Step Load Research Articles

Investigation of bending behavior of two and three-dimensional honeycomb and auxetic sandwich beams

The bending behavior of two (2D) and three-dimensional (3D) sandwich beams that have a negative Poisson’s ratio (auxetic) and conventional honeycomb was investigated for different geometries, sheet thicknesses, and cell thicknesses. The re-entrant and solid specimens were produced with a stereolithographic (SLA) 3D printer and subjected to a three-point bending test under the conditions specified in the ASTM C393 standard. The data obtained from the tests were used to verify finite element analysis (FEA) results. The radius of curvatures was calculated for each specimen depending on the load step. In addition, Poisson’s ratio was calculated for each auxetic sample. As a result, the 3D arrowhead beam, with a cell thickness of 1 mm and a sheet thickness of 2 mm, exhibits peak force values that are 497.340% and 461.500% higher than re-entrant and missing rib beams, respectively. Besides, the maximum strain energy values of same 3D arrowhead specimens (596.120 mJ) are higher than re-entrant (101.032 mJ) and missing rib (108.201 mJ) specimens. It was determined that the arrowhead is the most durable structure compared to other auxetic structure geometries. Therefore, when arrowhead and honeycomb 3D beams are compared, it was observed that the maximum strain energy of arrowhead specimens was higher in both horizontal (84.310%) and perpendicular (131.910%) positioned specimens. Comparing the arrowhead and honeycomb 2D beams with the highest maximum strain energy, it can be concluded that the arrowhead beam absorbs 20.000% more energy.

Sonochemical synthesis of hollow mesoporous silica spheres (HMSSs) and its effective utilization for one-step synthesis of curcumin-loaded HMSSs

Herein, HMSSs were successfully synthesized using the sonochemical method. Compared with a conventional synthesis under continuous stirring, HMSSs synthesized under ultrasound irradiation were uniform hollow spheres with a thinner mesoporous shell, resulting in higher pore volumes and larger specific surface areas. The hollow cores were acquired without the templates but from ethanol droplets generated in situ by ultrasound. The ethanol/water volume ratio (E/W) affected the structures of sonochemical synthesized particles, i.e., mesoporous silica particles were obtained at low E/W before transforming to HMSSs under moderate E/W and dense silica spheres at high E/W. Varying [NH4OH] base catalysts in an aqueous phase altered the core structure of HMSSs from hollow cores at high [NH4OH] to partial-filled cores at a moderate concentration and non-hollow cores at low [NH4OH]. Ultrasound was effectively utilized to prepare curcumin-loaded HMSSs in a single step. Stable ethanol droplets containing curcumin were produced from the ultrasound's homogenizing effect, leading to curcumin's encapsulation within HMSSs. While the cavitation induced the chemical effect, reducing the reaction time. The encapsulation of curcumin and the formation of HMSSs could occur concurrently; thus, not only is the drug loading step unnecessary, but the efficiency of loading drugs inside HMSSs is also enhanced.

Influence of load increase rate during incremental test on aerobic performance indicators

Maximum oxygen consumption is an informative indicator of aerobic performance, and its assessment is associated with a significant investment of time. The paper compares the results of maximum tests with a stepwise increasing load with different duration of one load step: 15, 30, 60, 120 and 240 s. Maximum oxygen uptake, total operating time, and pair-wise relationship of protocols were assessed. There was no statistically significant difference in maximum oxygen uptake rate in protocols of different duration. Also, all the studied protocols showed a high correlation with each other (r = 0.92–0.97).

Superelement-based acceleration of finite-element simulations of wind turbine rotors

Large numbers of wind turbine rotor finite-element simulations are required for blade bearing raceway and ring fatigue calculations. Finite-element rotor models come along with a complex nonlinear model behaviour and a high number of degrees of freedom due to the necessity of considering the blade bearing’s surrounding structures. For that reason, accelerating such simulations is of particular interest for the iterative design process. This study focuses on different superelement configurations for the rotor model of the IWT 7.5-164 reference turbine. The blade bearing’s resulting contact forces and contact angles are analysed for 18 load steps throughout an exemplary rotor rotation and the respective model results are compared to each other. The results show that implementing superelements in the rotor model significantly increases the computational efficiency with an acceptable loss of accuracy in terms of the blade bearing’s internal loads. Furthermore, it is shown that such models outperform the acceleration and especially accuracy achieved by the usage of a one-third rotor model.

