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The Mohr-Coulomb elastoplastic model is extensively employed in geotechnical engineering. Despite the well-known nonlinear behaviour of granular cohesionless materials under a wide range of normal stress or confining pressure, the Mohr-Coulomb elastoplastic model continues to be largely used in geotechnical engineering, due to its simplicity and large availability in numerous commercialised software. Its application requires the input of a key parameter, named Poisson's ratio. It is however a big challenge as its value changes with axial strain and confining pressure. In this study, the optimal Poisson's ratio of granular materials for the Mohr-Coulomb elastoplastic model is determined by reproducing the experimental results of stress-strain relationships through numerical modelling with the Mohr-Coulomb elastoplastic model. The results show that the Poisson's ratio μ0.02, corresponding to the slope of the volumetric strain against axial strain curve at 2% of the peak deviatoric stress can be used in numerical modelling with the Mohr-Coulomb elastoplastic model as long as the granular material does not exhibit dilation behaviour. When the material exhibits dilation behaviour, the Poisson's ratio μ0.36, corresponding to the slope of the volumetric strain against axial strain curve at 36% of the peak deviatoric stress, seems to be appropriate in numerical modelling with the Mohr-Coulomb elastoplastic model. In addition, the study also shows that the Mohr-Coulomb elastoplastic model can be used to simulate mechanical behaviour of granular material under small strain, but not appropriate under large strain conditions. [ABSTRACT FROM AUTHOR]

The backfill in the stope usually forms a composite structure with the surrounding rock in order to bear pressure together to support the goaf and ensure the safe mining of subsequent ores. Based on laboratory tests and theoretical analysis, the energy and damage evolution of the rock–backfill composite materials (RBCM) are studied deeply. The results show that: (1) The σp (peak stress), εp (peak strain), and E (elasticity modulus) decreased with the increase of the internal backfill diameter. When the diameter of the backfill increases from 10 mm to 40 mm, σp decreases from 50.15 MPa to 18.14 MPa, εp decreases from 1.246% to 1.017%, and E decreases from 7.51 GPa to 2.33 GPa. The UT shows an S-shaped distribution, the UE shows an inverted U-shaped distribution, and the UD first increases slowly and then increases rapidly. The UTp, UEp, UDp, UEp/UDp, and UEp/UTp decrease by 67.38%, 97.20%, 58.56%, 32.64% and 13.64% respectively, and the UDp/UTP increases by 20.93% with the increases of the backfill diameter. (2) A damage constitutive model of the RBCM is established based on the energy consumption characteristics. The damage evolution curve shows an S-shaped distribution, and the damage rate evolution curve shows an inverted U-shaped distribution. (3) The AE correlation fractal dimension decreases with the increase of the strain gradient and damage value, and the AE correlation fractal dimension presents linear and exponential functions with them, respectively. With the increase of stress, microcracks first appear and gather in the internal backfill of the RBCM, and then microcracks appear and gather in the peripheral rock, which together lead to the macro penetration failure of the RBCM. [ABSTRACT FROM AUTHOR]

The method of backfill in underground mining is important for ground control as well as material recycling and energy efficiency. Even though extensive testing and field studies of backfill have been conducted, less is known about the detailed damage and fracturing that occurs directly at the rock/backfill interface. In this paper, cylindrical specimens containing an inner diameter of backfill and an outer diameter of rock (RB) were tested under triaxial compression. Acoustic emissions (AE) were used throughout testing, and X-ray computed tomography (CT) scanning was conducted before loading was applied and after the specimens had failed. The high-resolution CT images were then converted into point clouds to isolate the fractures and visualize them in three dimensions. The point clouds clearly show that fracturing occurred both in the rock and along with the contact between rock and backfill, while very little fracturing was found to occur in the backfill. Based on the point cloud and AE results, a unique evolution of fracturing is found to occur that includes two stages of shear fracturing in the rock, tensile fracturing along with the rock/backfill interface, and final tensile fracturing in the rock after delamination from the backfill, all of which contributed to the nonlinear stress–strain response. This paper presents a novel approach for investigating the initiation and propagation of 3D fractures in laboratory testing and can offer a useful reference for further studies on the mechanics of bi-material structures. [ABSTRACT FROM AUTHOR]

A viscoplastic self-consistent model is implemented to study the plane strain deformation behavior and texture evolution of pure magnesium single crystals and polycrystals. The numerical simulations of single crystals are carried out based on seven channel-die compression tests. Due to the different deformation mechanisms of tensile twinning and compressive twinning, the hardenings caused by the twins are distinguished to better describe their effects on the whole deformation. The uniform parameters are calibrated with the channel-die experimental data in single crystal cases and then used to predict the polycrystalline deformation. The polycrystalline aggregates with 100 and 300 random orientations are developed for their plastic deformation. Five various self-consistent schemes are employed to investigate the influence of their linear hypotheses on stress–strain responses. A trend of hardening rate from weak to intense is shown for the five self-consistent schemes: Tangent, n eff = 10, Affine, Secant and Taylor. The same trend is also obtained in their corresponding texture evolutions and yield surfaces. At low plastic strains, a high asymmetry and a strong anisotropy between compressive and tensile yield strengths are exhibited by the yield surfaces. [ABSTRACT FROM AUTHOR]

