Dynamic Strength Enhancement Research Articles

Static and dynamic mechanical properties of magnesium phosphate cement modified by metakaolin after high-temperature treatment

Magnesium phosphate cement (MPC) is widely used in fields of construction and fire-retardant coatings due to its excellent properties. The hydration properties of MPCs at high temperatures are the points of interests. In this work metakaolin was used to improve workability and mechanical properties of MPC, and static and dynamic mechanical properties and hydration products of the MPC after high temperature treatment were investigated. The results showed that the incorporation of metakaolin in MPC allows extending and importantly tuning the setting time of MPC. Notably, 40% replacement of dead burnt magnesia with metakaolin in MPC could extend the setting time of MPC to about 24 min. While the fluidity of MPC significantly decreased with increasing metakaolin content. Typically, increasing the temperature leads the deterioration of mechanical properties of the MPC. However, static and dynamic strength enhancement was also observed at 600℃. The main hydration products of MPC with metakaolin are struvite (MgNH4PO4·6H2O), unreacted MgO, aluminum phosphate (APH) and possibly amorphous substance containing aluminum. Moreover, the impact resistance of MPC at high temperatures were significantly improved with incorporation of metakaolin.

Dynamic Characterization of Concrete using Split Hopkinson Pressure Bar

Concrete exhibits brittle behaviour and is weak under tensile and flexural loading. The response of concrete to dynamic loading is of interest in a variety of civilian and military applications. Understanding the response of concrete to impact or explosive loading is important for effective protection of defence and civil structures. The split Hopkinson pressure bar (SHPB) technique has been used widely to measure the dynamic strength enhancement of materials at high strain rates. Although, SHPB technique has been verified for metallic materials, but validity and accuracy of SHPB results for non-metallic, e.g. concrete materials have not been thoroughly studied so far. The present study examines the application of SHPB to determine the dynamic strength of concrete under compressive loading. The aim of this study is to understand the strain rate effect on the ultimate uniaxial compressive strength of concrete in SHPB tests for two different grades of concrete. The behaviour of concrete at strain rates of the order of 200 - 600 per second and pressures up to 0.38 MPa are studied experimentally. The strength of concrete is found to be increased with the increase in strain rates. Further, it is observed that due to the composite microstructure of concrete, deformation and stresses are non-uniform in the concrete specimens.

Effect of dry-wet cycles on dynamic mechanic and microstructure of cemented broken mudstone

The secondary utilization of abandoned mine is usually accompanied by water erosion, cyclic impact, and induced earthquake, which induces a great engineering challenge for the mudstone stratum reconstruction. Consequently, this paper carried out ultrasonic detection, split Hopkinson pressure bar (SHPB) impact, thermogravimetry, and mercury intrusion porosimetry tests to investigate the dynamic performance and microstructure of reinforcing mudstone under water erosion. The results show that the dynamic compression strength (DCS) and strain-rate enhancement effect of cemented broken mudstone (CBM) firstly increase and then decrease with the increase of dry-wet cycles, the maximum growth is 15.38% for CBM in 3 dry-wet cycles while the maximum drop is 19.63% for that in 9 dry-wet cycles. The dry-wet cycles mainly affect the DCS by changing the elastic limit strength, and promote the carbonization of Ca(OH)2 to produce up to 9.365% CaCO3 during the cemented matrix’s hydration. In the late dry-wet stage, the deterioration of internal mudstone particles induces pore cracks and causes up to 19.63% decrease in DCS. This study reveals the evolution law of dynamic mechanical properties of CBM under water erosion, and provides experimental reference for the reconstruction of abandoned mine.

Dynamic compression behavior of ultra-high performance concrete with hybrid polyoxymethylene fiber and steel fiber

