Effect Of Loading Frequency Research Articles

Evaluation of the effect of the loading frequency on Wöhler’s curve parameters of a high modulus asphalt concrete (HMAC)

The fatigue's behaviour of bituminous materials is affected by the loading frequency. For hot mix asphalt (HMA) with unmodified bitumen, previous research showed a decrease in fatigue life, under strain-control mode, by increasing loading frequency. However, the effect of loading frequency on polymer-modified HMA has not been investigated extensively and researchers have found opposite results using the same testing method in strain-controlled mode. This paper looks at how the loading frequency impacts Wöhler's law parameters (slope and intercept) of a high modulus asphalt concrete (HMAC) made with a polymer-modified bitumen. Under strain-controlled mode, Tensile-Compression tests (TC) on cylindrical samples were carried out at isothermal conditions (θ = 10°C) and three distinct frequencies (5, 10 and 25 Hz). The analysis of covariance (ANCOVA) was considered to compare Wöhler's law parameters from different frequencies. The results showed no statistical difference in the slope and the intercept values from all studied frequencies.

Low Cycle Fatigue Properties of FGH720Li Superalloy

Low cycle fatigue (LCF) properties of a powder metallurgy(PM) nickel base superalloy FGH720Li were systematically studied in this work, including smooth LCF and notched LCF tested at various temperatures and different stress. The relationship between the fatigue life and applied stress was analyzed both for smooth fatigue and notch fatigue tests. The effects of loading frequency and stress ratio on LCF behavior were also studied. As an important influencing factor of the fatigue life in powder metallurgy superalloy, the effect of inclusions on LCF life was also investigated. The results showed that the fatigue properties of FGH720Li alloy was excellent, when tested at the temperature of 450°C and applied stress of 1230MPa, the fatigue life could exceed 5×104 cycles. When tested at 650°C and 1150MPa, the average fatigue life was still beyond 2×105 cycles.

Effect of the loading frequency on the sand liquefaction behaviour in cyclic triaxial tests

This study was aimed at highlighting the effect of loading frequency on the sand liquefaction response by performing cyclic triaxial tests in both stress-controlled and strain-controlled modes. For the first case, dense (IDmat = 0.70) and medium-dense (IDmat = 0.55) sand specimens were subjected to alternative and non-alternative axial stresses; whilst dense (IDmat = 0.70) sand specimens were subjected to alternative axial strain over a wide range of loading frequencies in the second case. It was experimentally found that in the case of alternative loading for test specimens with IDmat = 0.70 in stress-controlled mode, the effect of the loading frequency was not notable for dry sand specimens. In contrast, the effect of the loading frequency was significant for saturated sand specimens in both the loading modes. This was because, upon shearing, transient deformation occurred with the presence of excess pore water pressure, during which the inertia force needed to be taken into account. In the axial direction, the inertia force opposing the external loading certainly increased the liquefaction resistance by suppressing the complete development of the sand liquefaction. As for non-alternative loading in stress-controlled mode for test specimens with IDmat = 0.55, the loading frequency played a vital role in pure extensional loading, which could also be logically attributed to the presence of the axial inertia force appearing in the large strain and transient deformation stages. However, the effect of the loading frequency on the compressive pattern was unclear due to the following two reasons: (i) the inertia force in the lateral direction appeared in the form of enhancing lateral confinement, thereby impeding the relative sliding; and (ii) during shearing, the rearrangement of the sand grains occurred in a more stable direction. These two aspects both provided beneficial contribution to sand liquefaction resistance, thereby preventing the occurrence of transient deformation. Under the circumstance, the sand specimens deformed only moderately, thereby eliminating the need to consider the role of the loading frequency.

Evaluating Energy Generation Capacity of PVDF Sensors: Effects of Sensor Geometry and Loading.

