Dynamic Splitting Research Articles

Dynamic Behavior and Constitutive Model for Two Tantalum-Tungsten Alloys under Elevated Strain Rates

The quasi-static and dynamic deformation behavior of two Ta-W alloys (Ta-2.5W and Ta-10W, mass fraction) was investigated by the quasi-static and dynamic compression test and Split Hopkinson pressure bar (SHPB) and Taylor impact tests. The results show that the yield stress of Ta-W alloys exhibits sensitivity to strain rate and W content. Based on the quasi-static and high strain-rate experimental data, the material constants in Johnson-Cook (JC) model were obtained for the two Ta-W alloys. In addition, validation of the derived constitutive model was carried out through comparison of Taylor impact inhomogeneous deformations under high strain rate (103∼104 s−1), obtained from simulations with their experimental counterparts. It is shown that the simulation results agree well with post-test geometries in terms of side profiles and impact-interface footprints for Taylor impact tests. To bridge the different spatial scale involved in the process of tantalum-tungsten alloy deformation, a meso-scale research was proposed via optical microscope (OM) image analysis. The results presented in this paper, provide new insights into the mechanisms suitable for the constitutive relations determination process.

Study of the dynamic fracture characteristics of coal with a bedding structure based on the NSCB impact test

To study the dynamic fracture deformation and failure characteristics of coal samples, the split Hopkinson bar (SHPB) impact loading system is used to test the dynamic fracture toughness of coal samples based on the notched semi-circular bend (NSCB) method under an impact load. The influence of the impact velocity and bedding angle on the dynamic fracture toughness, failure strain and strain rate of coal samples is preliminarily discussed, and a high-speed camera records and observes the dynamic fracture process of the coal sample. Finally, the distinct lattice spring model (DLSM) numerical analysis method is applied to analyze the sample dynamic splitting process and the change in the stress field. Meanwhile, the influences of the impact velocity, bedding angle, elastic modulus of bedding medium, bedding spacing and bedding width on the KIC test results are quantitatively analyzed.

Numerical simulation of rock failure under static and dynamic loading by splitting test of circular ring

The fracture propagation and failure process in rock are subjects of great interest in rock mechanics. In the present study, an approach combining the finite element method (FEM) and the discrete element method (DEM), named ELFEN, is adopted to simulate and investigate the failure process of a typical hard rock (Carrara marble) under static and dynamic splitting ring tests. The FDEM software is firstly validated by simulating the test of a Brazilian disk under static loading. Then, the failure of circular ring specimens is numerically studied under static and dynamic loading, with the ratio of internal to external diameter (represented by λ) of the specimen groups varying from 0.1 to 0.6, with increments of 0.1. Under static loading, with the increase in internal diameter, the failure mode is transformed from diametrical splitting to four-fan-shaped failure (when λ≥0.4). Under dynamic loading, four-fan-shaped failure occurs from geometric axial symmetry to axial asymmetry, which means that, with increasing internal diameter, the tensile cracks along the horizontal diametrical direction gradually deviate toward the top loading platen. The peak load of the circular ring has a descending trend with the increase in internal diameter under both loading conditions. A rational load value (peak load or crack initiation load) is suggested to calculate the tensile strength of circular rock ring specimens under the splitting ring test. Finally, the numerical results are compared with previous experimental results. The good agreement between the two sets of results, in terms of the failure modes and variation trend of the indirect tensile strength with increasing internal diameters, verify the accuracy and applicability of the FDEM approach.

Energy-Efficient Traffic Splitting for Time-Varying Multi-RAT Wireless Networks

This paper investigates the energy-efficient traffic splitting for time-varying wireless networks, which have been configured with multiple radio access technologies (multi-RATs). A single stream of the media content is split into multiple segments, which could be transmitted over multiple RATs simultaneously so that the complementary advantages of different RATs can be exploited. To address this problem, we formulate the traffic splitting as a long-term energy efficiency (EE) maximization problem with respect to the time-varying channel state information (CSI). An equivalent transformation method is proposed to convert the long-term nonconvex EE maximization problem into a concave optimization. To reduce the computational complexity, we develop a dynamic traffic splitting (DTS) algorithm, which iterates only one time when the network state changes. Then, we use the definition of tracking error to describe the difference between the DTS and the target optimal traffic splitting solution. After that, an adaptive-compensation traffic splitting (ACTS) algorithm is proposed to offset the tracking error so as to enhance the EE performance. More specifically, we give a sufficient condition for significantly eliminating the tracking errors of the ACTS algorithm. Simulation results show that the proposed ACTS algorithm obtains the EE performance comparable with the optimal solution at the overhead of only a single iteration at each timeslot of the network state acquisition.