Synthetic fibers are adopted to mix with conventional steel fibers (SF) for ultra-high performance concrete (UHPC) to improve chemical resistance and reduce self-weight. Polyoxymethylene fiber (POMF), as a type of lightweight, high-strength, corrosion-resistant synthetic fiber, is expected to replace SF generally used in UHPC. Due to the obvious difference of mechanical properties between POMF and SF, different POMF/SF hybrid ratios may affect the dynamic compression behavior of UHPC. To investigate the dynamic mechanical behavior of hybrid POMF/SF reinforced UHPC (HUHPC), axial compression and split Hopkinson pressure bar (SHPB) tests with strain rate from 40 to 170 s−1 of HUHPC were conducted. The fiber volume fraction was fixed at 3% and four different POMF/SF hybrid ratios (0:3, 1:2, 2:1, 3:0) were adopted. The effects of hybrid POMF/SF ratios on the dynamic compressive properties of HUHPC, including dynamic compressive strength, dynamic elastic modulus, dynamic increase factor (DIF), and impact toughness were discussed. The results show that HUHPC exhibits obvious strain rate sensitivity under impact loading. With the increase of strain rate, the dynamic compressive strength, dynamic elastic modulus, DIF and impact toughness of HUHPC increased. Based on the linear assumption of fiber volume fraction and dynamic compressive strength enhancement of UHPC, the calculated dynamic compressive strength of HUHPC is lower than the experimental values. As the strain rate increases, the propagation path of microcrack transforms from interface of fiber-matrix to penetrating the fiber, resulting in plastic deformation of SF and fracture of POMF, consuming a large amount of impact energy and improving the impact toughness resistance of HUHPC.

Mesoscopic investigation on the mechanism of concrete dynamic tensile strength enhancement based on the E(A, B) algorithm

The dynamic tensile strength of concrete shows a certain dynamic enhancement, whereas the potential mechanism of the dynamic enhancement has not been clearly revealed so far. This paper studies the mechanism of concrete dynamic tensile strength enhancement and identifies the contribution of various factors to the dynamic enhancement. In order to study the fracture of concrete under dynamic load, this paper first developed a three-dimensional polyhedral aggregate packing program by adopting the algorithm of Entrance block between A and B (E(A, B)), which greatly simplifies the calculation of contact or overlapping judgment between two blocks in the modeling process. A cohesive model is then proposed to describe the fracture in the microscopic concrete model. Through the verification simulations, appropriate material parameters and mesh size are selected. Finally, the effects of the micro-crack inertia, material heterogeneity, the structural inertia and the material viscosity on the concrete dynamic tensile strength enhancement are discussed. The results show that the dynamic tensile strength of concrete is not significantly affected by the micro-crack inertia but is highly related to the material heterogeneity, the structural inertia and the material viscosity. Specifically, the material heterogeneity has a more pronounced effect on the dynamic tensile strength at higher strain rate. The contribution of the structural inertia effect and the material viscosity to the enhancement of dynamic tensile strength rank the second and first respectively, and their influence degree increase with the increase of strain rate.

Simulation and Research on Dynamic Crushing of Rod Concrete

The dynamic mechanical properties of waste glass powder concrete are studied, and the dynamic compressive strength, peak strain, stress-strain curve and dynamic enhancement factor change with strain rate and waste glass powder content are analyzed. Based on the HJC model of concrete, the SHPB test was simulated by ANSYS/LS-DYNA software. With the increase of impact pressure, the phenomenon from block spalling to aggregate crushing appears. When the impact velocity is low, several radial through cracks appear in the test block, and the test block is separated into several blocks with larger volume. With the increase of impact velocity, multiple cracks develop simultaneously.

A Micromechanical Model for Simulation of Rock Failure Under High Strain Rate Loading

Quasi-brittle materials such as rock are rate sensitive materials and their behaviour under dynamic loading is not identical with that under static loading. In this study, numerical Brazilian tensile tests are conducted using a Split Hopkinson Pressure Bar system in an attempt to reproduce the dynamic increase factors (DIF) of the experimental tests. The rock is modelled by a bonded particle system made of spherical particles which interact at the contact points. The numerical results indicate that while the bonded particle system with a simple contact bond model can closely mimic the static behaviour of the sandstone specimens, it lacks what is needed for a rate dependent material. Therefore, a micromechanical model in which the contact bond strength is allowed to vary in proportion to the relative velocity of the involved particles is introduced. It is shown that the modified model can reproduce the physical tests data reported in the literature. In particular, with the application of strength enhancement coefficients in the range of 0–16 × 105, DIF values of 1.1–13 are obtained in the indirect tensile Brazilian tests, and the induced strain rate in the specimen is in 10–1000 s−1 range. Our preliminary study indicates that the model, consistent with the fact reported for the quasi-brittle materials, shows different rate-dependent sensitivity and dynamic strength enhancement in tension and compression. The micromechanical parameters in the proposed model can be adjusted to reproduce the physical rock strength, and that the shape of the reflected and transmitted numerical waves can be modified to approach those in the physical tests.