This paper focuses on the energy generating capacity of polyvinylidene difluoride (PVDF) piezoelectric material through a number of prototype sensors with different geometric and loading characteristics. The effect of sensor configuration, surface area, dielectric thickness, aspect ratio, loading frequency and strain on electrical power output was investigated systematically. Results showed that parallel bimorph sensor was found to be the best energy harvester, with measured capacitance being reasonably acceptable. Power output increased with the increase of sensor’s surface area, loading frequency, and mechanical strain, but decreased with the increase of the sensor thickness. For all scenarios, sensors under flicking loading exhibited higher power output than that under bending. A widely used energy harvesting circuit had been utilized successfully to convert the AC signal to DC, but at the sacrifice of some losses in power output. This study provided a useful insight and experimental validation into the optimization process for an energy harvester based on human movement for future development.

Experimental and numerical investigation on mechanical behaviour and moisture transport of pavement concrete under fatigue load and hydrodynamic coupling

To investigate the mechanical behaviour and moisture transport of pavement concrete under fatigue load and hydrodynamic coupling, a three-dimensional nonlinear numerical model was proposed and the distribution and change of the internal stress, strain and humidity were analysed at different concrete layers with the loading cycles. Furthermore, the proposed coupling model was validated by experimental results and the effect of stress level and loading frequency on the mechanical response of concrete were explored. The results showed that severer coupling conditions accelerate the deterioration of the concrete structure. After the stage, I loading, the water retained in the upper layer tends to transfer to the middle layer. 0.5 stress level can be seen as the threshold for pavement concrete from the elastic stage to the plastic stage. With the 0.7 stress level, the peak stress at the upper layer of the concrete is positively correlated with the loading frequency.

Steady-state response of flexible combined pile-raft foundation under dynamic loading

The present study proposes a closed-form solution of flexible combined pile-raft foundation (CPRF) resting on an elastic medium under dynamic loading. Two components of the foundation system, i.e. pile and raft are idealized as beam and plate, respectively, resting on an elastic two-parameter Pasternak soil medium. The governing differential equations of these components under horizontal harmonic loading is formulated and the subsequent closed-form solutions of the same are obtained by using Fourier transformation technique. Two simultaneous analyses are performed for piles and raft and displacement compatibility is satisfied at the junction of the components. The proposed method is validated with dynamic centrifuge test results, available in the literature. In further parametric studies, the effect of loading frequency on the dynamic displacement response of CPRF is investigated. Also it is perceived that with an increase in raft flexibility by 60%, the horizontal load sharing proportion of raft reduces by 75%–65% for loose to dense sands, correspondingly. This study contributes significantly in understanding the steady-state response of CPRF under dynamic loading from an analytical perspective.

Genetic programming to formulate viscoelastic behavior of modified asphalt binder

The objective of this research was to develop prediction models for complex shear modulus (G*) and phase angle (δ) of bitumens modified with crumb rubber, styrene-butadiene styrene, and polyphosphoric acid at low and moderate temperatures. The experiments consisted of three different dosages of each modifier added to the original bitumen followed by measurement of G* and δ of the original and modified bitumen using the dynamic shear rheometer (DSR) test in frequency sweep mode (21 loading frequencies from 0.1 to 100 Hz) at seven test temperatures: −22, −16, −10, 0, 10, 16 and 22 °C. Having the experimental database, a robust genetic programming (GP) method was used to develop an individual prediction model for each modifier based on temperature, loading frequency, the G* and δ of the original bitumen, and the dosage of the modifier. Results showed that GP successfully developed accurate and meaningful expressions for calculating G* and δ of the modified bitumen as two main constitutive components of the viscoelastic behavior of bituminous composites. Then, a parametric study and sensitivity analysis were performed on the developed models to better understand the effect of variables on the trend of the models. The modifier dosage is the most effective input variable of the model and the amount of G* and δ of the original bitumen accurately reflect the effect of temperature and loading frequency on viscoelastic behavior of the modified bitumen, as they behave linearly at the considered test temperatures.