Generation Control of Multi-Area Multi-Source Power System using Fire-Fly Algorithm

This article presents automatic generation control (AGC) of an interconnected three area power system having different production sources such as hydropower, thermal and gas power plants. The control areas are provided with single reheat turbine and generation rate constraints of 3%/min. An attempt has been made to apply a Proportional derivative–Proportional integral derivative (PD–PID) cascade controller in AGC. Controller gains are optimized simultaneously using more robust and powerful evolutionary computational technique Firefly Algorithm (FFA). Performance of classical controllers such as Proportional Integral (PI) and Proportional Integral Derivative (PID) controller are investigated and compared with PD–PID cascade controller. Investigations reveal that PI, and PID provide more or less same response where as PD–PID cascade controller provides much better response than the later. The system dynamic performances are studied with 1% and 2% step load perturbation in Area1. Sensitivity analysis reveals that the FFA optimized PD–PID Cascade controller parameters obtained at nominal condition of loading, size and position of disturbance and system parameter (Inertia constant, H) are robust and need not be reset with wide changes in system loading, size, position of disturbance and system parameters.

"Impact of hydride composite materials on thermochemical hydrogen compression"

The formation of a hydride from a metal or intermetallic alloy is an exothermic reversible chemical reaction. The thermodynamic properties governing hydride formation facilitate their thermochemical applications, such as hydrogen compression. Thermal transport inside the materials is of highest importance for system dynamics and its productivity. In this contribution, we demonstrate a two-stage thermochemical hydrogen compressor using the AB5 alloy LaNi5 in the first stage and an AB2 alloy TiMn2 (Hydralloy®) in the second stage. To evaluate the impact of the thermal conductivity of the materials, the productivity of the compressor was compared for pure metal hydride compacts (MH) and metal hydride composite materials (MHC). Compared to pure MH pellets, the MHC used in this study contain expanded natural graphite (ENG), a secondary phase with highest thermal conductivity. As the radial thermal conductivity of the MHC increased, the time required for the loading, temperature change and unloading steps was successfully reduced by 400%. Productivity was increased by over 320 % from 14 NlH2/(kgMaterial*h) to 44 NlH2/(kgMaterial*h). Overall, MHC have the potential to simplify handling, reactor design and reduce investment costs for thermochemical compression systems. Thus, MHC have highest impact and potential for thermochemical applications.

Dissipative and thermal aspects in cyclic loading of additive manufactured AISI 316L

This study presents a comprehensive understanding of the fatigue behavior of 316L stainless steel specimens produced using the laser powder bed fusion (PBF-LB/M) method. The investigation has been conducted through a multifaceted approach that includes surface roughness analysis, density measurements, microstructural examination, fatigue testing, strain measurements, and thermographic analysis. Thermographic data processing models, i.e. the bi-power law (TCM-modified) model and the TCM method, are applied to fatigue test results of standard samples to evaluate fatigue limit values. The fatigue limit of the samples was estimated using the Murakami Method (MM), Constant Amplitude Loading (CAL) and Step Loading (SL) tests. The proposed methodology allows exploration of the entire stress level range within a single test, which could allow evaluation of the fatigue limit of components within only one test. This study is the starting point for a rapid evaluation method for estimating the fatigue limit using thermography, offering a cost-effective, time-efficient, and non-destructive means of assessing the fatigue performance of materials produced using additive manufacturing processes.

