Step Load Research Articles

A variationally consistent contact formulation based on a mixed interpolation point method and isogeometric discretization

This work enhances the robustness of the standard displacement-based (penalty Gauss-point-to-segment) contact formulation by applying the so-called Mixed Interpolation Point method (MIP). We consider a smooth isogeometric discretization of the contact surface in order to avoid various artificial discontinuities of the normal contact gap function. Like any existing MIP-enhanced formulation in terms of implementation, the proposed MIP contact formulation only requires a minor modification from an existing implementation of the standard contact formulation. Nevertheless, the improvement in robustness is shown to be quite significant, which often enables the simulation of contact problems with a larger load step size for efficiency and/or a larger penalty parameter for accuracy in the contact constraint. The formalism of the proposed MIP contact method is based on the idea of relaxing the contact constitution at integration points. To this end, at first the contact pressure is considered as an additional unknown apart from the displacement field, and the perturbed Lagrange multiplier potential is used to enforce the contact constraint. The MIP method then eliminates the contact pressure unknown directly at integration points, instead of discretizing it as it is done in the standard mixed contact formulations. As a result, the residual vector is identical to the standard displacement-based contact formulation. However, the resulting tangent stiffness matrix is different, as the MIP tangent is now based on an extrapolation of the contact pressure iteratively over Newton iterations. Several challenging numerical examples are presented to illustrate the accuracy, robustness and efficiency of the proposed formulation.

Experimental Evaluation of KIEAC of a Carbon Steel Using the Pin-Loaded SENT Geometry

Abstract The experimental determination of the threshold stress intensity factor for environment-assisted cracking (KIEAC) is described by several standards, which allow the use of different specimen geometries and methodologies for crack length estimation. In some cases, the combination of structure, specimen size, or both, main loading direction, and crack orientations of the component that need to be characterized limit the use of standardized geometries. Consequently, alternative geometries must be used. In this study, fatigue pre-cracked pin-loaded single edge notched tension specimens as defined by BS 8571:2018, Method of Test for Determination of Fracture Toughness in Metallic Materials Using Single Edge Notched Tension (SENT) Specimens, were applied for the experimental evaluation of the KIEAC of a carbon steel. The specimens were tested in deaerated substitute ocean water solution saturated with carbon dioxide at 40°C and at 1 and 10 bar (100 and 1,000 kPa) under constant load conditions with incremental (step) loading. The crack length during the tests was monitored by direct current potential drop, which was helpful for defining the applied K corresponding to the onset of subcritical crack growth (KIEAC). Additionally, fractographic analysis of KEAC specimens and results from the fracture toughness evaluation of this material in air at room temperature have also been reported.

A new nonlinear control of an active rectifier for variable speed generating units

This paper deals with a newly conceived (state-feedback) nonlinear control strategy for an active rectifier that is supplied by a Permanent Magnet Synchronous Generator (PMSG). The aim of the proposed control strategy is properly regulate the DC-link voltage within a wide range of system operating conditions by: (i) taking into account a sufficiently rich model, in which the classical simplification – imposing to the DC voltage dynamics a relative degree equal to two – is avoided (so that the input power is not assumed to supply instantaneously the sum of load power and charging rate of the capacitor energy, with the resistance loss and the switching device loss being neglected); (ii) resorting to neither input–output linearizing strategies (involving the DC-link voltage regulation error) that lead to unstable regulation dynamics nor to simplifying design assumptions that overlook derivatives of intermediate reference signals; (iii) avoiding the computational-effort-requirements of Model Predictive Control (MPC) techniques. Simulation and Hardware in the loop (HIL) results illustrate the effectiveness of the proposed control – exhibiting a rather simple structure while guaranteeing local exponential stability for the resulting error system – in terms of Total Harmonic Distortion (THD) and controller response to load and DC voltage reference steps: high band-width and good steady-state waveforms are achieved. Robustness issues and control adaptiveness with respect to uncertain model parameters are also finally addressed.

