Loading Waveform Research Articles

Development of active power filter using rectifier boost technique

<span>The development of active power filter (APF) using rectifier boost technique has been identified to compensate for the pulsating nature of the distorted supply current waveform of non-linear load. In this work, investigation is carried out on the operation of rectifier without any filters function. This is then extended to operate the rectifier converter with an active power filter function. APF function is implemented by enabling the closed-loop control using standard proportional integral control to rectify the distorted supply current to become continuous, sinusoidal and in-phase with the supply voltage waveform. Consequently, the total harmonic distortion (THD) level was reduced to meet the acceptable limit defined in the standard of IEEE-519 1992. The selected simulation results obtained from MATLAB/Simulink are presented to justify the proposed filter structure.</span>

Recommendation of Suitable Loading Waveforms and Wavelength Equations for Dynamic Modulus Based on Measured Wheel Loads

Dynamic modulus is a key parameter in the pavement structure design and analysis. A haversine loading waveform is the most widely recommended waveform in the laboratory to obtain the dynamic modulus of asphalt mixtures by test protocols, which may not completely represent all field loading conditions. The aim of this study is to investigate the suitable vertical compressive stress pulse waveforms at different depths under different pavement structures and to obtain the best fitting waveforms by using the least squares method. Specifically, the vertical compressive stress of different pavement structures was calculated by utilizing a three-dimensional finite element program incorporating with the measured wheel loads. The results revealed that the vertical stress pulse waveforms at different depths in asphalt layer were different within different pavement structure combinations. Generally, the square waveform fitted the vertical stress pulse waveform better for the shallow depth of the surface layer. The haversine waveform became more suitable while the depth increased. The bell waveform was better when the depth went deeper (i.e., 10 cm). In addition, the method of choosing waveforms at different depths of different pavement structures was provided, and the equation of calculating wavelength was recommended. Moreover, dynamic modulus under different loading waveforms were analysed through laboratory test, and the process for obtaining dynamic modulus of asphalt layer in any depth was presented.

Modelling creep-fatigue behaviours using a modified combined kinematic and isotropic hardening model considering the damage accumulation

The low cycle fatigue and low cycle fatigue-creep behaviours of P92 steel with strain-control mode and fully inverse loading waveforms at the temperature of 630 °C were investigated. A typical cyclic softening behaviour of P92 steel was observed. As the hold time was introduced at the tensile peak and compressive valley strains, the failure cycle was reduced and the stress reduction became more obvious with the increasing hold time because of the improvement in creep deformation. Moreover, a modified combined kinematic and isotropic hardening model by introducing a non-linear interaction damage accumulation constitutive model was proposed to capture the loop stress-strain behaviour until fracture and predict the failure life as the applied strain amplitudes and duration periods changed. The validation results showed that the accelerated cyclic softening and the greatly reduced failure cycle as the duration periods prolonged under the low cycle fatigue-creep interaction conditions were well described by the modified model.

The foundations of fracture impedance spectroscopy

The foundations of Fracture Impedance Spectroscopy (FIS) as a theoretical framework for analyzing and predicting fatigue and corrosion fatigue crack growth rate in real structures under complex loading waveforms from laboratory data obtained under simple, “pristine” loading conditions is presented. FIS is based upon an analogy between current flow (crack growth rate) in an electrical circuit in response to an applied, time-dependent voltage (time-dependent stress intensity factor). In this paper, the framework of the theory and its application for two different aerospace alloys are presented.

