Dynamic Increase Factor Research Articles

Influence of friction on the dynamic increase factor of concrete compressive strength in a split Hopkinson pressure bar test

This study includes the experimental and numerical research conducted to establish a standardized lubrication technique for a concrete split Hopkinson pressure bar (SHPB) test. Concrete SHPB tests were conducted using different quantities of high-vacuum grease, petroleum jelly, and Teflon to determine a desirable lubrication method. Subsequently, follow-up concrete SHPB tests with different specimen dimensions were performed to validate the proposed lubrication method and obtain a dynamic increase factor (DIF) exclusively with respect to the rate effect (that is, pure rate DIF). Furthermore, the pure rate DIF was verified through finite element analysis of the SHPB tests. The experimental and numerical outcomes demonstrated that the frictional effect was eliminated in the concrete SHPB test when a lubricant more than or equal to 12 mg/cm2 was applied to the specimen irrespective of the lubricant type.

Investigation on the dynamic tensile behaviour of rubberized mortar using the Brazilian disc method

The use of tire rubber particles as a partial substitute for fine aggregate in cement-based materials has been proven to potentially have both environmental and economic benefits. Although the studies on the physical and mechanical properties of rubberized mortar are abundant, those related to the dynamic tensile behaviour are scarce. In this study, rubber powders with three sizes (0.125 mm, 0.180 mm, and 0.425 mm) are used to fabricate the Brazilian disc rubberized mortar specimen, with rubber volume percentages up to 40%. A 50 mm-diameter split Hopkinson pressure bar (SHPB) apparatus is then adopted to conduct the dynamic split tension tests, and the static tensile tests are also conducted for comparison purposes. Experimental results show that the dynamic split tensile strength of the rubberized mortar linearly increases with increasing the loading rate, whereas decreases with the increase of the rubber volume percentage. The mortars with smaller rubber powder at the same percentage exhibit more significant dynamic strength loss. As compared with the static tests, the reduction ratio of the dynamic split tensile strength of mortar with different rubber content is much smaller. The dynamic increase factors of the rubberized mortar are more sensitive to the loading rate, and positively correlated to both the rubber volume and the powder size. Besides, the occurring time of the primary crack initiation gradually decreases with increasing the loading rate and rubber size, whereas increases with increasing the rubber volume percentage. In addition, the dissipated energy density of the rubberized mortar is positively correlated with the rubber volume percentage, and the addition of the smaller rubber powder can enhance the dynamic energy absorption capacity.

Research on energy dissipation and damage evolution of dynamic splitting failure of basalt fiber reinforced concrete

Based on split Hopkinson bar (SHPB) loading system and digital image correlation (DIC) technology, a series of dynamic splitting experiments of basalt fiber reinforced concrete (BFRC) were carried out to reveal the effects of basalt fiber content and loading level on the dynamic tensile properties, energy dissipation and damage evolution. The results show that the dynamic tensile strength and dynamic increase factor (DIF) have obvious strain rate effect, and the appropriate basalt fiber content (0.39%) can remarkably improve the dynamic tensile strength of concrete. According to the changing trend of dissipated energy curve, the dynamic splitting process can be divided into compaction stage, elastic stage, plastic yield stage and softening stage. Appropriate basalt fiber content (0.39%) can effectively improve the dissipated energy and dissipated energy rate of BFRC. The loading level only promotes the dissipated energy of BFRC, but has no obvious effect on the dissipated energy rate. Adding an appropriate basalt fiber content (0.39%) can reduce the shear strain, thereby reducing the damage degree of the shear failure zone, and delaying the cracking time in the center of the specimen to a certain extent.

