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To enhance understanding of the dynamic tensile failure mechanism of MgO-C refractories, the Split Hopkinson Pressure Bar test combined with digital image correlation techniques were utilized. The dynamic failure behavior of specimens with different heat treatments and graphite content was investigated under various impact velocities. The MgO-C specimens exhibit severer damage and a more tortuous crack path with higher impact velocity. The SiC and forsterite phases generate during heat treatment are advantageous in inhibiting crack propagation, while the formation of oxide layer makes the material more susceptible to brittle fracture. Due to the presence of microcracks, high graphite samples lead to a reduction in tensile strength, but result in a wider fracture zone, which dissipates impact energy. Compared with the significant increase of strength under compressive impact loading, the critical tensile strain rate of MgO-C is easier to achieve after which the tensile strength decreases. [ABSTRACT FROM AUTHOR]

Tensile crack-induced collapse is widely distributed in major mountainous areas of China, often result in sudden and catastrophic consequences due to the rapid movement of the falling bodies. To better understand the development of tensile crack-induced collapses, the phase field method, originally used for studying brittle fracturing, has been further applied in this research. Firstly, the application of the phase field method in modeling rock tensile failure is validated, demonstrating its effectiveness. The rationality of applying the phase field method for analyzing tensile crack-induced collapses has been thoroughly investigated, and solutions to address phase field disorder have been proposed. By incorporating heterogeneity features and implementing a prefabricated crack, both approaches can effectively address the issue of phase field disorder. However, the latter method aligns more closely with engineering practices. The present study provides a comprehensive investigation into the development of tensile crack-induced collapses with varying cavity morphologies. The corresponding change laws of tensile crack-induced collapses are systematically analyzed and summarized. Moreover, novel stability evaluation parameters, including reduction coefficients and safety points, are proposed and successfully applied in calculations. Our research on different cave morphologies has shown that the depth and shape of a cave, as well as the thickness and center of gravity of protruding rock masses, significantly influence the development process of tensile crack-induced collapse. This approach facilitates a comprehensive examination of the fracture process and yields valuable insights into the mechanics underlying tensile crack-induced collapses. [ABSTRACT FROM AUTHOR]

The seismic responses of asphalt slabs in asphalt concrete-faced rockfill dams (ACFRDs) under the oblique incidence of seismic waves and the effect of a high strain rate on failure evaluation are still understudied. In this paper, formulas for the free field of SV waves at arbitrary incident angles are derived, a wave input model for SV wave spatial oblique incidence is established, and a method is proposed for assessing the failure of asphalt slabs. In this method, the real-time change in the tensile strength as it relates to the strain rate is considered. Taking an actual ACFRD as the research object, 16 obliquely incident SV wave conditions with different azimuthal and oblique incidence angles are considered, the influences of the incidence angle on the seismic response of an asphalt slab are analyzed in terms of the stress and acceleration, and the differences between the static and dynamic damage assessment methods are explored. The results show that the established SV-wave spatial oblique incident wave input model can accurately simulate the free-field half-space. The numerical solution of the wave input agrees well with the analytical solution. Compared with that of the vertical input, the maximum acceleration of the slab in the axial and vertical directions increased for 13 of the 15 oblique incidence conditions. The spatially oblique input leads to an increase in the dynamic stresses on the slabs, with the maximum tensile and compressive stresses increasing by 63 % and 153 %, respectively. Neglecting the oblique incidence of seismic motion may result in a significant underestimation of the seismic response of the slab. Moreover, the traditional method of increasing the static strength by 20 % as a criterion for determining the strength of slab tensile failure is overly strict. [ABSTRACT FROM AUTHOR]

Compressional wave velocity measurements and cone penetration tests were carried out for pressure cores recovered from the Krishna–Godavari Basin off the eastern coast of India, using an instrumented pressure testing chamber constructed by the Japanese National Institute of Advanced Industrial Science and Technology. For pressure cores with high gas hydrate saturations (S h) and exhibited high compressional wave velocities also display a high loading force that resulted in tensile failure. A pressure core sample with low compressional wave velocities and low gas hydrate saturations led to a low loading force, which resulted in radial shear. The cone factor connecting the cone resistance and the undrained shear strength, N k , was estimated using cone penetration testing and the uniaxial compressional results. We estimated its value as N k = 45 by using a uniaxial compression loading speed of 0.01%/min and a cone insertion loading speed of 0.25 mm/s. Undrained shear strengths, estimated from cone penetration results using N k = 45, were assumed to be equal to the tensile strengths between hydrate-hydrate and hydrate-particle interface when S h is less than 50% and increases when S h is more than 50% assuming that gas hydrate morphology was matrix supporting. • Compressional wave velocity measurements and cone penetration tests were applied to pressure cores. • Failure modes of pressure cores were assessed after cone penetration testing. • The cone factor for pressure cores was estimated to be 45. • Undrained shear strengths were estimated at 0.1–0.2 MPa under a 2400 m/s compressional wave velocity. [ABSTRACT FROM AUTHOR]

Tensile failure, hybrid failure (combined tension and shear) and shear failure are the three basic failure types of brittle rock. For a long time, little attention has been paid to the first two failure types except for the failure strength of direct/indirect tension, and the deformation characteristics and failure criterion of rock in triaxial extension (herein "triaxial extension" denotes axial tension/unloading-tension in confining pressure P c) are still unclear. This paper proposes a simple auxiliary apparatus to implement triaxial extension tests of cylindrical rock specimens combined with a servo-controlled MTS machine. Subsequently, the mechanics behaviors of sandstone in triaxial extension tests under different constant confining pressures (P c = 0, 5, 10, 15, 20, 30, 40, 50 and 60 MPa) are experimented and analyzed in detail, including the fracture angle, the failure surface morphology, the deformation and strength. With the increase of confining pressure, the failure type of sandstone presents a continuous transition from tensile failure to hybrid failure and to shear failure as a whole. The changes of failure morphology, elastic modulus, Poisson's ratio, maximum strain and failure strength with increase in P c show significant difference among three failure types. For the tested sandstone, the partitions of confining pressure between tensile failure and hybrid failure and between hybrid failure and shear failure are approximately at 20 MPa and 40 MPa, respectively. Hoek-Brown criterion with a tension cut-off and Fairhurst criterion are applicable to fitting the failure strength of triaxial extension test, and the latter presents a satisfying continuous transition from tensile failure to shear failure. [ABSTRACT FROM AUTHOR]