Coupled effects of high temperature and strain rate on compressive properties of hybrid fiber UHTCC

This study investigates the combined effect of the strain rate and temperature on the compressive properties of hybrid fiber ultra high toughness cementitious composites (UHTCCs) using a Split Hopkinson pressure bar. Specimens were first heated to different exposure temperatures, e.g. ambient temperature, 200, 400, 500, 600 and 800 °C, and subsequently, cooled to ambient temperature. Thereafter, the specimens were tested at four different strain rates. The test results show that the dynamic compressive strength of the UHTCC is enhanced at a temperature of 200 °C, and subsequently, decreases with the increase in exposure temperature. The strain rate sensitivity of UHTCC is largely enhanced with the increase in exposure temperature. The possible mechanism of this phenomenon was discussed based on the high-speed photography of the crack propagation process on the surface of the specimens and microscopic observation of fibers condition on their fracture surfaces. Moreover, an empirical relationship is established to express the dynamic strength enhancement of fire-damaged UHTCC as a function of strain rate.

Experimental study on dynamic mechanical property of cemented tailings backfill under SHPB impact loading

Cemented tailings backfill (CTB) have increasingly been used in recent years to improve the stability of mining stopes in deep underground mines. Deep mining processes are often associated with rock bursting and high-speed dynamic loading conditions. Therefore, it is important to investigate the characteristics and dynamic mechanical behavior of CTB. This paper presents the results of dynamic tests on CTB specimens with different cement and solid contents using a split Hopkinson pressure bar (SHPB). The results showed that some CTB specimens exhibited one to two lower stress peaks after reaching dynamic peak stress before they completely failed. The greater the cement-to-tailings ratio is, the more obvious the strain reaction. This property mainly manifested as follows. First, the dynamic peak stress increased with the increase of the cement-to-tailings ratio when the impact velocity was fixed. Second, the dynamic peak stress had a quadratic relationship with the average stress rate. Third, the cement-to-tailings ratio could enhance the increase rate of dynamic peak stress with strain rate. In addition, the dynamic strength enhancement factor K increased with the increase of strain rate, and its value was larger than that of the rock samples. The failure modes of CTB specimens under low-speed impact were tensile failure and X conjugate shear failure, where were nearly the same as those under static uniaxial and triaxial compression. The CTB specimens were crushed and broken under critical strain, a failure mode similar to that of low-strength concrete. The results of the experimental research can improve the understanding of the dynamic mechanical properties of CTB and guide the strength design of deep mining backfills.

Dynamic in-plane compression of Miura-ori patterned metamaterials

The responses of Miura-ori patterned metamaterials to in-plane dynamic compression are studied for several scenarios, namely: compression of uniform density metamaterials at a constant velocity and impulsive loading modelled as an impact with initial velocity, and a mass impact of a metamaterial with graded density. Analytical models of the dynamic strength enhancement, impact velocity attenuation and energy absorption are proposed for materials with uniform initial density and materials with positive density gradient. Attention is paid to the influence of the material topology governed by the initial dihedral angle γ0 on the dynamic response of origami-based stationary blocks with equal densities. The propagation of disturbances—velocity and nominal strains—along the samples is analysed and it is shown that the localisation is more pronounced for small values of γ0. Different from other open cell materials (e.g. foam and honeycomb), the propagation speed of the yield stress, which corresponds to the quasi-static strength, is not related to the elastic properties of the metamaterial in the form of precursor elastic wave as it is governed by the inertial properties of the cells due to the local structural softening. Therefore, the in-plane dynamic compression of Miura-ori patterned metamaterials cannot be directly interpreted by means of the shock wave theory commonly used for cellular materials with different topologies. Nonetheless it is shown that, similarly to other cellular materials, the Miura-ori patterned metamaterials exhibit increased energy absorption capacity when increasing the loading rate. However, the dynamic energy absorption capacity of the analysed metamaterials in not uniquely defined by their relative density and is strongly dependent on their topology. The analytical models are verified by numerical simulations.

