Application Of External Load Research Articles

Beam modeling in a floating frame of reference for torsion dynamics of helicopter rotor blades

AbstractIn the ongoing development of DLR’s Versatile Aeromechanics Simulation Tool, an elastic-beam model is integrated into the multibody system based on the floating frame-of-reference formulation. Although the application of this formulation for one-dimensional beam models has already been addressed in the literature, the challenge remains to properly model the torsion dynamics of rotor blades – especially under high centrifugal loads. To this aim, this work suggests the consideration of rotational shape functions in the inertia shape integrals and in the application of gravitational, inertial, and external loads. This modified approach is validated based on the structural analysis of a rotor blade with complex geometrical properties.

РЕКОМЕНДАЦИИ ДЛЯ МОДЕЛИРОВАНИЯ ЭЛЕМЕНТОВ ИЗ СТАЛЬНЫХ ТОНКОСТЕННЫХ ПРОФИЛЕЙ

Despite the increase in the growth rate of construction with the use of steel thin-walled sections, the building code for the design and calculation of these structures has a number of conventions and indicates the preferred use of the results of experimental studies in the design. Fullscale tests are generally not available during design, so numerical modelling is a correct alternative. The analysis of existing scientific literature shows that there is no consensus among researchers regarding the main parameters of the models and the modelling algorithm in general, so the aim of the paper is to develop recommendations for modelling thin-walled sections. The results of the study for object made from a coupled sigma profile under axial compressive force conditions let us develop key recommendations for modelling including the use of finite element type, finite element mesh parameters, the mechanism of application of boundary conditions, external loads and other model parameters. The results were verified by analytical methods of thin-walled sections calculation. The obtained convergence of the results indicates the possibility of using these recommendations in the design and calculation of thin-walled structures.

A second-order in time and space model to solve the coupled Reynolds–Rayleigh–Plesset equations for the dynamics of cavitated hydrodynamic journal bearings

Cavitation in hydrodynamic bearings is a fundamental factor in the behavior of the fluid film and in the equilibrium position the journal will have after the application of external loads. The calculation of these parameters by robust tools is an active research topic even though accuracy, stability and conservativeness are not so often discussed. This work shows the results obtained through the implementation of a novel model developed from first principles that allows characterizing the pressure field of a liquid–gas lubricant mixture avoiding the reported instabilities with the time derivative of the mixture density. The formulation also relies on a coupled transport equation describing the evolution of the liquid fraction, ensuring mass conservation. This model is applied to study cases with gaseous, vaporous or pseudo-cavitation through the use of the Rayleigh–Plesset equation to characterize the physical behavior of the bubbles formed taking into account the characteristic time of each phenomenon. High resolution schemes are used for the spatial treatment of the advection and a temporal discretization based on the Strang splitting method which allows for a second order convergence in space and time. The numerical results obtained are contrasted and discussed with experimental data found for static and dynamic bearings for the three types of cavitation mentioned, obtaining a very good agreement.

A significant review on the performance of microbial concrete in comportment of diverse nutrients

Concrete is substantially a prerequisite material being used but as the year’s pass, the concrete structures due to external load application might be subjected to the inevitable crack formation that can degrade their durability and strength. The addition of bacteria and the supplementary calcium source creates a pervious layer over concrete fissures similar to calcite precipitation in sealing pores and micro-cracks in the concrete. This review exemplifies the usage of several species of calcite-precipitating, alkali-resistant Bacillus bacteria as crack healing agents and nutrients added for bacteria sustainability in the concrete and mortar at diverse age periods. Various strategies have been proposed to endow self-healing in concrete in past decades. This review summarizes the effect of micro-capsules, hydrogels, cellulose fiber, polymers, mineral admixtures and bacteria type when employed in cementitious materials. This study exuviates light on the advantages of bio minerals produced via bacteria metabolism that improves mechanical properties, durability parameters and microstructure behaviour. It can be summarized that the inclusion of bacteria in concrete and mortar improves its properties resulting in crack healing, making it more sustainable and reducing maintenance cost. Furthermore, research can be a promising investigation into the longevity of the bacteria for its extensive practical outcome-based application.