Dynamic Properties of Mortar in High-strength Concrete

Mortar is the matrix in concrete; at the meso-scale, concrete is a composite consisting of mortar matrix, coarse aggregate particles, and the interfacial transition zones between them. In this investigation, the mortars of three high-strength concretes were subjected to uniaxial compression and splitting tension at different speeds, from static to dynamic, to examine the effects of strain rate on their stress-strain and failure behaviour. Notable strain rate sensitivity was observed in both the compressive and tensile behaviour of the mortars, with the tensile response displaying greater rate dependence. The rate dependence for both tensile and compressive responses, varies little with mortar strength; i.e. similar rate sensitivity was noted for the three mortars studied. The dynamic increase factor (DIF) was adopted to evaluate strain rate dependence. Both the tensile and compressive DIF display a two-stage variation with strain rate, i.e. an initial gradual increase up to a transition strain rate, followed by a rapid rise thereafter; the transition occurs at ∼1 s−1 for tension, and ∼30 s−1 for compression. By fitting experimental data, empirical formulae were proposed for both the tensile and compressive DIFs of mortar. Finite element (FE) simulations of the static and dynamic tension/compression tests were also performed, using the JH-2 model to describe mortar. Good correlation between the simulations and experiments provides substantiation that strain rate dependence of the tensile/compressive strengths measured in dynamic tests are actual material properties.

Effect of fiber content and fiber length on the dynamic compressive properties of strain-hardening ultra-high performance concrete

Strain-hardening ultra-high performance concrete (SH-UHPC) exhibits excellent mechanical properties at the quasi-static state and becomes a promising material for protective structures exposed to impact or blast threats. But its performance under loads with high magnitude and short duration remains unanswered. The focus of this study is to investigate the dynamic compressive properties of SH-UHPC under high strain rates, which are achieved by split Hopkinson pressure bar (SHPB) tests. SH-UHPC with different fiber contents of 1, 1.5, and 2 vol% and different fiber lengths of 6, 12, and 18 mm are tested. The considered strain rate ranges from ∼ 180 s−1 to ∼ 300 s−1. The results show that SH-UHPC exhibits good impact resistance in terms of integrity. Samples could remain good integrity at a strain rate of ∼ 240 s−1. The key parameters to measure the dynamic properties, i.e. energy absorption, dynamic compressive strength, and dynamic increase factor (DIF) of SH-UHPC show strain rate sensitivity and all of them increase with strain rate. Increasing fiber content leads to a slight improvement of the impact resistance of SH-UHPC while reducing fiber length impairs the impact resistance. Empirical DIF relations of SH-UHPC are firstly proposed in this study.

Dynamic Mechanical Properties of Slag Mortar with Alkali-Resistant Glass Fiber

To investigate the influence of alkali-resistant glass fiber on the dynamic mechanical properties and microstructure of slag cement mortar (SCM), five SCM blocks with different volumes of alkali-resistant glass fiber (0%, 0.25%, 0.5%, 0.75%, and 1%) were prepared. A split Hopkinson pressure bar (SHPB) test device with a diameter of Φ50 mm was used to test the impact compression under different loading pressures. In this way, the stress–strain curve of SCM under average strain rates corresponding to different loading air pressures is obtained, and the relationship between the ultimate toughness, the dynamic increase factor (DIF), and the fiber content of SCM can be comprehensively analyzed in combination with the dynamic and static mechanical parameters. Research results showed that, with the increase in fiber content, the static compressive strength of the SCM showed a trend of first increasing and then decreasing, where 0.75% can be the optimal one with the maximum compressive strength. Under the impact load, the dynamic compressive strength of the SCM had a positive correlation with the average strain rate. Within the strain rate range of 75.03 s−1~141.47 s−1, When the fiber volume content is 0.75%, the dynamic compressive strength, ultimate toughness and DIF of SCM are higher than those of other groups, showing good impact resistance. According to the analysis of SEM images, the bonding performance between alkali-resistant glass fiber and cement matrix was strong, and the fibers were interlaced in the mortar to form a network structure, which has a good effect on toughening and strengthening the SCM.