Dynamic splitting tensile behaviors of red-sandstone subjected to repeated thermal shocks: Deterioration and micro-mechanism

Thermal shock (TS) is common for rock engineering where the environment temperature changes sharply. As a typical kind of brittle material, rock fails mainly because that the tensile stress exceeds the tensile strength, which is influenced significantly by strain rate. In this work, physical detection, quasi-static and dynamic splitting tensile tests were carried out on red-sandstone free from and after 10, 20, 30, 40 artificial TS cycles. TS-induced deterioration of red-sandstone was obvious, with the rock density reduced, and the porosity increased obviously with the adding-up of TS cycles. Splitting tensile strength (STS) of red-sandstone was influenced by the coupling effects of TS-induced decay and strain rate strengthening. Considering the strain rate effect, deterioration model was built to predict the STS degradation of rock after repeated TS. The model parameters, decay constant (λ) and half-life (N1/2) were both expressed as functions of strain rate. Microscopic features of the splitting tensile fracture surface were scanned by SEM techniques to analyze the micro-mechanism of TS-induced deterioration and strain rate effects on the STS. As the SEM results show, trans-grain fracture during the splitting tension increases with the rise of strain rate, leading to the strengthening of STS, and TS cycles reduces the agglutination of cements and the gomphosis between grains and thus reducing the STS of TS weathered red-sandstone.

Dynamic data split: A crosstalk suppression scheme in TSV-based 3D IC

Through-silicon via (TSV) technology improves transmission bandwidth in three-dimensional integrated circuits (3D ICs) due to its short connection path. However, it is limited by crosstalk issues, especially in closely spaced TSV clusters. The present study proposed a cost-effective scheme called the “dynamic data split” for optimization. The presented scheme was compared with other schemes, and its memory access requests from the L1 cache to L2 cache were examined using benchmarks such as SPEC2006Int. The proposed scheme presented the best bandwidth improvement at a minimal cost, i.e., it achieved a 15.75% improvement on requests for instruction fetches, in contrast to the 7.01% improvement observed in other dynamic mapping schemes.

Dynamic splitting tensile test of short carbon fiber C/SiC ceramic matrix composites

In order to investigate the dynamic fracture mechanics behavior and damage morphology of C/SiC ceramic matrix composites, dynamic splitting tensile tests on the C/SiC composites with three different volume fractions of short carbon fiber (16.0%, 21.0%, 24.8%) were carried out by the split Hopkinson pressure bar, the destructive interface part of C/SiC composites was scanned by using scanning electron microscopy and the failure characteristics and toughening mechanism of C/SiC composites were analyzed. The experimental results show that the dynamic tensile strengths of the C/SiC composite specimens with the same short carbon fiber volume fraction increase with the increasing of the impact pressure in the dynamic splitting tensile failure process, the failure of the specimens is significantly correlated with impact pressure and short carbon fiber volume fraction. When the short carbon fiber volume fraction is 16.0%, the tensile strength of the C/SiC composite specimens is the lowest. After the impact, the overall destruction of the C/SiC composite specimens is related to impact pressure and short carbon fiber volume fraction.

Comparison of High-strain Rate Behaviour of OFHC Copper Using Dynamic Indentation and Split Hopkinson Pressure Bar Techniques

The material behaviour can vary with strain rate so it becomes necessary to understand the response of materials under dynamic loading that closely match the service conditions. The high-strain rate behaviour of OFHC copper has been evaluated using split Hopkinson pressure bar (SHPB) technique and dynamic indentation (DI) technique. A comparison of data obtained using these techniques indicated a good match. The effect of strain-rate and microstructure on behaviour of material and strain-rate sensitivity has been discussed.