Dynamic strength enhancement and strain rate sensitivity in ice-templated ceramics processed with and without anisometric particles

We investigated the effects of platelets and strain rate on the compressive response of ice-templated porous alumina, processed from ultrafine, equiaxed particles and from a mixture of ultrafine, equiaxed and large platelet particles. Results revealed a remarkable enhancement of the quasistatic and dynamic compressive mechanical properties of the templated ceramics in the presence of platelets. Specific compressive mechanical properties of the ice-templated alumina containing platelets are superior in comparison to those of the various metallic, syntactic and natural cellular solids. In the presence of platelets, compressive strength exhibited contrasting strain rate sensitivity between the elastic and inelastic stages of deformation.

Dynamic tensile strength enhancement of concrete in split Hopkinson pressure bar test

Split Hopkinson pressure bar technique has been widely used to measure the dynamic tensile strength of concrete materials. Most experimental results show that the tensile strength of concrete material increases with strain rates. However, the dynamic tensile strength derived from the split Hopkinson pressure bar test is affected by lateral inertia confinement, which may lead to the overestimation of dynamic mechanical properties of concrete materials. The true dynamic characteristics of concrete materials are not actually shown by experimental data. It is impossible to completely eliminate the influence of lateral inertia confinement in split Hopkinson pressure bar tests. In this study, a rate-insensitive material model is used in commercial finite element software to study how the lateral inertia confinement affects the dynamic tensile strength of concrete material at strain rates between 30/s and 150/s. Comparison of finite element results and split Hopkinson pressure bar test results shows that the dynamic tensile strength enhancement of concrete materials is strongly influenced by the inertial effect. The dynamic increase factor of concrete materials which remove the influence of lateral inertia confinement in split Hopkinson pressure bar tests can reflect the true dynamic characteristics of concrete materials. It is also found that the influence of lateral inertia confinement is related to the size of the specimen.

Inertial effect on concrete-like materials under dynamic direct tension

The inertial effect on the dynamic strength enhancement of concrete-like materials has been widely concerned and its influence on the dynamic tensile strength is particularly controversial, causing great disturbance to dynamic measurement. Therefore, both the experimental and the numerical analyses on the tubular specimens of concrete-like materials are conducted to further investigate the degree of inertial effect under dynamic direct tension. The inertial effect is one of the structural effects, and therefore the tubular specimens with different inner diameters are employed to demonstrate the different influences of inertial effect on the dynamic tensile strength of concrete-like materials. The experimental and numerical results indicate that the inertial effect has some influence on dynamic tensile strength, which increases with strain rate. In addition, the surface area of concrete-like materials will be greatly affected by inertial effect under dynamic tensile loading.

Strain Rate Effect in the Mesoscopic Modeling of High-Strength Steel Fiber-Reinforced Concrete

The research presented in this paper aims to investigate the behavior of a high-strength steel fiber-reinforced concrete (HSSFRC) mesoscopic finite element model, at compressive high strain rates. In order to produce a three-dimensional meso-scale finite element model, a computer code is developed to randomly produce mesoscopic models of SFRC specimen. The specimen is assumed to be reinforced by 0.6 percent volume fraction of hooked steel fibers (Dramix RC-65/35-BN) with random positions and orientations. Aggregates of the compound are assumed to have spherical shape and are produced according to Fuller grading curve. Based on the initial mesoscopic model, a finite element model is produced and used in the explicit dynamic simulation. The contribution of inertial confinement in the dynamic strength enhancement of concrete at high strain rates was investigated and its effective role was observed. Accordingly defining a Dynamic Increase Factor (DIF) for mortar matrix led to overestimation; nevertheless, the inertial confinement by itself could not justify the increment of specimen strength under the dynamic loading. Obtained results also show that steel fibers have a negligible influence on the strength, strength enhancement ratio (DIF) and post peak behavior of the model at high strain rates.