Experimental evaluation of stress history effect on compressibility characteristics of lime-stabilized expansive soils

ABSTRACT Lime stabilization has long been identified as an effective method for reducing the swelling potential of expansive soils. Despite numerous studies evaluating the volumetric behaviour of lime-stabilized expansive soils, the influence of stress history variations on deformation characteristics of these soils has not been adequately explored. This study specifically evaluates the impact of stress history conditions on compressibility characteristics of lime-stabilized expansive soils, with special attention to the rebound index at different loading stages. In this regard, one-dimensional consolidation-swell tests were conducted on samples containing different lime percentages (3% to 12%) under various stress history conditions. To include different stress history conditions, overconsolidation ratio (OCR), seating pressure, and loading step were varied during the induction of stress history in the samples. Results showed that OCR and seating pressure significantly influenced both the rebound (increased up to 118%) and compression indices (decreased up to 53%), while the loading step had a negligible impact on the compressibility properties of stabilized soils. Additionally, when comparing the relative importance of investigated parameters, it was revealed that, OCR had the highest influence on the rebound index (up to 118.0% increase), while the compression index was most significantly affected by lime percentage (up to 61.4% decrease).

Permeability evolution of fractured coal subject to confining stress and true triaxial stress loading: experiment and mathematical model

The accurate elucidation and prediction of coal permeability evolution under stress loading conditions are crucial for coalbed methane production. In this study, flow experiments were conducted on six cylindrical coal samples and four cubic coal samples under both confining and true triaxial stress loading conditions, respectively. The structure and characteristic parameters of the fractures inside each coal sample were obtained using the computed tomography scanning system and image processing technologies. The coal permeability under both types of loading processes was calculated through the transient pulse method. A mathematical model was developed to assess the evolution of coal permeability under true triaxial loading based on the current true triaxial permeability model and fractal theory. The results revealed that during the confining pressure loading, the coal permeability decreased exponentially with effective stress and was effectively described using the SD model. Additionally, the coal permeability initially rapidly decreased, followed by a gradual decrease, and eventually stabilized at a constant value. Particularly, during the first three loading steps, the fracture aperture and corresponding permeability of the six cylindrical coal samples decreased by ∼51.79%–57.83% and ∼38.06%–42.12%, respectively. However, during the final three loading steps, the fracture aperture and corresponding permeability of the six coal samples decreased by ∼18.26%–23.08% and ∼22.15%–26.93%, respectively. Moreover, owing to the various crossing angles of complex fracture networks with each principal stress, the effect of each principal stress on the coal permeability evolution was highly anisotropic during triaxial stress loading. Particularly, the permeability of the ST1 sample decreased by 43.08%, 14.84%, and 42.08% during the loading of each principal stress. Similarly, the permeability of the ST2 sample decreased by ∼65.74%, 14.29%, and 19.97%. The permeability reductions for the ST3 sample were ∼34.03%, 55.85%, and 10.12%, while those for the ST4 sample were ∼35.97%, 46.51%, and 17.52%. The SD model failed to describe these anisotropic effects. Compared with the SD model, the improved model, based on the current true triaxial permeability model and fractal theory, effectively described the anisotropic effect of each principal stress on the permeability of coal samples with complex fracture networks under triaxial stress conditions.

Effect of Step Load Based on Time under Tension in Hypoxia on the ACL Pre-Operative Rehabilitation and Hormone Levels: A Case Study.

The aim of the study was to determine the effect of step load in hypoxia on the effectiveness of preoperative rehabilitation (PR) and hormone levels based on a case study. Introduction: We assessed the impact of variables such as rate of movement and time under tension (TUT) in normobaric hypoxia on the levels of growth hormone (GH), insulin-like growth factor 1 (IGF-1), and erythropoietin (EPO). Additionally, the impact of step load on the hypertrophy and strength of knee extensors and flexors was assessed. Methods: The work uses a case study, the research subject of which was a 23-year-old female professional handball player. The tests included an isokinetic assessment of the peak torque of knee extensors and flexors as well as body composition analysis. Results: The results showed a more than (10.81-fold) increase in GH after the microcycle with time under tension (TUT). The deficit between the lower limbs was also reduced. Conclusions: Using a hypoxic environment based on an appropriate altitude, combined with changes such as a short rest break between sets and a controlled tempo of movement with an eccentric phase, TUT may offer an alternative to the PR process, especially among athletes who care about fast RTS.