A rate-dependent elastic–viscoplastic constitutive model is proposed to simulate the plane strain deformation of pure magnesium crystal. The observed plastic deformation mechanisms of slip, twinning and their interaction are accounted for. The basal, prismatic and pyramidal slip systems in the parent grain, compressive twinning (CT) and tensile twinning (TT) are incorporated in the model. Due to the differences of generation mechanisms of twins and their effects on the deformation, the constitutive descriptions of CT and TT are distinguished to better characterize their effects on the overall hardening of magnesium single crystals. For investigating the generation of different slips and twins, the plane strain numerical schemes of seven different orientations for loading and fixed boundary are considered and their various mechanisms and characteristics of plastic deformation are discussed. The contributions of different deformation modes to the macroscopic plastic deformation of magnesium single crystals in the seven cases are presented. These computational predictions are carefully compared with their corresponding macroscopic experimental observations (stress-strain responses) and other numerical results. These results prove that it is necessary to distinguish different twinning systems and their associated hardening laws for the plastic deformation of magnesium and its alloy. [ABSTRACT FROM AUTHOR]

The mechanical responses of hexagonal close-packed (HCP) materials are highly anisotropic, as well as strain rate and temperature-dependent, because of inherently asymmetric slip and twinning in constituent crystalline grains. How slip and twinning are affected by strain rate/temperature and how they alter the deformation response of HCP materials are still not well understood. To provide greater insights, a novel crystal plasticity model is proposed to describe the mechanical behavior and microstructure evolution of single-crystal and polycrystalline HCP materials under different loading conditions. A new flow rule is developed, which incorporates the effects of strain rate and temperature on the activation of specific deformation mechanisms. Both dislocation density-based and empirical hardening laws are adopted to describe the overall hardening contributed by slip, twinning, and their interaction, taking into consideration loading conditions and the difference between tension and compression twinning. The proposed model is implemented in the commercial FE package ABAQUS via a user-defined subroutine. The mechanical response and microstructure evolution of single-crystal titanium with different orientations, and polycrystalline titanium comprising grains with 350 random orientations, subjected to deformation at different strain rates and temperatures, are numerically explored. The numerical results display good agreement with published experimental data. The simulations indicate that an increase in strain rate or decrease in temperature will retard the activation of slip systems, thereby promoting twinning instead. A higher twinning volume fraction leads to a larger amount of lattice reorientation, more pronounced hardening, and higher stress levels. This effort has yielded a greater understanding of the influence of strain rate and temperature on the mechanical response of HCP materials and facilitated mesoscale simulations that couple slip and twinning. • A novel flow rule considering the effect of strain-rate and temperature is proposed. • Tensile twinning and compressive twinning are distinguished in calculations. • Different hardening laws are used to take strain rate and temperature into account. • The macroscale and mesoscale deformation behaviors of α -Ti are investigated. [ABSTRACT FROM AUTHOR]

Abstract: The dynamic properties of TiC particulate-reinforced titanium matrix composites (TiCp/Ti) prepared by pre-treatment melt process at elevated temperatures were characterized and tested by using split Hopkinson tensile bar (SHTB). The tensile properties of these materials were investigated to determine all the parameters of the Johnson–Cook constitutive mode. By considering the strain hardening and the coupled effects of temperature and strain rate, a modified Johnson–Cook constitutive model was proposed to predict the dynamic behavior of TiCp/Ti. The model predictions are in good agreement with the experimental results, which indicates that the modified model can be used to describe the flow stress for the studied composite at elevated temperatures. [Copyright &y& Elsevier]

The shear performance of the rock-CPB interface is the basis for in-depth understanding of the arch effect of CPB. In this study, the rock-CPB sample was prepared for direct shear test to study the influence of different mix proportion, curing time and JRC on rock-CPB interface properties, and the microstructure was analyzed. The results show that the presence of saw-tooth structure accelerates the increase rate of shear stress. Due to the large strength difference between the rock structure and the CPB structure, the failure mainly occurs at the interface between the two structures and in the CPB structure, and no obvious deformation and failure phenomenon is observed in the rock structure. The shear failure envelope of the rock-CPB composite samples follows the Mohr-Coulomb failure criterion. The adhesion of the rock-CPB interface increases with the increase of solid content, c/t ratio, curing time and JRC , and the friction angle of the rock-CPB interface increases with the increase of solid content, c/t ratio and curing time. 1H NMR test results showed that with the increase of solid content, c/t ratio and curing time of CPB, the proportion of macropores and secondary pores in CPB decreased, while the proportion of micropores pores increased. There is obviously excessive porosity near the rock-CPB interface, and the porosity decreases gradually with the increase of the distance from the interface. • The shear mechanical properties of rock-CPB interface assessed. • Mix proportion, curing age and JRC dependent evolution of the shear properties studied. • Shear failure of rock-CPB mainly occurs at the interface and CPB structure. • A typical excess of porosity of CPB is found near rock-CPB interface. [ABSTRACT FROM AUTHOR]