Investigation into Pure Rate Effect on Dynamic Increase Factor for Concrete Compressive Strength

Abstract Dynamic increase factor (DIF) has been used to consider rate effect on compressive strength of concrete in both design and analysis of concrete structures loaded with high rate. Until now, a variety of DIFs have been suggested by various researchers, and these DIFs are adopted in design guidelines and model codes, e.g., ACI 349-13, ACI 370R-14, fib MC2010, and UFC 3-340-02. However, the DIFs includes the axial and radial inertia effects, which cause confining effect and resistance to deformation. Therefore, additional strength enhancement due to the inertia effects is included in the DIFs, so applying the DIFs to design and analysis can lead to nonconservative results. In this study, in consideration of the strain acceleration and geometry of a specimen, a new DIF, which is exclusive of the inertia effects, was suggested based on concrete split Hopkinson pressure bar (SHPB) test results. Furthermore, to validate the new DIF, finite element analyses to which the existing and new DIFs were applied were performed for concrete SHPB tests. The verification results indicated that the DIF proposed in this study improve predictions of the dynamic strength enhancement of concrete compared with the existing DIFs.

Theoretical and Experimental Justification of Low-Rigid Components’ Use for Concrete Dynamic Strength Enhancement

The article presents methods of applied mechanics for solving problems in materials science, in particular to predict the dynamic strength of concrete with low-rigid dispersed materials. Various low-rigid dispersed materials which were included in the composition of cement concrete for replacement of a part of sand for assessment of their effect on its dynamic strength were studied. Besides, the simulation task of low-rigid components effect mechanism on a dynamic strength was solved by means of numerical methods.The computing tool, that is capable to take into account explicitly the elastic properties of structure elements in conjunction with their interaction in space, is needed to simulate system operation in the dynamic mode of loading. The finite element method (FEM) used on the basis of structurally-simulated models corresponds to this criterion to the maximum extent.The model that reproduces the mechanism of crack formation in a structure under load was developed. This model allows assessment of the influence of different factors, both separately and in combination, for determination of the main regularities of dynamically strong concrete obtaining on the basis of use of components with reduced rigidness.

Further Investigation on the Real Rate Effect of Dynamic Tensile Strength for Concrete-Like Materials

AFFECTED BY THE INERTIA EFFECT, REAL RATE EFFECT AND END FRICTION EFFECT (ONLY EXISTED IN SPLITTING AND SPALLING TESTS), THE EXPERIMENTAL TENSILE STRENGTH OF CONCRETE-LIKE MATERIALS GREATLY INCREASES WITH THE LOADING-RATE OR STRAIN-RATE. TO FURTHER INVESTIGATE THE INFLUENCE DEGREE OF REAL RATE EFFECT FOR CONCRETE-LIKE MATERIALS IN DYNAMIC TENSILE TESTS, THIS PAPER SYSTEMATICALLY CONDUCTS EXPERIMENTAL RESEARCH TO STUDY DYNAMIC BEHAVIORS OF CONCRETE-LIKE MATERIALS IN TENSILE EXPERIMENTS, VIZ. DIRECT TENSILE TESTS, SPLITTING TESTS AND SPALLING TESTS. AT THE SAME TIME, FINITE ELEMENT SIMULATIONS ARE EMPLOYED TO ANALYZE THE MECHANICAL CHARACTERISTICS OF CONCRETE-LIKE MATERIALS, WHERE A HYDROSTATIC PRESSURE DEPENDENT MODEL, THE DRUCKER-PRAGER CONSTITUTIVE MODEL, IS USED FOR SPECIMENS, WHICH CAN CONSIDER THE INFLUENCE OF INERTIA EFFECT. IN THE NUMERICAL MODEL, THE SPECIMEN IS SET WITH RATE-INDEPENDENCY, THUS THE PREDICTED DYNAMIC TENSILE STRENGTH OF SPECIMENS IS FREE OF THE REAL RATE EFFECT. THE END FRICTION EFFECT IS ALSO TAKEN INTO ACCOUNT IN THE NUMERICAL ANALYSIS OF DYNAMIC SPLITTING AND SPALLING TESTS. IT IS FOUND THAT THE DYNAMIC TENSILE STRENGTH ENHANCEMENT OF CONCRETE-LIKE MATERIALS IN NUMERICAL SIMULATIONS DOES NOT VARY OBVIOUSLY WITH LOADING-RATES, WHICH INDICATES THAT THE INERTIA EFFECT AND END FRICTION EFFECT MAKE LITTLE CONTRIBUTIONS TO THE DYNAMIC INCREASE FACTOR (DIF) OF CONCRETE-LIKE MATERIALS. THEREFORE, THE REAL RATE EFFECT DOMINATES THE DYNAMIC TENSILE STRENGTH ENHANCEMENT OF CONCRETE-LIKE MATERIALS IN LABORATORY TESTS, BUT THE INERTIA EFFECT AND END FRICTION EFFECT DO NOT.

