Dynamic Strength Increase Research Articles

An Experimental Study of the Dynamic Split Tension Properties of Reinforced Concrete

Dynamic split tensile tests of reinforced concrete were carried out using the split Hopkinson pressure bar experimental technique to determine the failure modes of reinforced concrete at different strain rates, and the effect of reinforcement ratio and reinforcement layouts on the dynamic performance. The specimens with nine reinforcement ratios were used in the tests. Experimental results show that the tensile strength of reinforced concrete exhibits a critical strain rate, beyond which a larger increase in dynamic strength of specimens occur. The dynamic split tension strength of reinforced concrete is demonstrated to be greater than the plain concrete with the same strength grade over the range of tested strain rates. The results also indicate that the dynamic split tension strength of specimens enhances with the increase in the reinforcement ratio. These findings are instrumental to guide the structural design of reinforced concrete in engineering applications.

Elastic Bands in Combination With Free Weights in Strength Training: Neuromuscular Effects.

This study compared the effects of a variable vs. a constant lower limb resistance training program on muscle strength, muscle activation, and ballistic muscle performance at different knee angles. Thirty-two females were randomized to a constant resistance training free-weight group (FWG) or a variable resistance training group using free weights in combination with elastic bands (EBG). Two variations of the squat exercise (back squat and split) were performed 2 days per week for 10 weeks. Knee extensor maximal voluntary isometric contraction (MVC) and countermovement jump were assessed at knee angles of 60, 90, and 120° before and after the intervention. During the MVCs, muscle activation of the superficial knee extensor muscles was measured using surface electromyography. The FWG increased their MVCs at 60 and 90° (24 and 15%, respectively), whereas the EBG only increased significantly at 60° (15%). The FWG increased their jump height significantly at all angles (12-16%), whereas the EBG only improved significantly at 60 and 90° (15 and 10%, respectively). Both groups improved their 6-repetition maximum free-weight squat performance (EBG: 25% and FWG: 23%). There were no significant changes in muscle activation. In conclusion, constant and variable resistance training provided similar increases in dynamic and isometric strength, and ballistic muscle performance, albeit most consistently for the group training only with free weights.

Numerical investigation of rock heterogeneity effect on rock dynamic strength and failure process using cohesive fracture model

In this study, a cohesive fracture model (CFM), which assumes rock material as assemblage of deformable grains joining together at their cohesive boundaries, is developed to simulate rock fracture and failure behavior in terms of mineral and cement properties. To better represent the structural bonds between the particles and the heterogeneity of rock material, a user-defined material model (UMAT) for the cohesive element with stochastic strengths is incorporated into the developed CFM. The capability of the developed model for rock fracture problems under both static and dynamic loads is first explored. Based on the developed model, the heterogeneity effect of rock on its dynamic failure process and dynamic strength is then studied. The results demonstrate that the developed CFM can be used to investigate the rock fracture problems under both static and dynamic loads. The heterogeneity of rock material not only affects the rock dynamic failure process but also leads to an increase of rock dynamic strength.

Impact resistance of reactive powder concrete

The impact behaviour of three types of reactive powder concretes (RPC) was studied using the split Hopkinson press bar (SHPB) testing method. These RPC were prepared with steel fiber volume fraction of 0%, 3%, and 4%, respectively. The stress-strain relationship, strain rate sensitivity threshold value, dynamic strength increase factor, modulus of elasticity and failure pattern of these RPC specimens subjected to impact load were investigated. From the tests, the strain rate sensitivity threshold value of 50 s−1 was obtained. The experimental results showed that when the strain rate increased from the threshold value to 95 s−1, the maximum stress of RPC increased by about 20% and the modulus of elasticity of RPC increased by about 30%. The failure pattern of RPC specimens with steel fiber reinforcement was very different from that of the RPC matrix specimen when subjected to impact loading. Under similar impact loading rate, cracks developed in the steel fiber reinforced RPC specimens, whilst the RPC matrix specimens were broken into small pieces.

