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Mechanical performance of coral reef limestone (CRL) is becoming an important topic in recent years due to the boom in offshore construction. However, as there is a lack of rock samples in specific regions, very limited progress has been made in terms of the compressive behaviour (both static and dynamic) of the CRL. Specially, proper descriptions and accurate quantifications of the strain-rate effect upon the compressive performance of the CRL remain to be performed, which will be the main emphasis of this paper. Four types of CRL, namely, coral framework (CF), coral boulder (CB), coral gravel (CG), and coral calcarenite (CC) limestone, were obtained from a human-made coral reef island in South China Sea. Static uniaxial and dynamic split Hopkinson pressure bar (SHPB) compression tests are conducted to obtain the strengths at very low and medium to high compressive strain rates, respectively. The strain-rate effect of the compressive strength is well described with the Herschel-Bulkley model, parameterising the static strength, the viscosity coefficient, the ‘shear-thinning’ index, and the reference strain rate. The overall quality of the CRL rock mass is then classified by the basic quality method, which is based on the uniaxial compressive strength and joint distributions of the rock mass. Variations of the Young’s modulus of the CRL rock mass under different strain rates are also discussed. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of brittle rock failure by the quaternion-based bonded-particle model in discrete element method (DEM). Unlike traditional approaches that utilize Euler angles or rotation matrices, this model employs unit quaternions to represent the spatial rotations of particles. This method simplifies the representation of 3D rotations, providing a more intuitive framework for modelling complex interactions in granular materials. The numerical model was validated by the uniaxial compression tests on rock, with good agreement with well-documented experimental data in terms of the rock uniaxial compression strength (UCS) and failure mode. During loading, the rock sample demonstrated a linear-elastic response at an axial strain of smaller than 0.45%. However, as internal bond breakage accumulated, this linear relationship weakened, and the stress-strain curve began to deviate from its initial linear trajectory. The bond breakage and the overall deformation of the rock were primarily controlled by the shear bonding force. The UCS was achieved at an axial strain of 0.625%, at which point the internal shear bonding force chains were predominantly aligned vertically. The brittle failure occurred when the internal damage of solids nucleated to form an interconnected failure plane, accompanied by a sharp rise in the internal damage ratio. The area of failure plane increased with the loading strain rate, gradually transforming the failure pattern from the local damage to a complete fragmentation. [ABSTRACT FROM AUTHOR]

This work proposes a unified damage model for concrete within the framework of stochastic damage mechanics. Based on the micro-meso stochastic fracture model (MMSF), the nonlinear energy dissipation process of the microspring from nanoscale to microscale is investigated. In nanoscale, the rate process theory is adopted to describe the crack growth rate; therefore, the corresponding energy dissipation caused by a representative crack propagation can be obtained. The scale gap from nanoscale to microscale is bridged by a crack hierarchy model. Thus, the total energy dissipated by all cracks from the nanoscale to the microscale is gained. It is found that the fracture strain of the microspring can be derived from the above multi-scale energy dissipation analysis. When energy dissipation is regarded as some microdamage to the microspring, the constitutive law of the microspring is no longer linearly elastic, as previously assumed. By changing the expression of the damage evolution law from fracture strain to energy dissipation threshold, the new damage evolution model is derived. The proposed model can not only replicate the original static model but also extend to cases of rate dependence. By deriving the fracture strain under different strain rates, the rate sensitivity of concrete materials can be reflected. The model parameters can be conveniently obtained by identifying them with experimental data. Finally, several numerical examples are presented to verify the proposed model. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes a rate-dependent cohesive zone model for simulating the evolution and growth of cracks under dynamic loading. This model takes into consideration the effects of cohesive separation rate and crack growth speed on the cohesive traction-separation relationship to reflect the rate-dependent characteristics of both material viscosity and energy dissipation of micro-crack branches. The evolution algorithm of the cohesive law is presented, wherein separation rate and crack growth speed are evaluated using the backward difference method explicitly in incremental computation, leading to the determination of rate-dependent critical traction and fracture toughness. The proposed model was implemented in ABAQUS software via UEL subroutine and utilized in an example of interfacial crack growth simulation, with satisfactory agreement found among the present results, experimental data, and numerical results from the literature. The effectiveness of the rate-dependent model and numerical algorithm in simulating high speed crack growth is validated through further numerical tests. [ABSTRACT FROM AUTHOR]

