy and hysteresis dissipated energy-based damage parameter increases with cycle number, and the fatigue limit stress decreases from 70% tensile strength at room temperature to 25% tensile strength at 800 °C in air. [ABSTRACT FROM AUTHOR]

This study deals with ductile fracture prediction of a marine steel using new damage functions. The stress triaxiality and Lode angle are known to be governing parameters on the ductile fracture. Recent research has reported that loading path is also one of the important parameters. A three-dimensional fracture strain surface for EH36 steel was developed based on a series of tension/compression tests subjected to the tension, shear, shear-tension, and compression. To account for the non-proportional stress effects, the linear and non-linear damage evolution models were presented with a combination of the fracture strain surface. The material constants associated with the damage evolution were identified from calibration with the experimental data. Validation of the damage-based fracture models was conducted by comparing predicted fracture initiation with the results of structural test results. The fracture model with the non-linear damage evolution predicts slightly more improved predictions than the linear damage-based fracture model. [ABSTRACT FROM AUTHOR]

To propose more accurate constitutive models for low-temperature concrete structures subjected to cyclic loading, this paper experimentally studied the cyclic compressive behaviours of normal weight concrete (NWC) at low temperatures. Firstly, cyclic compression tests on 36 NWC prisms of C25, C45, and C55 grades were performed at four low-temperature levels, i.e., 20, −30, −60, and − 80 °C. By comparing with monotonic results, it was confirmed that the low-temperature cyclic envelope curves of NWC can be predicted by the corresponding monotonic constitutive models. Furthermore, the effects of low temperatures and grades of NWC on key indexes in constitutive models of NWC under cyclic compression were discussed and analyzed in detail, e.g., plastic strain, stress deterioration ratio, endpoint strain, and damage variable. Through the regression analysis of test data, the cyclic constitutive models composed of the envelope, unloading, and reloading curves were established to describe cyclic stress-strain behaviours of NWC at 20 ∼ −80 °C. Finally, the validations proved that the developed models provided reasonable predictions on low-temperature cyclic response of NWC. • Arctic low temperatures improve cyclic compressive behaviours of NWC. • NWC prisms show diagonal shear failure at different low temperatures. • Low temperatures improve ultimate strength, peak strain, and modulus of elasticity of NWC. • Low temperatures have limited influences on cyclic parameters of NWC. • Developed models reasonably predict low-temperature cyclic compressive σ-ε curves of NWC. [ABSTRACT FROM AUTHOR]

The behavior of steel fiber reinforced concrete (SFRC) under cyclic loading plays an important role in prediction of SFRC structural response. This paper deals with the stress-strain behavior and damage evolution of SFRC under uniaxial cyclic compression. A total of 36 specimens are tested for different fiber volume fractions and aspect ratios. Acoustic emission (AE) technique is used to characterize the damage progression and reveal the failure mechanism of SFRC during the whole loading process. The results show that slight strength degradation for SFRC specimens under cyclic loading is observed in comparison with monotonic loading cases. The increase in volume fraction of steel fiber can lead to a remarkable decrease in the plastic strain accumulation as well as an increase in the elastic stiffness ratio, while the effect of fiber aspect ratio is undiscernable. In addition, it is indicated from the AE parameters analysis that the total AE activities of SFRC specimens are higher than that of plain concrete specimen and become greater with an increase in the fiber volume fraction, while the opposite is true for increasing the fiber aspect ratio. Meanwhile, as substantiated by AE, the failure of SFRC mainly demonstrates a shear cracking mode that is induced by fiber pull-out and fiber sliding events, the amount of which is proportional to both fiber volume fraction and aspect ratio. Finally, based on the test results of stiffness degradation, an analytical formulation for the damage evolution law of SFRC is developed and the prediction yields a close estimation of SFRC damage progression. [ABSTRACT FROM AUTHOR]