The structural temporal approach to dynamic and quasi-static strength of rocks and concrete

This work presents results of an experimental and theoretical study on dynamic and quasi-static failure of rocks and concrete. The results of dynamic compression and splitting of rocks (gabbro, granite, marble), as well as dry, water-saturated and frozen limestone and concrete are discussed. The tests were conducted using the Split-Hopkinson pressure bar with the diameter of 20 mm. It is shown that one material (or its condition) can have a lower dynamic strength for a higher static strength compared to the other material (or its condition). Also, it is shown a dependence of the threshold limit stress on the stress pulse duration. An unified interpretation of the experimental results, based on the structural–temporal approach is presented.

Dynamic failure of dry and fully saturated limestone samples based on incubation time concept

This paper outlines the results of experimental study of the dynamic rock failure based on the comparison of dry and saturated limestone samples obtained during the dynamic compression and split tests. The tests were performed using the Kolsky method and its modifications for dynamic splitting. The mechanical data (e.g. strength, time and energy characteristics) of this material at high strain rates are obtained. It is shown that these characteristics are sensitive to the strain rate. A unified interpretation of these rate effects, based on the structural–temporal approach, is hereby presented. It is demonstrated that the temporal dependence of the dynamic compressive and split tensile strengths of dry and saturated limestone samples can be predicted by the incubation time criterion. Previously discovered possibilities to optimize (minimize) the energy input for the failure process is discussed in connection with industrial rock failure processes. It is shown that the optimal energy input value associated with critical load, which is required to initialize failure in the rock media, strongly depends on the incubation time and the impact duration. The optimal load shapes, which minimize the momentum for a single failure impact, are demonstrated. Through this investigation, a possible approach to reduce the specific energy required for rock cutting by means of high-frequency vibrations is also discussed.

Mechanical properties and behaviour of concrete reinforced with spiral-shaped steel fibres under dynamic splitting tension

Concrete exhibits excellent resistance to compressive forces, but is brittle and weak in tension. Various types of fibres have been investigated by many researchers to improve the ductility and energy absorption capability of concrete materials and structures under static and blast and impact loadings. Spiral-shaped steel fibres were recently proposed as reinforcement in a concrete matrix and it was found that spiral fibre reinforcement can significantly improve the ductility, crack control ability and energy absorption capacity of concrete material under static and impact compressive loads. This paper presents an experimental study of the static and dynamic properties of steel fibre reinforced concrete (SFRC) materials under splitting tension. SFRC materials mixed with spiral-shaped steel fibres of different volume fractions were prepared and tested. A high-speed camera was used to capture the deformation, failure and crack opening process of the tested specimens. The contribution of spiral fibres to the mechanical properties and behaviour of concrete at high strain rate under splitting tension was investigated. Analyses of the test results revealed the effectiveness of spiral fibres in improving the performance of SFRC (e.g. crack control, energy absorption capability and more pronounced rate sensitivity under dynamic splitting loading). Moreover, crack opening and closing (pull-back by spiral fibres) processes were observed, demonstrating the excellent bonding and outstanding performance of spiral steel fibres in maintaining the integrity of the concrete material, thus resulting in significant improvements in impact resistance and energy dissipation.

Optimal Secure Multicast With Simultaneous Wireless Information and Power Transfer in Presence of Multiparty Eavesdropper Collusion

In this paper, secure transmission with simultaneous information and energy multicast in the presence of multiparty eavesdropper (EVE) collusion is investigated. In the considered system, legal users simultaneously decode information and harvest energy through dynamic power splitting (PS), whereas EVEs form multiple eavesdropping-collusion parties to cooperatively decode signals. A novel optimization problem is formulated to maximize the overall achievable secure multicast capacity while satisfying the maximum transmit power constraint at the transmitter, guaranteeing the minimum harvested energy requirements at each legal user and restricting the wiretapping capability of each EVE party. Since the formulated problem is nonconvex and hence difficult to solve, it is decomposed into a power minimization subproblem and a rate optimization subproblem and then solved in a nested manner. By leveraging the properties of the objective function, it is proved that through the proposed algorithm the optimal secure multicast capacity can be achieved, with the optimal transmission strategy, i.e., optimal beamforming matrix, optimal artificial noise (AN) covariance matrix, and optimal PS factors, obtained. Moreover, the case of imperfect channel state information (CSI) is considered in which a robust secure multicast scheme is designed by considering the channel uncertainty of EVEs and maximizing the worst-case achievable secure capacity. Simulation results confirm the optimality of the proposed algorithm, show its capability of adapting to the dynamic joining and departure of multicast users and EVE parties, and verify its superiority over existing schemes for both the perfect CSI and imperfect CSI cases.

