Increasing Loading Rate Research Articles

Experimental investigation of dynamic bond behaviors between LRS-FRP and concrete

The dynamic bonding behaviour at the interface between a large-rupture-strain fibre-reinforced polymer (LRS-FRP) and concrete was investigated via drop weight impact tests of notched beam specimens. The main experimental variables included the concrete strength grade, number of LRS-FRP layers, and loading rate. With increasing loading rate, the location of the peeling damage at the interface between the LRS-FRP and concrete shifted from the interior of the concrete layer to the concrete–binder layer interface, binder layer, and even binder layer–FRP interface. The ultimate debonding load of the LRS-FRP strips increased with increasing strain rate, reaching more than 1.7 times the static value at a strain rate of approximately 4/s, while the effective bond length did not show a clear increasing or decreasing tendency with increasing strain rate. For the local bond-slip relationship, both the peak interfacial bond stress and interfacial fracture energy increased with increasing strain rate, and the influences of the concrete strength and number of FRP layers on the strain rate sensitivity were insignificant. Based on the experimental results, improved models to predict the ultimate debonding load and effective bond length at the LRS-FRP–concrete interface under both static and dynamic loading conditions were proposed. Furthermore, dynamic increase factor (DIF) formulas for the peak interfacial bond stress and interfacial fracture energy for bond-slip modelling of the interface between a polyethylene naphthalate (PEN)-FRP and concrete were established.

Improved conductive and self-sensing properties of cement concrete by PDMS/NCB-impregnated recycled fine aggregate

Polymer emulsion impregnation is a common method for strengthening recycled fine aggregate (RFA). By mixing Nano carbon black (NCB) into the polymer emulsion, highly conductive modified RFA (MRFA) can be simultaneously obtained. In this paper, highly conductive and self-sensing concrete was prepared by using the MRFA impregnated with the NCB and polydimethylsiloxane (PDMS) solution. The microstructure of MRFA and MRFA concrete physico-mechanical properties were studied. The effects of replacement ratio of MRFA, temperature, water content, and loading rate on the conductivity and self-sensing of prepared MRFA concrete was explored. The results show that the PDMS fills the pores of the RFA surface. The water absorption decreases by 2.53%, the apparent density and fineness modulus increased by 1.5% and 8.3%, respectively. With the increase of MRFA replacement ratio, the resistivity of concrete gradually decreases, and a sharp decline appears from 60% to 70%. The percolation threshold of concrete is calculated to be 3.37 vol% of NCB. Meanwhile, the MRFA concrete has good temperature sensitivity and humidity adaptability. Under the stress cycle loading, the stress sensitivity of the MRFA concrete with 70% replacement ratio MRFA is as high as 4.45%/MPa. Besides, the stress sensitivity of MRFA concrete decreases as the loading rate increases.

Dynamic impact experimental and global cohesive element method to shale fracture characterization

The fracture behavior and various of mechanical mechanisms of shale are greatly influenced by the bedding surface. To study the effects of bedding angles on the crack propagation patterns and dynamic fracture properties, the shale notched semi-circular bend (NSCB) specimens were studied by the modified split Hopkinson pressure bar (SHPB) device, three-dimensional digital image correlation (3D-DIC) system with the high-speed (HS) photography and ABAQUS finite element software. The research results showed that at the same impact velocity, the 30°, 45° and 60° beddings played an obvious deflexion effect on the shale crack propagation. However, the 0° and 90° bedding did not deflect the crack propagation. The dynamic fracture toughness of shale NSCB specimens increased with the increase of impact loading rate but the increase magnitude decreased with the higher impact rate. The kinetic energy during specimen fracture was calculated by the extracting displacement data from the 3D-DIC, and it was found that the kinetic energy accounted for less than 6% of the absorbed energy during the dynamic compression of the shale. The crack propagation characteristics of ABAQUS simulation was approximately consistent with the experimental results. It is conducive to the study of shale gas fracturing and the formation of complex seam networks in shale and it is of great research significance for the efficient extraction of shale gas.