Highlights • Investigated intermediate strain rate tensile behavior of transparent polyurethane. • Large tensile deformation behavior and tensile failure behavior were analyzed. • Digital Image Correlation technique was adopted to acquire specimen strain. • Strain rate and temperature dependence and their equivalence were observed. • A phenomenological analysis of mechanical quantities was carried out. Abstract Transparent polyurethane has been widely applied in laminated windshield glasses as the interlayer material to enhance the reliability due to its outstanding impact resistance. Under impact loading such as bird strike, the interlayer undergoes large tensile deformation mainly at intermediate strain rates (at the order of magnitudes from 100 to 103, excluding 1000/s). In addition, the interlayer is on service over a wide range of temperatures for a plane traveling around the world. The mechanical behavior of transparent polyurethane under these conditions is not fully understood. In this study, systematical experiments were performed on transparent polyurethane. The viscoelasticity of the material was firstly verified by several quasi-static cyclic tests. Then a series of large tensile deformation and tensile failure experiments were conducted under intermediate strain rates and at temperatures of − 40 ° C to 40 ° C using a servo-hydraulic high-speed tensile machine. All strain data were acquired by the DIC technique. The experimental results show that tensile stress–strain curves and failure behaviors are significantly temperature and strain rate dependent. The rate-temperature equivalence was also observed. Finally, a phenomenological analysis of mechanical quantities of the material was carried out. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the in situ stress state of rock mass is fundamental for engineering, geological and geophysical applications. In situ stress state determination requires in principle the evaluation of the three principal stresses and the related principal directions, but it is widely recognized in the literature that the maximum horizontal stress is the most difficult component to accurately estimate. In the context of borehole methods, this paper proposes a step-by-step analytical procedure to estimate some bounds to the maximum horizontal stress, starting from a geomechanical description of the rock and relying on information generally available in the engineering practice. The procedure is divided in substeps, each one requiring additional information about the mechanical properties of the rock and on the geometrical properties of the failed portion of rock: more information available implies a lower uncertainty on in situ stress estimate. Furthermore, since the proposed procedure is analytical, it allows a complete and very easy implementation in a spreadsheet. The aim of the work is thus to provide a rigourous but simple analytical tool that can be used in engineering practicte to estimate some bounds to the maximum horizontal in situ stress state. The approach is finally validated by means of both numerical simulations, performed with a sophisticated numerical tool, and experimental field data coming from the literature. Highlights • Determination of in situ stress state is crucial for any underground application in many fields of engineering. • Borehole methods have been accepted to be one of the most reliable method to estimate in situ stress state. • A step-by-step analytical procedure is presented to estimate the in-situ stress state relying on drilled borehole information. • Any step of the procedure reduces of the bounds of the maximum horizontal stress, at the cost of more information required. • The procedure is validated by means of both simulations with advanced numerical methods and in situ literature data. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • A dynamic spring element model was implemented in a parallel computing framework. • Fibre failure leads to dynamic stress waves. • The dynamic stress waves increase the probability of failure of the fibres. • The dynamic effects are increased when the matrix is considered linear elastic. • The dynamic effects change the cluster formation and material failure. Abstract A spring element model that takes into account the dynamic effects associated with fibre failure in composite materials is presented. The model is implemented in a parallel environment to allow a better performance in the prediction of the complex mechanisms associated with longitudinal tensile failure. The model is used to identify the changes in the stress fields around a broken fibre representing fibre failure as a dynamic phenomenon. In light of these changes in the stress fields, the cluster formation and failure development is analysed and the results are compared with the static spring element model. It is observed that the stress redistribution around a broken fibre is strongly dependent on the dynamic effects and it varies with the material in study, specially if the matrix is considered linear elastic. The changes in local stress redistribution are seen to affect the materials tensile behaviour and cluster formation, being this changes higher for a material with an elastic matrix. [ABSTRACT FROM AUTHOR]

• Responses of jointed rigid pipes under normal faults with various crossings are studied. • Analytical solutions for pipe bending moment and joint shear force at different crossings are proposed. • Excessive joint deflection dominates the pipe safety for relatively shallow burial depth and low soil density. • Increasing the burial depth and soil density, pipe failure mode changes to barrel bending and bell cracking. • Fault-crossing at around 7/10 of the pipe segment is the worst scenario for rigid pipes. The structural safety of jointed rigid pipes can be jeopardized under abrupt ground settlements. Most previous studies have investigated the response and safety of jointed rigid pipes under abrupt ground subsidence using numerical and experimental methods at two specific positions, i.e., the shear band caused by the ground movements crosses the pipe at a joint and at the midspan of a pipe segment. In this work, a closed-form analytical solution for the deformation and mechanical response of jointed rigid pipes under abrupt ground subsidence with various crossing positions is presented. The pipe bending moment and the joint shear force are calculated, which match quite well with the results of finite element analysis. Then the maximum allowable settlements are examined, and the corresponding failure modes are identified. It shows that with the increase of burial depth and soil density, the main failure mode gradually changes from the excessive joint deflection to the barrel bending and bell cracking, and the maximum allowable subsidence distance decreases remarkably. The worst crossing position is at nearly 7/10 of the pipe length. The results offer new insights into the failure mechanism of jointed rigid pipe, and provide a basis for the protection of rigid pipes under abrupt ground subsidence. [ABSTRACT FROM AUTHOR]

In order to theoretically clarify the effect of Cu on Al/Mg 2 Si interface, the calculation method of first principles is employed to determine a stable interface in Al(100)/Mg 2 Si(111) through surface convergence test and adhesion work. This study aims to investigate the stability, electronic structure, and fracture failure of Cu-doped Al(100)/Mg 2 Si(111) interfaces with varying concentrations of Cu. The interfacial bonding strength is the highest when the doping concentration of Cu is 1.85 % according to adhesion work. Comparing with orbital hybridization degree of density of states and pseudogap width of interface total density of states, it can be known that the generation of Mg–Si and Mg–Cu bonds and the best stable Al(100)/Mg 2 Si(111)-1.85 at.%Cu interface, respectively. The difference charge density shows that the charge density between Al–Cu atoms and Mg–Cu atoms is significantly higher than Al–Si atoms and Mg–Si atoms. The average numbers of transfer per atom at the Al(100)/Mg 2 Si(111)-1.85 at.%Cu interface are higher than that at the Cu-undoped Al(100)/Mg 2 Si(111) interface, indicating an increase in ionic bond strength. Tensile simulations of Al(100)/Mg 2 Si(111) and Al(100)/Mg 2 Si(111)-1.85 at.%Cu interfaces were performed. According to tensile stress-strain curve and difference charge density, cracks are initiated in Mg 2 Si due to the fracture of Mg–Si and Mg–Cu bonds. • Cu-doped Al(100)/Mg 2 Si(111) interface models were studied using density functional theory. • From the perspective of adhesion energy and elastic structure, Al(100)/Mg 2 Si(111)-1.85 at.%Cu interface is more suitable. • The tensile strength of Al(100)/Mg 2 Si(111)-1.85 at.%Cu interface is the highest in contrast to Al(100)/Mg 2 Si(111). • The cracks are initiated in Mg 2 Si surface. [ABSTRACT FROM AUTHOR]

In recent decades, numerous geotechnical hazards, including landslides, foundation settlements, and tunnel collapses, have been linked to block-in-matrix rock (bimrock), resulting in substantial damage. The investigation and analysis of the engineering properties and mechanical behavior of both the rock and soil have become increasingly intriguing research areas. However, analyzing bimrock remains a formidable challenge due to its inherent heterogeneity. In this study, to investigate the tensile behavior of the bimrock, the numerical method that couples discontinuous deformation analysis (DDA) and smoothed particle hydrodynamics (SPH) is improved. First, a tension damage model is implemented in SPH for simulating the tensile behavior of the soil. The effectiveness of the presented model is verified through the direct tensile test model and the Brazil disc split model as an indirect tensile test. The contact algorithm of DDA-SPH is then modified by adding a tensile contact spring to introduce tensile strength at the interface between the matrix and the rock in the bimrock. Through a simple pulling numerical model, the accuracy of this modification has been verified, and the appropriate tensile contact stiffness is discussed. Furthermore, using the proposed method, the overall tensile strength of the bimrock with respect to the interface tensile strength is investigated. Finally, the proposed numerical method is applied to the simulation of geoengineering problems, demonstrating the capacity to analyze the stability and large deformation of bimrocks. • A tension damage model is added to the soil-particle based SPH method for simulating the tensile behavior of the matrix. • The contact algorithm of DDA-SPH is then modified by adding a tensile contact spring to introduce tensile strength at the interface. • The overall tensile strength of the bimrock with respect to the interface tensile strength is investigated. • The proposed method is applied to analyzing the bimrock slope and bimrock shallow tunnel. [ABSTRACT FROM AUTHOR]