Resistance of submicrocrystalline aluminum alloys to high-rate deformation and fracture after dynamic channel angular pressing

A comparative study of the deformation behavior of submicrocrystalline (formed by dynamic channel angular pressing) and coarse-grained commercial aluminum alloys (AMts and V95), and commercial aluminum A7 has been performed under impact compression conditions. Dynamic elastic limit (Hugoniot elastic limit) σHEL, yield strength Y, and spall strength σsp at a deformation rate of (1.2–3) × 105 s−1 were determined by analyzing the experimental velocity profiles of the free surface of samples. It has been found that the dynamic elastic limit and yield strength increase after all the materials studied have transformed into a submicrocrystalline state (crystallite size of 200–700 nm). Submicrocrystalline alloy V95 has the highest value of Y and submicrocrystalline aluminum A7 has the lowest one. The effect of grain size on spall strength is ambiguous. The values of σsp for submicrocrystalline alloys are 1.32–1.36 GPa. The submicrocrystalline V95 alloy and commercial A7 aluminum demonstrate a decrease in the spall strength, whereas the spall strength of the alloy AMts slightly increases with decreasing grain size.

Effects of Thermal Treatment on Tensile Strength of Laurentian Granite Using Brazilian Test

The effect of thermal treatment on several physical properties and the tensile strength of Laurentian granite (LG) are measured in this study. Brazilian disc LG specimens are treated at temperatures of up to 850 °C. The physical properties such as grain density, relative volume change per degree, and P-wave velocity are investigated under the effect of heat treatment. The results indicate that both the density and the P-wave velocity decrease with the increase in heating temperature. However, the relative volume change per degree is not sensitive below 450 °C, while a remarkable increase appears from 450 to 850 °C. All cases are explained by the increase in both number and width of the thermally induced microcracks with the heating temperature. Brazilian tests are carried out statically with an MTS hydraulic servo-control testing system and dynamically with a modified split Hopkinson pressure bar (SHPB) system to measure both static and dynamic tensile strength of LG. The relationship between the tensile strength and treatment temperatures shows that static tensile strength decreases with temperature while the dynamic tensile strength first increases and then decreases with a linear increase in the loading rate. However, the increase in dynamic tensile strength with treatment temperatures from 25 to 100 °C is due to slight dilation of the grain boundaries as the initial thermal action, which leads to compaction of rock. When the treatment temperature rises above 450 °C, the quartz phase transition results in increased size of microcracks due to the differential expansion between the quartz grains and other minerals, which is the main cause of the sharp reduction in tensile strength.

Strain rate effects for spallation of concrete

Appropriate triaxial constitutive laws are the key for a realistic simulation of high speed dynamics of concrete. The strain rate effect is still an open issue within this context. In particular the question whether it is a material property - which can be covered by rate dependent stress strain relations - or mainly an effect of inertia is still under discussion. Experimental and theoretical investigations of spallation of concrete specimen in a Hopkinson Bar setup may bring some evidence into this question. For this purpose the paper describes the VERD model, a newly developed constitutive law for concrete based on a damage approach with included strain rate effects (1). In contrast to other approaches the dynamic strength increase is not directly coupled to strain rate values but related to physical mechanisms like the retarded movement of water in capillary systems and delayed microcracking. The constitutive law is fully triaxial and implemented into explicit finite element codes for the investigation of a wide range of concrete structures exposed to impact and explosions. The current setup models spallation experiments with concrete specimen (2). The results of such experiments are mainly related to the dynamic tensile strength and the crack energy of concrete which may be derived from, e.g., the velocity of spalled concrete fragments. The experimental results are compared to the VERD model and two further constitutive laws implemented in LS-Dyna. The results indicate that both viscosity and retarded damage are required for a realistic description of the material behaviour of concrete exposed to high strain effects (3).

Influence of static pre-loading on the dynamic bending strength of concrete with particle element modeling

Experimental tests show that static pre-loading has a significant effect on the dynamic strength of concrete. Based on meso-scale particle element model, numerical simulations of dynamic bending tests with pre-loading are performed. Complete stress-strain relationships are then obtained. Significant increase in dynamic strength is found when the pre-loadings are imposed within the elastic limit of concrete. However, when the imposition of pre-loadings reaches the plastic or softening range, dynamic strengths may gradually decrease along with the increase in pre-loadings. The distribution of energy components and the failure modes are discussed to explain the mechanisms of the phenomena.

Study on the Mechanical Properties of High Performance Hybrid Fiber Reinforced Cementitious Composite (HFRCC) under Impact Loading

The dynamic material properties of high performance hybrid fiber reinforced cementitious composites (HFRCC) with various volumetric fractions of steel and polyvinyl alcohol (PVA) fibers were studied by the Split Hopkinson Press Bar (SHPB) test. The results show that HFRCC with higher volumetric fraction of steel fibers are more sensitive to stain rate and the dynamic compressive strength increase more prominently with the strain rate increasing, but peak strain shows the opposite trend. The PVA fibers increase the ductility of HFRCC more effectively than steel fibers. Compared to PVA fiber reinforced cementitious composites (FRCC), HFRCC present better dynamic material properties under impact loading.