Poisson's ratio testing method and multi-factor influence quantification model of asphalt pavement materials

The current standard in China does not specify the standard test method for determining Poisson's ratio, and the Poisson's ratio used in pavement design is often roughly fixed, which can result in large errors in the calculations and analysis of road structure. In this paper, the research on different test methods for Poisson's ratio is carried out and the non-linear characteristics of tensile and compressive Poisson's ratio on pavement materials under different influencing factors are analyzed. The results indicate that the absolute value of the ratio between transverse rebound strain and longitudinal rebound strain under 50% of the maximum destructive load can objectively characterize the mechanical properties of Poisson's ratio, and it is the optimal measurement method for Poisson's ratio in asphalt pavement materials. The Poisson's ratio of two pavement materials both decreases with the increase of loading rate and gradually tends to a constant value. Cement dosage and temperature are the most significant factors affecting the tensile and compressive Poisson's ratio of cement stabilized crushed stone and asphalt mixture, respectively. Specifically, when cement dosage increases by 1%, the Poisson's ratio of cement stabilized crushed stone decreases by approximately 10%, and the temperature has a significant influence on the difference in tension and compression for Poisson's ratio of asphalt mixture (up to 40%). The asphalt content corresponding to the minimum Poisson's ratio is basically consistent with the optimal asphalt content obtained by the Marshall volume method. Based on these, a quantification value-taking model is established of the tensile and compressive Poisson's ratios of typical asphalt pavement materials varying with multiple factors. The research results can serve as a reference for the scientific value-taking of pavement structural design parameters considering the different mechanical characteristics of materials in tension and compression.

Enhancing anammox process with granular activated carbon: A study on Microbial Extracellular Secretions (MESs)

Granular activated carbon (GAC), a porous carbon-based material, provides increased attachment space for functional microorganisms and enhances nitrogen removal by facilitating extracellular electron transfer in the anammox process. This study investigates the effects of GAC on the biosynthesis of microbial extracellular secretions (MESs) and explores the roles of these secretions in anammox activities. Four lab-scale reactors were operated: two downstream UASB reactors (D1 and D2) receiving effluents from the upstream UASB reactors (U1: no-GAC, U2: yes-GAC). Our results indicate that MESs were enhanced with the addition of GAC. The effluent from U2 exhibited a 59.62 % higher amino acid content than that from U1. These secretions contributed to an increase in the nitrogen loading rate (NLR) in the downstream reactors. Specifically, NLR in D1 increased from 130.5 to 142.7 g N/m3/day, and in D2, it escalated from 137.5 to 202.8 g N/m3/day, likely through acting as cross-feeding substrates or vital nutrients. D2 also showed increased anammox bacterial activity, enriched Ca. Brocadia population and hao gene abundance. Furthermore, this study revealed that D2 sludge has significantly higher extracellular polymeric substances (EPS) (48.71 mg/g VSS) and a larger average granule size (1.201 ± 0.119 mm) compared to D1 sludge. Overall, GAC-stimulated MESs may have contributed to the enhanced performance of the anammox process.

Mechanical Behavior of Polymer Braided Stent at Rapid Radial Loading

Stent implantation is the mainstream treatment for high-incidence vascular diseases. The stent is implanted into the blocked vessel with minimal trauma to restore blood flow. Polymer braided stents with superior biocompatibility and flexibility have broad application prospects in stent implantation. An ideal polymer stent should have suitable radial supporting capacity to withstand the cyclic radial load from vessels. Especially in the case of accelerated vasoconstriction caused by emotional excitement, drinking or fever, etc. However, there are currently limited studies on the mechanical properties of stent at rapid radial loading. In this work, the radial supporting capacity and fatigue properties of polymer stent at rapid loading rate were investigated by experiment and simulation. With the increase of radial loading rate, the stent has different deformation tendency and fatigue resistance. This study is helpful to study the structural changes of the polymer braided stent at different loading rates, and provide ideas for the evaluation and the optimization of the polymer braided stent.

Pullout Response of the Pre-displacement Bladed Anchor in Cohesionless Soil

This study presents a new alternative technique, the pre-displacement pile, which aims to improve the uplift capacity of the regular pile. This modified pile intends to offer a bearing area underneath the sand during uplift using the blades. This method does not hinder the piles from being driven or penetrated the ground. An experimental study was conducted to investigate the uplift response of the modified anchor. The influence of the number of blades, the loading rate, and the relative density of the sand on the uplift capacity of the anchor pile with blades is studied. The ultimate pullout response of the pre-displacement anchor implanted in sand improves with an increase in blade number and loading rate. The percentage improvement in the pullout load for the anchor with four blades and a loading rate of 1 mm/s was (301%, 289%, and 249%) at sand relative densities of (30%, 50%, and 80%), respectively, compared to the unbladed pile. The existence of blades led to the creation of an ideal anchoring system. This piling modification is practical, suitable for marine projects, and easy to install for offshore applications.