An experimental study is conducted to describe rate-dependent shear strength in a submerged granular medium to understand the mystery of submarine landslides with extremely small slide angles and long run-out distances. The experimental apparatus allows a long-span shear strain rate, γ̇, for five orders of magnitude from 10−4 to 101 s−1. It is observed that (a) submerged sand under higher shear tend to have bigger yield strength; this positive response of rate effect is significantly affected by the magnitudes of shear strain rates. (b) the residual strength of soil is clearly affected negatively by shear strain rate, decreasing as shear strain rate increases; even small variations under lower rate cause notable differences in residual strength, indicating a novel weaking rate-dependent. The yield strength and residual strength are corresponding to the shear state of soil. Hence, it is enough experimentally to explain that as long as the submarine mass flow speeds up, the slope sliding can be kept by only a small amount of force along the slide direction, which can be calculated as the gravity component even with a small slide angle. [ABSTRACT FROM AUTHOR]

Monotonic and pulsating cyclic compression tests were performed systematically based on high density polyethylene (HDPE). The compressive creep behaviors were further implemented to compare with the ratcheting deformation under the same peak stress level. Monotonic compression experiments reveal that HDPE shows obvious four-stage characteristics which are sensitive to temperature, but independent of the stress rate when the temperature is greater than 80 °C. Interestingly, the strain range during the stage III always starts at 7.4% and ends at 62.6% under various temperatures and stress rates. Moreover, the compressive ratcheting behavior of HDPE significantly depends on temperature T, peak stress σp, stress rate σ˙, stress amplitude Δσ and stress ratio R, especially when T ≥ 80 °C, σp ≥15 MPa, σ˙ ≤0.1 MPa/s and R ≥ 0.8. Additionally, decreasing the stress rate or increasing the temperature will decrease the compressive modulus of HDPE. An approximate linear relationship between the creep strain rate and the ratcheting strain rate under different temperatures and stress ratios was observed. The compressive deformation characteristics of HDPE were compared with that of some other gasket materials, a universal phenomenon is found that the compressive ratcheting evolution with time can be characterized by creep approximately under the same loading peak stress, stress rate and temperature for various gasket materials when the stress ratio is greater than zero, which indicates that the ratcheting deformation for gaskets under cyclic loads may be estimated by the corresponding static creep strain in practical engineering.

BACKGROUND: Collagen type II, silk fibroin and hyaluronic acid, which can provide an ideal microenvironment for cells, have become ideal scaffold materials for cartilage repair. OBJECTIVE: To evaluate physicochemical properties and biomechanical properties of cartilage tissue engineering scaffolds prepared from collagen type II, silk fibroin and hyaluronic acid. METHODS: Collagen type II-silk fibroin-hyaluronic acid composite scaffolds were prepared by low-temperature 3D printing technology. The microstructure, porosity and water absorption of scaffold were detected. Different strain rates were used to compress the scaffold, and the rate dependent properties of the scaffold were investigated. Constant stress level or constant strain was applied on the surface of scaffold for 3 600 s, and the creep behavior and stress relaxation behavior were investigated. RESULTS AND CONCLUSION: (1) The prepared scaffolds had three-dimensional porous structure, uniform pore size and good connectivity. The porosity was (85.1±1.6)%. The water absorption expansion rate was (1 071.7±131.6)%. (2) Under different strain rates, the compressive stress-strain curves of cartilage scaffolds did not coincide, which indicated that the compressive mechanical properties of cartilage scaffolds were rate dependent. With the increase of strain rate, the Young’s modulus increased. (3) When the stress level was constant, the creep strain first increased rapidly and then increased slowly. When the stress level increased, the creep strain also increased. (4) When the compression strain was constant, the stress of scaffold decreased rapidly at first and then slowly with the relaxation time. With the increase of the compression strain, the stress at different times increased. (5) The mechanical properties of the collagen type II-silk fibroin-hyaluronic acid composite scaffolds are similar to those of host cartilage, which means that the prepared scaffold is a kind of nonlinear viscoelastic material. [ABSTRACT FROM AUTHOR]