Revealing the factors resulting in incomplete recovery of perfluoroalkyl acids (PFAAs) when implementing the adsorbable and extractable organic fluorine methods

Per- and polyfluoroalkyl substances (PFASs) are environmental contaminants of concern. Techniques that quantify total organic fluorine (TOF) such as the adsorbable organic fluorine (AOF) and extractable organic fluorine (EOF) methods are important for PFAS risk assessments. The objective of this study was to systematically evaluate each step of the AOF (loading, washing, combustion) and EOF (loading, washing, elution, combustion) methods for the recovery of ten ultrashort-, short-, and long-chain unsubstituted perfluoroalkyl acids (PFAAs). We measured the overall recovery of fluoride for each method for each PFAA, and the recovery of each PFAA around the loading, washing, and elution steps. We also measured the combustion efficiency of each PFAA by direct combustion. The overall AOF and EOF recovery ranged from 9.3%–103.3% to 21.0%–108.1%, respectively, with higher recoveries measured for PFAAs with increasing chain length in both methods. The three ultrashort-chain PFAAs (trifluoroacetic acid, perfluoropropionic acid, and perfluoropropanesulfonic acid) exhibited the lowest overall recoveries from 9.3–25.2% for AOF and 21.0–51.5% for EOF. We found that decreases in the overall recovery are the result of losses of ultrashort- and short-chain PFAAs during the washing step and the incomplete mineralization of perfluoroalkyl sulfonic acids during combustion for AOF and incomplete elution of short- and long-chain PFAAs and the loss of ultrashort-chain PFAAs during the washing step for EOF. Our data suggest that the EOF method is more appropriate than the AOF method for measuring TOF in samples containing ultrashort- and short-chain PFAAs and that methodological improvements are possible with a focus on the washing, elution, and combustion steps.

A Low-Power Capacitorless LDO Regulator with Transient Enhancement Structure

In this paper, a portable low-power low dropout regulator (LDO) with no output capacitance has been designed in 0.18-[Formula: see text]m CMOS technology. In this paper, a PMOS power transistor is used and combined with an overshoot and undershoot suppression circuit to improve the transient response of the circuit. A modified Class-AB operational transconductance amplifier (OTA) is used to reduce power consumption and act as an error amplifier to eliminate low frequency poles. At the same time, a super source follower and a Miller capacitor are used for frequency compensation of the system. Finally, the system can provide a stable voltage for a load step from 0.2 to 25[Formula: see text]mA in 500[Formula: see text]ns edge-time for 140[Formula: see text]pF-load capacitance. The test results show that the circuit achieves a good voltage regulation rate and high transient response.

A multi-depth spiral milli fluidic device for whole mount zebrafish antibody staining

Whole mount zebrafish antibody staining (ABS) is a common staining technique used to localize protein information in a zebrafish embryo or larva. Like most biological assays, the whole mount zebrafish ABS is still largely conducted manually through labor intensive and time-consuming steps which affect both consistency and throughput of the assay. In this work, we develop a milli fluidic device that can automatically trap and immobilize the fixed chorion-less zebrafish embryos for the whole mount ABS. With just a single loading step, the zebrafish embryos can be trapped by the milli fluidic device through a chaotic hydrodynamic trapping process. Moreover, a consistent body orientation (i.e., head point inward) for the trapped zebrafish embryos can be achieved without additional orientation adjustment device. Furthermore, we employed a consumer-grade SLA 3D printer assisted method for device prototyping which is ideal for labs with limited budgets. Notably, the milli fluidic device has enabled the optimization and successful implementation of whole mount zebrafish Caspase-3 ABS. We demonstrated our device can accelerate the overall procedure by reducing at least 50% of washing time in the standard well-plate-based manual procedure. Also, the consistency is improved, and manual steps are reduced using the milli fluidic device. This work fills the gap in the milli fluidic application for whole mount zebrafish immunohistochemistry. We hope the device can be accepted by the zebrafish community and be used for other types of whole mount zebrafish ABS procedures or expanded to more complicated in situ hybridization (ISH) procedure.Graphical