Experimental study on dynamic fracture of UHPC-NC specimens after high temperature burning treatment

For the ultra-high performance concrete (UHPC) overlaying normal concrete (NC) structures subjected to high-temperature burning, the reliable interface performance between UHPC and NC is the foundation for achieving their good co-work behavior. This paper focused on experimental and theoretical research on crack propagation at the interface of these UHPC-NC configurations under stress waves. A bi-material notched semi-circle bend (BNSCB) configuration was proposed, and the impact experiments were conducted on the BNSCB specimens using a split Hopkinson pressure bar system (SHPB). Three temperatures (20℃, 100℃, and 300℃) were designed in this test, examining the interfacial impact performance of the BNSCB specimen subjected to the high-temperature burning. Based on the finite element method and crack propagation gauge, the dynamic fracture toughness at the crack tip of the interface of the BNSCB specimen was calculated. The results show that: (1) the K1 values for the untreated BNSCB specimens increased by 23.3% and 22.6%, respectively, compared to the heat-treated BNSCB specimens. (2) the parameter K2 at the crack tip of the BNSCB specimen is much smaller than K1, and the parameter K1 plays a dominant role in crack initiation; (3) with the increase in loading rate from 580 GPa/s to 740 GPa/s, both untreated and heat-treated BNSCB specimens show significant improvement in the parameters G and K, with a more pronounced improvement observed in untreated compared to heat-treated BNSCB specimens.

Оценка влияния скорости нагружения на прочность скальных горных пород при одноосном сжатии и характер их разрушения

When studying physical and mechanical properties of rocks, one of the main parameters to be determined is their uniaxial compressive strength. During laboratory experiments, the value of this parameter can be influenced by both loading conditions and the shape of the prepared samples. By studying the impact of these factors, it is possible to give an adequate assessment of changes in the strength of rocks, as well as to predict their failure behavior. The paper presents the results of experimental studies of hard rocks from the Kola region deposits under uniaxial compression. The values of the rock strength at loading rates from 0.01 to 50 kN/s were established. On the basis of the obtained data it was revealed that the V.A. Fokin criterion can be used to determine the rated strength of rocks. It is shown that the values of uniaxial compressive strength of brittle elastic rock (urtite) and non-brittle elastic-plastic rock (carbonatite) will also increase with the increasing loading rate. It is established that for hard rocks prone to dynamic failure, an increase in the loading rate will lead to an increase in the intensity of their failure. Increasing the loading rate for non-brittle elastic-plastic rocks can cause them to start failing dynamically. The obtained conclusions indicate that the failure behavior of rock samples depends on the time factor of loading.

Fracture characteristics of cement-stabilized macadam using three-point bending test

Semi-rigid base is the main component of highway pavements in China and is prone to cracking damage under vehicle loading and environmental effects. In this paper, three-point bending (TPB) tests on prefabricated cracked specimens of CSM beams were conducted to study the damage evolution and fracture characteristics of cement-stabilized macadam (CSM) under different loading rates. The acoustic emission (AE) technique and scanning electron microscope (SEM) analysis were utilized. The results indicate that the fracture energy and fracture toughness of CSM were enhanced at increasing loading rates, and the amplitude distribution of AE events became wider and more energetic. The damage modes of CSM can be classified into interface transition zone (ITZ) damage, cement mortar damage, and aggregate damage. The fracture surfaces were mainly ITZ damage. The brittle structure of CSM was caused by incomplete hydration of cement particles in ITZ. This study can provide a better understanding of the fracture mechanism of CSM at different loading rates. The findings can also guide the design, construction, operation and maintenance of semi-rigid roadbed pavement structures.