Static and dynamic response of pyramidal lattice sandwich plate with composite face sheets reinforced by graphene platelets

For the pursuit of lightweight structures with excellent mechanical performance, sandwich structures with lightweight cores and strengthened face sheets have become popular in the advanced engineering fields in recent years. In this paper, the nonlinear static and dynamic responses of the sandwich plate constructed with the aluminum pyramidal lattice core and functionally graded graphene platelets reinforced composite (FG-GPLRC) face layers are analyzed. The partial differential equations of the pyramidal lattice sandwich plate considering the geometric nonlinearity are achieved based on the Reddy’s third order shear deformation theory (TSDT), which are solved by the combination of finite element model, Raphson’s method and Newmark numerical integration method. Four different types of the impact loads including the step loading, triangular loading, half-sine loading and exponential loading with the same peak value and duration time are investigated. The influences of the thickness-to-truss radium ratio and the angle of the truss core, as well as the weight fraction and distribution patterns of graphene platelets (GPLs) over the face sheets under different impact loads and different boundary conditions are analyzed. According to the numerical results, it is concluded that the sandwich plate with higher weight fraction of GPLs, lower thickness-to-truss radium ratio, and higher angle of the truss core can exhibit better performance in both static and dynamic loading conditions. The FG-X distribution of GPLs presents the best dynamic performance as compared to the other GPL distributions. The combination of lightweight truss core with the advanced GPLRC face sheets is instructive for potential applications and further research.

Characterizing damage evolution in fiber reinforced composites using in-situ X-ray computed tomography, deep machine learning and digital volume correlation (DVC)

In-situ X-ray computed tomography (CT) techniques enable to examine microstructural evolution of composite materials continuously and nondestructively under loading for better understanding the initiation and propagation of microscopic damages. This study explores the tensile damage behavior of notched woven carbon/epoxy composites under successive loading process. Since the image contrast of different composite material phases is normally low and conventional image segmentation techniques are hard to identify microstructures and defects in different directions distinctively, a U-Net image segmentation model based on deep machine learning algorithms is employed to characterize microstructures and material damages precisely and consistently. Digital volume correlation (DVC) approach is proposed here tocharacterize the three-dimensional (3D) deformation fields of carbon/epoxy composites at different loading steps, and the DVC results are used to predict location of damage initiation and propagation. It is found that the damage modes of notched carbon/epoxy composites mainly include fiber breakage, longitudinal cracks in warp fiber tows, and transverse cracks in weft fiber tows. The individual fiber breakage occurs in warp fiber tows at the intermediate loading level, and longitudinal and transverse cracks appear in further loading levels. Longitudinal cracks and fiber breakage events take place in a region tangent to the notch tip and exhibit a strong interaction. The development of longitudinal cracks promotes the initiation and propagation of transverse cracks. In addition, the strain concentration regions in 3D deformation fields match well with the microscopic cracks segmented by the U-Net deep learning model. This study demonstrated that the integration of X-ray computed tomography, digital volume correlation and image semantic segmentation enables to identify damage initiation, evolution, extent and mechanism.

Autologous Fat Grafting Air-valve Loading Technique.

Autologous fat transfer is fast becoming a common adjunct procedure in plastic surgical operations, especially in breast reconstruction. Postharvest fat processing can be performed actively or passively with various techniques. All techniques require the final step of loading the harvested fat into a syringe for injection. We describe here an innovative and elegant technique for safer and more efficient loading of the plunger back into the barrel of a fat-filled syringe before injection. This technique improves ease of loading, reduces loss of harvested fat, and eliminates the risk of bodily fluid splash.

The Effect of Load Steps and Initial Crack Length on Stress Distribution of Fiber Metal Laminates

Fiber Metal Laminates (FMLs) are advanced composite materials composed of alternating layers of fiber-reinforced composites and metal sheets, offering a unique blend of high strength, stiffness, and fatigue resistance. Understanding the behavior of FMLs under different loading conditions, particularly in the presence of initial cracks, is crucial for predicting their structural integrity and performance. This study investigates the effect of initial crack length and load steps on the stress distribution within FML structures through numerical simulation. A glass fiber-reinforced epoxy (GFRE) composite is constituted by glass fibers in off-axis directions of 0o/90o. The crack length of 3 mm, 6 mm, 9 mm, and 12 mm is initially defined on the side of fiber metal laminates. The tensile load was gradually applied to the laminates for five different ratios. The ratios are defined by dividing the tensile load with the area of the laminates which is then called stress ratio. The results show that the greater the initial crack length and load step ratio, the greater the stress around the crack tip. The stress distribution in each layer of the laminates shows different characteristics between tensile and compressive. This will affect the crack propagation behavior of the laminates. Moreover, the findings also provide valuable insights for optimizing FML design, improving damage tolerance, and extending service life in diverse engineering applications.