Characteristics of 3D-printing ultra-high performance fibre-reinforced concrete under impact loading

3D-printing concrete exhibits anisotropy under static loads, owing to its unique additive manufacturing process, while its dynamic performance study is still insufficient. In particular, the dynamic properties of 3D-printing ultra-high performance fibre reinforced concrete (3DP-UHPFRC) have not been studied yet. Therefore, this study explores the characteristics of 3DP-UHPFRC under impact loads using the SHPB tests. Three impact velocities of 3.886, 6.026, and 8.538 m/s were studied in the tests. The impact process was recorded by a high-speed camera. The dynamic mechanical characteristics of 3D-printing ultra-high performance concrete (3DP-UHPC) without fibre, 3DP-UHPFRC and reference specimens were investigated in terms of fibre type, fibre content, preparation method, loading direction, and impact velocity. The characteristics of strain rate, dynamic compressive stress, dynamic increase factor (DIF), energy absorption capacity and failure process were evaluated. The findings of this study indicated that the degree of failure of 3DP-UHPC was similar in all directions, while the degree of failure of 3DP-UHPFRC in all directions was different. The degree of failure in the X-direction was the worst, followed in decreasing order by the degrees of failure in the Y- and Z-directions. At the same impact velocity, the elastic modulus and strain rate effect of the 3D-printing specimens exhibited anisotropic characteristics, owing to the different elastic modulus of the 3D-printing specimens in each direction. Furthermore, the specimens were more susceptible to deformation in the X-direction than that in the Y- and Z-directions. As the impact velocity was increased, the dynamic peak stresses for 3DP-UHPFRC were isotropic at the same impact velocity, owing to the strain rate effect. Finally, the DIF of the 3D-printing specimens was observed to be anisotropic, and in the X-direction the specimens exhibited the most significant strain rate sensitivity.

Numerical investigation of the dynamic increase factor of ultra-high performance concrete based on SHPB technology

This work numerically investigates the dynamic increase factor (DIF) of ultra-high performance concrete (UHPC) under compressive impact by employing a split-Hopkinson pressure bar (SHPB). The DIF test values of UHPC are collated at six different sizes of SHPB devices, and the effectiveness of the formulas to calculate the DIF proposed by the specification for normal concrete (NC) is evaluated. An analysis model simulating the SHPB test of UHPC in the LS-DYNA is established by calibrating the Karagozian and Case concrete (K&C) model, and the DIF-strain rate curves of UHPC are further supplemented and improved. The expressions of DIF under six different specimen sizes for UHPC are obtained by regression analysis. The results indicate that (1) the proposed formulas by the specification for NC will overestimate the strain rate effect of UHPC under impact compression; (2) the calibrated K&C model can simulate well the dynamic stress–strain curve of UHPC in compression under different specimen sizes; (3) the DIF value increases with the increase of the specimen size and decreases with the increase of the uniaxial compressive strength, while the variation trends of the threshold of strain rate in the DIF-strain rate curves are opposite to those of the DIF value; (4) the proposed formulas to calculate the DIF can match well the numerical results, and avoid overestimating the DIF of UHPC to a certain extent.

Machine learning method to predict dynamic compressive response of concrete-like material at high strain rates

Machine learning (ML) methods with good applicability to complex and highly nonlinear sequences have been attracting much attention in recent years for predictions of complicated mechanical properties of various materials. As one of the widely known ML methods, back-propagation (BP) neural networks with and without optimization by genetic algorithm (GA) are also established for comparisons of time cost and prediction error. With the aim to further increase the prediction accuracy and efficiency, this paper proposes a long short-term memory (LSTM) networks model to predict the dynamic compressive performance of concrete-like materials at high strain rates. Dynamic explicit analysis is performed in the finite element (FE) software ABAQUS to simulate various waveforms in the split Hopkinson pressure bar (SHPB) experiments by applying different stress waves in the incident bar. The FE simulation accuracy is validated against SHPB experimental results from the viewpoint of dynamic increase factor. In order to cover more extensive loading scenarios, 60 sets of FE simulations are conducted in this paper to generate three kinds of waveforms in the incident and transmission bars of SHPB experiments. By training the proposed three networks, the nonlinear mapping relations can be reasonably established between incident, reflect, and transmission waves. Statistical measures are used to quantify the network prediction accuracy, confirming that the predicted stress-strain curves of concrete-like materials at high strain rates by the proposed networks agree sufficiently with those by FE simulations. It is found that compared with BP network, the GA-BP network can effectively stabilize the network structure, indicating that the GA optimization improves the prediction accuracy of the SHPB dynamic responses by performing the crossover and mutation operations of weights and thresholds in the original BP network. By eliminating the long-time dependencies, the proposed LSTM network achieves better results than the BP and GA-BP networks, since smaller mean square error (MSE) and higher correlation coefficient are achieved. More importantly, the proposed LSTM algorithm, after the training process with a limited number of FE simulations, could replace the time-consuming and laborious FE pre- and post-processing and modelling.