Quantitative investigation of rock dynamic failure using Voronoi-based discontinuous deformation analysis

Dynamic failure widely exists in rock engineering, such as excavation, blasting, and rockburst. However, the quantitative measurement of the dynamic damage process using experimental methods remains a challenge. In this study, a SHPB modeling technique is established based on Voronoi-based DDA to study the damage evolution of Fangshan granite under dynamic loading. The assessment of cracking along the artificial joints among Voronoi sub-blocks is conducted using the modified contact constitutive law. A calibration procedure has been implemented to investigate the rock dynamic properties quantitatively. The dispersion and damping effect can be effectively eliminated by regular discretization in SHPB bars, based on which the dynamic stress equilibrium can be satisfied. To reproduce the loading rate effect of the dynamic compressive strength, which has been observed in the experiment, a modification strategy considering the influence of the rate effect on the strength meso-parameters is proposed. Using this strategy, the peak stresses of the transmitted waves predicted by DDA match well with those obtained from experiments conducted at different loading rates. The simulation results show that more microcracks are generated and the proportion of tensile cracks decreases as the loading rate increases. Furthermore, the dynamic mechanical behavior and fracturing process have also been discussed and compared with the experiments. The results show that the established SHPB system is a powerful tool for quantitative analysis of rock dynamics problems and can handle more complex problems in the future.

Dynamic enhancement induced by interface for additively manufactured continuous carbon fiber reinforced composites

Additive manufacturing (AM) has become one advanced manufacturing method for continuous carbon fiber reinforced polymer (CCFRP) composites, which is promising to work as aerospace materials. To promote their efficient design and extensive application, series of tensile and fiber-pullout tests were implemented to investigate the rate-dependent mechanical behavior and fracture mechanism. Both tensile strength and fracture strain significantly increase with the increase of strain rate in the studied range of 0.0001–1600 s−1. Fiber-pullout test also confirms that the interfacial shear strength of composites increases with the increasing loading rate. Dynamic enhancement is analyzed quantitatively, and it is mainly contributed by the rate-dependent behavior of fiber-matrix interface. Fracture analysis indicates the stronger interfacial bonding under dynamic loading, which is attributed to the impeded interfacial crack propagation by high strain rate. A constitutive model is built to predict the rate-dependent strength of CCFRP composites, and it fits well with the experimental results.

Experimental Study on Mixed-Mode (I–II) Fracture Toughness of Freshwater Ice

In recent years, the issue of aircraft icing has gained widespread recognition. The breaking and detachment of dynamic ice can pose a threat to flight safety. However, the shedding and fracture mechanisms of dynamic ice are unclear and cannot meet the engineering needs of ice-shedding hazard assessment. Therefore, studying the fracture toughness of ice bodies has extremely important practical significance. To address this issue, this article uses a centrally cracked Brazilian disk (CCBD) specimen to measure the pure mode I toughness and pure mode II fracture toughness of freshwater ice at different loading rates. The mixed-mode (I–II) fracture characteristics of ice are discussed, and the experimental results are compared and analyzed with the theoretical values of the generalized maximum tangential stress (GMTS) criterion considering the influence of T-stress. The results indicated that as the loading rate increases, the pure mode I toughness and pure mode II fracture toughness of freshwater ice decrease, and the fracture toughness of freshwater ice is more sensitive to the loading rate. In terms of fracture criteria, the theoretical value of the ratio of pure mode II fracture toughness to pure mode I fracture toughness based on the GMTS criterion is in good agreement with the experimental value, while the theoretical value based on the maximum tangential stress (MTS) criterion deviates significantly from the experimental value, indicating that the GMTS criterion considering the influence of T-stress can better predict the experimental results.

Long-term effects of running exercises on natural grass, artificial turf, and synthetic surfaces on ground reaction force components in individuals with overpronated feet: A randomized controlled trial

