Strain Rate Effect Of Concrete Research Articles

A modified bond-based peridynamic approach for rigid projectile perforation on concrete slabs

Peridynamic (PD) has a unique advantage in describing the crack growth and fragmentation of brittle materials. Concerning the dynamic behaviors and failure patterns of concrete slabs under projectile perforations, a modified bond-based PD approach maintaining both the easy implementation and computational stability characteristics was firstly developed from the following three aspects, (i) a rate-dependent PD constitutive model was proposed for describing the dynamic behaviors of concrete; (ii) a progressive damage criterion considering the tension-compression anisotropy, softening behavior, and strain rate effect of concrete was incorporated to more accurately reproduce the damage and failure of concrete; (iii) an improved micro-modulus function related to bond length was introduced to reveal the internal length effect of bond force. Then, numerical simulations of projectile perforation on concrete slabs by utilizing the developed modified bond-based PD approach, as well as the corresponding sensitivity analyses of discretization parameters including horizon size and particle spacing were performed. Based on the recommended horizon size and particle spacing, the predicted residual velocity of projectile and failure patterns of concrete slabs exhibited an excellent agreement with the test data. Furthermore, by comparisons of the traditional bond-based PD and classical finite element methods, the superiority of developed approach in describing the perforation damage of concrete targets against projectile impact was demonstrated. Finally, the modified bond-based PD approach was employed to blind simulate the projectile normal and oblique perforating multi-layered spaced concrete target plates. It was found that the modified PD model reasonably predicted the terminal ballistic trajectory, deflection angle, and residual velocity of projectile, as well as the failure patterns of target plates. The present work provides a new way to predict the terminal ballistic effect of projectile and dynamic behaviors of concrete slabs.

Mechanical properties and strain rate effect of graphene oxide grafted carbon fiber modified concrete under dynamic impact load

In order to improve the shortcomings of carbon fiber, such as smooth surface, poor bonding with concrete matrix and easy to pull out under dynamic impact load, graphene oxide is grafted onto the surface of carbon fiber by chemical bonds to prepare graphene oxide grafted carbon fiber (CF-GO). The dynamic impact test on CF-GO modified concrete (CF-GOMC) is carried out by the split hopkinson pressure bar (SHPB) test system, and the mechanical properties and strain rate effect of CF-GOMC under dynamic impact loads are studied. The modification mechanism of CF-GO is analyzed by microscopic tests. The results show that compared with carbon fiber, the surface roughness and surface area of CF-GO increase and CF-GO is more suitable for bonding with concrete matrix. CF-GO can improve the mechanical properties of concrete under dynamic impact load and enhance the strain rate effect of dynamic mechanical properties indicators by preventing crack, bridging, filling and promoting hydration. With the increase of CF-GO content, the dynamic mechanical properties and strain rate sensitivity of CF-GOMC increase first and then decrease. The optimal content of CF-GO is 0.3 %. The improvement effect of CF-GO on the dynamic mechanical properties and strain rate effect of concrete is better than that of carbon fiber. The graphene oxide on the surface of CF-GO makes the interface of CF-GO/concrete matrix compact by improving the mechanical bite force of fiber/concrete matrix and promoting the formation of hydration products on the surface of fiber. The addition of CF-GO does not change the composition of cement hydration products, but promotes the crystal growth of hydration products and optimizes the pore structure of concrete.

Similarity law of global and local responses of reinforced concrete beams subjected to impact loading

In the experiment of scaled models to dealing with the impact of reinforced concrete large structures, the strain rate hardening effect can lead to significant differences between the dynamic responses of prototype and model. In the present study, this thorny problem is solved by adjusting the impact velocity. Based on the mechanisms of different dynamic responses, the similarity laws for the global and local responses of reinforced concrete beams subjected to impact loading are proposed by considering the strain-rate effect of materials through Buckingham Π-theorem, where constitutive models of different materials are employed for inferring dynamic stress. In dimensional analysis, multiple control factors need to be considered simultaneously, such as the strain-rate effects of steel and concrete, since reinforced concrete components are typical composite structures. The similarity law cannot be unified when considering multiple control factors. In view of this, it is recommended to estimate the dynamic response of reinforced concrete components subjected to impact loading through the upper and lower bounds of similarity law. The upper bound of similarity law is based on the strain-rate effect of concrete, while the lower bound of similarity law is based on the strain-rate effect of steel bar. The numerical models of reinforced concrete beams are established based on the experimental background. The numerical simulations with different scaling factors indicate that the model modified by the lower bound of similarity law can better predict the global response of prototype, while the model modified by the upper bound of similarity law can more accurately predict the local response of prototype. In addition, the influence of dynamic elastic modulus of reinforced concrete on the similarity law is discussed through comparative analysis.