With advancements in space exploration, operations in low-gravity environments such as the lunar surface are expected to be conducted. Geotechnical engineering problems such as those associated with the foundation bearing capacity considerably influence the feasibility and safety of these operations. In this study, the findings of experiments conducted by Japan Aerospace Exploration Agency (JAXA) scientists on parabolic flight, aimed at measuring the ultimate bearing capacity of shallow foundations under 1/6 g to 2 g of gravity, are analysed. Specifically, the results are analysed to calculate the ultimate friction angle based on the classical Terzaghi limiting equilibrium solution in soil mechanics. The friction angle of foundation sand increases as the gravity level decreases. This finding is verified through advanced arbitrary Lagrangian–Eulerian (ALE) finite element simulations based on a simple Mohr–Coulomb model. Moreover, the underlying mechanism for this phenomenon is examined considering an ALE finite element simulation based on a newly developed rheological model known as the Tsinghua–MiDi sand model. The pressure-sensitive and rate-dependency constitutive behaviour of sand is clarified. Notably, this phenomenon increases the viscous shear stress and ultimate friction angles in low-gravity conditions. The coupled effects of the loading rate and low-gravity level on the bearing capacity of foundation sand are predicted. The findings can provide a novel theoretical prospective for geotechnical studies in space exploration engineering in low-gravity conditions. [ABSTRACT FROM AUTHOR]

Nitrogen (N) and phosphorus (P) are the dominant limiting nutrients in alpine meadows, but it is relatively unclear how they affect the soil microbial community and whether their effects are rate dependent. Here, N and P addition rates (0, 10, 20, and 30 g m–2 year–1) were evaluated in an alpine meadow and variables related to plants and soils were measured to determine the processes affecting soil microbial community and enzyme activities. Our results showed that soil microbial biomass, including bacteria, fungi, gramme-negative bacteria, and actinomycetes, decreased along with N addition rates, but they first decreased at low P addition rates (10 g m–2 year–1) and then significantly increased at high P addition rates (30 g m–2 year–1). Both the N and P addition stimulated soil invertase activity, while urease and phosphatase activities were inhibited at low N addition rate and then increased at high N addition rate. P addition generally inhibited peroxidase and urease activities, but increased phosphatase activity. N addition decreased soil pH and, thus, inhibited soil microbial microorganisms, while P addition effects were unimodal with addition rates, achieved through altering sedge, and available P in the soil. In conclusion, our studies indicated that soil microbial communities and enzyme activities are sensitive to short-term N and P addition and are also significantly influenced by their addition rates. [ABSTRACT FROM AUTHOR]

Nitrogen (N) and phosphorus (P) are the dominant limiting nutrients in alpine meadows, but it is relatively unclear how they affect the soil microbial community and whether their effects are rate dependent. Here, N and P addition rates (0, 10, 20, and 30 g m–2 year–1) were evaluated in an alpine meadow and variables related to plants and soils were measured to determine the processes affecting soil microbial community and enzyme activities. Our results showed that soil microbial biomass, including bacteria, fungi, gramme-negative bacteria, and actinomycetes, decreased along with N addition rates, but they first decreased at low P addition rates (10 g m–2 year–1) and then significantly increased at high P addition rates (30 g m–2 year–1). Both the N and P addition stimulated soil invertase activity, while urease and phosphatase activities were inhibited at low N addition rate and then increased at high N addition rate. P addition generally inhibited peroxidase and urease activities, but increased phosphatase activity. N addition decreased soil pH and, thus, inhibited soil microbial microorganisms, while P addition effects were unimodal with addition rates, achieved through altering sedge, and available P in the soil. In conclusion, our studies indicated that soil microbial communities and enzyme activities are sensitive to short-term N and P addition and are also significantly influenced by their addition rates.