Coherent and dynamic beam splitting based on light storage in cold atoms.

We demonstrate a coherent and dynamic beam splitter based on light storage in cold atoms. An input weak laser pulse is first stored in a cold atom ensemble via electromagnetically-induced transparency (EIT). A set of counter-propagating control fields, applied at a later time, retrieves the stored pulse into two output spatial modes. The high visibility interference between the two output pulses clearly demonstrates that the beam splitting process is coherent. Furthermore, by manipulating the control lasers, it is possible to dynamically control the storage time, the power splitting ratio, the relative phase, and the optical frequencies of the output pulses. With further improvements, the active beam splitter demonstrated in this work might have applications in photonic photonic quantum information and in all-optical information processing.

Noise property of a four-level system in vee + ladder configuration

We present the results of a theoretical study of the output amplitude noises of a four-level atomic system in vee + ladder configuration. The difference and connection of electromagnetically induced transparency and Autler–Townes splitting effects are investigated theoretically. The output amplitude noise of the probe field of two effects were compared, the quantum properties of the input field of the thin medium and the thick medium under Autler–Townes splitting or electromagnetically induced transparency (EIT) condition are maintained, the higher fidelity is obtained in the storage of the thick medium under the condition of EIT in non-classical states. The noise characteristics of the squeezed vacuum field after four-level coherent medium are studied; the noise of the output field is lowest when the detection light is far away from detuning. The double split of EIT window was made by the dynamic Stark splitting on the ground state of control field, the quantum properties of the input field in the strong-control field and strong-coupling field were maintained. The output noise spectrum is divided with the increase in the field strength, the maximum output squeezing is far away from the resonance.

Dynamic Brazilian test of concrete using split Hopkinson pressure bar

To research the fracture pattern of concrete specimens, the dynamic splitting tensile tests are conducted in different arc loading angles and impact velocities. The stress state of the specimens can be calculated by analyzing the strain gauges data on the split Hopkinson pressure bar. The specimens under the lower impact velocity achieves the stress equilibrium in loading direction, and the stress-state of the specimen is similar to that of quasi-static condition, in which the initial crack occurs at the center of the specimen and propagates along the loading diameter direction. When the impact velocity increases, the stress equilibrium is difficult to attain, and multiple cracks sometimes even ribbon fracture fragments appear at the center of specimens. The impact velocity plays a significant role in the failure pattern of concrete specimens, and different angles arc loading affect the local stress distribution of the specimens. The suitable load angle can reduce the local failure and improve the failure pattern of specimens. The stress state and failure pattern of specimens simulated by LS-DYNA coincide with the test results.

Static and dynamic tensile failure characteristics of rock based on splitting test of circular ring

Static and dynamic splitting tests were conducted on ring marble specimens with different internal diameters to study the tensile strength and failure modes with the change of the ratio of internal radius to external radius (ρ) under different loading rates. The results show that the dynamic tensile strength of disc rock specimen is approximately five times its static tensile strength. The failure modes of ring specimens are related to the dimension of the internal hole and loading rate. Under static loading tests, when the ratio of internal radius to external radius of the rock ring is small enough (ρ<0.3), specimens mostly split along the diametral loading line. With the increase of the ratio, the secondary cracks are formed in the direction perpendicular to the loading line. Under dynamic loading tests, specimens usually break up into four pieces. When the ratio ρ reaches 0.5, the secondary cracks are formed near the input bar. The tensile strength calculated by Hobbs' formula is greater than the Brazilian splitting strength. The peak load and the radius ratio show a negative exponential relationship under static test. Using ring specimen to determine tensile strength of rock material is more like a test indicator rather than the material properties.