The influence of the experimental methodology on evaluating the hydrogen embrittlement susceptibility of AISI 4340 steel manufactured by different routes

High strength low alloy steels are widely used in the offshore and marine industries for the manufacturing of fasteners. In this context, the occurrence of hydrogen embrittlement is a risk to the service life of these components. The scope of this work is to evaluate the impact of the steelmaking process on the hydrogen embrittlement susceptibility of AISI 4340 steel through different experimental methodologies. The effects of the presence of segregation bands and inclusions on the hydrogen interaction with the microstructure of the steel were studied through hydrogen electrochemical permeation tests and mechanical characterization. The hydrogen embrittlement susceptibility was investigated by performing uniaxial standard tensile tests with prior hydrogen charging, slow strain rate tests with in-situ hydrogen charging, and incremental step loading tests with notched specimens and in-situ hydrogen charging. Fractographic analyzes were carried out to support the interpretation and discussion of the results. It was observed that continuous casting route results in a microstructure more susceptible to hydrogen embrittlement, due to the presence of coarser segregation bands, with a greater density of impurities and inclusions.

Effect of Fractionation Columns on the Elution of Rare Earth Elements Recovered from Acid Mine Drainage

Rare earth elements (REE) can be found in expressive contents in different secondary sources, such as acid mine drainage (AMD). This work evaluated separation of light and heavy rare earth elements (REE) from an acid mine drainage (AMD) generated in a former uranium mine in Brazil by using ion exchange. This AMD presents pH 3.50, total REE content of 97 mg L−1 and 1.3 g L−1 of sulfate and was used in the REE loading experiments. Loading experiments were carried out in columns using a commercial strong acid cation (SAC) exchange resin. Elution was performed with 0.01 mol L−1 NH4EDTA in systems with one, two and three columns. Regarding the loading step, the resin presented a total loading capacity of 0.58 mmol g−1. The resin proved to be more selective for light REE with adsorption efficiency of 78% and 48% for heavy REE. Regarding elution, high efficiencies between 90 and 100% were achieved for REE. The final REE solution is approximately 10 times more concentrated in the liquor related to the acid mine water. Better fractionation results were achieved for the system with three columns. Although the complete separation of the REE into pure elements was not possible, two distinct fractions of heavy and light REE could be obtained, and La was completely separated from the other REE. In order to improve fractionation and separate the REE into individual ones, the concentrated fractions can proceed to subsequent ion exchange systems.

ClosedLoop Control of Pulse-Density-Modulated Wireless Power Transfer with Fast MEPT

Efficient wireless power transfer (WPT) requires closedloop control to perform maximum efficiency point tracking (MEPT) against the dynamic changes of operating conditions. In this paper, we took the latest deltasigma pulse-density-modulated (PDM) WPT system as the plant and presented a systematic control design procedure. The WPT system was modeled by the reducedorder dynamic phasor method and further decoupled under the tuned condition. It was identified that the coupled resonators behave like a secondorder system, and its natural frequency is much lower than the resonant frequency. This was considered in the closedloop design to derive a suitable loop gain and ensure stability. The nonlinearity caused by the dualside PDM control is limited to a small inner loop so that the entire loop is linear and simple. Experimental results showed that the closedloop voltage regulation and MEPT took only about 10 ms after load stepping. The system DC output voltage was always tightly regulated, and the steadystate efficiency was very close to the theoretical maximum value under different coupling conditions.