The research for the features of similar material mechanics, the law of acoustic emission (AE) responses and fracture evolution characteristics can reflect the regularity of deformation, stress and fracture of coal and rock, revealing the evolution mechanism of dynamic disasters. We studied the mechanical properties of loaded sand-paraffin (SP) samples associated with different mixture ratio and the damage evolution of internal cracks based on AE. Results show that (1) with the increase of paraffin content, mechanical parameters of SP samples including compressive strength, elastic modulus, tensile strength, Poisson's ratio and internal friction angle show the overall upward trend, while failure pattern transitions to wedge splitting type and brittle shear failure from extrusion powder flexible broken; (2) the AE test can reflect the internal damage degree of SP samples and law of its space-time evolution. Along with the load increase, the AE response can be divided into three stages: slow growth, rapid growth and saturation, while also the damage variable reduces first then increases rapidly and grows slowly at last; (3) using the quadratic polynomial fitting, fragment expressions of damage evolution mechanism of SP are obtained. Based on Weibull distribution, we get the AE parameters (cumulative pulses) coupling mechanical parameters (stress and strain) expression, which fits better with the experimental data. By studying the mechanical parameters, the main failure form, AE response characteristics and its internal damage evolution rule, the paper evaluate the mechanical properties of the SP material from the macroscopic appearance to the microscopic mechanism, which provides the scientific basis for better selection and matching the material of analog simulation. [ABSTRACT FROM AUTHOR]

The mechanical properties of concrete under cyclic tensile loading using square waveform, sine waveform and ramp waveform are studied. The experiments are performed on a closed-loop electro-hydraulic servo-controlled material testing system (MTS). The axial strain, dissipated energy per loading cycle, the damage evolution law and deformation modulus are mainly studied. The results show that the three-stage evolution law of axial strain and damage variable of concrete under ramp waveform and sine waveform are more obvious than those under the square waveform. The dissipated energy changes at different stages of fatigue life. At the beginning and end of the fatigue life, the rate of dissipated energy is higher than that at the medium stage of the fatigue time, which is attributed to the formation of cracks. The evolution of deformation modulus of concrete subjected to cyclic tensile loading using three loading waveforms also shows three stages: fast increase in the damage-increase at a slow constant rate-and accelerated increase in damage until failure. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to develop a confinement enhanced damage-plasticity model for concrete that can characterize the improvement of strength and ductility under high confining stress. The influence mechanisms of confinement in damage and plasticity sub-space are discussed separately. In the damage subspace, a confinement influence function considering the difference between uniaxial and multiaxial damage evolution is suggested based on the strain energy equivalence confinement. A confinement enhanced damage evolution model is established, which can capture the strength increase under high confining stress. In the plasticity subspace, a confinement enhanced phenomenological plasticity model, which includes equation of state of the spherical space, is provided to reproduce the high ductility at the principal stress direction under lateral confining stress. The simulated strength and stress–strain curves under active confinement agree well with the experimental results. Furthermore, the predicted three-dimensional ultimate stress points are consistent with the classic strength surface, validating good performance of the proposed model. • A confinement enhanced damage-plasticity model for concrete is developed. • A confinement influence function is introduced in the damage sub-space. • A confinement enhanced phenomenological model is built in the plasticity sub-space. • The model is validated to reproduce hardening and ductility under confinement. [ABSTRACT FROM AUTHOR]

A coupled viscosity-damage model of concrete under high strain rate is proposed based on the bi-scalar plastic-damage model. A rheological model is provided to explain the time delay mechanism of the damage under dynamic loading. A rate-dependent damage evolution is established by introducing the Duvaut–Lions type of the damage driving stress rate, and damage and viscosity are coupled. The operator splitting method is used to design an explicit calculation procedure. It is illustrated how the damage evolution predicted by the model differs for dynamic loading and quasi-static loading. Excellent comparisons between the simulated and experimental results show that the model can be used to reflect the strain rate effect of concrete under impact. • A coupled viscosity-damage model with dynamic effect is developed for concrete. • A rheological model is presented to explain relationship of damage and viscosity. • The Duvaut–Lions type of rate dependent damage driving stress is constructed. • The viscosity-damage coupling is established, and dynamic damage evolution is built. • A series of simulations of concrete dynamic tests validate the proposed model. [ABSTRACT FROM AUTHOR]