Abstract Hydraulic fracturing can be considered one of the most effective stimulation methods for fractured network formation. The interaction between induced and natural fractures greatly affects the success of the treatment. In this paper, the extended finite element method (XFEM) and distinct element method (DEM) have been combined to identify the propagation of hydraulic fractures in a porous medium with naturally fractured blocks. DEM is used to investigate hydraulic fractures in a natural-fracture network and evaluate the production flow rate under different apertures and lengths of hydraulic fracture. XFEM is used to simulate the deformation of the natural-fracture interface during the approaching phase of induced fractures as a no-remeshing tool. The results of DEM reveal that length of hydraulic fracturing has a significantly greater effect on production than aperture. Results from XFEM show that tensile and shear debonding of natural fractures change as a function of angle and distance from an induced fracture. Highlights • Hydraulic fracturing in natural fracture reservoirs. • Effective stimulation methods for fractured network formation. • The distinct element method (DEM) and the extended finite element method (XFEM). [ABSTRACT FROM AUTHOR]

Abstract An experimentally validated multiscale computational model for predicting the progressive damage and failure analysis (PDA) of 3D woven textile composites (3DWTCs), is presented. The 3DWTCs are made through a 3D textile weaving process and the dry fiber tows are infused with SC-15 polymer matrix into a single composite material. A thick symmetric configuration of hybridized (carbon, glass and kevlar tows) textile architecture is examined at the entire coupon level to determine the progression of damage under tensile loading and to understand the benefits of hybridization and the resulting performance enhancements. Results also include micro-CT analysis of the laboratory scale coupon to study the effect of microstructure imperfections response. A finite element (FE) model is generated directly from micro-CT data using the software, Simpleware [1]. A three-scale modeling strategy is adopted, where the meso-scale representative volume elements (RVEs) are modeled explicitly in the failure prone gage-area to consider the tow architecture and a 2-layer concentric cylinder (2CYL) [2,3] micromechanics model is used within the fiber tows to consider the fiber/matrix scale and the pre-peak nonlinearity, as caused by matrix microdamage. The analytical subscale model results in a computationally efficient framework for PDA of 3DWTCs. [ABSTRACT FROM AUTHOR]

The dynamic compression responses of four glass spheres with diameters 7.71 mm, 11.90 mm, 17.88 mm, and 24.87 mm are investigated by the modified split Hopkinson pressure bar device (SHPB). Based on the high speed images, the dynamic compression of the spheres can be divided into three stages, e.g. the quasi-elastic deformation stage, the non-linear deformation stage, and the brittle failure stage. The formation and growth of the shadow areas observed at the impact end and the support end in the spheres are the main causes of the catastrophic failure. The movement of the plane-like fronts of the shadow areas is activated and driven by local velocity gradients. Dynamic breakage of the spheres is then analyzed by the particle size distribution, the fractal properties, etc. The results show that there are two main failure mechanisms in the process of sphere breakage, e.g. the tensile failure mechanism and the shear failure mechanism. In order to investigate the transition of these two failure modes, a bimodal Weibull distribution model is preliminarily established on the Weibull statistical strength. The model shows both good simulations to the experimental data and obvious the transition process of two failure mechanisms. The scaling laws and the strain rate effects of breakage strength are then discussed. The results are helpful for the controlling of pulverization for brittle particles. [ABSTRACT FROM AUTHOR]

Horizontal borehole instability raises concern due to negative impacts on success of drilling, reaming and pipe installation during Horizontal Directional Drilling. Shear failure (blow-out) and tensile failure (hydraulic fracture) are the two main failure mechanisms controlling borehole instability in saturated clay. Criteria for categorizing shear failure versus tensile failure are given which are functions of the ratio of undrained shear strength to vertical effective stress S u / σ v ′ and the Lateral Earth Pressure Coefficient at Rest K 0 ′ . Given that S u / σ v ′ and K 0 ′ can be expressed as a function of soil parameters like friction angle, Over Consolidation Ratio (OCR) and Liquidity Index (LI), new forms of these criteria are developed for situations where S u / σ v ′ and K 0 ′ cannot be obtained directly. It is confirmed that tensile failure generally controls in brittle clays like heavily overconsolidated and/or low liquidity index materials. Cases from the laboratory and the field as reported in the literature are used to examine the effectiveness of the new criteria, with inconsistency between theory and the physical evidence explained. [ABSTRACT FROM AUTHOR]

A seismic response analysis of a piled raft foundation combined with cement deep mixing walls (DMWs), supporting a 12-story base-isolated building under strong earthquakes, was conducted using a three-dimensional finite element model. To evaluate the induced internal stresses in the DMWs against their capacity, a nonlinear elastic model with tensile and shear criteria was applied for the stabilized soil. In order to verify the deformation parameters of the DMWs and the soil, a numerical simulation was carried out using the moderate earthquake motion recorded during the 2011 Tohoku Earthquake. As the strong earthquake motion, Level 2 earthquakes with a mean return period of approximately 500 years were generated using two sets of phase data. Four numerical cases were conducted considering the effects of the presence of the DMWs on the seismic response of the piled raft system. Based on the dynamic analysis results, it is seen that the bending moments near the pile head were decreased significantly by the DMWs. This occurred because the amplification of the lateral ground displacements below the raft was restrained by the DMWs, and the shear force acting at the pile head was very small because the lateral external force acting at the bottom of the raft was carried mostly by the DMWs. It is also seen that the amplification of the acceleration response spectra of the raft with DMWs was substantially lower than that of the raft without DMWs in long periods of 1–2 s. This is probably due to the difference in the kinematic interaction effects between the foundation and the soft soil. Tensile and shear failure occurred in the DMWs, and the extent of the tensile failure was dominant. Nevertheless, it was found that the grid-form DMWs were quite effective for reducing the sectional force of the piles to an acceptable level, even if such partial failure occurred. The DMWs could be designed more rationally by following the principles of a performance-based design (PBD), because minor damage to DMWs can be tolerated under strong earthquakes provided that the required foundation performance has been satisfied. [ABSTRACT FROM AUTHOR]

Drained triaxial tension tests were conducted to investigate the tensile and shear failure behaviors of cement-treated soils under effective confining pressures. In the tests, tensile force was applied on saturated cement-treated soil specimens at effective confining pressures. The experimental result for a cement-treated soil shows that tensile failure occurred at low effective confining pressures, while shear failure occurred at high effective confining pressures. Based on experimental evidence, a failure criterion for cement-treated soil is discussed. In this study, tensile failure is assumed to be dominated by the effective minor principal stress and the Mohr-Coulomb failure criterion is adopted for shear failure. The observed stress states at failure lie on the failure criterion consisting of the tensile and shear failure surfaces, indicating that the proposed failure criterion is suitable for cement-treated soils. The experimental results provide a good understanding of the failure behavior of cement-treated soils that exhibit tensile and shear failure modes. [ABSTRACT FROM AUTHOR]

• The zero-thickness cohesive element model is used to simulate the RCC layers. • Failure modes of a RCC dam subjected to underwater contact explosion are obtained. • Effects of the properties of RCC layers on the dam's blast responses are studied. Considering the construction technology of roller compacted concrete (RCC) gravity dams, there are layers in dams inevitably, which may affect the blast responses of dams. This paper is aimed to investigate the effects of RCC layers on the failure modes and dynamic responses of RCC gravity dams bearing underwater contact explosion loading. Initially, the zero-thickness cohesive element model is presented to simulate the crack behavior of the RCC layers. An impact test of a concrete beam is selected to verify its reliability. The concrete damage plasticity model considering strain rate effects is utilized to simulate the various responses of concrete under blast load. The dam–reservoir–foundation interactions are described using the coupled Eulerian-Lagrangian method. The numerical methods are validated by a field blast test of a concrete structure. Subsequently, numerical models were established for gravity dam-reservoir-foundation systems, with and without considering RCC layers respectively. Based on validated algorithms, these models were utilized to explore the effects of RCC layers on the blast performance of the dam. Finally, the influence of the parameters of RCC layers on the nonlinear responses and failure modes of the dam is further investigated, including the maximum traction, the fracture energy, and the layer thickness. The results reveal that the layers significantly change the failure modes. The RCC layers are the weak link of the dam and they will lower the anti-explosion performance of the dam. Therefore, it is crucial to take the layers into account when conducting anti-explosion research, and RCC dams cannot be simply simplified as normal concrete dams. To guarantee the safety of the dam, it is important to ensure the layers with sufficient strength and fracture energy, as well as an appropriate thickness. [ABSTRACT FROM AUTHOR]