Dynamic behaviour of normal-strength carbonate aggregate concrete at temperatures up to 800°C

The impact resistance of normal-strength carbonate concrete at elevated temperatures up to 800°C was experimentally studied using a split Hopkinson pressure bar (SHPB). The SHPB impact tests focused on the effects of concrete design strengths, high temperature and strain rate on the dynamic behaviour of concrete at elevated temperatures. The stress–time and strain–time curves of the tested specimens were recorded to analyse the impact behaviour of concrete at elevated temperatures. The influences of static compressive strength, high temperature and strain rate on the dynamic peak strength of concrete are discussed. The test results show that the strain rate effect of concrete at elevated temperature is significantly different from that of concrete at room temperature. Within the scope of the SHPB impact tests in this work, the strain rate effects of concrete at 200°C and 300°C are obviously higher than at room temperature. Concrete heated to 400°C was found to recover to around the same level of the dynamic strength increase factor (DIF) values of concrete at 20°C. The strain rate effects of concrete at 600°C and 800°C deteriorated sharply compared with those of concrete at room temperature and lower than 600°C. This paper also shows that a previously recommended formula for the DIF is not suitable for predicting the dynamic behaviour of concrete at temperatures higher than 400°C.

Nonlinear fracture dynamics of laminates with finite thickness adhesives

Finite thickness interfaces, such as structural adhesives, are often simplified from the modeling point of view by introducing ideal cohesive zone models that do not take into account the finite thickness properties in the evaluation of the interface stiffness and inertia. In the present work, the nonlinear dynamic response of those layered systems is numerically investigated according to the finite element method. The weak form of the dynamic equilibrium is written by including not only the contribution of cohesive interfaces related to the virtual work exerted by the cohesive tractions for the corresponding relative displacements, but also considering the work done by the dynamic forces of the finite thickness interfaces resulting from their inertia properties. A fully implicit solution scheme both in space and in time is exploited and the numerical results for the double cantilever beam test show that the role of finite thickness properties is not negligible as far as the crack growth kinetics and the dynamic strength increase factor are concerned.

Resistance exercise training modulates acute gene expression during human skeletal muscle hypertrophy

We sought to determine whether acute resistance exercise (RE)-induced gene expression is modified by RE training. We studied the expression patterns of a select group of genes following an acute bout of RE in naïve and hypertrophying muscle. Thirteen untrained subjects underwent supervised RE training for 12 wk of the nondominant arm and performed an acute bout of RE 1 wk after the last bout of the training program (training+acute). The dominant arm was either unexercised (control) or subjected to the same acute exercise bout as the trained arm (acute RE). Following training, men (14.8 ± 2.8%; P < 0.05) and women (12.6 ± 2.4%; P < 0.05) underwent muscle hypertrophy with increases in dynamic strength in the trained arm (48.2 ± 5.4% and 72.1 ± 9.1%, respectively; P < 0.01). RE training resulted in attenuated anabolic signaling as reflected by a reduction in rpS6 phosphorylation following acute RE. Changes in mRNA levels of genes involved in hypertrophic growth, protein degradation, angiogenesis, and metabolism commonly expressed in both men and women was determined 4 h following acute RE. We show that RE training can modify acute RE-induced gene expression in a divergent and gene-specific manner even in genes belonging to the same ontology. Changes in gene expression following acute RE are multidimensional, and may not necessarily reflect the actual adaptive response taking place during the training process. Thus RE training can selectively modify the acute response to RE, thereby challenging the use of gene expression as a marker of exercise-induced adaptations.

Dynamic Nonlinear Crack Growth at Interfaces in Multi-layered Materials

Finite thickness interfaces, such as structural adhesives, are often simplified from the modelling point of view by introducing ideal cohesive zone models that do not take into account the finite thickness properties in the evaluation of the interface stiffness and inertia. In the present work, the nonlinear dynamic response of those layered systems is numerically investigated according to the finite element method. The weak form of the dynamic equilibrium is written by including not only the contribution of cohesive interfaces related to the virtual work exerted by the cohesive tractions for the corresponding relative displacements, but also considering the work done by the dynamic forces of the finite thickness interfaces resulting from their inertia properties. A fully implicit solution scheme both in space and in time is exploited and the numerical results for the double cantilever beam test show that the role of finite thickness properties is remarkable as far as the crack growth kinetics and the dynamic strength increase factor are concerned.