Canalicular fluid flow induced by loading waveforms: A comparative analysis

Dynamic loading on the bone is beneficial in prevention and cure of bone loss as it encourages osteogenesis (i.e., new bone formation). Loading parameters such as strain magnitude, frequency, cycles, and strain rate (depending on loading waveform) affect the new bone formation. In-vivo studies suggested an optimal and osteogenic range of strain magnitude, frequency, and cycles to elicit the maximum new bone response. Still, there is no consensus on the selection of loading waveform. Animal studies on bone adaptation considered sinusoidal, and non-sinusoidal (e.g., trapezoidal, sawtooth, and triangular) loading waveforms according to physiological loadings (e.g., walking, running, and jumping etc.) without considering the relative effect of these waveforms on the loading-induced mechanical environment. The present study attempts to bridge this gap. Accordingly, this work hypothesizes that bone being a biphasic material (solid and fluid phases) experiences the same strain distribution for the different loading waves of the same amplitude, however, other components of the mechanical environment such as pore-pressure and interstitial fluid motion regulating the bone adaptation may differ. An in-vivo cantilever bending study is selected to substantiate the hypothesis. A poroelastic model is used to estimate the pore pressure and fluid motion developed in mouse tibia subjected to the: (i) trapezoidal, (ii) sawtooth, and (iii) triangular bending waves. Furthermore, poroelastic response of pore-pressure and fluid motion induced by these loading waveforms are compared and analyzed. This work also investigates how bone loss associated alterations in the microstructural environment of cortical bone affect the canalicular fluid motion induced by these waveforms. Overall results may be useful in designing optimal biomechanical interventions such as physical exercises to improve the bone health.

Experimental and Numerical Analyses of the Rational Loading Waveform in SHPB Test for Rock Materials

Aiming at the determination of the rational loading waveform for rock materials, the comparative impact tests under the loadings of rectangular and half-sine stress waves were performed on red sandstone using an Ø50 mm SHPB apparatus. Experimental results with the rectangular stress wave affirm that the waveform dispersion and stress-strain curve oscillation frequently exist during the test of rock materials, which signifies that the accuracy of test results derived from the rectangular stress wave loading cannot be guaranteed. Under the loading of the half-sine stress wave, the phenomenon of wave dispersion during the tests has been eliminated radically, and there is no oscillation in the stress-strain curves. To further demonstrate the rationality of the half-sine wave loading in the SHPB test, by utilizing the three-dimensional numerical simulation approach, the propagations of rectangular, triangular, and half-sine stress waves travelling in the axial and radial directions of the SHPB with four elastic bar diameter sizes are analyzed and compared. The results show that the waveform dispersion of the rectangular and triangular stress waves always exists and will be more and more serious with increasing diameter size and propagation distance. For the half-sine stress wave, the waveform dispersion effect is very weak and not affected by the bar diameter size and propagation distance. The half-sine stress wave is the rational loading waveform for rock SHPB tests with different bar diameters.

Numerical study on the competitive cracking behavior in TC and interface for thermal barrier coatings under thermal cycle fatigue loading

Thermal barrier coatings (TBCs) have complex structures and service in the extremely high temperatures. The cracking phenomenon is usually observed at both the top coat and the interface between the bond coat and the thermally grown oxide layer under the thermal cyclic fatigue (TCF) loading. In this study, the competitive cracking behavior in top coat and interface for TBCs were numerical investigated by using a cohesive zone model (CZM) and extend finite element method (XFEM). The dominance stress controlling cracks initiation and propagation was analyzed. The initiation and propagation of TC cracks were controlled by tensile stress, while the interfacial cracking was led by tensile and shear stress commonly. Then, the results showed that the cracks in the TC layers could decrease the stress and delay the interfacial cracks propagation. In addition, this paper also analyzed the key factors affecting the cracking behaviors in TBCs. Changing of the thermal load waveform could lead different cracks propagation path in top coat and failure mode of TBCs. Moreover, the initiation location of TC cracks was independent on the interfacial roughness, but stress concentration of TBCs with the rougher interface was more evident. The thinner initial TGO layer could prolong the lifetime of TBCs. The results demonstrate that the load waveform should be designed to characterize the service environment.