BackgroundOverpronated feet are associated with alteration of lower limb mechanics. Also, quantifying ground reaction force-related changes due to exercise on different surfaces is important for understanding the potential risk of injuries. Research questionWhat are the long-term effects of running exercises on natural grass, artificial turf, and synthetic surfaces on ground reaction force components in individuals with overpronated feet? MethodsOf 82 contacted individuals with overpronated feet aged 18–30, 22 were excluded because they did not meet inclusion criteria (20), and two declined to participate. Sixty individuals were included in randomization, the control (n = 15), hard court or synthetic surface (n = 15), natural grass (n = 15), and artificial turf (n = 15) groups. There was a sample loss of 8 individuals because of personal problems (2 in each group). The intervention groups performed running exercises on natural grass, artificial turf, and synthetic surfaces over eight weeks, three sessions per week. No training or test-related injuries were reported throughout the study. A force plate was embedded midway through the 18-running concrete path to collect ground reaction force data while running on stable ground before and after interventions. ResultsFindings demonstrated significant group-by-time interactions for vertical loading rate (p = 0.016, ETA=0.297). Post-hoc analyses showed increased loading rate amplitude in two natural grass and artificial turf groups (but not in the control and synthetic surface groups) post-intervention. SignificanceRunning training on natural grass and artificial turf surfaces may place individuals with overpronated feet at a higher risk of injury while running on a stable surface.

Molecular Dynamics Simulation and Experimental Study of Mechanical Properties of Graphene-Cement Composites.

To investigate the mechanical properties of graphene (G) and calcium silicate hydrate (C-S-H) composites in different directions, molecular dynamics (MD) simulations and experiments were used, and the effects of temperature, loading rate, and graphene defects were also investigated. The experimental results show that the addition of graphene can improve the flexural, compressive, and tensile strength of the composite. The results of molecular dynamics simulation show that the addition of graphene in x and z directions can enhance the tensile strength of G/C-S-H in three directions, while the addition of graphene in y direction can reduce the tensile strength of G/C-S-H. At the same time, the tensile strength of G/C-S-H decreases with the increase in temperature and increases with the increase in loading rate. Meanwhile, the mechanical properties of G/C-S-H can be improved using a certain concentration of monatomic vacancy defects, diatomic vacancy defects, and S-W defects.

On the dynamic performance of additively manufactured visco-elastic meta-materials

Additive manufacturing (AM) has revolutionized the production of structures with tailored material properties, including elastomer polyurethanes (EPU) which exhibit exceptional mechanical performance. EPU possesses unique characteristics, such as high elongation at break, efficient energy dissipation, and superior specific strength, making it well-suited for applications requiring resilience to dynamic loadings. By combining the advantages of AM and EPU, enhanced and customized meta-materials can be created, surpassing the mechanical performance of traditional bulk materials. However, because of the non-linear stress–strain response of both the constitutive material and the structure, designing such meta-materials for high strain-rates can be challenging. In this work, therefore, quasi-static and dynamic experiments were conducted to evaluate a meta-material architecture. The investigation revealed a strong positive rate dependency. The mechanical performances, including strength, and dissipated energy, increased with increasing loading rate. The EPU meta-materials demonstrate their suitability for dynamic applications where high energy dissipation is crucial for reducing transmitted loads.

Research on dynamic splitting tensile damage characteristics of steel fiber reinforced concrete under freeze-thaw cycle environment based on AE

This paper aims to investigate the dynamic splitting tensile damage characteristics of steel fiber reinforced concrete (SFRC) and achieve early warning of the dangerous stage under freeze-thaw cycle environment through acoustic emission (AE) technology, Mel-frequency cepstral coefficients (MFCCs) and genetic algorithm-back propagation (GA-BP) neural network. The effects of fiber content and loading rate on mechanical properties, wavelet energy spectrum coefficient and wavelet time-frequency-energy maximum, as well as MFCCs of AE signals were studied herein. Then a GA-BP neural network was established to realize the classification of the damage degree of SFRC under freeze-thaw cycling with MFCCs as the signal characteristic. The results indicate that the increasing loading rate leads to more serious brittle failure, but the appropriate steel fiber content can make the cracks generated develop fully during the dynamic splitting tensile failure process by analyzing the wavelet energy spectrum coefficient and wavelet time-frequency-energy maximum. It is also found that the damage degree of SFRC can be identified by analyzing the MFCCs. Furthermore, the GA-BP neural network performs well in the classification of the damage degree of SFRC with the recognition over 90%, which is more suitable for concrete conditions with a proper loading rate and fiber content.