Mechanical properties of ceramics–cement based porous material under impact loading

Cementitious materials and ceramic aggregates used as basic materials, ceramics–cement based porous material (CCPM) has been prepared. Φ100mm SHPB has been improved by wave shaping techniques, which can guarantee the availability of the tests. Quasi static compression test and impacting compression test have been carried out, the damage process of specimen under loading has been analyzed, and mechanics parameters under different strain rates have been obtained, moreover, based on this, the mechanical properties of CCPM under impact loading, including strength property, deformation property, impacting toughness, have been studied, in addition, the prospect of CCPM’s application has also been discussed. The results indicate that, the quasi static and impact compressive stress–strain curve of CCPM includes a strain plateau, which helps to better absorb energy; the dynamic strength increase factors of CCPM and the natural logarithm of relative strain rate are of a linear relationship; the relationship between the dynamic peak strain increase factors and the related strain rate can be described with an exponential linear, which shows obvious “damage softening” effect; with the increase of average strain rate, the impacting toughness of CCPM gets strengthened continuously and the impact toughness indexes are in a logarithm relationship with strain rate; CCPM is more strain rate sensitive than ordinary cement based composite materials. Thus it can be seen, CCPM possesses the advantageous mechanical properties of both porous materials and ordinary cement based composite materials. Besides, the material is easy to prepare and simple to make. Along with its high plasticity and low density, CCPM has a promising future to perform its potential advantages in engineering, especially in national defense engineering.

Research on the dynamic compressive test of highly fluidized geopolymer concrete

Slag and fly ash were used in the fabrication of geopolymer concrete (GC), sodium hydroxide (NaOH, NH) and sodium carbonate (Na2CO3, NC) were used for slag and fly ash activation, then highly fluidized C30 GC was prepared. Φ100mm SHPB apparatus was improved by the pulse shaper technique, and the dynamic stress equilibrium and nearly constant strain rate loading were realized. Based on this, the dynamic compressive mechanical properties, including strength, deformation and energy absorption property, of GC were tested, in addition, the mechanism of dynamic properties of the GC was primarily analyzed. The results indicate that, dynamic compressive mechanical properties of GC exhibit strong strain rate dependency; dynamic strength increase factor increases approximately linearly with the logarithm of the average strain rate; the peak strain increases firstly and decreased later with the rising of stain rate, 66.7s−1 is the critical strain rate between impact toughness and impact embrittlement; from an energy perspective, the correlation between specific energy absorption and stain rate is obvious and the equation type is binomial curve.

Dynamic compressive behavior of ceramic fiber reinforced concrete under impact load

Dynamic properties of ceramic fiber reinforced concrete (CRFRC) are investigated using a 100-mm-diameter split Hopkinson pressure bar (SHPB) system. Different ways of choosing strain rate were adopted to express the strain rate effects on dynamic compressive stress, elastic modulus and impact toughness of CRFRC. The results show that the dynamic compressive strength and impact toughness increase with strain rate, while the elastic modulus decrease with strain rate. The addition of ceramic fiber can significantly improve the dynamic strength and elastic modulus of concrete. 0.1% and 0.2% volume fraction of ceramic fiber improve the impact toughness at higher strain rate. And the optimum volume fraction of ceramic fiber is 0.2%. The dynamic damage model was established based on the improved parallel bar system (IPBS) model. The result reveals that the model can well describe the relation between stress and strain of CRFRC.

Strength-Training with Whole-Body Vibration in Long-Distance Runners: A Randomized Trial

A parallel group randomized trial was designed to analyze the impact of 6 weeks of strength training programs performed with or without whole-body vibration on muscular and endurance performance parameters in long-distance runners. 22 endurance runners were allocated into strength with whole-body vibration (n=8), without (n=8), and control (n=6) groups. Before and after the experimental period the subjects performed the following tests: a) maximum dynamic strength test, b) maximal incremental treadmill test, and c) time to exhaustion at velocity corresponding to maximal oxygen uptake. The fractions of the aerobic and anaerobic contribution in time to exhaustion test were also calculated. Both strength trained groups showed a similar increase in maximum dynamic strength (~18%). The aerobic contribution was enhanced for strength training group without whole-body vibration (~25%) after experimental period. No statistical differences were observed in any other variable. These results suggest that 6 weeks of strength training performed with or without whole-body vibration improve similarly the maximum dynamic strength in long-distance runners. In addition, both training modes studied had no deleterious effects on the traditional parameters of endurance performance, traditional strength training program results in increased aerobic contribution during high-intensity aerobic exercise.