In this paper, effects of strength attenuation on stress development tendency (strain hardening or softening) under different strain rate were analysed. A series of triaxial creep and constant strain rate compression tests were conducted on frozen sand samples. Based on test results analysis, it was indicated that strength attenuation function is usually underestimated with the creep test results. After correcting the function with constant strain rate test results, it was verified that the calculation accuracy of a rate-dependent model with strength attenuation proposed by the authors is improved greatly. Further theoretical analysis indicates that the development tendency of stress (strain hardening or softening) under different constant strain rates is dependent upon relative position between stress point and failure line in p−q space, and strain softening degree is dependent on applied strain rate and the law of strength attenuation. [ABSTRACT FROM AUTHOR]

Ice gouging is a phenomenon that occurs when ice drifts to shallow depths due to current, wind and tides, thus resulting in a large deformation. Many previous studies have conducted numerical analysis based on centrifuge model experiment to understand the mechanism of ice gouging. Strain softening and rate dependence are the two important factors that influence ice gouging. However, the effect of these two important factors on the seabed subjected to large deformation was not considered in previous studies. Therefore, in this study, these factors are simulated, and the occurrence of large deformation in soil due to ice gouging is analyzed. In addition, numerical modeling based on two cases from among knowledge gained via various centrifuge model experiments is implemented. To simulate the large deformation of the soil, return mapping algorithm is applied to the ABAQUS subroutine VUMAT in conjunction with the formulas that define strain softening and rate dependence. These factors are reflected in ice gouging simulations and the consistency is reviewed by comparing and analyzing the results of the centrifuge model experiments implemented in previous studies. Based on this, the kinetic energy of ice keel is defined and a subroutine expressing kinetic energy reduction according to seabed soil resistance is additionally reflected. In addition, this method is applied to seabed slope in cold region to analyze the behavior of the slope. • Numerical analysis on centrifuge model experiment helps to understand ice gouging. • Strain softening and rate dependence are simulated to analyze soil deformation. • The dissipation of kinetic energy of an iceberg during ice gouging is analyzed. • Proposed method is applied to cold region seabed slope to analyze its behavior. [ABSTRACT FROM AUTHOR]

Summary: This paper presents a constitutive model for time‐dependent behaviour of granular material. The model consists of 2 parts representing the inviscid and viscous behaviour of granular materials. The inviscid part is a rate‐independent hypoplastic constitutive model. The viscous part is represented by a rheological model, which contains a high‐order term denoting the strain acceleration. The proposed model is validated by simulating some element tests on granular soils. Our model is able to model not only the non‐isotach behaviour but also the 3 creep stages, namely, primary, secondary, and tertiary creep, in a unified way. [ABSTRACT FROM AUTHOR]

Granular materials can behave as harmless sand dunes or as devastating landslides. A granular avalanche marks the transition between these distinct solid-like and fluid-like states. The solid-like state is typically described using plasticity models from critical state theory. In the fluid regime, granular flow is commonly captured using a visco-plastic model. However, due to our limited understanding of the mechanism governing the solid-fluid-like transition, characterizing the material behavior throughout the life cycle of an avalanche remains an open challenge. Here, we employ laboratory experiments of transient avalanches spontaneously generated by a rotating drum. We report measurements of dilatancy and grain kinematics before, during, and after each avalanche. Those measurements are directly incorporated into a rate-dependent plasticity model that quantitatively predicts the granular flow measured in experiments. Furthermore, we find that dilatancy in the solid-like state controls the triggering of granular avalanches and therefore plays a key role in the solid-fluid-like transition. With the proposed approach, we demonstrate that the life cycle of a laboratory avalanche, from triggering to run out, can be fully explained. Our results represent an important step toward a unified understanding of the physical phenomena associated with transitional behavior in granular media. [ABSTRACT FROM AUTHOR]