• The experimental results indicate that the specimens undergo three types of local compressive failure modes. • The calculation model for the local bearing capacity of concrete with spiral stirrups F l is formulated. • To ensure simultaneous tensile and compressive failures, a critical reinforcement ratio of the spiral stirrup ρ v u is proposed. • The calculation model is highly accurate. This study evaluates the mechanical and failure modes of 30 concrete specimens under local compression. The experimental results indicate that the specimens undergo three types of local compressive failure modes. These modes primarily depend on the strengths of the tensile and compression regions. Based on the test results, a calculation model for the local bearing capacity of concrete with spiral stirrups F l is formulated. In the model, the local compressive failure of concrete with spiral stirrups is classified into tension and compression. The yielding of spiral stirrups in the tensile region is regarded as tensile failure, and compressive failure is considered to have occurred when the strain value in the compression region reaches 0.002. To simplify the model calculation, the spacing s , diameter d , and yield strength f y of spiral stirrup as well as the local compression area ratio A b / A l (ratio of the concrete bearing area A b to the loading area A l) are comprehensively analyzed. Based on the analysis results, a formula for the bearing capacity of the reinforcement N s is derived. The independent variables of the formula are yield force of spiral stirrup f y A ssl , stirrup spacing s , and ratio of specimen side length to loading plate side length h / a. A simplified formula for calculating the tensile failure of the specimen is derived. To ensure simultaneous tensile and compressive failures, a critical reinforcement ratio of the spiral stirrup ρ v u is proposed based on the calculation model. Test data from similar axial local compression tests are collected to verify the accuracy of the calculation model. The local bearing capacity of concrete is recalculated according to the calculation method proposed in this study. Results show that the calculation model is highly accurate. Accordingly, it may be used as reference for calculating the local bearing capacity. [ABSTRACT FROM AUTHOR]

The present paper investigates a 3-D numerical analysis method, combining peridynamics (PD) and the discrete element method (DEM), which can simultaneously handle both the fracture and post-fracture behavior of materials assumed to be soils. With geomaterials, it is very important to treat the process from fracture to post-fracture in a continuous manner. The bond-based PD model is employed to reproduce the behavior from the initiation of cracks to fracturing. At the same time, the DEM model is also introduced so that the contact forces among the fragmented pieces can be calculated after the fracturing. The computational process of the physical contact between the wall element and the computational nodes of the PD is also solved based on the DEM so that plane geometry boundaries can be used. For the failure criteria of the connection between the computational nodes of the PD, a bilinear type of bond model is introduced to obtain a nonlinear stress–strain relationship and numerical stability when the calculation process switches from PD to the DEM. To verify the proposed approach, numerical simulations of the diametral compression test are performed on flawless cylindrical specimens, and the results are compared with theoretical solutions for the stress distribution inside the specimens. Based on the results, a simulation of the compressive failure of multiple soil-like crushable objects is performed as an example of the application of the coupled PD-DEM. The obtained results show that the proposed method will serve as a computational framework that can treat the continuous fracture behavior of soil-like materials in three dimensions. [ABSTRACT FROM AUTHOR]

Considerable research has recently been undertaken to investigate the simulation of brittle fracture mechanisms using distinct element grain boundary models. However, to date, few studies have addressed the limitations of these models when used for predictive analysis. Our research suggests that mesh geometry dependencies can impart irreducible uncertainties into the method despite apparent calibration. In addition, Voronoi meshing routines can limit the kinematic freedom, and increase the degree of interlocking and localized tensile failure; whereas, more recently introduced triangular mesh geometries can have an opposite effect, increasing kinematic freedom, and predisposing models towards shear failure mechanisms. [ABSTRACT FROM AUTHOR]

In this work, fracture mechanisms of a newly developed carbon fiber/epoxy laminate [+75/0/-75] s were assessed by scanning electron microscopy (SEM). The composite strain-stress curves were plotted with the displacement information simultaneously acquired from both a tensile testing machine crosshead and an extensometer. The strain-stress curve plotted with the displacement data from the machine test showed an average slope change from E 1m = 22.783 GPa to E 2m = 13.170 GPa on about 65% of the total strain before global failure, while strain-stress curves plotted with displacement data from the extensometer showed one single slope. While results reported in literature related to composite failure mechanisms, where some authors reported a slope change in strain-stress curves associated to progressive failure, experimental evidence in this work for strain-stress curves showed one single slope, indicating that such slope change is related to the strain measuring technique, and not to a progressive failure. The fracture surface was studied, and four main features were observed, which were related to failure mechanisms during the uniaxial test. The identified failure mechanisms occurred on a stage above 93% of the total strain before global failure. [ABSTRACT FROM AUTHOR]

Natural fractures and laminations are common features in many unconventional resources which require hydraulic fracturing for economic production. To better understand the effect of such features on rock failure, we have conducted indirect tensile or Brazilian testing on several rock samples from Green River and Niobrara formations. Brazilian experiments were conducted at various orientations of laminations or natural fractures with respect to the loading direction. In contrast to isotropic rocks, we observed that rock failure can occur away from the vertical central line of rock parallel to lamination or natural fracture when the approaching angle between the lamination and loading direction is less than 30° in the tested samples. It was also determined that the tensile strength parallel to laminations or calcite-filled fractures is lower than the tensile strength perpendicular to lamination and/or the natural fractures; for instance the tensile strength of Niobara calcite-filled fractures were almost one third of the rock matrix. [ABSTRACT FROM AUTHOR]

The ground stability of gateroads is a major concern in underground coal mines, especially when the surrounding strata are weak and fractured. This paper presents a novel numerical modelling technique for gateroad stability analysis based on a case study conducted at Zhaogu No. 2 mine, China. Considering the occurrence of fracturing and its weakening effect on the stiffness of the rock mass, a tension-weakening model is implemented into FLAC3D, whereby the stiffness of the rock mass is progressively decreased with the failure state. A relationship between the intensity of fractures and the residual strength of the rockmass is built in the numerical model. A parametric study of the tension-weakening model is carried out, and the results are compared to elastic-perfectly plastic and strain-softening models. It is shown that the tension-weakening model exhibits noticeable effects on ground deformation and rock support loading, which are observed from field measurements. Thus, the tension weakening model offers a more realistic simulation of the gateroad behavior than elastic-plastic and strain softening models. The proposed model can be utilized for other applications involving rock reinforcement of mine openings under similar geotechnical conditions. [ABSTRACT FROM AUTHOR]

The influence of defect band on the tensile failure in high pressure die casting of AM60B magnesium alloy was studied by in situ scanning electron microscope. Results showed that the size and distribution of the externally solidified crystals (ESCs) had a significant influence on the tensile behavior induced by defect band in the specimen. In the specimen with few ESCs, the crack would initiate at the gas-shrinkage pore in the center of the specimen. Further crack propagation at the defect band was hindered by the surrounding dense microstructure, and the effect of defect band on the tensile failure was just reducing the efficient force bearing area. In the specimen with large and complex ESCs, the microstructure around the defect band would become coarse and comprised of large porosities with complex morphology. The defect band would act as crack initiation site and facilitate the crack propagation to both center and surface of the specimen, leading to the final fracture. [ABSTRACT FROM AUTHOR]