Consolidation of calcareous and siliceous sandstones by hydroxyapatite: Comparison with a TEOS-based consolidant

Abstract A new consolidating treatment for limestone, based on the reaction between diammonium hydrogen phosphate (DAP) and calcite in the stone to form hydroxyapatite (HAP), has recently been proposed. Thanks to the formation of HAP at grain boundaries, so that grains are more effectively bonded, HAP-treated limestone undergoes a significant improvement in mechanical properties. Considering that the HAP treatment has the advantage of not substantially altering the transport properties of treated stones, unlike many other consolidating products currently used, in this paper, the effectiveness of HAP as a consolidant for sandstones with varying carbonate content and porosity was investigated and compared with that of a TEOS-based consolidant. Two lithotypes were used: Giallo Siena (a highly porous calcareous sandstone) and Pietra Serena (a quartzitic sandstone with low porosity). After preliminary artificial weathering according to previously developed methodologies, each lithotype was treated: (i) with a 1 M aqueous solution of DAP; (ii) with a solution of TEOS oligomers in isopropyl alcohol, each applied by brushing until apparent refusal. The effects of the two consolidating treatments were assessed by comparing microstructural, physical and mechanical properties of treated and untreated stones. HAP-treated specimens experienced an increase in dynamic elastic modulus and tensile strength substantially similar to that achieved by TEOS-treated ones. However, differently from samples consolidated with TEOS, HAP-treated stones experienced very low variations in porosity and pore size distribution, leading to substantially no variation in the rate of water sorption and only a small decrease in water vapor permeability. Considering the good performance on both lithotypes, comparable to that achieved by a TEOS-based treatment, HAP is confirmed as a promising consolidating treatment, which has the advantage of being effective after just two days, not substantially altering the transport properties of treated stones and not being hazardous for human health, thanks to its aqueous solvent.

Modeling of strain rate effects for concrete with viscoelasticity and retarded damage

Especially the tensile strength of concrete may by far exceed the quasistatic values under high strain rate conditions. Due to present knowledge this is predominantly caused by the higher resistance of water in the capillary system of concrete on one hand and by the limited speed of the evolution of micro and macro cracks on the other hand. This contribution combines a viscoelastic approach for the first case with a gradient damage approach for the second, whereby damage is retarded due to inertial effects under high strain rates. The combined material law includes quasistatic behavior as a special case and covers dynamic material behavior up to a strain rate in a magnitude of 103 s−1. The material law is implemented in a Finite Element Method and first of all applied to uniaxial wave propagation problems. Thus, typical experimental results for dynamic tensile strength increase factors may be reproduced within a calibration procedure for the three dynamic material parameters of the material law. Furthermore, the structural behavior of a plain concrete beam under impact loading is investigated. The effects of structural mass inertia and the material's strain rate sensitivity to the dynamic load bearing capacity are exemplarily outlined. Finally, it is demonstrated how the dynamic load bearing capacity depends on load duration beneath load amplitude.

Neuromuscular adaptations during combined strength and endurance training in endurance runners: maximal versus explosive strength training or a mix of both

This study compared the effects of mixed maximal strength and explosive strength training with maximal strength training and explosive strength training combined with endurance training over an 8-week training intervention. Male subjects (age 21-45years) were divided into three strength training groups, maximal (MAX, n=11), explosive (EXP, 10) and mixed maximal and explosive (MIX, 9), and a circuit training control group, (CON, 7). Strength training one to two times a week was performed concurrently with endurance training three to four times a week. Significant increases in maximal dynamic strength (1RM), countermovement jump (CMJ), maximal muscle activation during 1RM in MAX and during CMJ in EXP, peak running speed (S (peak)) and running speed at respiratory compensation threshold (RCT(speed)) were observed in MAX, EXP and MIX. Maximal isometric strength and muscle activation, rate of force development (RFD), maximal oxygen uptake [Formula: see text] and running economy (RE) at 10 and 12kmhr(-1) did not change significantly. No significant changes were observed in CON in maximal isometric strength, RFD, CMJ or muscle activation, and a significant decrease in 1RM was observed in the final 4weeks of training. RE in CON did not change significantly, but significant increases were observed in S (peak), RCT(speed) and [Formula: see text] Low volume MAX, EXP and MIX strength training combined with higher volume endurance training over an 8-week intervention produced significant gains in strength, power and endurance performance measures of S (peak) and RCT(speed), but no significant changes were observed between groups.