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Series of terrorist attacks in Europe recently using vehicles lead to significant demands to improve the impact resistance of current anti-ram bollards by replacing conventional materials with composite materials. Composite bollards comprised of an outer steel hollow tube, an inner aluminium alloy hollow tube and a crushable foam core, are studied against conventional steel ones. The current design of bollard barrier systems is based on the crash-test certification, which often leads to an overly conservative design with excessively stiff bollards. In addition, the foundation of the bollard system is designed against unnecessarily high reaction forces. In this work, the performance of fixed bollard systems is optimised using the numerical approach which incorporates the rate-dependent effect of the hybrid metal/Divinycell H250 PVC foam cores. The bollard deflection is used as the indicator to quantify the relative impact resistance of the system along with the recorded peak reaction forces. The results indicate that an improvement, in the form of increased resistance and reduced reaction forces requirement, of the typical hollow tube system can be achieved by incorporating other energy dissipating materials in the bollard structure. This study highlights the importance of an optimised composite bollard by reducing the reaction force transfer requirement. [ABSTRACT FROM AUTHOR]

Monotonic and pulsating cyclic compression tests were performed systematically based on high density polyethylene (HDPE). The compressive creep behaviors were further implemented to compare with the ratcheting deformation under the same peak stress level. Monotonic compression experiments reveal that HDPE shows obvious four-stage characteristics which are sensitive to temperature, but independent of the stress rate when the temperature is greater than 80 °C. Interestingly, the strain range during the stage III always starts at 7.4% and ends at 62.6% under various temperatures and stress rates. Moreover, the compressive ratcheting behavior of HDPE significantly depends on temperature T , peak stress σ p , stress rate σ ˙ , stress amplitude Δ σ and stress ratio R , especially when T ≥ 80 °C, σ p ≥15 MPa, σ ˙ ≤0.1 MPa/s and R ≥ 0.8. Additionally, decreasing the stress rate or increasing the temperature will decrease the compressive modulus of HDPE. An approximate linear relationship between the creep strain rate and the ratcheting strain rate under different temperatures and stress ratios was observed. The compressive deformation characteristics of HDPE were compared with that of some other gasket materials, a universal phenomenon is found that the compressive ratcheting evolution with time can be characterized by creep approximately under the same loading peak stress, stress rate and temperature for various gasket materials when the stress ratio is greater than zero, which indicates that the ratcheting deformation for gaskets under cyclic loads may be estimated by the corresponding static creep strain in practical engineering. [ABSTRACT FROM AUTHOR]

To obtain reliable stress and strain fields using finite-element simulations, the applied constitutive equations, in general formulated in terms of evolution equations for the stresses and a set of state variables, that should be able to describe hardening properties as well as micro structural aspects with sufficient accuracy. The viscoplastic response of materials requires the time integration of system of non-linear first order differential equations. In viscoplasiticy, the constitutive equation might become very complicated to evaluate depending upon their degree of non-linearity. The constitutive model includes a yield criterion and multi-component formulations of non-linear isotropic and kinematic hardening. Both the types of the loading and the complexity of the constitutive laws ask a stable and efficient algorithm. The elasto-viscoplastic constitutive equations of the Chaboche model have been developed and modified many times. In this study Chaboche viscoplastic constitutive model is presented in strain-space. This model is further implemented in the framework of finite element. The performance of the new algorithm is compared to that is presented in existing literature results. [ABSTRACT FROM AUTHOR]