A dynamic FEM-DEM multiscale modeling approach for concrete structures

The dynamic behavior of concrete is strain-rate-dependent, which affects the failure modes of structures. In this study, a multiscale modeling approach is developed for concrete structures under dynamic loading, which couples the macroscale finite element method (FEM) and the mesoscale discrete element method (DEM). The dynamic response of structures is analyzed using the macroscale FEM, and a representative volume element (RVE), composed of aggregate, mortar, and interface, is simulated using the mesoscale DEM to capture the strain rate effects of concrete. By fully taking the advantages of macroscale and mesoscale simulations, it is no longer essential to use complex constitutive laws or input rate-dependent parameters for the macroscale FEM modeling because the concrete fracture can be explicitly illustrated through the breaking process of the mesoscale RVE. This approach is applied to concrete beams and the Koyna dam to verify its applicability. These practical comparisons demonstrate that the seismic tensile strength increases with the strain rates under seismic loading and that the actual repeated crack-closure process of concrete is adequately simulated by the proposed method.

Determining Dynamic Mechanical Properties for Elastic Concrete Material Based on the Inversion of Spherical Wave.

The paper presents a new method to study the dynamic mechanical properties of concrete under low pressure and a high strain rate via the inversion of spherical wave propagation. The dynamic parameters of rate-dependent constitutive relation of elastic concrete are determined by measured velocity histories of spherical waves. Firstly, the particle velocity time history profiles in the low stress elastic region at the radii of 100.6 mm, 120.6 mm, 140.6 mm, 160 mm, and 180.6 mm are measured in the semi-infinite space of concrete by using the mini-explosive ball and electromagnetic velocity measurement technology. Then, based on the universal spherical wave conservation equation and the fact that the accommodation relationship in state equation satisfies linear elastic law, the inverse problem analysis of spherical waves in concrete (called "NV + T0/SW") is proposed, which can obtain the dynamic numerical constitutive behavior of concrete in three-dimensional stress by measuring the velocity histories. The numerical constitutive relation is expressed in the form of distortion, and it is found that the distortion law has an obvious rate effect. Finally, the rate-dependent dynamic parameters in concrete are determined by the standard linear solid model. The results show that the strain rate effect of concrete cannot be ignored with the strain rate range of 102 1/s. This study can provide a feasible method to determine the dynamic parameters of rate-dependent constitutive relation of concretes. This method has good applicability, especially in the study of the dynamic behavior of multicomponent composite materials with large-size particle filler.

Research on Dynamic Strength and Inertia Effect of Concrete Materials Based on Large-Diameter Split Hopkinson Pressure Bar Test.

The Split Hopkinson Pressure Bar (SHPB) test device is an important tool to study the dynamic characteristics of concrete materials. Inertial effect is one of the main factors that cause inaccurate results in SHPB tests of concrete materials. To solve this problem, Large-diameter SHPB tests on concrete and mortar were performed. A dynamic increase factor (DIF) model considering strain rate effect and inertia effect was established. This model provides a scientific reference for studying the dynamic mechanical properties of concrete materials. The experimental results indicate that the strain rate effect of concrete is more sensitive than that of mortar, but the inertia effect of mortar is more sensitive than that of concrete. Under the same strain rate, the energy utilization rate, average fragment size, and impact potentiality of mortar are higher than concrete.

Numerical and Experimental Study on Strain Rate Effect of Ordinary Concrete under Low Strain Rate

The strain rate effect of ordinary concrete is researched through the particle discrete element method. A random construction method of three-dimensional aggregate is used, and the numerical sample consistent with the experimental specimen is constructed using particle flow code. Moreover, the simulation method that can properly reflect the strain rate effect with the parallel bond model is proposed. The meso-parameters are calibrated by triaxial compression test. Based on the proposed simulation method and calibrated meso-parameters, the numerical tests of direct tension, uniaxial compression, and cyclic loading under strain rates from 10−5 s−1 to 10−1 s−1 are carried out. The results show that the strain rate effect of concrete can be simulated with particle flow code by assuming that the micromechanical properties of materials vary with the strain rate, and the strength and failure characteristics of numerical samples under different strain rates are described well by the proposed method. In addition, the different mechanical responses of the samples to the strain rate in the compression test and the tensile test are obtained, and the changes of mechanical parameters and damage degree with strain rates in the cyclic loading test are also successfully simulated. This study can provide a feasible numerical method for the follow-up research of dynamic mechanical behavior of concrete and offer theoretical guidance for the stability assessment of concrete engineering.