Size effect on dynamic splitting tensile strength of concrete: Mesoscale modeling

The size effect on the dynamic strength of concrete has been recognized, but the main causes of this phenomenon are not well understood. In this study, a mesoscale particle model was established to simulate the dynamic splitting tensile behavior of concrete and calibrated with experimental data. An equivalent momentum scheme based on mesoscale modeling was proposed for the quantitative division of the dynamic increase factors. The results showed that the inertia effect and multiple cracks improved the tensile strength, owing to the rate effects. However, the inertia factor played a dominant role, whereas the multiple crack factor minimally influenced the dynamic size effect of concrete.

Test of Dynamic Mechanical Properties of Ambient-Cured Geopolymer Concrete Using Split Hopkinson Pressure Bar

The application of geopolymer concrete (GPC) in construction could reduce a large amount of carbon dioxide (CO2) emission, which is greatly beneficial to environmental sustainability. Structures made of GPC might be subjected to extreme loading such as impact and blast loads. Therefore, a good understanding of the dynamic properties of GPC is essential to provide reliable predictions of performance of GPC structures subjected to dynamic loading. This study presents an experimental investigation on the dynamic compressive and splitting tensile properties of ambient-cured GPC using split Hopkinson pressure bar (SHPB), with the strain rate up to 161.0 s−1 for dynamic compression and 10.3 s−1 for dynamic splitting tension. The failure mode and damage progress of GPC specimens, energy absorption, and dynamic increase factor (DIF) were studied. Test results showed that ambient-cured GPC exhibited strain rate sensitivity. The compressive and splitting tensile DIFs increased with the strain rate and the ambient-cured GPC with lower quasi-static compressive strength exhibited higher DIFs under both dynamic compression and splitting tension. Empirical formulas were proposed to predict the DIF of ambient-cured GPC. Furthermore, the specific energy absorption of ambient-cured GPC under dynamic compression increased approximately linearly with the strain rate.

Effects of fibres on ultra-lightweight high strength concrete: Dynamic behaviour and microstructures

The use of ultra-lightweight high strength concrete (ULHSC) for prefabricated structures has been recognized as a promising method of construction. Adding fibres commonly enhances the mechanical properties of ULHSC but increases its density. In order to balance the strength and self-weight of ULHSC with different fibres, an extensive investigation was carried out to understand the effects of fibre incorporation on ULHSC made with fly ash cenospheres. This study focuses on the dynamic compressive response and failure mechanism of ULHSC with varied fibre contents. Impact tests were conducted by using a Φ100-mm splitting Hopkinson pressure bar apparatus with different strain rates ranging from approximately 20 s−1 to 120 s−1. The results showed that the compressive properties of ULHSC exhibited a strong strain rate dependency, and the addition of fibres increased the strain rate sensitivity of ULHSC, especially at high strain rates. It was interesting to find that the dynamic compressive strength increased with increasing fibre content, and the dynamic increase factor (DIF) showed the same tendency. These findings indicated that the tested ULHSC with 1 vol% end-hooked steel fibre maintained an excellent balance between the dynamic strength and density. Finally, a recalibrated model considering the effect of fibres was proposed to predict the DIFs of ULHSCs under impact loads.