Inverse design and additive manufacturing of shape-morphing structures based on functionally graded composites

Shape-morphing structures possess the ability to change their shapes from one state to another, and therefore, offer great potential for a broad range of applications. A typical paradigm of morphing is transforming from an initial two-dimensional (2D) flat configuration into a three-dimensional (3D) target structure. One popular fabrication method for these structures involves programming cuts in specific locations of a thin sheet material (i.e. kirigami), forming a desired 3D shape upon application of external mechanical load. By adopting the non-linear beam equation, an inverse design strategy has been proposed to determine the 2D cutting patterns required to achieve an axisymmetric 3D target shape. Specifically, tailoring the localised variation of bending stiffness is the key requirement. In this paper, a novel inverse design strategy is proposed by modifying the bending stiffness via introducing distributed modulus in functionally graded composites (FGCs). To fabricate the FGC-based shape-morphing structures, we use a multi-material 3D printer to print graded composites with voxel-like building blocks. The longitudinal modulus of each cross-sectional slice can be controlled through the rule of mixtures according to the micro-mechanics model, hence matching the required modulus distribution along the elastic strip. Following the proposed framework, a diverse range of structures is obtained with different Gaussian curvatures in both numerical simulations and experiments. A very good agreement is achieved between the measured shapes of morphed structures and the targets. In addition, the compressive rigidity and specific energy absorption during compression of FGC-based hemi-ellipsoidal morphing structures with various aspect ratios were also examined numerically and validated against experiments. By conducting systematical numerical simulations, we also demonstrate the multifunctionality of the modulus-graded shape-morphing composites. For example, they are capable of blending the distinct advantages of two different materials, i.e. one with high thermal (but low electrical) conductivity, and the other is the other way around, to achieve combined effective properties in a single structure made by FGCs. This new inverse design framework provides an opportunity to create shape-morphing structures by utilising modulus-graded composite materials, which can be employed in a variety of applications involving multi-physical environments. Furthermore, this framework underscores the versatility of the approach, enabling precise control over material properties at a local level.

On the use of strength criteria of anisotropic materials

One of the distinctive properties of composite materials is their anisotropy, which allows, in particular, to use of their strength properties maximum. Thanks to new technologies, in particular 3-D printing, there are new opportunities for purposeful formation of the structure of composite materials and their properties. An important issue here is ensuring strength, which is determined depending on the structure of the composite as a whole and the mechanical characteristics of its individual components. The article reviews well-known strength criteria (Goldenblatt-Kopnov, Mises-Hill, Fisher, Hoffman, Tsai-Wu, and others), the relationship between them, and data on the reliability of their application under certain load conditions. Some of them assume the same properties of materials under conditions of compression and tension and are obtained by generalizing the classical theories of strength and plasticity of isotropic bodies. The criteria that take into account the different resistance of the composite material to tension and compression, which is inherent in many composite materials, have gained greater practical application. When choosing a strength criterion, from the point of view of structure, it is worth considering separately layered and reinforced composite materials, and from the point of view of load - the static or cyclic nature of its application. The criteria listed above do not take into account the damage accumulation in the material that occurs under the influence of external loads. The number of such known criteria is limited, because today there is no method for predicting the stage of scattered fracture of composite materials taking into account the anisotropy of mechanical properties, and the choice of the damage parameter is not justified. The perspective of developing such criteria for clarifying the received conclusions regarding the load-bearing capacity of products made of composite materials under different types of application of external loads is noted.

Tunable ferroelectric auxetic metamaterials for guiding elastic waves in three-dimensions

In this paper, we conduct a computational investigation of both static and dynamic behavior of three-dimensional, dynamically tunable metamaterials. We consider the dynamical characteristics of metamaterials with effective Poisson’s ratios varying among positive, zero, and negative values. We analyze their effectiveness in controlling elastic waves in all directions and investigate different avenues to tune their transmission characteristics by an application of external loads, effects of poling, and a combination of the two. We demonstrate the utility of the metamaterials through tunable multiplexing and guiding of waves at different frequencies. Our findings can open the door to advanced multi-functional and robust acoustic devices.