Mechanical Sensing Properties of Embedded Smart Piezoelectric Sensor for Structural Health Monitoring of Concrete

ABSTRACT An embedded smart piezoelectric sensor was fabricated, and the encapsulation material was prepared with cement, epoxy resin, curing agent, and improvement additives. Structural health monitoring (SHM) methods based on dynamic stress-sensing capability of piezoelectric sensor were presented. Mechanical Testing & Simulation (MTS) amplitude-scanning and frequency-scanning dynamic loadings were designed. Mechanical performance of encapsulation material, i.e., strength, Young modulus, and stress transmitting loss; the effects of different loading frequencies on output voltages; and stress sensitivities (V/MPa), were investigated. The electromechanical impedance and mechanical responses of embedded sensors with various loadings were studied in concrete. Theoretical formula indicates that output voltage is mainly related with external stress and area of Piezoelectric Lead Zirconate Titanate (PZT) ceramic. The optimized ratio of 4:2:0.5:1.6–4:2:0.5:2 is satisfactory and it can ensure optimal mechanical performance of encapsulation material. Stress sensitivities increase with the areas of PZT ceramic, and the effects of thickness on sensitivities are not obvious. The impedance response curve has left shifting tendency with the increase of dynamic cycles and loading values. The three-point bending destruction during concrete static loading can be in real-time reflected. The embedded sensors were suitable for dynamic mechanical monitoring in concrete. The excellent mechanical sensing performance exhibits great application potentials for SHM of concrete in civil engineering.

Experimental and numerical evaluation on the geogrid cyclic and post-cyclic behavior by considering pre-cyclic pullout load effect

In this study, the cyclic and post-cyclic behaviors of geogrid have been evaluated by considering the effect of pre-cyclic loading. In order to investigate this behavior, monotonic and multistage pullout tests have been conducted on PET geogrid in poorly graded sand. Also, numerical modeling using three-dimensional finite element method was carried out to simulate actual shape of geogrid and pullout test in order to investigate the effect of pre-cycle load, activated geogrid length, loading amplitude, frequency, and geogrid stiffness during monotonic and multistage test. The obtained results showed that numerical method estimated monotonic and post-cyclic phase conservatively although the unloading and reloading loops were not simulated due to the used constitutive soil model. Also, it was found that the pre-cyclic loading increased the pullout load, accumulated incremental and average strain during cyclic loading. Furthermore, it was shown that the effect of loading amplitude and frequency on the pullout behavior during multistage test was affected by the pre-cycle load value. On the other hand, the results indicated that the rate and value of pullout load and interface coefficient changes at post-cyclic phase due to the geogrid axial stiffness were influenced by the pre cyclic load value.

Frequency-related viscoelastic properties in high translucent CAD-CAM resin-based composites

ObjectiveThe study aims to quantify the viscoelastic properties of representative dental CAD/CAM resin-based composites (RBC) and to determine the effects of loading frequencies on the viscoelastic material response in comparison to clinically established CAD/CAM glass ceramics. MethodsEight RBCs, one leucite-reinforced, and one lithium disilicate glass-ceramics were selected. The quasi-static (indentation hardness HIT, indentation modulus, EIT) and viscoelastic (storage modulus E′, loss modulus E″, loss factor tan δ) material behavior was monitored by a depth-sensing indentation test equipped with a DMA module. A low-magnitude oscillating force was therefore superimposed onto a quasi-static force (Fmax = 1000 mN) at 20 different frequencies in the range 1–50 Hz. One and multiple-way analysis of variance (ANOVA), the Tukey honestly significant difference (HSD) post-hoc tests (α = 0.05), and a Pearson correlation analysis were used for data analysis. ResultsThe quasi-static parameters increased with the crystalline phase in glass ceramics and with the amount of inorganic filler in RBCs. The tan δ, which is related to the damping capacity of a material, increased with the increasing amount of glass phase in glass ceramics or with the amount of organic phase in RBCs. A pronounced influence of the frequency on the measured parameters and their patterns of variation was observed. HIT was up to ten time higher in glass ceramics compared to RBCs and highest at the lowest frequency (1 Hz). Parameters EIT and E′ differ less and were lowest at the lowest frequency. E″ distinguished three different patterns of variation with frequency. The tan δ decreased rapidly with frequency in glass ceramics, while the decrease in RBCs was gradually. Frequency influenced stronger tan δ (p < 0.001, ηP2 = 0.85), followed by E′ (p < 0.001, ηP2 = 0.773), EIT (p < 0.001, ηP2 = 0.772), and E″ (p < 0.001, ηP2 = 0.714), and less HIT (p < 0.001, ηP2 = 0.384). ConclusionsAll materials sowed viscoelastic behavior related to their microstructure and the internal friction created by grain or interphase boundary relaxation. RBCs have better damping capabilities over a wider frequency range. The deviations from the ideal elasticity were significantly lower in the glass ceramics than in the RBCs.