• A novel neural network is proposed to predict mechanical response of lattice-based metamaterial. • A simple method based on GCN for graph-level feature extraction was proposed. • The nonlinear stress–strain relationship of lattice metamaterials is precisely predicted. • The proposed network outperforms traditional Artificial Neural Networks. Predicting the stress–strain curve of lattice-based metamaterials is crucial for their design and application. However, the complex nonlinear relationship between the mesoscopic structure of lattice materials and their macroscopic mechanical behavior makes prediction challenging. In this study, beam element models of over 20,000 lattice structures were established using Python scripts, and calculations were performed by ABAQUS to obtain training and testing datasets. The spatial features of each lattice-based metamaterial were then encoded into a graph, a data structure recognizable by machine learning algorithm. Utilizing machine learning methods, a Structure to Sequence Neural Network was constructed and trained, achieving rapid prediction of the compressive stress–strain curves for lattice-based metamaterials. Afterwards, several lattice structures were randomly selected and 3D printed. The accuracy of the simulation results as well as machine learning predictions was validated through quasi-static compression experiments. It is revealed that the proposed Neural Network model outperforms the traditional Artificial Neural Networks as the errors are reduced while the Coefficient of Determination is higher. The results demonstrate the accurate fitting between the complex spatial features of the lattice-based metamaterials and their stress–strain curves, which provides a potential methodology for inverse optimization of the lattice-based metamaterials in the future. [ABSTRACT FROM AUTHOR]

• Phase gradient architectures can optimize the atomic structure topology of CuZr-based metallic glasses to increase the strength even further. • Phase gradient invalidates the autocatalytic activation mechanism of shear transformation zones. • Phase gradient encourages branches of shear band and nucleation of multiple shear bands, which mechanism improves the ductility of metallic glasses. • The mechanical synergy of the phase gradient metallic glasses can be further tuned. Increasing strength is usually at the cost of sacrificing ductility in structural materials. The tradeoff becomes even conspicuous in a category of metallic amorphous materials, the so-called metallic glasses (MGs) featured without any atomic-scale translational symmetry. Therefore, there remains little room for simultaneous optimization of strength and ductility in MGs through tailoring the morphology and kinetics of structural imperfections. Here we propose an alternative strategy for modulating the mechanical properties of MGs through introducing proper content of compositional or phase gradient inspired by the mechanistic strain gradient theory. We design two types of CuZr-based phase gradient metallic glasses (PGMGs) with different compositional concentration gradient directions in either continuous or stepped gradient form. Extensive molecular dynamics simulations demonstrate that phase gradient raises the concentration of mechanically stable icosahedral and icosahedron-like Voronoi polyhedra and, thus, increases the strength of MGs. In terms of plastic deformation, free volume mismatch between phases invalidates the autocatalytic activation mechanism of shear transformation zones, resulting in greater resistance to shear band propagation. The phase gradient also encourages branches of shear band and nucleation of multiple shear bands, which mechanism delocalizes deformation and postpones failure. These mechanisms lead to an improvement in the overall ductility of the PGMGs. The present strategy sheds light on evading the long-standing strength-ductility tradeoff in amorphous metals through extrinsic chemical and geometrical modulation that can be handled by appropriate thermal processing and fabrication technique. [Display omitted] [ABSTRACT FROM AUTHOR]

• Triaxial loadings have been conducted in the rock-backfill interface behavior. • Fracturing is monitored by traditional methods and modern CT scan technology. • Change of non-linear stress-strain behavior and fracturing has been presented. In the mining industry, backfill is used to stabilize mined-out underground excavations, and backfill made from mine tailings has the added advantage of being sustainable and eliminating the hazards of storing tailings on the surface. Triaxial compressive tests were conducted on laboratory specimens containing both rock and backfill, in order to better understand the effects of rock/backfill volume fraction and confining pressure on the strength and fracturing in rock-backfill systems. The importance of the interface between rock and backfill was also studied by conducting before and after CT scanning. The results are very interesting and show that shear and tensile fracturing in the rock as loading progresses occurs concurrently with slip and failure of the rock-backfill interface, resulting in unique stress-strain behavior, crack dilatancy, and failure types. The backfill shows very little fracturing but has a major effect on the effective strength and elastic properties of the combined specimens. The results provide important details on the failure process of rock-backfill systems and provide guidelines for the safe design of underground excavations in rock. [ABSTRACT FROM AUTHOR]