Fatigue damage evolution of extruded AZ31B magnesium (Mg) alloy is investigated under strain-controlled tension-compression loading along the extrusion direction at various strain amplitudes, and the different cyclic deformation behaviors are observed. At the strain amplitude of 2%, the tensile peak stress displays significant cyclic softening, whereas the compressive peak stress shows consistent cyclic hardening. At 1%, moderate cyclic hardening is observed at both the tensile peak and compressive peak stresses. At 0.5%, the tensile peak stress presents stable cyclic hardening, whereas the compressive peak stress almost keeps constant. The microstructure morphologies associated with the cyclic deformation are analyzed by scanning electronic microscope (SEM). The degree of deformation twins is evaluated by analyzing X-ray diffraction (XRD) using a normalized parameter λ . The results show the fatigue crack initiation modes and its propagation modes are dependent on the strain amplitude. At 2%, grain boundary (GB) cracking and triple joint cracking are detected after 1st loading cycle. At 1%, fatigue crack initiates at grain boundary (GB cracking), twin boundary (TB cracking) and triple joint of three neighboring grains. Both grain boundary induced (GB-induced) intergranular and persistent slip band induced (PSB-induced) transgranular propagation modes play an important role in the early-stage crack growth. At 0.5%, crack initiation modes are similar to that at 1%, but GB-induced intergranular propagation mode dominates the early-stage crack growth. The effects of the microstructure (texture, grain size and uniformity) on the fatigue damage behavior are discussed. [ABSTRACT FROM AUTHOR]

The effect of stress state on the initiation of damage for leaded free cutting steel has been investigated under hot rolling conditions. Double notched (DN) circumferential tension samples were designed and used to simulate damage development at different stress states and deformation conditions using a Gleeble (3800) thermal-mechanical testing system. Two DN sample geometries with varying notch profiles were used to account for different states of stress. To simulate the conditions of hot rolling the samples were tested at high temperatures (900–1200 °C) and moderate strain rates (0.1–1 s−1). After testing to failure, which normally occurs at one notch of the specimen, the unfailed notch of each sample was sectioned to examine the sites where damage occurs since the material has been captured in a state very close to failure. Two of the cases examined have shown definitive damage paths occurring from ‘outside–in’ for a sharp notch deformed atT = 900 °C and from ‘inside–out’ for a blunt notch tested atT = 1200 °C for the same strain rate of 0.1 s−1. The experimental results of the failure initiation sites were compared with computed values of the stress fields around the notch profiles, obtained from FE analysis using a set of viscoplastic constitutive equations calibrated for free cutting steel. The temperature profiles from high temperature mechanical testing were used in the FE calculations of the stress state. [ABSTRACT FROM AUTHOR]

Damage evolution is detected via the degradation of modulus of concrete immersed in sulfate solution. The mechanism of the damage is investigated by using SEM method, and it is proved that the internal expansion stress induced by delayed ettringite leads to the nucleation of micro-cracks. The criterion of damage nucleation is proposed in terms of the tension strength of concrete. The differential equation with respect to the internal expansion stress is derived by virtue of chemical reaction rate on delayed ettringite formation, and a new chemo-mechanical model of corrosion damage in concrete under sulfate attack is then proposed. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on circular stirrup-confined concrete specimens under uniaxial and monotonic load. The effects of stirrup volume ratio, stirrup yield strength and concrete strength on damage evolution of stirrup-confined concrete were investigated. The experimental results showed that the strength and ductility of concrete are improved by appropriate arrangement of the stirrup confinement. Firstly, the concrete damage evolution can be relatively restrained with the increase of the stirrup volume ratio. Secondly, higher stirrup yield strength usually causes larger confining pressures and slower concrete damage evolution. In contrast, higher concrete strength leads to higher brittleness, which accelerates the concrete damage evolution. A plastic strain expression is obtained through curve fitting, and a damage evolution equation for circular stirrup-confined concrete is proposed by introducing a confinement factor (C) based on the experimental data. The comparison results demonstrate that the proposed damage evolution model can accurately describe the experimental results. [ABSTRACT FROM AUTHOR]