Criteria for accurate measurement of thermal diffusivity of solids using the Angstrom method

This paper presents criteria for accurately measuring the thermal diffusivity of a solid material using the Ångström method. In this technique, a periodic heat load is supplied at one end of the sample and temperatures are measured at different locations along the sample. The thermal diffusivity is estimated from the measured amplitude and phase of the temperature oscillations at various locations along the sample. Criteria are presented for important measurement parameters namely as periodic heat load power, waveform and frequency. Since more than two sensors are used, the accuracy is further improved since the linearity of the phase and amplitude data can be verified. We experimentally establish that a sinusoidal input heat load with a frequency as defined by the criteria established in this paper results in improved measurement accuracy. Based on these criteria, the experiment was performed with a range of materials spanning low to high thermal diffusivities namely Teflon, G10, Titanium Alloy (Ti6Al4V), Stainless steel (SS316) and Aluminium Alloy (Al6061-T6). The results indicate that the measured thermal diffusivity values deviate from the literature data at room temperature by less than 2.1%.

Modeling erythrocyte electrodeformation in response to amplitude modulated electric waveforms

We present a comprehensive theoretical-experimental framework for quantitative, high-throughput study of cell biomechanics. An improved electrodeformation method has been developed by combing dielectrophoresis and amplitude shift keying, a form of amplitude modulation. This method offers a potential to fully control the magnitude and rate of deformation in cell membranes. In healthy human red blood cells, nonlinear viscoelasticity of cell membranes is obtained through variable amplitude load testing. A mathematical model to predict cellular deformations is validated using the experimental results of healthy human red blood cells subjected to various types of loading. These results demonstrate new capabilities of the electrodeformation technique and the validated mathematical model to explore the effects of different loading configurations on the cellular mechanical behavior. This gives it more advantages over existing methods and can be further developed to study the effects of strain rate and loading waveform on the mechanical properties of biological cells in health and disease.

Effects of loading waveforms on rock damage using particle simulation method

The particle simulation method is used to study the effects of loading waveforms (i.e. square, sinusoidal and triangle waveforms) on rock damage at mesoscopic scale. Then some influencing factors on rock damage at the mesoscopic scale, such as loading frequency, stress amplitude, mean stress, confining pressure and loading sequence, are also investigated with sinusoidal waveform in detail. The related numerical results have demonstrated that: 1) the loading waveform has a certain effect on rock failure processes. The square waveform has the most damage within these waveforms, while the triangle waveform has less damage than sinusoidal waveform. In each cycle, the number of microscopic cracks increases in the loading stage, while it keeps nearly constant in the unloading stage. 2) The loading frequency, stress amplitude, mean stress, confining pressure and loading sequence have considerable effects on rock damage subjected to cyclic loading. The higher the loading frequency, stress amplitude and mean stress, the greater the damage the rock accumulated; in contrast, the lower the confining pressure, the greater the damage the rock has accumulated. 3) There is a threshold value of mean stress and stress amplitude, below which no further damage accumulated after the first few cycle loadings. 4) The high-to-low loading sequence has more damage than the low-to-high loading sequence, suggesting that the rock damage is loading-path dependent.

A high-repetition-rate bipolar nanosecond pulse generator for dielectric barrier discharge based on a magnetic pulse compression system

Magnetic pulse compression (MPC) systems are suitable for generating dielectric barrier discharges (DBDs) owing to their capability of producing high-amplitude, short pulse voltage waves. This paper proposes a high-frequency, bipolar magnetic compression system to study DBD plasma characteristics. First, the principle of bipolar MPC is explained [a bipolar MPC system comprises a full bridge inverter circuit, pulse transformer (PT), and magnetic switch (MS)]. Additionally, the design of the PT and MS is described. Then, the waveform of the resistive load is tested and compared with PSpice simulation results. It was found that the nanosecond pulse generator produces a pulse on a resistor with an amplitude of 0-13 kV, a rise time of approximately 100 ns, and a repetition frequency of 0 to several kHz. Finally, this paper studies the plasma characteristics under the application of a high-frequency bipolar pulse, and the charge-voltage Lissajous figure of the discharge waveform is analyzed. Combining discharge photographs and theoretical calculation results yields the relationship between the discharge characteristics and the frequency, which enriches the theoretical study of high-frequency bipolar discharges.