Dynamic response of asphalt pavement under vibration rolling load: Theory and calibration

Vibrating compaction is a critical procedure to guarantee the quality of asphalt pavement, and the dynamic response of asphalt pavement is a complicate problem in the process of vibration rolling. This paper aims to theoretically investigate the dynamic response regulation and influencing factors of pavement under the moving vibration load. Firstly, based on the viscoelastic theory of multi-layer system, the dynamic response governing equation of asphalt pavement is established under the vibration load, the governing equation was then simplified by the two-dimensional Fourier transform method. Further, the stiffness matrices of the single-layer and multi-layer pavement structures are obtained by using the dynamic stiffness method. Consequently, the exact solution is obtained for the plane strain problem by programming and validated with the finite element results, which is proved to be more efficient in the calculation. Moreover, the model is calibrated by the field test to determine the wave number and dynamic modulus during the vibration rolling process. Subsequently, the effects of material and load frequency are investigated on the dynamic response in terms of acceleration and displacement. The results show that both of the acceleration and displacement of pavement surface vibrate periodically with the vibrating load, and the changing regulations of the acceleration and displacement with loading time can be divided into three stages corresponding to the engineering practice such as rapid decline, slow decline and stable stages. Finally, the influencing factors are analyzed in terms of the modulus of materials (i.e., surface course and base course) and the frequency of load. It shows that the peak values of acceleration and displacement of the pavement surface change significantly and nonlinearly with the material modulus in the form of power function. Meanwhile, the influence of vibrating frequency is also significant and a quadratic relationship between the acceleration/displacement and frequency. This paper provides an insight into understanding the dynamic response of pavement under vibration rolling condition to some extent, and potentially provides theoretical and technical support for accurately determining and modifying some indexes that characterize the compaction degree of asphalt pavement.

Experimental study of particle migration under cyclic loading: effects of load frequency and load magnitude

The study of particle migration in porous media under cyclic loading is the key to understand the mechanism of mud pumping hazard in railway embankments. This paper presents a series of particle migration tests, in which soil particles migrate into an overlying gravel layer under cyclic loading. The results show that the increase in loading frequency and load magnitude leads to more particle migration upwards at a greater rate, implying that the train speed and axle loads affect the extent of mud pumping. The slurry turbidity in the gravel layer increases to a steady state value with time. Soil particles smaller than 5 μm have the potential to diffuse into the entire gravel layer, and larger particles tend to aggregate in the bottom layer of the gravel. The backward erosion gradually develops deeper into the soil layer, and there is a maximum erosion depth associated with each load frequency and load magnitude. As for the mechanism, the pore water pressure oscillates because of liquid sloshing. Its amplitude is much larger in the gravel layer than that in the soil layer due to their difference in permeability. The axial hydraulic gradient acts as a pumping effect to stimulate the migration of soil particles. Increasing load frequency is conducive to the generation of a stronger pumping effect at the gravel–soil interface. Increasing load magnitude does impact not only the extent of pumping effect, but also the development of an interlayer which plays an important role in promoting particle migration.

Effect of Loading Frequency on the Recovery Process of Disuse Muscle Atrophy

Effect of Loading Frequency on the Recovery Process of Disuse Muscle Atrophy

Permanent deformation characteristic of asphalt mixture under coupling effect of moisture, overload and loading frequency