Fibre-reinforced composites are rapidly increasing their market share in structural applications. Nevertheless, this increase would be much stronger if reliable failure predictions were available. These predictions are not only insufficiently reliable for complex loading of multidirectional composites, but even for longitudinal tensile failure of unidirectional (UD) composites. Since composite failure usually coincides with longitudinal failure of a 0° ply, the reliability often hinges on longitudinal failure predictions of UD composites. Despite great progress in the state-of-the-art models, significant obstacles remain in collecting the necessary input data and understanding the influence of the modelling assumptions. This review therefore surveys the mechanics, chemistry and physics involved in tensile failure of UD composites and highlights potential areas for improvement. Specific proposals are made to advance the state-of-the-art strength models, which could catalyse the use of composites in structural applications. [ABSTRACT FROM AUTHOR]

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impact has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. In this contribution, a rate-dependent stress-based nonlocal damage model has been introduced for the simulation of dynamic tensile failure of quasi-brittle materials. The recently proposed stress-based nonlocal criterion has been updated in order to be consistently combined with a rate-dependent version of the well-known Mazars damage model. The model was implemented in LS-DYNA using a fully explicit computational scheme. Two sets of numerical examples have been presented. First, one-dimensional numerical analyses were conducted to evaluate the model capabilities, applicability and limitations. Second, the model has been validated against experimental results. It has been shown that the proposed model, in addition to correcting spurious mesh sensitivity, also provides a more realistic representation of damage initiation and growth, in particular around discontinuities (notches and free boundaries) and damaged areas. [ABSTRACT FROM AUTHOR]

• The wave input method for P-wave spatial oblique-incidence is established. • A new core safety evaluation method based on instantaneous tensile stress and strength is proposed. • The responses of an asphalt concrete core embankment dam under P-wave spatial oblique incidence are investigated. • The damage mechanism of the core caused by spatial oblique incidence is revealed. • The tensile weak zone of the core under different incident modes are clarified. The research on asphalt concrete core embankment dam under the spatial oblique incidence of seismic wave and the dynamic tensile failure of the core is still insufficient. This paper considers the spatial arbitrariness of P-wave incident azimuth angle γ and oblique incident angle α , constructs the non-uniform free field on the foundation boundary based on the wave field superposition principle, and establishes a wave input method for P-wave oblique-incidence in three-dimensional space. Then, an empirical formula for the change in the instantaneous tensile strength of asphalt concrete with the strain rate is established on the basis of the test results. Furthermore, a new core safety evaluation method based on instantaneous tensile stress and strength is proposed to discriminate element tensile failure. Finally, the influence law of incident azimuth and oblique incident angles on the acceleration and stress spatial distribution of asphalt concrete core is analyzed. The damage mechanism of the core caused by the surge of local tensile stress caused by spatial oblique incidence is revealed. The traditional static strength discrimination method has an error in discriminating the core failure compared with the method proposed in this paper. The distribution characteristics of tensile weak zone and failure zone of the core under different incident modes are clarified. Results show that compared with the vertical incident, the core accelerations in the water flow and dam axis directions increased by a maximum of 9.6 times and 68% under spatial oblique incidence (γ ∈ [0°, 90°] and α ∈(0°, 75°]), and the core acceleration in the vertical direction can be reduced by 69%. The core compressive and tensile stresses increased by a maximum of 31.3% and 4.0 times under spatial oblique incidence (γ ∈ [0°, 90°] and α ∈(0°, 75°]). Neglecting spatial oblique incidence will underestimate the core accelerations and stresses seriously. The more the incident direction deviates to the dam axis and the larger the oblique incident angle is, the more prone it is to tensile failure on the wave-facing side of the core. When the incident direction is consistent with the dam axis and the incident angle is 75°, the tensile failure on the wave-backing side of the core is more serious than that on the wave-facing side. The use of the traditional static strength discrimination method will result in the large crack failure error of the core. [ABSTRACT FROM AUTHOR]

In this work, the effects of acetylation modification on the plasticization properties of citrates (triethyl citrate (TEC), acetyl triethyl citrate (ATEC), tributyl citrate (TBC) and acetyl tributyl citrate (ATBC)) were investigated by using a combination of characterization techniques and molecular dynamics (MD) simulation. The tensile test show that the acetylated ATEC and ATBC have better plasticizing efficiency compared to their non-acetylated counterparts, and consequently the tensile strength and elongation at break of ATEC/PVC (ATBC/PVC) are 13.9% (18.7%) and 8.3% (2.2%) higher than those of TEC/PVC (TBC/PVC), respectively. The MD simulation results corroborate these observations and further reveal the reason for these changes. The research mainly includes three aspects: 1. Effect mechanism of acetylation modification on the interaction between citrate and PVC; 2. The changes of microstructure, energy and motion state of PVC composites during tensile process; 3. Tensile failure mechanism of PVC composites under large tensile force. In addition, the effect of acetylation treatment on toxicity of citrate was also studied. Thus, our study contributes to better understanding of the plasticizing mechanisms of citrate and proposes an optimized citrate structure with higher performance. [Display omitted] • Focus on the influence of acetylation modification on the plasticization properties of citrate. • To study the mechanism of plasticization and tensile failure of citrate/PVC composites. • Reveal the microstructure and energy changes of PVC composites during elastic deformation and plastic deformation. [ABSTRACT FROM AUTHOR]

The failure strain of fibre-reinforced composites can be increased by fibre hybridisation. A recently developed model for unidirectional composites was extended to hybrid composites to analyse this synergetic effect, called the hybrid effect. The model predicts individual fibre breaks and interactions among clusters of fibre breaks. Three key parameters were studied to understand how they can maximise the hybrid effect, namely low elongation fibre strength scatter and hybridisation fibre stiffness and failure strain. Larger strength scatter of the low elongation fibres leads to larger hybrid effects, as the scatter spreads out the cluster development over a larger strain interval. The failure strain ratio of the two fibre types should be above 2 for the properties used here, but a higher ratio did not yield any additional benefits. Increasing the stiffness of the hybridisation fibre reduces the stress concentrations on the low elongation fibre and may also enlarge the hybrid effect. These conclusions provide guidelines for designing optimal hybrid composites. [ABSTRACT FROM AUTHOR]

Unstable failure of brittle rocks is a hazardous problem in deep mining and tunneling projects. Failure of brittle rocks around excavation boundaries is dominated by tensile fracturing. Homogeneous models cannot capture this rock failure mechanism while heterogeneous models are proven capable of capturing it. A question that may arise is how a change in failure mechanism influences the failure intensity. In this paper, an explicit FEM tool (Abaqus) is employed to simulate failure of homogeneous and heterogeneous rocks. Material heterogeneity is introduced into Abaqus models using Python scripts and the simulation results demonstrate that heterogeneous models can capture splitting rock failure. The effect of material heterogeneity on rock failure intensity in unconfined and confined compression tests is investigated. The simulation results from both types of models show that rock failure is more violent when the loading system is softer and the confinement is lower. However, it is observed that when two materials have the same peak strength, the heterogeneous model has more released energy than the homogeneous model due to the difference in the failure mode. The tensile splitting failure mode of the heterogeneous model releases more energy than the shear failure mode of the homogeneous model. This can be an important point to be considered for rockburst support design. Furthermore, the simulation results show that for two models with similar material composition, the strength of the more heterogeneous model is lower than the less heterogeneous model; as a result, the failure of the more heterogeneous model will be less violent if it fails in an unstable fashion, and this is in good agreement with field observation results. [ABSTRACT FROM AUTHOR]