Effect of Initial Static Load and Dynamic Load on Concrete Dynamic Compressive Failure

Concrete structures often suffer from multiple dynamic loads, or prior to being subjected to dynamic loads have already withstood initial loads. The mechanical properties of concrete materials under dynamic loads may be closely linked to initial loading history that concrete suffers. This paper deals with the investigation of the effect of initial static load and initial dynamic load on dynamic compressive failure of concrete materials based on numerical simulations. A meso-scale modeling method was established and a total of 147 three-dimensional concrete cubic models were simulated. It is demonstrated in research results that the strain rate effect of concrete is relatively weak under low strain rates, while the strain rate effect is significantly enhanced under high strain rates. Dynamic compressive strength under different dynamic loads gradually decreases with the increasing initial static load and the increasing strain rate can weaken the influence of initial static load on dynamic compressive strength of concrete. When dynamic load changes in hardening stage, tangent modulus has a sudden increase and compressive stress reaches a new peak strength. A sudden increase of dynamic load in softening stage reverses the postpeak softening to postpeak hardening followed by a second peak strength. Initial dynamic load plays an important role in dynamic compressive failure of concrete, especially under low strain rates. The numerical results compare well with the existing test results.

Experimental study and numerical simulation of impact compression mechanical properties of high strength coral aggregate seawater concrete

Based on the mix proportion design principle of high performance lightweight aggregate concrete, rich slurry concrete theory and fiber reinforced toughening technology, high strength Coral Aggregate Seawater Concrete (CASC) and sisal fiber reinforced CASC (SFCASC) with 28d cube compressive strength of 74.6 MPa and 77.3 MPa respectively were prepared. The Ф75 mm split Hopkinson pressure bar (SHPB) was used to carry out impact compression tests on CASC and SFCASC with sisal fiber content of 3 kg/m3, and the finite element software LS-DYNA was used to carry out numerical simulation analysis on CASC impact tests. The results show that high strength CASC has obvious strain rate effect, DIF is related to strain rate (ε˙) and concrete compressive strength (fcu), the relationship between DIF and strain rate recommended by the European Concrete Committee (CEB) is not applicable to high-strength CASC, the parameters of CEB model are re-fitted and the relationship between DIF and strain rate is established on the basis of the above ε˙ and fcu; It is found that impact incident energy, reflection energy, transmission energy and absorption energy are positively correlated with strain rate. The strain rate effect of concrete is explained from the angle of energy, and the relation between DIF and absorption energy is fitted. When the DIF of CASC and SFCASC are the same, the absorption energy of SFCASC is 49.06%–140.29% higher than that of CASC; The finite element software LS-DYNA is used to simulate three working conditions with strain rates of 62.3 s−1, 93.1 s−1and 137.4 s−1. The stress-strain curves and failure patterns obtained are in good agreement with the experimental results. The change of Poisson's ratio with strain rate is analyzed according to LS-DYNA numerical calculation. It is found that Poisson's ratio of CASC has no strain rate effect within the strain rate range of this paper (3 × 10−6–1.8 × 102 s−1).

Improved finite difference analysis of dynamic responses of concrete members reinforced with FRP bars under explosion

An improved finite difference (FD) procedure to predict the dynamic response of concrete members reinforced with fiber reinforced polymer (FRP) bars under explosion is proposed. This method no longer needs the empirical formulae of the moment-curvature relationship, which is only suitable for concrete members reinforced with steel bars but inappropriate for those reinforced with FRP bars. The equilibrium of the horizontal force of the structural element, considering the strain rate effects of the concrete and the reinforcing bars, is established to calculate the depth of the neutral axis based on a numerical solution of nonlinear equations. A layered-section-based analysis model is used to calculate the strain rate effect of the compressive concrete. The Euler-Bernoulli’s beam theory is solved numerically using an explicit FD scheme. The accuracy of the FD model is validated by the experiments of the concrete members reinforced with steel bars and FRP bars, respectively. The FD results are also compared with those obtained by single-degree-of-freedom (SDOF) analysis and finite element (FE) analysis, respectively. The FD procedure is accurate, applicable and convenient to predict the dynamic responses of the flexural concrete members reinforced with FRP bars, as well as those of the members reinforced with steel bars.

820 コンクリート材の衝撃引張り強度とひずみ速度依存性

820 コンクリート材の衝撃引張り強度とひずみ速度依存性

Compressive Strain Rate Effects of Concrete

ABSTRACTSince good constitutive laws are required to model correctly the behaviour of concrete under impact loading, it is necessary to determine the complete stress-strain response of concrete at varying strain rates. Conflicting evidence emerges about whether the critical compressive strain (defined as the strain observed at maximum stress) increases or decreases with an increasing strain rate. In this paper, a comprehensive description is given of the brittle fracture process for plain concrete under static and impact loading. The strain rate dependance of tensile microcrack growth is used to explain both the increase in strength and the increase in critical compressive strain that can occur at high strain rates. More extensive experimental results are required to determine the fundamental changes in behaviour that occur as the loading rate is increased and, thus, facilitate the development of a more precise failure model for concrete.