Considering the effect of shear loading, an original GTN damage model is extended by combining the damage mechanics concept and the porous plasticity model. The extended model in terms of various stress triaxiality was deduced in detail. The model is extended by coupling two damage parameters into the yield function. The implementation is achieved by the development of the user material subroutine. Internal damage evolution was observed by scanning electron microscopy (SEM). Parameters were determined from experimental data and previous studies. The damage model was applied to in-situ tensile tests to compare its predictability in various specimens. The obtained numerical results were in good agreement with experimental results, showing better ductile fracture predictions. [ABSTRACT FROM AUTHOR]

In the present work, a constitutive model of materials undergoing the plastic strain induced phase transformation and damage evolution has been developed. The model is based on the linearized transformation kinetics. Moreover, isotropic damage evolution is considered. The constitutive model has been implemented in the finite element software Abaqus/Explicit by means of the external user subroutine VUMAT. A uniaxial tension test was simulated in Abaqus/Explicit to compare experimental and numerical results. Expansion bellows was also modelled and computed as a real structural element, commonly used at cryogenic conditions. [ABSTRACT FROM AUTHOR]

This is the first of a two-part paper that develops a validated 2-level parametrically-upscaled continuum damage mechanics (PUCDM) model for multiscale modeling of damage evolution in plain weave woven composites, characterized by multiple levels of hierarchy. The two-level PUCDM models, viz. PUCDM-1 and PUCDM-2, bridge three levels of composite hierarchy. The PUCDM-1 model corresponds to the constitutive model for yarns in the mesoscopic woven RVE with representation of the morphology and mechanisms of underlying unidirectional fibers in level-1 of the hierarchy. The PUCDM-2 model is used for modeling the structural scale composite behavior with morphological and mechanisms representation from the underlying levels 2 and 1. Functional forms of constitutive coefficients in the PUCDM-1 and PUCDM-2 models, in terms of representative aggregated micro- and meso-structural parameters (RAMPs) , material properties, and damage states, are generated by machine learning (ML) methods. The ML algorithms operate on databases of volume-averaged response variables, obtained from the simulation of lower-level RVEs. The validation tests, comparing PUCDM model results with homogenized variables from lower scale analysis demonstrate very good accuracy. The two-level PUCDM models are capable of predicting the stress–strain and damage evolution with high computational efficiency and accuracy. • Two-level Parametrically-upscaled continuum damage mechanics (PUCDM) model. • Multiscale modeling of deformation and damage in plain-weave woven composites. • PUCDM-1 model for mesoscale fiber yarn modeling incorporates microscale descriptors. • PUCDM-2 model for macroscale modeling bridges mesoscale descriptors and properties. • High accuracy and computational efficiency in multiscale analysis of woven composites. [ABSTRACT FROM AUTHOR]

Theoretical and experimental studies on the nonlinear mechanical properties of lightweight foamed concrete under uniaxial compression are conducted over the temperature range of 223–343 K and the strain rate range of 0.001–118/s in this paper. The experimental results show that the mechanical properties of the materials under uniaxial compression are strongly dependent on the density and temperature, but are weaker on strain rate. Based on the experimental results, the characteristics of density effect and temperature effect are analyzed and a calculation model is developed to describe the nonlinear mechanical behavior of lightweight foamed concrete. The model takes into account the effect of material damage, density and temperature. The experimental verification and the error analysis show that the model is shown to be able to predict the nonlinear deformation behavior of lightweight foamed concrete over a wide range of temperatures and densities. [ABSTRACT FROM AUTHOR]