Refractory brittleness is often reduced by the introduction of micro-cracks during the production process. Using the finite element method, crack initiation and formation in a magnesia-spinel refractory during the cooling stage of the firing process are simulated in a 2D representative volume element. The microstructure is described by a random spatial distribution of spinel and magnesia grains surrounded by a magnesia matrix. The grain size distribution was chosen according to the Dinger–Funk equation. Crack formation due to a thermal misfit between the magnesia and spinel phases was evaluated using the concrete damaged plasticity model in ABAQUS. After cooling, the model displayed reduced brittleness and a crack pattern typical of this type of material. The obtained mechanical properties from the subsequently simulated tensile test were compared with literature values. The simulation provided reasonable values and is capable of reproducing micro-cracking in the presented refractory. [ABSTRACT FROM AUTHOR]

The paper presents a semi-analytical solution for the stability of inclined boreholes drilled in isotropic and anisotropic formations. Conventional analytical or semi-analytical models are designed to yield no borehole collapse failure along the borehole wall. However, in deep wells, where there are high hoop stresses acting on the borehole wall, it is difficult to apply the safe mud weight produced by these ubiquitous models. The proposed solution in this work imposes a constraint on how much failure will occur along the borehole wall. This risk-controlled stability analysis will produce a safe mud window that can be realistically achieved during drilling operations. Analytical solutions for stress distribution for isotropic and anisotropic rocks are presented. In addition, a solution for the upper limit for the mud window to prevent tensile failure is developed. The initial poroelastic or “undrained” drilling effect on pore pressure is also incorporated in the discussed models. [ABSTRACT FROM AUTHOR]

We perform numerical studies on vertical fracture propagation induced by tensile hydraulic fracturing for shale gas reservoirs. From the numerical simulation, we find that tensile fracturing occurs discontinuously in time, which generates saw-toothed responses of pressure, the fracture aperture, and displacement, and that fracture propagation is sensitive to factors such as initial condition of saturation, a type of the injection fluid, heterogeneity, tensile strength, elastic moduli, and permeability models. Gas injection induces faster fracturing in shale gas reservoirs than water injection, for the same mass injection, because of high mobility of gas. However, water injection to highly water-saturated formations can contribute to fast pressurization and high mobility of water, resulting in large fracturing. For moderate initial water saturation, complex physical responses within the fracture result from strong nonlinear permeability and multiphase flow with gravity. Pressure diffusion and pressurization within the fracture are also affected by permeability. High intrinsic and high relative permeabilities result in fast fluid movement of injected fluid, followed by fast fracturing. High Young׳s modulus and high Poisson׳s ratio do not seem favorable to fracture propagation, although they are not significantly sensitive. For heterogeneity, a geological layer of high strength between near surface and above the shale gas reservoirs can prevent vertical fracture propagation, changing the direction of fracturing horizontally. [ABSTRACT FROM AUTHOR]

A cohesive fracture model considering tension, compression and shear material behaviour is implemented into the hybrid continuum–discrete element method, i.e. Universal Distinct Element Code (UDEC), to simulate possible multiple fracture in soft rocks. The fracture model considers both elastic and inelastic (decomposed to fracture and plastic) displacements. The norm of the effective inelastic displacement is used to control the fracture behaviour. Three numerical examples, including Mode-I, Mode-II and Mixed-mode tests, are conducted to verify the model and its implementation in UDEC. The model is subsequently applied to simulate uniaxial compression and Brazilian disc tests on soft rocks and the results are compared with experimental results. The results indicate that the cohesive fracture model is capable of realistically simulating the combined tensile, shear and mixed-mode failure behaviour applicable to geomaterials. [ABSTRACT FROM AUTHOR]

Abrupt ground movements induced by earthquake, landslide, tunnelling and excavation, etc., pose significant threats to the safety of underground jointed pipelines. Most previous studies have investigated the response of buried pipelines through experimental tests and numerical simulations, but analytical solutions for the safety evaluation of jointed pipelines are limited. In this study, enhanced analytical solutions of pipe bending moment and joint shear force are derived, in which the effects of different pipe-fault crossing configurations and pipe-soil interactions are considered. Then, the allowable offset distances are investigated using the proposed solutions, respectively, for three different failure modes, i.e., joint shear failure, pipe bending failure, and joint pull-out failure. The results show that the joint pull-out failure governs the pipe safety when the burial depth is relatively shallow, while the pipe bending failure dominates when they are buried deeply. The worst pipe-fault crossing position is not at the pipe joint or the midspan of the pipe segment, but at one tenth of the pipe segment adjacent to the joint. The results offer new insights into buried DI pipe failure mechanism and provide a basis for the protection of buried DI pipes under abrupt transverse ground movement conditions. • Enhanced solutions are proposed for internal forces of ductile iron pipes under various fault-pipe crossing conditions. • Different failure modes for ductile iron pipes under under various strike-slip fault-pipe crossing scenarios are assessed. • Maximum allowable offset distances for ductile rion pipes under various strike-slip faulting conditions are evaluated. [ABSTRACT FROM AUTHOR]

Discontinuities remarkably dominate in jointed or blocky media, yielding an assemblage of blocks, and the mechanical properties of joints can greatly affect the behavior of a blocky system. Discontinuous deformation analysis (DDA) is promising method to analyze the failure process of a blocky system. In three-dimensional (3-D) blocky systems, face-to-face contact between adjacent blocks is common, especially for an ideal rock block system at the very early stage. The accurate modelling of the face-to-face contact is critical. The original 3-D DDA adopts the concentrated spring face-to-face contact model, in which a face-to-face contact is simply converted as the combination of several dominant contacts each with a couple of normal and shear springs. However, such simplification is not always accurate and can yield unreasonable results when simulating the progressive tensile failure between blocks under asymmetrical tension conditions. To overcome this problem, a multi-spring face-to-face contact model incorporated into 3-D DDA is proposed. In the proposed model, multiple couples of normal and shear springs are applied for a face-to-face contact to simulate the distributed force induced between contacting blocks. Several verification examples involving tensile failure are studied to show the advantages of the improved 3-D DDA. The simulated results obtained from the improved 3-D DDA converge to the theoretical solutions or experimental results while the original 3-D DDA gets unreasonable results. Thus, the improved 3-D DDA can provide more reasonable results when analyzing the tensile failure problems. • Improved 3-D DDA by incorporating a multi-spring face-to-face contact model. • Showed limitations of the original concentrated spring face-to-face contact model. • Presented the advantages of the proposed contact model in 3-D DDA. • Improved 3-D DDA can more reasonably simulate tensile failure problems. [ABSTRACT FROM AUTHOR]

The behavior of transversely isotropic rock material was studied under Brazilian test conditions for nine different rocks (two sandstones, one shale, two slates, one schist, and three gneiss). Both the variation of the strength and the final fracture patterns induced by testing were examined as a function of the inclination angle of the weak planes. The combination of the observations for both parameters illustrated clearly the complexity of the failure of such rocks, which was summarized by assuming four different trends. The four trends for the variation of the strength as a function of the inclination angles range from little or no variation (trend 1, i.e., isotropic behavior for strength) to a sharp decrease of the strength from very small angles onwards, followed by a leveling off (trend 4). Trend 2 is characterized by a constant value between 0° and about 45°, followed by a linear decrease, while trend 3 corresponds to a decrease of the strength over the entire interval, but a rather systematic decrease, approximating a linear variation. Different variations were observed among these four trends in terms of the relative lengths of the respective fractures along the weak planes and in any other direction. For the rocks investigated, there is a cross-over from dominant fractures in directions other than the weak planes to dominant fractures along the weak planes. For the four trends, there are systematic changes in the position of this cross-over point, i.e., on average, at about an inclination angle of 75° for trend 1 and at about 15° for trend 4. For inclination angles larger than the cross-over point, the rock specimens failed primarily by splitting between both loading lines. [ABSTRACT FROM AUTHOR]