Numerical investigation on the cracking behaviors of thermal barrier coating system under different thermal cycle loading waveforms

The influence of a thermal cycle fatigue (TCF) loading waveform on cracking behaviors of an atmospheric plasma-sprayed thermal barrier coating (TBC) system was numerically studied using the commercial software. Results showed that the TCF loading waveform considerably affects stress distributions in the TBCs and is the leading factor for the initiation and propagation of cracks in TBCs samples. Under triangular load waveform, the cracks initiate and grow in top-coat (TC) layers. However, under trapezoidal load waveform, the cracks are likely to initiate at an off-valley location and propagate along the TC/thermally-grown-oxide (TGO) interface, causing the complete spallation of the TC layer. Furthermore, the influences of the prior time and holding time on the crack initiation and propagation under the trapezoidal loading conditions were discussed, respectively. Interfacial cracks are likely to initiate when the prior time is prolonged. Under the trapezoidal loading conditions, the location of crack initiation moves from the peak to the off-valley location of a TC/TGO interface as holding time increases. Under a long holding time of the trapezoidal loading conditions, an improved spray quality of TC/TGO interface can increase the service life of TBCs.

Effect of Loading Waveform Pattern and Rest Period on Fatigue Life of Asphalt Concrete Using Viscoelastic Continuum Damage Model

Fatigue cracking is one of the most critical types of distress in asphalt pavements and is due to actions of repetitive traffic loading over time. The fatigue life of asphalt concrete is often estimated from laboratory experiments where the performance depends directly on the test method, loading conditions, temperature, rest period, and aging in addition to the composition and properties of the mixture itself. The uniaxial fatigue test has become a popular method for developing constitutive models that describe the fatigue behavior of asphalt concrete mixture owing to the uniform states of stress across the specimen section. This study investigates the effect of the loading waveform (sinusoidal versus haversine) and rest period (continuous versus intermittent) on the laboratory fatigue life of asphalt concrete mixtures. The fatigue analysis was performed using the simplified viscoelastic continuum damage (S-VECD) approach where the damage characteristic (C-S) curves were established for all the cases, and then used to estimate the fatigue laws through simulated predictions. The proposed uniaxial fatigue test and analysis method were able to determine the fatigue life relationships of asphalt concrete mixture at different waveform and rest period conditions with a reduced testing time compared to other traditional testing and analysis methods. Overall, both rest period and waveform pattern were found to affect the laboratory fatigue life of asphalt concrete mixture. Model predictions show that pulse-rest loading yields an equivalent fatigue life to continuous loading at strain values that are approximately four times greater.

High temperature fatigue and creep-fatigue behaviors in a Ni-based superalloy: Damage mechanisms and life assessment

The low cycle fatigue (LCF) and creep-fatigue behaviors of Ni-based GH4169 superalloy are investigated by uniaxial strain-controlled fully-reversed testing at 650 °C. Compared with LCF tests, the effects of tensile and compressive strain hold times on creep-fatigue lifetimes are experimentally explored with varying total strain ranges in the present work. In order to elucidate the damage mechanisms under complex loading waveforms, an additional series of tests with both tensile and compressive hold times are carried out at a given total strain range of 2.0%. Posterior to the cyclic tests, main-crack-failure modes and secondary cracking modes are studied via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. Main-crack failure mechanisms are examined by the fracture appearance observations. Cracking modes are explored through quantitative characterization on the distributions of secondary cracks in the longitudinal cross sections under different loading waveforms. Moreover, a generalized life model based on linear damage summation (LDS) framework and energy dissipation criterion (EDC) is elaborated to estimate the damage mechanisms of fatigue, creep and oxidation. The prediction results can well establish the correlations between the reductions of numbers of cycles to failure and the presences of different damage mechanisms under respective loading waveforms.