To evaluate the influence of vehicle load and driving speed on permanent deformation characteristic of asphalt mixture in high temperature and rainy areas, the Hamburg wheel tracking test was conducted under different test conditions by changing the wheel load, loading frequency and test environment. The permanent deformation characteristic of asphalt mixture under coupling effect of moisture, overload and loading frequency was discussed. Meanwhile, three kinds of anti-rutting agents commonly used in engineering were selected, and the influence of various anti-rutting agents on permanent deformation characteristic of asphalt mixture was also studied by rutting test and Hamburg wheel tracking test. The results show that the moisture, load and loading frequency are important factors affecting the permanent deformation characteristic of asphalt mixture. Under coupling effect of moisture, overload and low loading frequency, the ability of asphalt mixture to resist the generation and development of permanent deformation is poor. The influence of coupling effect of moisture, overload and low loading frequency needs to be considered when evaluating the anti-rutting performance of asphalt mixture. After the addition of anti-rutting agent, the dynamic stability of asphalt mixture increases significantly first, and then tends to be gentle when the dosage of anti-rutting agent exceeds 0.45%. The difference of improvement effect of various anti-rutting agents on rutting resistance of asphalt mixture is obvious, and the improvement effect is in order of PR. PLASTS (PR) > RESIN ALLOY (RA) > Duroflex (Df). The anti-rutting agent PR with dosage of 0.45% is recommended to improve the resistance of asphalt mixture to permanent deformation. The research results can provide some references for improving the rutting resistance of asphalt pavement in high temperature and rainy areas.

Liquefaction resistance behaviours of gravel steel slag

Due to the continuous scarcity of sand and gravel under environmental protection policies, a new geo-backfill material with the good liquefaction resistance is urgently needed. Steel slag is a kind of industrial solid waste. The aged eight-month steel slag usually could be used as backfill material. It is important to investigate the liquefaction resistance behaviours of steel slag. According to the soil classification method, the waste steel slag is also divided into gravel steel slag, coarse steel slag and fine steel slag. Dynamic triaxial tests were carried out to investigate the liquefaction resistance of gravel steel slag in the paper by considering the effects of consolidation stress ratio, loading frequency, confining pressure and gravel-size particle contents. The variations of pore water pressure and cyclic shear resistance were analysed. The calculation model of pore water pressure of saturated sand proposed by Seed and Finn could be applied to analyse the dynamic pore water pressure development of gravel steel slag. Compared with conventional soils, the cyclic liquefaction resistance of gravel steel slag is better than that of fine sand, gravel soil, sand-gravel soils and sand pebbles. Gravel steel slag can be used to replace some conventional cohesionless soils as a geo-backfill material.

Energy-based model for predicting liquefaction potential of sandy soils using evolutionary polynomial regression method

This study presents a comprehensive, energy-based model for the evaluation of the liquefaction potential of all types of sandy soils, including clean, silty, and clayey sands. Unlike previous energy-based capacity models, this study considers the effect of Plasticity Index (PI) and loading Frequency (Frq) in the proposed model. Introducing an innovative method, the model was developed in two steps. At first, a small-scale model was developed using Evolutionary Polynomial Regression method (EPR), considering PI, Frq, Fine Content (FC), mean grain size (D50), and uniformity coefficient (Cu). For this aim, 63 laminar shaking table tests were performed on different fine-grained soils mixed with sand, with a wide range of PI (from 0 to 26) to provide a comprehensive data bank for EPR modeling. Then, by introducing an appropriate scaling factor, the model was scaled up to the field condition considering effective stress (σv′) and relative density (Dr). The model was successfully verified using a number of 100 case-histories worldwide with various soil properties. The results showed that the present model is considerably superior to previous energy-based models. Parametric analysis demonstrated that the liquefaction capacity energy of sandy soils with high fine content is mainly influenced by FC and PI, respectively.

Performance of sludge drying reed beds for the leachate purification: Effects of sludge loading frequencies and plant species

Sludge drying reed beds (SDRBs), as a natural biological technology, have positive effects on surplus sludge treatment. However, few studies focus on the sludge leachate purification in the SDRBs regarding the wetland plant species and sludge loading rates. In this study, four SDRBs planted with two wetland plant species (Phragmites australis, Typha angustifolia) were investigated for leachate purification under six sludge loading volumes and feeding frequencies (9L/3d, 6L/3d, 4L/d, 3L/d, 2.5L/d, and 2L/d). Results showed that the lowest Escherichia coli content of 630 number/mL was determined in the P. australis SDRBs, with 86.2–92.8% lower than those in the T. angustifolia controls. However, similar removal efficiencies of chemical oxygen demand (COD), total nitrogen (TN), ammonium (NH4+), total phosphorus (TP), and heavy metals were obtained in the SDRBs with both plant species. Moreover, the optimum sludge loading volume and feeding frequency of 3L/d was determined in the P. australis system, with pollutants (COD, TN, NH4+, and TP) mass removal efficiencies in the leachate being over 94.9%. In addition, the principal component analysis indicated that water loss and oxidation-reduction potential had positive influences on pollutant removal in the planted SDRBs. Overall, the best leachate removal efficiency was obtained in the P. australis SDRBs under the sludge loading volume and feeding frequency of 3L/d.