Effect of aggregate characteristics on properties of cemented sand and gravel

Abstract To improve the stability of cemented sand and gravel (CSG) dam construction materials, artificial aggregates can be selected to replace missing natural aggregates, and aggregate grading optimization can be carried out to meet the needs of engineering applications. This article uses finite-element analysis software to explore the influence of aggregate characteristics on the performance and destruction characteristics of CSG materials through numerical simulation. The results show that (1) with the increase of circular natural sand gravel aggregates, the peak stress and elastic modulus of the sample increase, while the strength also increases. (2) Compared to circular aggregates, polygonal aggregates have more edges and corners, which exacerbate the deformation disharmony between mortar and aggregates; the phenomenon of stress concentration is more obvious, so under the same loading step, the degree of damage of polygonal aggregates is greater than that of circular aggregates. (3) After the freeze–thaw cycle test, the deterioration of parameters in the CSG resulted in more severe damage and strength loss of the crushed stone aggregate than the circular aggregate sample. This conclusion can provide a reference for the design of CSG mix ratio in engineering sites.

Centrifugal and gyroscopic effects on dynamic response of rotating cantilever beams under step loading

In this paper, a simplex finite element model for beams (FEMB) with a complete formulation is presented to study the dynamics of rotating cantilever beams subjected to distributed external loads. In our approach, the finite element method (FEM) and Timoshenko's beam theory is used. We will, particularly, examine the centrifugal and gyroscopic effects on the linear vibration of a rotating cantilever beam in stationary regime. In this model, extension, bending, and torsion degrees of freedom (DOF) are combined using a parameterization of the 3D motion of the beam by Euler's three-angle sequence. That allows identifying all the terms of the gyroscopic coupling in a more compact equations system. To ensure convergence, a sufficient number of finite elements are required in this model. The considered beam in the numerical simulation is pre-twisted and linearly tapered with a rectangular section. Results show that the dynamic centrifugal effect decreases the natural frequencies of the beam. Furthermore, the gyroscopic coupling induces rapid extension and torsion beatings when the beam undergoes a step bending load. Without damping, these beatings can persist and cause material fatigue.

A combined experimental and finite element analysis of the human elbow under loads of daily living

Elbow trauma is often accompanied by a loss of independence in daily self-care activities, negatively affecting patients’ quality of life. Finite element models can help gaining profound knowledge about native human joint mechanics, which is crucial to adequately restore joint functionality after severe injuries. Therefore, a finite element model of the elbow is required that includes both the radio-capitellar and ulno-trochlear joint and is subjected to loads realistic for activities of daily living. Since no such model has been published, we aim to fill this gap.For comparison, 8 intact cadaveric elbows were subjected to loads of up to 1000 N, after they were placed in an extended position. At each load step, the displacement of the proximal humerus relative to the distal base plate was measured with optical tracking markers and the joint pressure was measured with a pressure mapping sensor.Analogously, eight finite element models were created based on subject-specific CT scans of the corresponding elbow specimens. The CT scans were registered to the positions of tantalum beads in the experiment. The optically measured displacements were applied as boundary conditions.We demonstrated that the workflow can predict the experimental contact pressure distribution with a moderate correlation, the experimental peak pressures in the correct joints and the experimental stiffness with moderate to excellent correlation. The predictions of peak pressure magnitude, contact area and load share on the radius require improvement by precise representation of the cartilage geometry and soft tissues in the model, and proper initial contact in the experiment.

Permeation kinetics of dimethyl sulfoxide in porcine corneoscleral discs

The cornea is the transparent tissue in front of the eye that bends light to help the eye focus. More than five million people's vision can be restored by a corneal transplant (keratoplasty), but there is a scarcity of suitable donor tissue. Cryopreservation could potentially increase the on-demand availability of corneas by reducing expiration and contamination during hypothermic storage, and allow equitable distribution. Understanding the transport of water and cryoprotectants across the tissue is important in developing effective cryopreservation protocols. Here, we first measured the shrinking and swelling kinetics at 22 °C and 0 °C of porcine corneoscleral discs when exposed to phosphate-buffered saline and to a cryoprotectant vehicle solution containing 2.5% chondroitin sulfate and 1% dextran. Other valuable measurements were made including the density and osmolality of the vehicle solution at 0 °C, and the water fraction of porcine cornea and sclera. Using the knowledge gained from this first part to minimize background swelling, we then examined permeation kinetics of dimethyl sulfoxide (Me2SO) in porcine corneoscleral discs at 0 °C, the temperature at which cryoprotectant loading typically occurs. The concentration data obtained as a function of time were fitted to a Fick's law model of one-dimensional diffusion to measure an effective diffusion coefficient of Me2SO, which was found to be 5.306×10−11 m2/s. We further quantified permeation kinetics of Me2SO in sclera alone at 0 °C to support our hypothesis that our measurements for corneoscleral discs will not be affected by the presence of the sclera. The obtained effective diffusion coefficient can be used in modelling aimed at developing cryopreservation protocols that minimize the exposure time of the corneas during the cryoprotectant loading step.