Experimental study on granite and the determination of its true strain-rate effect

To accurately determine the true strain-rate effect of granite in split Hopkinson pressure bar (SHPB) tests, systematic experimental studies from quasi-static to dynamic loading on the same batch of granite samples is required. Therefore, firstly, splitting, uniaxial and triaxial compression tests were used to study the mechanical response of granite under different static stress conditions with the MTS rock mechanics test system, and the impact compression tests were performed at different strain-rates by the split Hopkinson pressure bar (SHPB). The test results show that the compressive strength increases with the increase of confinement, but the increase rate decreases as the confinement gets larger. The axial failure strain also increases with the increase of confinement. Failure is related to the composition and structure of granite, as well as the stress state. With increasing confinement, the sample is more constrained, the elastic limit strain becomes smaller, and the elastic modulus becomes larger accordingly. In addition, shear slip failure takes place under triaxial compression. In the dynamic compression tests, the strain-rate affects not only the strength of granite, but also the degree of fragmentation and the breaking pattern. Also, it has been found that the dynamic compressive strength enhancement of rocks under impact loading is due to the combined effects of the material strain-rate, lateral inertia and end friction; however, in SHPB tests they are coupled together and could not be separated from each other. To determine the material strain-rate effect of rocks in the SHPB tests, the dynamic compressive strength enhancement caused by the lateral inertial effect and end friction effect needs to be removed. Assuming that the effect of the material strain-rate, lateral inertia and end friction is uncoupled, the numerical simulation method has been employed to simulate the SHPB tests on granite. The true strain-rate effect of granite in SHPB tests is thus determined.

Effects of Plyometric Training Volume and Training Surface on Explosive Strength

The purpose of this study is to examine the effects of different volume and training surfaces during a short-term plyometric training program on neuromuscular performance. Twenty-nine subjects were randomly assigned to 4 groups: control group (CG, n = 5), moderate volume group (MVG, n = 9, 780 jumps), moderate volume hard surface group (MVGHS, n = 8, 780 jumps), and high volume group (HVG, n = 7, 1,560 jumps). A series of tests were performed by the subjects before and after 7 weeks of plyometric training. These tests were measurement of maximum strength (5 maximum repetitions [5RMs]), drop jumps (DJs) of varying heights (20, 40, and 60 cm), squat and countermovement jumps (SJ and CMJ, respectively), timed 20-m sprint, agility, body weight, and height. The results of the present study suggest that high training volume leads to a significant increase in explosive performance that requires fast stretch-shortening cycle (SSC) actions (such as DJ and sprint) in comparison to what is observed after a moderate training volume regimen. Second, when plyometric training is performed on a hard training surface (high-impact reaction force), a moderate training volume induces optimal stimulus to increase explosive performance requiring fast SSC actions (e.g., DJ), maximal dynamic strength enhancement, and higher training efficiency. Thus, a finding of interest in the study was that after 7 weeks of plyometric training, performance enhancement in maximal strength and in actions requiring fast SSC (such as DJ and sprint) were dependent on the volume of training and the surface on which it was performed. This must be taken into account when using plyometric training on different surfaces.

Semi-empirical equations for the dynamic strength enhancement of concrete-like materials

SHPB tests are often employed to characterize the dynamic compressive strengths of concrete-like materials which are very important for the construction of reliable constitutive equations used in numerical simulations. The SHPB test data, however, need to be interpreted correctly as recent studies have revealed that the apparent compressive strength enhancement found in the SHPB tests might be due to a combination of effects such as material inherent behavior (rate sensitivity) and inertial confinement. To this end, semi-empirical equations are suggested for the dynamic strength enhancement of concrete-like materials. The equations are formulated on the basis of the recent numerical analysis that dynamic tensile tests on concrete-like materials have no inertial confinement effects and the assumption that the increments of the tensile and compressive strengths due to strain rate effects only are equal at the same strain rate. It is shown that the present semi-empirical equations are in reasonable agreement with available test data for concrete materials.