THE FLEXURAL BEHAVIOR OF SUSTAINABLE LIGHTWEIGHT CONCRETE HOLLOW CORE SLABS

Hollow slabs are reinforced concrete slabs with voids that enable less concrete to be used. To promote sustainability goals, this type of slab reduces material use and enhances insulating characteristics. This paper presents an experimental program for studying the flexural behavior of the sustainable hollow Slab elements of lightweight concrete with three different ratios of mixes using additives to choose the best ratio, which is then compared to a solid model with the same concrete mix with the best characteristics to clarify the variations in the structural behavior of voided slabs. In this investigation, hollow slabs with circular plastic tubes of 50mm diameter and 1020 mm length were used, organized as continuous voids with 30 mm spacing between the tubes. Three slabs were created, each measuring 1020 mm long, 420 mm wide, and 100 mm thick. All three slabs were tested via four-point loading. The best mix ratio was chosen and used to build a robust model. To analyse the key advantages of this technology over typical solid slabs, this solid model was compared to the best-performing model among the three hollow core slabs. Several properties such as load-carrying capability, deflection, crack patterns, and failure modes were observed in all loading steps. The results showed that using uniformly distributed uniaxial voids in the slab reduced concrete use by 23% as compared to solid slabs, Notably, the specimen with 200 kg/m3 lightweight aggregate demonstrated greater crack distribution and crack breadth.

Deformation characteristics and creep behaviour of rigid particulates-EPS beads composites

The compression behaviour of the mixture of glass beads (representing rigid particles) and EPS beads (representing deformable particles) during the loading-unloading process is systematically examined through performing two sets of large-size oedometer experiments, including incremental step-by-step and one-step loading scenarios. At each step during the loading-unloading cycle, the void ratio (e) and the at-rest coefficient of lateral earth pressure (K0) are measured for pure rigid samples and rigid-soft particle mixtures. To consider the creep effect, the overburden pressure at the final loading step is maintained on the sample for 24 h prior to unloading. The results show that at a given overburden pressure, with the addition of soft particles to the pure rigid aggregates, the values of e and K0 decrease. Additionally, for both pure rigid samples and rigid-soft particle mixtures, with increasing the overburden pressure, e decreases whereas K0 augments. Moreover, due to the creep behaviour during the constant loading step, K0 decreases over time for both samples; a phenomenon which is observed to be more pronounced for pure rigid aggregates compared to rigid-soft particle mixtures. Finally, a well-established creep model is used to simulate the creep behaviour of pure rigid samples and rigid-soft particle composites.

Multi-criteria parametric optimization of the displacement and weight of a shell of minimal surface on a circular contour consisting of two inclined ellipses under thermal and power loading with consideration of geometric nonlinearity

Thin shells are well suited to optimal design problems, and the finite element method and gradient descent method make it possible to solve inverse problems in structural and applied mechanics. The calculation process takes into account geometric nonlinearity. In modern numerical studies, a formulation taking into account geometric nonlinearity in the finite element method is used, namely, a stepwise loading procedure. It becomes necessary to take into account the relations between displacement vectors and their derivatives and strain increments. These relations help to determine the stiffness matrix of the finite element at each loading step, which leads to a qualitative study of real displacements in the structure. The geometrically nonlinear calculation of a shell of minimum surface on a circular contour consisting of two inclined ellipses in a multi-criteria parametric optimization is performed by the finite element method. The finite element method is a universal variational method that is focused on solving the most complex problems of elasticity theory and applied and structural mechanics using separate calculation complexes. Stiffness matrix - for the whole body is formed on the basis of the finite element stiffness matrix. The system of solving equations of the finite element method is formed using the Lagrange's variational principle, according to which the total potential energy P of a finite element model of a body in a state of stability and equilibrium has a minimum value. As part of the sensitivity analysis, gradients of the design variables of the structure, displacements in the form of partial derivatives of these characteristics along the design variables, and shell thickness are calculated. The sensitivity information serves as the basis for building an optimal design algorithm using the gradient descent method of the objective function. Standard multi-criteria parametric optimization allows for an average of 10% steel savings, with geometric nonlinearity increasing to 20%. At this study site, 24.4% of sheet steel was saved, which is a significant relative saving. This methodology of the authors shows high results for innovative design and production of steel thin shells in Ukraine and around the world, and also makes it possible to apply several types of optimization to one research object.