Computational modelling of built-up cold-formed steel columns and parametric studies

The paper develops a detailed finite-element model for the reliable nonlinear collapse analysis of built-up cold-formed steel columns with discrete fastener connections. First, the main components of the computational model are discussed, including their available options and comparison between their results, followed by a rational justification for the options chosen in this study. Specifically, a detailed theoretical background is provided with conclusive comparisons of the options for enforcing end support conditions, applying contact conditions between the constituent plates, selecting connection elements for representing intermediate screw fasteners, and finally, the incremental load stepping and nonlinear solution schemes. The accuracy and performance of the proposed FE modelling strategy are verified against the experimental data, contributing to the calibration of the influencing parameters. The predictions of the proposed methodology are shown to be in excellent agreement with the test data in terms of the failure mode and the inelastic capacity curve up to ultimate capacity and beyond. Upon its successful calibration, the FE model is utilised to perform extensive parametric studies into the effects of various design parameters on the ultimate strength of built-up columns. This includes practical variations of cross-section dimensions, sectional slenderness, built-up section geometry, and fastener spacing. The parametric study results suggest a meaningful dependency of the ultimate capacity of built-up sections on the fastener spacing ratio, the critical buckling half-wavelength of the built-up section and the relative number of restrained components in the built-up configuration undergoing sectional buckling.

Evaluation Method for the Bearing Capacity of Reinforced Concrete Beams Based on the Kaiser Effect

The Kaiser effect is an important phenomenon that occurs in acoustic emission. The estimation of reinforced concrete structures based on acoustic emission is receiving widespread attention. Cumulative acoustic emission parameters were used for the preliminary estimation of Kaiser points through the step loading experiments on four simply supported reinforced concrete beams under bending load. Taking this Kaiser point as the centre, an appropriate interval value was determined. After functional fitting of the “load-acoustic emission parameter” curve, the difference between the estimated value and the measured value was calculated. The Kaiser point was modified in accordance with the “load-difference” curve. The modified Kaiser point was used to calculate the Felicity ratio. The ratio of the maximum load to the ultimate bearing capacity of the beams was calculated. The curves of the ratio and the Felicity ratio were obtained. Fitting of the corresponding relational function was performed. Based on the relational function, the ultimate bearing capacity of the beams was estimated according to the mean of the experiment. The results showed that if there was a significant difference between the preliminarily estimated value and the measured value of the Kaiser point, modifications can be conducted to avoid any significant difference in the Kaiser point caused by human factors. Using the Felicity ratio and based on function fitting, the maximum difference in the ultimate bearing capacity of the beams was estimated to be no more than 10%. Moreover, it was concluded that the estimated ultimate bearing capacity of the beams was characterized by high stability in the case of different sample data. This method will provide a theoretical basis for evaluating the ultimate bearing capacity of reinforced concrete beams under bending load based on the Kaiser effect.

Experimental investigation on PEM fuel cell flooding mitigation under heavy loading condition

Water flooding is a crucial factor affecting the lifetime of a proton exchange membrane (PEM) fuel cell. In this paper, A laboratory-scale (25 cm2) single PEM fuel cell was tested to investigate the characteristics of fuel cell under a load shedding process with various air flow rates. The performance was assessed using output power and electrochemical impedance spectroscopy, while the cathode flooding was identified using voltage and cathode pressure drop. The results show that reducing the load within a specific range can lower the impedance and lessen floods. The results also demonstrate that there is an optimal inlet flow rate interval considering its effect on system efficiency, durability, and water flooding. In addition, a stepped load shedding pattern was proposed to solve the flooding problem of a fuel cell while taking the power demand into account. The effect of load shedding cycle and air inlet flow rate on flooding was evaluated by comparing the flooding severity coefficient and output power. The results prove that a longer current reduction cycle is more favorable in alleviating flooding problems. The optimal mode for the fuel cell in this paper was determined according to the flooding situation and power variation. Finally, the approach to obtaining the optimal load shedding pattern for other PEM fuel cells system was summarized. The flooding mitigation approach proposed in this paper can be utilized as a guide for developing strategies to extend the life of PEM fuel cells.