The effect of supplementary cementitious material systems on dynamic compressive properties of ultra-high performance concrete paste

Supplementary cementitious materials (SCMs) have been widely used to replace cement in UHPC, which benefits to reducing CO2 emissions and declining construction costs. However, the influence of SCMs on the dynamic compressive properties of Ultra-high performance concrete (UHPC) paste has not been well understood. Here, the workability, pore structure, quasi-static compressive strength and dynamic compressive properties of UHPC paste with different SCM systems are comprehensively studied. The results show that the substitution of cement by ground granulated blast furnace slag (GGBS) and limestone powder (LP) increases workability. With the substitution rate of 10% LP, the microstructure of M2 becomes denser. With the further increasing replacement of GGBS and LP, the porosity increases, while the quasi-static compressive strength and dynamic compressive properties trend to decrease, but the extent is limited. The compressive properties such as dynamic compressive strength, dynamic increase factor (DIF), peak strain and toughness are highly dependent on the strain rate. An applicable DIF model for UHPC is established at the strain rate of 53.9 - 170.7 s−1. The fractal dimension increases with the rising of GGBS and LP. Additionally, a positive linear relationship is observed between the fractal dimension and the denary logarithms of the strain rate.

Dynamic mechanical properties of hybrid fiber reinforced concrete

In this study, the dynamic mechanical properties of hybrid fiber reinforced concrete (HFRC) are analyzed with respect to failure mode, dynamic increase factor (DIF), and peak strain by means of a SHPB testing apparatus. The factors that influence the dynamic mechanical properties include fiber type and fiber content. It is concluded that the best dynamic mechanical properties of fibers are CS-PHFRC at medium and low strain rates and AS-PHFRC at a high strain rate. Within a certain range, the higher the fiber content is, the larger the DIF of the corresponding HFRC and the more obvious the increase in dynamic compressive strength. AS-CSHFRC improves the dynamic compressive deformability of the HFRC. The polypropylene fiber causes plasticity, as shown in the failure mode of concrete. The Ottosen nonlinear elastic model, modified by introducing the damage factor, can better describe the dynamic mechanical properties of HFRC.

ИССЛЕДОВАНИЕ ДИНАМИЧЕСКИХ СВОЙСТВ ДРЕВЕСИНЫ ПРИ ОДНООСНОМ СЖАТИИ

The paper presents the results of an experimental study of dynamic deformation and fracture of pine samples under compression under uniaxial stress conditions. The experiments were carried out using the classical version of the Kolsky method. Dynamic tests of pine samples cut in three directions relative to the fibers were carried out: along and across and at an angle of 45 degrees. At least three modes of loading intensity (strain rate) are implemented for each orientation of the sample. Within each mode, at least five repeated tests were carried out with the averaging of the data obtained and the determination of the scatter characteristics. As a result of the experimental study for each tested sample, time dependences of strain, stress, strain rate of the samples were obtained, deformation diagrams were constructed. The analysis of the data obtained made it possible to determine the strain rate dependences of the ultimate strains, ultimate stresses, as well as the time of the beginning of fracture (lifetime). For each direction of sample cutting, the dependences of the dynamic increase factor on the strain rate are obtained. It is shown that the strain rate significantly affects the strength and deformation characteristics of pine. A tendency to increase the ultimate values of strains and stresses with an increase in the strain rate is obtained. At the same time, the time of the beginning of fracture decreases. The influence of the loading direction of the sample relative to the orientation of the fiber was also significant. The most durable is the direction along the fiber. The dynamic increase factor for the studied directions on the right limit of the realized strain rate range was about 3. The data presented in the work can be used in the tasks of assessing the dynamic strength of structural elements made of pine.