Strength of Compressed Reinforced Concrete Elements Reinforced with CFRP at Different Load Application Eccentricity

Currently, many studies are devoted to the use of polymer composite materials to increase the strength and stability of concrete elements. In compressed reinforced concrete elements, the bearing capacity depends on the eccentricity of the external application of the external force and the corresponding stress-strain state, as well as the location and number of composite materials glued to the surface of the structure. The choice of a scheme for placing composite materials depending on the stress state of the structure is an urgent scientific problem. At the same time, the issue of central compression and the compression of columns with large eccentricities has been well studied. However, studies conducted in the range of average eccentricities often have conflicting results, which is the problem area of this study. The primary aim of this study was to increase the strength and stiffness of compressed reinforced concrete elements reinforced with composite materials, as well as a comparative analysis of the bearing capacity of ten different combinations of external longitudinal, transverse, and combined reinforcement. The results of testing 16 compressed columns under the action of various eccentricities of external load application (e0/h = 0; 0.16; 0.32) are presented. It is shown that the use of composite materials in strengthening structures increases the bearing capacity up to 41%, and the stiffness of the sections increases up to 30%. Based on the results of the study, recommendations are proposed for improving the calculation method for inflexible columns reinforced in the transverse direction, which take the work of concrete under the conditions of a three-dimensional stress state into consideration.

NdFeB Magnets with Well‐Pronounced Anisotropic Magnetic Properties Made by Electric Current‐Assisted Sintering

Electric current‐assisted sintering (ECAS) technologies are highly promising for processing of NdFeB magnets. Due to the combination of direct Joule heating and application of external load, even powders, whose particle size distribution and morphology are not optimum for conventional powder processing like melt‐spun powders or magnet scrap, can be easily sintered to high densities. A systematic study is done to demonstrate the potential of field‐assisted sintering technique/spark plasma sintering (FAST/SPS) and flash spark plasma sintering (flash SPS) for sintering of NdFeB powders. Melt‐spun, commercial NdFeB powder (Magnequench MQU‐F) is used as starting material. Its platelet‐like shape makes this powder extremely difficult to sinter by conventional methods. This study clearly reveals that especially in the case of flash SPS application of external pressure in combination with short cycle times enables to achieve well‐pronounced anisotropic magnetic properties without the need of subsequent upset forging. Optimized flash SPS parameters are applied to NdFeB magnet scrap with broad particle size distribution, demonstrating the general potential of ECAS technologies for recycling of waste magnet materials. Finally, the results are benchmarked with respect to established NdFeB processing technologies and electrodischarge sintering (EDS), another promising ECAS technology with very short cycling time.

Comparing two types of loading during inspiratory muscle training in patients with weaning difficulties: An exploratory study

BackgroundInspiratory muscle training improves respiratory muscle function and may improve weaning outcomes in patients with weaning difficulties. Compared to the commonly used pressure threshold loading, tapered flow resistive loading better accommodates pressure–volume relationships of the respiratory muscles, which might help to facilitate application of external loads and optimise training responses. ObjectiveThe objective of this study was to compare acute breathing pattern responses and perceived symptoms during an inspiratory muscle training session performed against identical external loading provided as pressure threshold loading or as tapered flow resistive loading. We hypothesised that for a given loading, tapered flow resistive loading would allow larger volume expansion and higher inspiratory flow responses and consequently higher external work of breathing and power than pressure threshold loading and that subsequently patients perceived fewer symptoms during tapered flow resistive loading than during pressure threshold loading. MethodsIn this exploratory study, 21 patients (maximal inspiratory pressure: 35 ± 14 cmH2O and vital capacity:0.85 L±0.37 L) performed two training sessions against external loads equalling 42 ± 15% of maximal inspiratory pressure provided either as pressure threshold loading or as tapered flow resistive loading. During these training sessions, breath-by-breath data of breathing parameters were collected, and patients rated their perceived breathing effort, dyspnoea, and unpleasantness. ResultsCompared to pressure threshold loading, tapered flow resistive loading allowed significantly larger volume expansion (0.53 ± 0.28 L versus 0.41 ± 0.20 L, p < 0.01) and inspiratory flow responses (0.43 ± 0.20 L/s versus 0.33 ± 0.16 L/s, p = 0.01). Tapered flow resistive loading was perceived as less unpleasant (3.1 ± 1.9 versus 3.8 ± 2.4, p = 0.048). No significant differences in breathing effort, dyspnoea, work of breathing, and power were observed. ConclusionsFor a given loading, inspiratory muscle training with tapered flow resistive loading allowed larger volume expansion and higher inspiratory flow responses than pressure threshold loading, which led patients to perceive tapered flow resistive loading as less unpleasant. This might help us to facilitate early implementation of inspiratory muscle training in patients with weaning difficulties. Clinical trial registration numberClinicaltrials.gov identifier: NCT03240263