Development of a verification device for axle (wheel) load scales based on the automatic loading

During the verification process of the special axle (wheel) load scales for motor vehicles test using the standard dynamometer method, it is necessary to first apply pressure vertically to the load receptor of the axle (wheel) load scales through the axis of the force sensor, which is a prerequisite for subsequent parameter verification work such as indication error. Aiming at the problem of low automation in the lifting and lowering work of force sensors using jacks, this paper introduces a verification device for axle (wheel) load scales based on the automatic loading function and proposes specific automatic loading methods and steps. According to the experimental data after implementation, the verification device has the advantages of high automation and high efficiency, which can replace the existing jack to achieve the lifting and lowering of the force sensor. With this verification device and its automatic loading method, the measurement of the axle (wheel) load scales can meet the requirements of JJG1014-2019 “Special Axle (Wheel) Load Scales for Motor Vehicle Test”.

HTS Tape Mechanical Behavior Sensitivity on Material Properties and Thickness of Material Layers

Multilayer 2G high-temperature superconductor (HTS) tapes undergo various mechanical loading steps as part of a superconducting magnet operation. The mechanical loading steps include cool-down to cryogenic temperatures and Lorentz forces when powering up the magnets. Studying the mechanical behavior of the constituent layers in a HTS tape can reveal the strain or stress level in each layer and help in predicting the probability of mechanical failure or critical current degradation. A detailed finite element method (FEM) -based simulation models enable us to estimate the mechanical behavior in each layer. However, defining the model parameters for material mechanical properties or thickness of each material layer can have consid-erable uncertainty due to variation in manufacturing processes and missing measurements of material properties within these tapes. The material properties in thin layers may not exactly comply with bulk materials of the same kind. In this paper we present a sensitivity analysis for mechanical behavior dependence on material properties variation in the constituent layers of a HTS tape. The results give important insight about the accuracy and reliability of mechanical simulation results and guide the priorities in future material characterization. In addition, we will investigate the effect of the thickness of Hastelloy layer on effective macroscopic mechanical behavior of the tape. A nonlinear elastoplastic FEM model is used for performing the simulations.

Model predictive control of mixing controlled compression ignition operation for low reactivity fuels

Using gasoline or other low reactivity fuels with a pilot injection or port fuel injection in a compression ignition engine has shown great potential in reducing NOx emissions while keeping high thermal efficiency compared to diesel. However, excessive combustion noise is caused by a high maximum pressure rise rate in the cylinder due to the higher fractions of premixed charge of the low-reactivity fuel. This noise can result in structural damage to engine components and as such, combustion noise limits the range of the operating parameters and makes the control of such engines challenging. In this study, a simulation environment was built up in MATLAB/Simulink leveraging a physics-based zero-dimension combustion model to capture the in-cylinder pressure time traces as well as metrics relevant to thermal efficiency and combustion noise. In order to also facilitate the control of emissions, machine learning models were investigated to capture NOx emissions. A kernel-based extreme learning machine (K-ELM) performed best and had a coefficient of correlation (R-squared) of 0.998. The combustion and NOx emission models are valid for not only conventional gasoline fuel but also oxygenated alternative fuel blends at three different pilot injection strategies. In order to track key combustion metrics while keeping noise and emissions within constraints, a model predictive control (MPC) was applied for a compression ignition engine operating with a range of potential fuels and fuel injection strategies. The MPC is validated under different scenarios, including a load step change, fuel type change, and injection strategy change, with proportional–integral (PI) control as the baseline. The simulation results show that MPC reduces about 26% of ringing intensity in the transient process and 17% at the steady state for E30. Generally, MPC can optimize the overall performance through modifying the main injection timing, pilot fuel mass, and exhaust gas recirculation (EGR) fraction.