Investigation on the Effect of Dynamic Frequency on Fracture Evolution in Preflawed Rock under Multistage Cyclic Loads: Insight from Acoustic Emission Monitoring

This study is aimed at revealing the effect of dynamic loading frequency on the fracture evolution behavior in preflawed rock samples under multistage cyclic loading conditions. The fracture evolution characteristics were investigated using stress-strain descriptions and in situ acoustic emission techniques. It is shown that rock strength, deformation, AE pattern, and fatigue life are strongly affected by the applied dynamic loading frequency. Rock fatigue strength and lifetime increase with the increase of dynamic loading frequency. The AE count and energy output both increase with the increase of the applied loading frequency. Six kinds of cracking modes were revealed by AE spectral frequency analysis. It is shown that large-scaled cracks are easy to be formed for rock subjected to high-frequency loads, reflected as the deceasing of AE signals with high-frequency–high-amplitude signal feature. It is suggested that applied dynamic loading frequency has obvious impact on the crack coalescence at the rock bridge segment. The testing results are helpful to enhance the cognitive of the influence of dynamic frequency on the crack communication behavior and can be expected to predict the stability of rock mass structures where rock mass is subjected to fatigue loading.

Joint fatigue-failure: A demonstration of viscoelastic responses to rate and frequency loading parameters using the porcine cervical spine

Fatigue-failure in low back tissues is influenced by parameters of cyclic loading. Therefore, this study quantified the effect of loading rate and frequency on the number of tolerated compression cycles. Energy storage and vertical deformation were secondarily examined. Thirty-two porcine spinal units were randomly assigned to experimental groups that differed by loading rate (4.2 kN/s, 8.3 kN/s) and loading frequency (0.5 Hz, 1 Hz). Following preload and range-of-motion tests, specimens were cyclically loaded in a neutral posture until fatigue-failure occurred or 10800 cycles were tolerated. Macroscopic dissection was performed to identify the fracture morphology, and measurements of energy storage and vertical displacement were calculated throughout the specimen lifespan (1%, 10%, 50%, 90%, 99%). Given the differences in compression dose—force-time integral—between experimental conditions, the number of sustained cycles were assessed following linear and nonlinear dose-normalization via correction factors calculated from existing risk-exposure approximations. Without dose-normalization, an 8.3 kN/s loading rate and 0.5 Hz loading frequency reduced the fatigue lifetime by 3541 and 5977 cycles, respectively (p < 0.001). Linear and nonlinear dose-normalization resulted in a significant rate × frequency interaction (p < 0.001). For a 1 Hz loading frequency, the number of sustained loading cycles did not differ between loading rates (padj ≥ 0.988), but at 0.5 Hz, spinal units compressed at 8.3 kN/s sustained 99% (linear) and 97% (nonlinear) fewer cycles (padj < 0.001). These findings demonstrate that the interacting effects of loading frequency and loading rate on spinal fatigue-failure depend on the normalization of dose discrepancies between experimental groups.

Experimental study on the liquefaction process of iron concentrate on swing table

Liquefaction of the bulk particle cargoes is a serious hidden risk for the ore transportation ship. To further understand the liquefaction mechanism of the particle cargoes, the model experiments have been carried out, by adopting saturated iron concentrate samples and the six degrees of freedom swing table. The effect of external load frequency on pore water pressure and excess pore pressure ratio was investigated. Results show that the external load is the main factor causing liquefaction of the cargo, and the possibility of liquefaction of different parts of the whole cargo is different. The bottom part of the cargo changes the most under different external loads, while the top does not.