The failure behaviour of WC–Co hard metals with binder contents of 6 and 12 wt.% was investigated under repeated dynamic loadings. The materials were consolidated by field-assisted sintering and exhibited WC grain sizes of about 200 nm. Static and dynamic strength tests on a servo-hydraulic testing machine and a modified split Hopkinson pressure bar (MSHPB), respectively, resulted in compressive fracture strengths between 4370 MPa and 6660 MPa depending on the Co content and the applied strain rate. Moreover, the hard metal samples were repeatedly loaded in the MSHPB below their compressive strength at different stress levels, and subsequently investigated by eddy current testing after each loading step. The measured eddy current signal was dependent on the stress level and the number of impacts. Additionally, the damage of the samples was examined with the help of images from the scanning electron microscope (SEM). Static strength tests of the pre-damaged material revealed that the residual strength of WC–6Co decreases with an increasing number of impacts. In contrast, WC–12Co shows strain hardening caused by the multiple-impact loadings. The size of the fragments revealed intensified microcrack initiation. [ABSTRACT FROM AUTHOR]

Metallic adhesively bonded joints have a wide range of applications in engineering fields. The damage and the stress evolution in the bondlines of the joints would occur simultaneously when the joints are subjected to external loads. In the present research, the influence of the bondline thickness on the damage and stress evolution of metallic adhesive bonding structures are investigated, with the cohesive interface model employed to equivalently simulate the bondline with various thicknesses. A prediction approach is employed to determine the cohesive parameters for the bondline with the thickness varied. Then a numerical example is presented to explore the bondline thickness-dependence strength and stress distribution in the bondline, followed by the validation with the existing experimental and theoretical results. Furthermore, as the main part of this paper, the influences of the bondline thickness on the damage and stress evolution in the bondline are investigated, involving the situations of the extremities and the whole bondline. The results show that, no matter in the extremities or in the whole bondline, the damage and stress evolutions are mutually influential processes, both of which are affected by the bondline thickness significantly. [ABSTRACT FROM AUTHOR]

Fully-reversed, strain-controlled isothermal tests were conducted on the Type 316FR at 650 °C for the strain ranges of Δ ɛ = ±0.4%, ±0.8%, ±1.0% and ±1.2%. A constitutive model based on isotropic and nonlinear kinematic hardening rules was used to replicate numerically the cyclic structural behaviour of the material until stabilisation. Finally to demonstrate the damage initiation and the damage evolution, a hysteresis energy-based phenomenological model was implemented in a user subroutine combined with a creep damage model based on the time-fraction law to replicate the experimental results. [ABSTRACT FROM AUTHOR]

Hydraulic slotting technique has been widely adopted in China for underground enhanced coal bed methane (ECBM) recovery. However, the related fundamental research of this technique is insufficient enough to get a comprehensive understanding of its action mechanism. In the current work, numerical models of specimens with various flaw inclination angles are built with particle flow code in two dimensions ( PFC 2D ). The relationship between mechanical properties and flaw inclination angle is analyzed, and the results show that (1) with an increase in the flaw inclination angle θ, the peak axial strain appears a decreasing trend after an initial increase and the peak lateral strain decreases gradually. With regard to the peak volumetric strain, when θ < 30°, it is positive, which implies that the specimen is in the state of volume shrinkage. In contrast, when θ > 30°, the peak volumetric strain is negative and the dilatancy occurs. (2) The elastic modulus and the initial damage coefficient show opposite variation trend with θ. Both of the relationships satisfy the Boltzmann function. (3) The peak strength and the initiation stress increase with θ. Furthermore, the damage constitutive equation of specimens with different flaw inclination angles under uniaxial compression is established with the consideration of the initial damage, loading damage, the stress–strain curves and crack number. The theoretical curves match well with the numerical curves and the achievements reported by other authors. The results of this study are expected to provide crucial guidance for the application of hydraulic slotting in ECBM recovery. [ABSTRACT FROM AUTHOR]