Abstract: A modified distinct element Voronoi method, which we refer to as “Trigon” logic, is proposed for use in both two and three-dimensional modelling. A rock material is represented by an assembly of triangular (in 2D) or tetrahedral (in 3D) blocks bonded through their contacts. To evaluate the ability of the proposed Trigon logic in simulating brittle fracture, a series of unconfined and confined compression and Brazilian tests is performed illustrating the sample failure mechanisms under two and three dimensional conditions. Typical failure fracture patterns (damage) experienced by brittle rock are explicitly simulated including axial tensile failure under unconfined compression, shear failure under confined compression, and tensile crack extension during indirect tension (Brazilian) testing. A ‘damage parameter’ D and a ‘damage intensity’ D 21 measure are proposed to allow the evaluation of the simulated rock damage in a more quantitative manner. At the mine opening scale, the process of roof shear failure is successfully captured using the Trigon logic. In comparison with the conventional Voronoi model, the proposed Trigon model is less mesh-sensitive and predicts both a more realistic friction angle and failure pattern under varied loading conditions. [Copyright &y& Elsevier]

Highlights: [•] We simulate the cover cracking behavior at meso-scale using Finite-element method. [•] The numerical results have a good agreement with the available experimental data. [•] We study some influencing parameters on the cracking behavior of cover concrete. [•] The heterogeneous model can well simulate the cracking behavior of cover concrete. [ABSTRACT FROM AUTHOR]

• Shale's tensile strength increases as bedding angle, while coal not. • Inverted Trumpet-shaped trend lines are established for shale and coal. • Ultimate loading rate exists for shale and coal is different. • Microstructures is the main cause of strength and failure anisotropy. • Bedding effects weakens as loading rate increases. Shale and coal samples were selected to clarify the tensile failure of layered sorptive rocks under the co-influence of bedding planes and loading conditions. Results indicate that, for these two rocks, tensile strength exhibited anisotropy. As loading rates rises, inverted Trumpet-shaped trend lines is proposed to reflect bedding effects with loading effects. The tensile strength of shale increases linearly with the bedding angle rises, while the value of coal shows great dispersion. The former is greater than the latter. An ultimate loading rate exists for shale and coal. Empirical relations were established to describe tensile strength with varying loading rate and bedding angle. Under the co-influences of bedding plane and loading conditions, more complex failure modes happened for coal and shale samples. Hydraulic fracturing along the direction of 60° (for shale reservoirs) or 45° (for coal reservoirs) to the bedding plane is more likely to produce a complex fracture network. [ABSTRACT FROM AUTHOR]

Abstract: We developed a hydraulic fracturing simulator by coupling a flow simulator to a geomechanics code, namely T+M simulator. Modeling of the vertical fracture development involves continuous updating of the boundary conditions and of the data connectivity, based on the finite element method for geomechanics. The T+M simulator can model the initial fracture development during the hydraulic fracturing operations, after which the domain description changes from single continuum to double or multiple continua in order to rigorously model both flow and geomechanics for fracture–rock matrix systems. The T+H simulator provides two-way coupling between fluid-heat flow and geomechanics, accounting for thermo-poro-mechanics, treats nonlinear permeability and geomechanical moduli explicitly, and dynamically tracks changes in the fracture(s) and in the pore volume. We also fully account for leak-off in all directions during hydraulic fracturing. We first test the T+M simulator, matching numerical solutions with the analytical solutions for poromechanical effects, static fractures, and fracture propagations. Then, from numerical simulation of various cases of the planar fracture propagation, shear failure can limit the vertical fracture propagation of tensile failure, because of leak-off into the reservoirs. Slow injection causes more leak-off, compared with fast injection, when the same amount of fluid is injected. Changes in initial total stress and contributions of shear effective stress to tensile failure can also affect formation of the fractured areas, and the geomechanical responses are still well-posed. [Copyright &y& Elsevier]

Abstract: Transitional failure envelopes of hybrid single-walled carbon nanotubes functionalized by functional groups and filled with butane molecules under combined tension–torsion are predicted using classical molecular dynamics simulations. The observations reveal that while the tensile failure load decreases with combined torsion, the torsional buckling moment increases with combined tension. As a result, the failure envelopes under combined tension–torsion are definitely different from those under pure tension or torsion. In such combined loading, there is a multitude of failure modes (tensile failure and torsional buckling), and the failure therefore exhibits the feature of transitional failure envelopes. In addition, the functionalization by functional groups decreases both tensile failure load and torsional buckling moment, while filling with butane molecules increases only the torsional buckling moment. Consequently, the transitional failure envelopes of functionalized and filled nanotubes are absolutely different relative to what is predicted for pristine nanotubes. [Copyright &y& Elsevier]

Abstract: This paper deals with the problem of rapid ascent mechanism of kimberlite magmas with a multi-directional approach: 1) the kinetics of diamond–graphite transition; 2) settling velocity of diamond phenocrysts in magmas and 3) formation of ruptures required for magma ascent with a high speed. Based on the diamond–graphite transition, we present an estimate of the ascent rates from high pressure and temperature experiments using Walker-type multi-anvil apparatus. The experiments were conducted with diamond, placed within a synthetic kimberlitic assemblage, keeping an initial pressure and temperature of 6GPa and 1350°C, respectively. It was observed that the volume fraction of diamond to graphite conversion strongly depended on the ascent rates. Using electrical resistivity and X-ray diffraction studies, we measured the degree of graphitization as a function of the ascent rate (u). For u <3m/s, diamond underwent almost complete graphitization (conversion>90%), whereas it remained nearly intact (conversion<10%) when u >10m/s. Our theoretical calculations of the settling velocity of mantle xenoliths again confirm that diamond can exist when u >3m/s. We performed numerical experiments with finite element (visco-elastic) models to analyze the dynamics of tensile failure at the tip of magma pools, leading to dilatational vertical fractures for magma transport. Considering the tensile strength of mantle in the order of 0.5kb, our models show this failure process as a function of the critical shape (Ar : ratio of vertical and horizontal dimensions) and density contrast (Δρ) of magma pools. The critical Δρ is estimated nearly 200kg/m3 when Ar is very large (>4). [Copyright &y& Elsevier]

Damage accumulation in compact bovine femur subjected to uniaxial tensile loading was examined by strong light illumination effects of microcracking. Imaging was done using a high-speed camera capturing image at 200 to 1500 FPS. The tensile tests were performed in a multipurpose tensile testing system with cross-head speeds ranging from 0.5 to 10 mm/min which leads to strain rates of 0.0001 to 0.0012 s-1 (physiologically relevant to walking and running Hansen et al., 2008). The post-failure images were then examined in a scanning electron microscopy (SEM) and effects of microstructure, strain rate, and orientation were evaluated. Correlation of the high-speed images with stress-strain curves indicated that optically visible microcracks were most likely initiated at yielding, and the specimens with dispersed microcracks exhibited a higher energy-absorption capacity compared to the specimens with coalesced local cracks. It was found that damage accumulation negatively correlates to strain rate and that transverse specimens exhibited a different failure pattern compared to the longitudinal specimens. Strain hardening and softening were found in the longitudinal and transverse specimens respectively. The microcracking in the transverse specimens instantly increased to peak after yielding compared to the gradual growth until failure in the longitudinal specimens. The average Young's modulus (21.5 GPa) and ultimate stress (93.5 MPa) of the specimens loaded in the longitudinal direction were more than twice that of the specimens (10.9 GPa and 36.2 MPa respectively) loaded in the transverse direction. The current technique has shown potential in relating damage accumulation real time in bone samples subjected to tensile loading condition. This information will be helpful in relating the role of micro damage accumulation in initiating failure and/or remodeling in bone. [ABSTRACT FROM AUTHOR]