Statistical Analysis of Dynamic Splitting Tensile Strength of Concrete Using Different Types of Jaws

AbstractThe indirect Brazilian test is widely used in engineering practice to indirectly determine the tensile strength of concrete. The Brazilian test (splitting tension test) is performed by applying a concentrated compressive load across the diameter of a disc-specimen. A series of dynamic Brazilian disc tests for concrete specimens were conducted using the split Hopkinson pressure bar (SHPB) technique. The objective of this paper is to investigate the standard Brazilian test by comparing the results obtained using the test with those obtained using a loaded arc, together with a comparison of the experimental results and the statistical analysis. The effect of loading angle on the dynamic splitting tensile strength was studied by one-way analysis of variance. By using linear regression method, the Weibull statistical model was introduced to obtain the relationship between the loading angle and dynamic splitting tensile strength of concrete. It was concluded that dynamic tensile strength of concrete is ...

Controlling slow and fast light and dynamic pulse-splitting with tunable optical gain in a whispering-gallery-mode microcavity

We report controllable manipulation of slow and fast light in a whispering-gallery-mode microtoroid resonator fabricated from Erbium (Er3+) doped silica. We observe continuous transition of the coupling between the fiber-taper waveguide and the microresonator from undercoupling to critical coupling and then to overcoupling regimes by increasing the pump power even though the spatial distance between the resonator and the waveguide was kept fixed. This, in turn, enables switching from fast to slow light and vice versa just by increasing the optical gain. An enhancement of delay of two-fold over the passive silica resonator (no optical gain) was observed in the slow light regime. Moreover, we show dynamic pulse splitting and its control in slow/fast light systems using optical gain.

A Modified Overstress Model to Simulate Dynamic Split Tensile Tests and Its Experimental Validation

The tensile strength of rock and rock-like materials is universally acknowledged to be much lower than their compressive strength and shearing strength (Meyers and Chawla 1999). Furthermore, the stability and reliability of rock structures are correlated remarkably well with the dynamic tensile strength of rock materials. Therefore, dynamic split tensile tests under high strain rate loads have been an active area of investigation in the field of rock mechanics. Carneiro (1943) first used Brazilian disc specimens to determine the tensile strength of brittle materials. Wang et al. (2004) improved these experiments by using flattened Brazilian discs (FBDs) instead to avoid stress concentration at the loading end. Wang’s method ensures that the initial crack only occurs in the central part of a specimen, which is vital for the validation of a split test. Recently, the split Hopkinson pressure bar (SHPB) has come into use as a general apparatus to determine the dynamic tensile strength of rock-like materials (Saksala et al. 2013; Xu et al. 2014). However, among the failure processes, crack propagation is difficult to observe, which makes it very difficult to judge whether or not the initial crack starts from the center. Finite element methods (FEMs) are widely used to simulate failure in brittle materials (e.g., Hang et al. 2015; Saksala et al. 2015). Compared with laboratory experiments, it is much easier and cost-effective to observe crack propagation in a simulation as long as an appropriate material model is built for the specimens. In this article, we build 3D models of FBD specimens in an SHPB apparatus and use these models to conduct numerical simulations of rock dynamic split tests. A modified overstress constitutive model is applied to describe the rock-like material. The failure patterns resulting from dynamic split tensile tests, as well as crack initialization and propagation, are studied with these simulations. The simulation results are verified by laboratory experiments using the SHPB and a high-speed camera.

Finite element modelling of mesoscale concrete material in dynamic splitting test

Dynamic tensile strength is one of the key factors of concrete material that needs to be accurately defined in analysis of concrete structures subjected to high-rate loadings such as blast and impact. It is commonly agreed that dynamic testing results of concrete material are influenced by the inertia effect, which is very much dependent on the specimen size and loading rate. It is therefore very important to remove the inertia effect in testing data to derive the true dynamic concrete material properties. On the other hand, coarse aggregates in concrete material are usually neglected due to testing limitation or numerical simplification. It has been acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In this study, a three-dimensional mesoscale model of concrete specimen consisting of cement mortar and coarse aggregates is developed to simulate splitting tensile tests and investigate the behaviour of concrete material at high strain rate. The commercial software LS-DYNA is used to carry out the numerical simulations of dynamic splitting tensile tests. The reliability of the numerical model in simulating the dynamic splitting tensile tests is verified by comparing the numerical results with the laboratory test data from the literature. The influence of inertia effect in dynamic splitting tensile tests is investigated and removed. An empirical formula to represent the true dynamic increase factor relations obtained from dynamic splitting tensile test is proposed and verified.