Uso de resíduos de frutas e verduras para gerar hidrogênio por meio da fermentação escura

ABSTRACT Fruit and vegetable waste (FVW) are sugar-rich substrates that can result in hydrogen through dark fermentation. The success of dark fermentation depends on changing operation parameters, which vary based on the characteristics of the substrate. This study aimed to determine how FVW from a central horticultural wholesaler should be fermented to produce the maximum amount of hydrogen. The following goals were listed as determining the pre-treatment method (chemical, thermic, and acid) for brewery sludge and evaluating the fermentation of the liquid fraction of FVW in an anaerobic structured bed reactor (AnSTBR). The AnSTBR operation started with sucrose as substrate at a hydraulic retention time (HRT) of 6 h. Then, the sucrose was gradually replaced by FVW at the HRT of 6 h. The FVW was fed as the sole carbon source (5 g COD/L) at HRTs of 6, 12, and 3 h for at least 20 days on each condition. The heat treatment resulted in the highest cumulative hydrogen production and hydrogen production rate (HPR) (90 mL H2 and 6.6 mL H2/h). In the AnSTBR operation, the highest values of HPR were observed at 6 h (2094 L H2/m3 reactor.d). The attempt to decrease the HRT to 3 h caused a reduction in the HPR to 216 L H2/m3 reactor.d. It is highly encouraging the increase of the organic loading rate and the use of co-fermentation, aiming to reduce the size of the reactor and water expended on dilution by increasing substrate concentration in future studies.

Deformation localization and crack propagation of sandstone containing different flaw inclination angles under different loading rates

Defects with varying geometric distribution in the rock mass play a crucial role in determining the stability of engineered rock masses. Previous studies have primarily investigated the initiation, propagation, and coalescence behaviors of flaws within rock masses. Nevertheless, there has been limited analysis of flawed rock masses under different loading rates. In this study, we present the deformation localization and cracking process of three types of inclined flawed sandstone specimens through conducting uniaxial compression and acoustic emission tests at four different levels of loading rate. The results supported the following findings: 1) Sandstone specimens with different flaw inclination angles exhibit the loading rate strengthening effect, and the strengthening effect gradually decreases with the increase of loading rate. 2) As the loading rate increases, the type of crack emergence changes from wing cracks to anti-tensile cracks, and the time of flaw initiation is shortened. 3) The cumulative acoustic emission counts were higher for the low-loading rate specimens than for the high-loading rate specimens. 4) Tensile cracks typically occur as the initial cracks. Anti-tensile cracks often coexist with wing cracks in rock specimens that have undergone tensile damage. Coplanar secondary cracks are the primary indication of shear damage formation in rock specimens. 5) The increase in loading rate promotes the transition of rock specimens from the mixed tensile-shear damage mode to the shear damage mode. These research results are of great theoretical significance and engineering value for understanding the failure mechanism of rock mass containing flaws and proposing effective measures to prevent cracking and ensure the safety of brittle solid structures.

Experimental study on mechanical and acoustic emission characteristics of sedimentary sandstone under different loading rates

In the field of rock engineering, complexity of stress environment is an important factor affecting its stability. Thus, in view of fracture mechanism of rock under different loading rates within the scope of quasi-static strain rate, four groups of uniaxial compression tests with different strain rates were carried out on sandstone specimens, and strength, deformation, failure modes and acoustic emission characteristics of specimens were compared and analyzed. Furthermore, the fracture mechanism was discussed from the perspective of fracture characteristics based on fractal dimension, crack propagation law inverted through acoustic emission b-value, and micro fracture morphology. The results showed that as the strain rate increased from 10 to 5 s−1 to 10−2 s−1, the fractal dimension of rock fragments increased, and the fractal dimension of rock fragments increased by 9.66%, 7.32%, and 3.77% successively for every 10 times increase in strain rate, which means that the equivalent size of fragments was getting smaller, and the fragmentation feature was becoming increasingly prominent. The crack propagation process based on acoustic emission b-value showed that with the increase of loading rate, the specimen entered the rapid crack propagation stage earlier, in order of 68%, 66%, 29%, and 22% of peak stress. Moreover, the microscopic fracture morphology showed that with the increase of loading rate, transgranular phenomenon was clear, and the fracture morphology changed from smooth to rough. That meant that the fracture of sandstone rock at high loading rates was mainly caused by the propagation of large cracks, which was different from the slow process of initiation, convergence and re-propagation of small cracks at low strain rates.