Stress measurements in the weak layer during snow stability tests

Abstract The snow compression test is a snow stability test where an isolated column of snow is progressively loaded by tapping on it to induce failure in a possible weak layer. The test result provides valuable information about the propensity of failure initiation within the snowpack. However, different persons might tap with different force and thus reduce the reproducibility of the test results. The aim of this work was to quantify the influence of different test persons and different snowpacks on snow compression test results. We therefore let 62 persons tap on a stress measurement plate during a workshop of the European Avalanche Warning Services. Moreover, in the field, we performed stress measurements during 116 snow compression tests with 13 persons at eight different locations in the Alps. Data on persons’ body features and snow properties were also collected. Our results show that the stresses that reach a weak snow layer due to tapping are influenced by both the snowpack as well as different persons. Still, the data's scattering is surprisingly small for lower loading steps and decreases with depth. Therefore, we can deduce that, especially when avalanche conditions are particularly dangerous, snow compression test results are quite reproducible.

Transient Response of Turbocharged Compression Ignition Engine under Different Load Conditions

<div>In urban roads the engine speed and the load vary suddenly and frequently, resulting in increased exhaust emissions. In such operations, the effect of air injection technique to access the transient response of the engine is of great interest. The effectiveness of air injection technique in improving the transient response under speed transient is investigated in detail [<span>1</span>]; however, it is not evaluated for the load transients. Load step demand of the engine is another important event that limits the transient response of the turbocharger. In the present study, response of a heavy-duty turbocharged diesel engine is investigated for different load conditions. Three cases of load transients are considered: constant load, load magnitude variation, and load scheduling. Air injection technique is simulated and after optimization of injection pressure based on orifice diameter, its effect on the transient response is presented. The results reveal that air injection into the intake manifold is an effective technique to improve the turbo lag of a heavy-duty turbocharged diesel engine under the transient conditions of load.</div>

A Self-Triggered Digitally Assisted Hybrid LDO with 110 ns Settling Time in 65 nm CMOS

This article presents a self-triggered digitally assisted hybrid low-dropout regulator (LDO). The proposed architecture uses an analog LDO for steady-state operation and a digital LDO to track large output current changes. The dual loop has a loop controller for coherent operation, and the digital loop is only triggered when there is a large load step. Therefore, the proposed LDO inherits some of the advantages of both parts. It achieves a high power supply rejection ratio (PSRR) from the analog part. The digital loop has a faster settling time and consumes less static power than the analog loop. In this design, the maximum load is 200 mA. For heavy load conditions, PSRR is −40 dB at 1 MHz. The quiescent current is 200 μA. The undershoot/overshoot with the corresponding settling time measured under a load current step of 200 mA/10 ns are 82 mV/89 ns and 112 mV/110 ns, respectively. The proposed LDO achieves a competitive 4.48 ps figure of merit. In the TSMC 65 nm process, the active area is approximately 0.027 mm2.

Modeling, Simulation and Digital Control of Full-Bridge LLC Resonant Converter

In order to demonstrate the performance of an LLC resonant converter with digital control, in this paper, we analyze the operating principles of the full-bridge LLC resonant converter. According to the operating characteristics of the full-bridge LLC resonant converter at different frequency bands, a simulation model was established using MATLAB Simulink software, and simulations were carried out. The digital control system of full-bridge LLC resonant converter was then designed based on the STM32 microcontroller. The simulation and experimental results demonstrated that the LLC resonant main circuit was always operating within the zero voltage switching (ZVS) state of the primary MOSFET and the zero current switching (ZCS) state of the secondary rectifier diode, while soft-switching was achieved in the full load range. The digital control of the full-bridge LLC resonant converter was ideally realized based on STM32. The input disturbance load test result validated the closed-loop concept of the proposed LLC resonant converter. The step response results in both simulation and load tests show that the power supply system has the desired dynamic response characteristics with low overshoot, short rise time and settling time, while the load step response results in both tests showed that the system has a strong anti-interference ability and robustness.