Stress Intensity Factor Based Damage Prediction Model for Plain Concrete under Cyclic Loading

Concrete highways and airfield pavements are subjected to a large number of traffic loads, which may cause failure of the pavement due to fatigue. In the present work, an experimental study has been carried out on plain concrete beam specimens to investigate the crack propagation and incremental damage characteristics of concrete under cyclic loading. A compliance test was conducted on beam specimens with different notch depths, and a relationship between notch depth and compliance was established. The fracture toughness of concrete was also estimated in a beam bending test. Fatigue tests were conducted on different sets of beam specimens under different load levels. Vertical displacement and crack mouth opening displacement were measured at each load cycle. The variation of stress intensity factor with load repetitions was observed using the crack lengths estimated from the compliance values corresponding to different load repetitions. The ratio of stress intensity factor to critical stress intensity factor (at failure), termed as toughness ratio in this work, has been found to influence the fatigue life. The fatigue life, as expected, also depended on stress ratio. A fracture mechanics–based damage prediction model was proposed for estimating the progressive damage at any intermediate load cycle. The damage prediction model was validated using the experimental results available in literature. Predicting fatigue damage at intermediate stages and due to variable sequence of loading is convenient for the design and evaluation of concrete pavements. Although the effects of frequency (in the ranges applicable for highway pavements) on the fatigue performance may not be significant, the applicability of the present model may be limited by the specific specimen size and loading waveform considered in this study.

In vitro mechanical stimulation facilitates stress dissipation and sealing ability at the conventional glass ionomer cement-dentin interface

ObjectiveThe aim of this study was to evaluate the induced changes in the chemical and mechanical performance at the glass-ionomer cement-dentin interface after mechanical load application. MethodsA conventional glass-ionomer cement (GIC) (Ketac Bond), and a resin-modified glass-ionomer cement (RMGIC) (Vitrebond Plus) were used. Bonded interfaces were stored in simulated body fluid, and then tested or submitted to the mechanical loading challenge. Different loading waveforms were applied: No cycling, 24 h cycled in sine or loaded in sustained hold waveforms. The cement-dentin interface was evaluated using a nano-dynamic mechanical analysis, estimating the complex modulus and tan δ. Atomic Force Microscopy (AFM) imaging, Raman analysis and dye assisted confocal microscopy evaluation (CLSM) were also performed. ResultsThe complex modulus was lower and tan delta was higher at interfaces promoted with the GIC if compared to the RMGIC unloaded. The conventional GIC attained evident reduction of nanoleakage. Mechanical loading favored remineralization and promoted higher complex modulus and lower tan delta values at interfaces with RMGIC, where porosity, micropermeability and nanoleakage were more abundant. ConclusionsMechanical stimuli diminished the resistance to deformation and increased the stored energy at the GIC-dentin interface. The conventional GIC induced less porosity and nanoleakage than RMGIC. The RMGIC increased nanoleakage at the porous interface, and dye sorption appeared within the cement. Both cements created amorphous and crystalline apatites at the interface depending on the type of mechanical loading. Clinical significanceRemineralization, lower stress concentration and resistance to deformation after mechanical loading improved the sealing of the GIC-dentin interface. In vitro oral function will favor high levels of accumulated energy and permits micropermeability at the RMGIC-dentin interface which will become remineralized.