Abstract: Data on the strength of the utero–placental interface (UPI) would help improve understanding of the mechanisms of placental abruption (premature separation of the placenta from the uterus) during motor-vehicle crashes involving pregnant occupants. An ovine model was selected for study because like the human, its placenta has a villous attachment structure. Uteri with intact placentas were obtained from three sheep as by-products of another research study. The samples were harvested between 102 and 119 days of the 145-day gestational period. Rectangular specimens with areas measuring 15 mm × 5 mm were cut through the thickness of the placenta and uterus. Each subject provided eight samples, of which four were tested at a nominal strain rate of 0.10 strains/sec and the remainder was tested at a nominal strain rate of 1.0 strains/sec. Sutures were used to secure the uterine side of the specimens to the test fixture, while mechanical clamps were used to attach the placenta side. A FARO arm scanner recorded the initial geometry of the tissue, and a random dot pattern applied to the placenta and uterus tissue allowed visualization of displacement. For the structure of the UPI, mean tensile failure strain and standard deviations are 0.37 (0.11) and 0.37 (0.18) for the 0.10 and 1.0 strain rates, respectively (p-value = 0.970) while the associated failure stresses are 6.5 (1.37) and 15.0 (5.08) kPa, (p-value = 0.064). The results from sheep UPI testing provide the first estimate of the human UPI structural failure tolerance. [Copyright &y& Elsevier]

Abstract: Rubber toughened epoxy/CNT nanocomposites were manufactured at different weight percents between 0 and 1% of multiwall carbon nanotube (MWNT) using a high intensity ultrasonic liquid processor with a titanium probe. Mechanical properties of manufactured dog bone samples were measured in tension and the results indicated a maximum of 23% increase in the elastic modulus at 0.6% by weight of MWNT. However, the fracture strength showed a maximum decrease of about 11% as a function of increasing MWNT loading. Scanning Electron Microscopy (SEM) images from the neat samples revealed a distinct circular pit at the top left edge of the specimen with an overall tearing deformation causing the fracture paths. Comparatively, all nanocomposite samples on an average seemed to show a prominent brittle fracture with little or no evidence of circular pit formation. The amount of tearing deformation seemed to be enhanced in the nanocomposite specimens as compare to the neat ones. Finally, Transmission Electron Microscopy images indicated that different states of dispersion exist in all of the nanocomposite samples. The data showed that agglomeration of nanotubes increases as a function of weight percent. In addition to mechanical property characterization, thermal conductivity of all the samples was determined as a function of temperature between 30°C and 100°C using the 3ω method. The tested samples showed an almost 16% increase in thermal conductivity. The minimal enhancement in thermal conductivity has been analyzed from the standpoint of the Effective Medium Theory. Interfacial thermal resistances exhibit no order of magnitude changes explaining the conductivity results. [Copyright &y& Elsevier]

Abstract: For the potential use of ultrafine grained materials for high performance kinetic energy penetrators of caliber projectile, the shear failure of 4130 steel having two different ultrafine grained (UFG) structures was examined under the tensile mode. 4130 steel with a lamellar UFG structure and with an equiaxed UFG structure was fabricated by equal channel angular pressing with routes A and Bc, respectively. Yield strength of UFG steel was enhanced almost twice compared to that of coarse grained counterpart while the former exhibited drastic ductility loss due to a lack of strain hardenability. Tensile failure of the steel having a lamellar UFG structure is manifested in sequence by formation of few sharp shear bands inclined by 45° to the stress axis, propagation of only one shear band of them, broad conjugate shear band formation and propagation, and failure in the shear mode along the first-formed shear band. In addition, large cracks parallel to the tensile stress were simultaneously developed due to its lamellar nature. The steel with an equiaxed UFG structure undergoing tensile rupture is characterized by a series of sequential processes of formation of one broad shear band inclined by 45° to the stress axis, its propagation, another broad conjugate shear band formation and propagation, and failure in the opening mode with the zigzag pattern. These characteristics were discussed with the aid of the models suggested for enhanced shear formability of nanocrystalline and/or UFG materials. [Copyright &y& Elsevier]

Abstract: A 1-m-thick pillar was subject to coupled excavation- and thermal-induced stresses to induce brittle rock mass yielding. The yielding strength of the heterogeneous and fractured rock mass consisting of Äspö diorite was evaluated at eighteen discrete locations using data from the displacement, acoustic emission, and thermal monitoring systems. The average rock mass yielding strength was determined to be 0.59 of the uniaxial compressive strength. The onset of dilation in uniaxial laboratory tests, determined from strain gauge data, was found to occur at approximately 0.45 of the uniaxial compressive strength. It was shown that that the onset of acoustic emission events in situ also occurred when the tangential stress exceeded 0.43 of the uniaxial compressive strength. For sites with absence of in situ data it is recommended that this lower-bound value determined from laboratory data may be used for assessing the in situ rock mass yielding strength. Visual observation and displacement monitoring showed that extent of rock mass yielding is sensitive to small changes in the tangential stress magnitudes. It was determined using three-dimensional modelling that changes in the tangential stress magnitude of approximately 1MPa was sufficient to cause yielding of the pillar to propagate in what appeared to be intact rock. Observations suggest that without this small stress change yielding of the rock mass would not occur. In other words, there appeared to be a well defined boundary, and if the stresses reached this boundary yielding was observed. However, if stresses were only slightly below this boundary yielding or time-dependant processes were not observed over the monitoring period used in the experiment. [Copyright &y& Elsevier]

Abstract: It is evident from a series of tests on simply supported reinforced concrete slabs that the failure of the slabs at large deflections is due to the formation of individual large cracks. This failure mode was also observed in the Cardington full-scale fire tests. Previous research indicates that the global behaviour of concrete slabs subject to large deflections can be well predicted by the smeared cracking model; however, the model cannot quantitatively predict the openings of individual cracks within the slabs at large deflections. For the discrete approach it is usually assumed that the cracks are formed along element edges, therefore continuous re-meshing is required during the analysis. Consequently, the results are mesh-dependent and the computing cost is high. In recent years, mesh independent finite element procedures, such as embedded (EFEM) and extended (XFEM) approaches, were widely used for modelling of the crack initiation and growth in structural members. However, most of the meshless models developed are either based on in-plane loading conditions or confined to thin shells with assumed full-depth cracks, which form apparent displacement jumps within an element. For a reinforced concrete slab, an out-of-plane load causes coupled stretching and bending of the slab, cracks are usually initiated at discrete positions and then propagated, until at last some individual full-depth cracks are formed. Pure stretching or assumed full-depth cracking is inadequate for modelling this kind of failure. Therefore, in this research, a non-linear layered procedure with embedded weak discontinuity is developed to quantitatively model the progressive tensile failure of reinforced concrete slabs subjected to large deflections. The current model inherits the advantage of the smeared approach, and at the same time, introduces the opening width of crack explicitly by taking the advantage of the better description of the kinematic characteristics of the EFEM approach. A series of validations have been conducted against test data at both ambient and elevated temperatures, and the research shows that the model developed in this paper is not sensitive to the FE mesh size and the aspect ratio of the slab. The results predicted by the model developed agreed well with the test data in terms of deflection and crack open width, also agreeing well with those modelled by the smeared model. Hence, this new approach provides a numerical method to predict the load capacity as well as identifying the occurrence and severity of crack failure in reinforced concrete slabs subjected to extreme loading conditions, such as fire. [Copyright &y& Elsevier]