Progressive collapse analysis of corner-supported composite modular buildings

In this study, the alternate load path analysis is carried out to investigate the progressive collapse resistance of multi-storey modular buildings constructed using corner-supported composite modules. Three sudden column loss scenarios are considered: a corner column, an edge column at the long side, and an edge column at the short side. The non-linear dynamic analysis is first conducted to investigate the peak dynamic response of the locally damaged structures. The dynamic analysis results show that the 4-storey building prototypes remain elastic whilst partial yield in the joint regions is developed in the 12-storey counterparts; however, no collapsing tendency has been observed in cases under design loads. In the non-linear static analysis, the capacity curve and the overload factor for each case are then identified by increasing the gravity load until failure occurs. Four possible failure mechanisms are identified, varying among the different column loss scenarios and the building height. The suitable dynamic increase factors (DIFs) for the proposed composite modular buildings are identified on the basis of achieving the same maximum vertical displacement at the point of column removal when the non-linear static method is carried out to predict the peak dynamic response to sudden column removal. The DIFs of 1.90 and 1.60 are recommended for the 4- and 12-storey modular buildings, respectively.

Dynamic Increase Factors of Concrete-Like Materials at Very High Strain Rates

Dynamic Increase Factors of Concrete-Like Materials at Very High Strain Rates

Machine learning applications for assessment of dynamic progressive collapse of steel moment frames

A highly efficient implementation of machine learning (ML) framework is developed for assessing the dynamic increase factor (DIF) used in nonlinear static analysis (pushdown). Analysis datasets with a total of 3992 samples consisting of training (70%) and testing (30%) are generated by the Monte Carlo simulation that carried out by Sap2000 program application programming interface (API) link with Python in PyCharm software. The generated datasets are evaluated by the correlation matrix and relative feature importance of features that consist of damping ratio (Zeta) range, period of vertical vibration (Tv), ratio of duration of a column removal to Tv (Tau), ratio of factored moment to yielding moment (MMR) and ratio of factored rotation to yielding rotation (TTR). The current study incorporates implementation of K-nearest neighbors algorithm (KNN), extreme gradient boosting (XGBoost), back-propagation neural network (BPNN), and one-dimensional convolutional neural network (1DCNN). The results confirm the ability of the developed framework to efficiently implement regression analysis for the DIF.

State-of-the-art review on ultra high performance concrete - Ballistic and blast perspective

The world is experiencing multiple blasts and ballistic attacks in the last few decades leading to huge loss of civilians and warriors in fringe territories. In this context, this paper reviews the state-of-the-art information on ultra-high-performance concrete (UHPC) against blast and ballistic impact. The first part briefly discusses the ingredients with special emphasis on fibres behaviour in UHPC. Secondly, the review of dynamic characterization of UHPC carried with the aid of Split Hopkinson Pressure Bar (SHPB), covering pulse shaper technique, dynamic increase factor (DIF) and high strain behaviour. Thirdly, from ballistic perspective, various prediction models, penetration depth analysis and various research to tackle high-velocity impact are addressed. Further, shock tube studies and in-field blast testing of UHPC is summarized under blast perspective. Performance of reinforcement bars in UHPC for mitigation of ballistic and blast attacks is reviewed. Subsequently, numerical simulation and technology to enhance ballistic and blast resistant efficiency, such as inducing prestressing, use of metallic foam and blast resistant design are discussed. This review endeavours to help researchers identify areas of improvement needed in UHPC and highlight the benefits of UHPC for designers and consultants to use in structures to mitigate the effect of impact and blast.

Modeling strain rate effect on tensile strength of concrete using damage plasticity model

Experimental researches show that the strength of material increases with strain rate. However, dynamic response of quasi-brittle materials is significantly different under tensile and compressive loading. In order to simulate this phenomenon, damage plasticity model is adopted and enhanced with strain rate effect. Two different state variables as equivalent strain rates are defined, one for compression and other one for tension. Corrected strain rates are used instead of instantaneous ones, in order to consider retardation of damage evolution. Simulations of Split Hopkinson pressure bar (SHPB) are conducted to investigate dynamic tensile strength of concrete specimens. Performance of the proposed model is demonstrated by comparing with experimental results. Furthermore, a parametric study is conducted and effects of different parameters, such as dilatation angle, friction, thickness of the specimen, shape of impulse and impedance mismatch, on the dynamic increase factor are discussed in detail.