Molecular dynamics simulations of ejecta production from sinusoidal tin surfaces under supported and unsupported shocks

Micro-ejecta, an instability growth process, occurs at metal/vacuum or metal/gas interface when compressed shock wave releases from the free surface that contains surface defects. We present molecular dynamics (MD) simulations to investigate the ejecta production from tin surface shocked by supported and unsupported waves with pressures ranging from 8.5 to 60.8 GPa. It is found that the loading waveforms have little effect on spike velocity while remarkably affect the bubble velocity. The bubble velocity of unsupported shock loading remains nonzero constant value at late time as observed in experiments. Besides, the time evolution of ejected mass in the simulations is compared with the recently developed ejecta source model, indicating the suppressed ejection of unmelted or partial melted materials. Moreover, different reference positions are chosen to characterize the amount of ejecta under different loading waveforms. Compared with supported shock case, the ejected mass of unsupported shock case saturates at lower pressure. Through the analysis on unloading path, we find that the temperature of tin sample increases quickly from tensile stress state to zero pressure state, resulting in the melting of bulk tin under decaying shock. Thus, the unsupported wave loading exhibits a lower threshold pressure causing the solid-liquid phase transition on shock release than the supported shock loading.

The effect of thermal loading waveform on the failure mechanism of atmospheric-plasma-sprayed thermal barrier coating system

The hot-end components of gas turbine engines in the serviced condition usually suffer from the thermal cycle loading effect. Thermal barrier coating (TBCs) have been used as a protective coating to prevent degradation during practical applications. In the current work, the influence of a loading waveform on the failure mechanism of an atmospheric plasma-sprayed TBCs is investigated at an elevated temperature. The samples under the trapezoidal loading condition present a lower thermal cycling fatigue lifetime than that under the triangular loading condition. Moreover, cracks in the TBCs samples under the triangular loading condition are initiated from the defects on the top coat formed during preparation, and cracks under the trapezoidal loading condition preferentially occur at the thermally grown oxide-top coat interface due to higher stress at the interface. Our results demonstrate that the thermal loading waveform should be carefully designed to characterize the in-service condition of hot-end components.

Static mode fatigue crack propagation and generalized stress intensity correlation for fatigue–brittle polymers

Stable fatigue crack propagation is predominantly described by the Paris power law correlation of the crack growth rate with the amplitude cyclic stress intensity. The Paris relationship works well for most ductile materials but does not capture the response for fatigue–brittle materials lacking a cyclic damage mechanism, including ceramics and many polymers. Instead, crack growth rate of fatigue–brittle materials correlates to the peak cyclic stress intensity factor, $$\hbox {K}_{\mathrm{max}}$$ . This work shows that $$\hbox {K}_{\mathrm{max}}$$ correlation of fatigue crack growth is derived directly from static mode crack tip behavior with constant correlation coefficients, and that $$\Delta \hbox {K}$$ correlations are not generally applicable for static mode crack propagation in fatigue–brittle polymers. This derivation predicts load ratio, frequency, and waveform effects, which are included in a general static mode fatigue crack propagation law. Fatigue crack propagation data of a known fatigue–brittle polymer are presented to demonstrate static mode crack propagation behavior correlation with $$\hbox {K}_{\mathrm{max}}$$ with constant parameters.

A Load Identification Algorithm of Frequency Domain Filtering Under Current Underdetermined Separation

Non-intrusive load identification is important to load monitoring and smart power utilization. In order to effectively identify load status and obtain detailed power data, this paper explained a load identification algorithm based on load decomposition and frequency domain filtering. It realized load decomposition through solving an underdetermined problem of a single current signal under non-intrusive collection mode and then further achieved load identification. The underdetermined problem was optimized into a 1-D underdetermined problem. The solving model was constructed as separating the collecting current into two channels. According to the sparseness of the current frequency domain signal, the optimal solution was obtained by a two-step iterative shrinkage threshold algorithm. Therefore, each operation load can be decomposed independently. The frequency-domain filter group was conducted to filter the decomposition current, and load identification was realized based on the decision of the frequency component after filtering. The algorithm simplified the identification complexity, with a fast convergence rate and high identification accuracy. The performance was validated by the actually measured data. The load decomposition and identification was effective, which could accurately determine the load status, and obtain the current waveform of the independent load.