Tensile tests on miniature spruce specimens have been performed by means of acoustic emission (AE) analysis. Stress was applied perpendicular (radial direction) and parallel to the grain. Nine features were selected from the AE frequency spectra. The signals were classified by means of an unsupervised pattern recognition approach, and natural classes of AE signals were identified based on the selected features. The algorithm calculates the numerically best partition based on subset combinations of the features provided for the analysis and leads to the most significant partition including the respective feature combination and the most probable number of clusters. For both specimen types investigated, the pattern recognition technique indicates two AE signal clusters. Cluster A comprises AE signals with a relatively high share of low-frequency components, and the opposite is true for cluster B. It is hypothesized that the signature of rapid and slow crack growths might be the origin for this cluster formation. [ABSTRACT FROM AUTHOR]

This paper presents a series of cyclic loading tests on structural steel materials under cyclic tension–compression at large inelastic strains with amplitude up to 20% (± 10%). A total of six different cyclic loading protocols were considered in this study. The effects of the loading history on the cyclic response of structural steels including strain hardening, stress–strain response, damage evolution and plastic energy dissipating capacity have been investigated. The test results show that structural grade steels of Q345B and Q420D have the same trend in cyclic hardening or softening behavior and the loading history has quite obvious effect on the behavior of cyclic hardening or cyclic softening. The effect of loading history on cyclic stress–strain response is obvious especially at low amplitude. The effect of pre-cyclic strain loading is more obvious than those of pre-single strain loading. The steel Young's modulus E generally decreases with the number of cycle increases and the degradation rate becomes faster and faster with the increment of strain amplitude level. The energy dissipating capacity is nearly the same for all loading protocols. The cumulative dissipated hysteretic energy is only related to the total experienced strains but irrelevant to the loading paths for structural steels under fully reversed cyclic tension–compression at large inelastic strains with amplitude up to 20% (± 10%). [ABSTRACT FROM AUTHOR]

The mechanical behavior of the brain plays a vital role in regulating brain morphology and brain function. The experimental results show that the brain tissue behaves with strong nonlinearity, heterogeneity, loading rate and direction dependence, and complicated damage behavior. However, the underlying mechanism attributed to the interlacement of extremely soft matrix and complex axonal fibers network in white matter brain tissue is still not well understood. In this work, the nonlinear mechanical behavior of white matter brain tissue is analyzed by developing a mesoscale visco-hyperelastic constitutive model that considers isotropic matrix and anisotropic axonal fibers separately. Based on the diffusion tensor imaging data, the generalized structure tensor is introduced to reflect axonal fiber's orientation and distribution characteristics in different parts of white matter. Furthermore, the damage models of matrix and axonal fibers are established to characterize their damage evolution differences. In particular, a damage initiation criterion based on the equivalent strain of axonal fibers stretch is proposed by analogy with the Tsai-Hill criterion, according to the distribution characteristics of axonal fibers. Then the mesoscale constitutive model is applied to reveal the effects of matrix and axonal fibers on the deformation and damage of white matter. On the one hand, it is found that the "S-shaped" nonlinear stress curve of white matter in uniaxial tension is the superposition of the convex stress curve of matrix and the "J-shaped" stress curve of axonal fibers, and the nonlinearity is stronger with the increase of strain rate. On the other hand, with the change of the distribution of axonal fibers from uniaxially oriented to dispersed, the anisotropy of white matter caused by the stress reinforcement of axonal fibers gradually evolves from transverse isotropy to isotropy. With the increase of strain rate, the reinforcement effect of axonal fibers tends to be more isotropic, and the proportion of isotropic matrix stress is increased, resulting in the isotropic tendency of white matter. Moreover, the asynchrony of matrix and axonal fibers damage also results in the complex transition of white matter deformation behavior. Along with the increase of deformation, the anisotropic mechanical behavior of white matter rapidly degenerates to isotropy with the damage of axonal fibers. Overall, the strong coupling of nonlinear, anisotropic, and strain rate-dependent mechanical behaviors of white matter is well explained by combining the asynchronous deformation and damage of matrix and axonal fibers in this study. It will help understand brain injury, brain growth and development, physiological brain diseases, and other relevant problems. [ABSTRACT FROM AUTHOR]