Stress recovery of laminated non-prismatic beams under layerwise traction and body forces

Emerging manufacturing technologies, including 3D printing and additive layer manufacturing, offer scope for making slender heterogeneous structures with complex geometry. Modern applications include tapered sandwich beams employed in the aeronautical industry, wind turbine blades and concrete beams used in construction. It is noteworthy that state-of-the-art closed form solutions for stresses are often excessively simple to be representative of real laminated tapered beams. For example, centroidal variation with respect to the neutral axis is neglected, and the transverse direct stress component is disregarded. Also, non-classical terms arise due to interactions between stiffness and external load distributions. Another drawback is that the external load is assumed to react uniformly through the cross-section in classical beam formulations, which is an inaccurate assumption for slender structures loaded on only a sub-section of the entire cross-section. To address these limitations, a simple and efficient yet accurate analytical stress recovery method is presented for laminated non-prismatic beams with arbitrary cross-sectional shapes under layerwise body forces and traction loads. Moreover, closed-form solutions are deduced for rectangular cross-sections. The proposed method invokes Cauchy stress equilibrium followed by implementing appropriate interfacial boundary conditions. The main novelties comprise the 2D transverse stress field recovery considering centroidal variation with respect to the neutral axis, application of layerwise external loads, and consideration of effects where stiffness and external load distributions differ. A state of plane stress under small linear-elastic strains is assumed, for cases where beam thickness taper is restricted to 15^{circ }. The model is validated by comparison with finite element analysis and relevant analytical formulations.

РАБОТА КОЛЕСНОГО ДИФФЕРЕНЦИАЛА В ПРОЦЕССЕ РАЗГОНА ПАХОТНОГО АГРЕГАТА

The use of ballast on wheeled tractors leads to over-compaction of the soil, including sub-arable horizons. When overclocking arable units with buried working bodies at operating speed, the engine will inevitably enter the overload mode. This leads to a decrease in productivity, fuel overspending, and deterioration of environmental indicators. The authors proposed for the first time to equip the driving wheels with built-in differentials. The basis of the wheel differential development concept is the eccentric application of external loads and the driving torque to the wheel. The purpose of the research is to identify structural and operational factors affecting the efficiency of the wheel differential, to establish the nature of the relationships between them. The operation of the driving wheel tire occurs under conditions of a deformed support surface. Acceleration of a mobile power vehicle with a built-in differential in the wheels is presented as a two-stage process. The first stage of acceleration corresponds to the operation of the gearbox in the planetary gearbox mode, the second stage - in the differential gearbox mode. The translational movement of the wheel, its rotation along the bearing surface is possible if the torque on the wheel rim is provided by the moment of friction force in the contact spot. The rotation of the bearing gear along the epicyclical gear, the formation of the tilting and lever moments in the wheel reducer contribute to the advancement of the pressure center along the length of the contact spot in the oncoming sector of the wheel, an increase in the friction force and tangential force, ensuring smooth starting of the tractor unit. After overcoming the moment of inertia of the rotating and translationally moving masses of the unit, the value of the required moment decreases. The driving bearing gear, remaining in the lower position, rotates the wheel. The gearbox has switched to differential operation. At the same time, the driver performs angular oscillations, the magnitude of which depends on the conditions of movement of the wheel and the traction resistance of the unit. The amount of torque required depends on the design parameters of the built-in differential, operating conditions, and gravity applied to the bearing drive gear of the planetary gearbox

Analysis of Stress-Strain State and Contact Pressures on Surfaces of Kaplan Runner Blade Bearing Bronze Bushes

An analysis of the existing and prospective blade seal designs for Kaplan runners was performed. The selected design type provides the maximum ecological safety for Kaplan runners. A 3D model of runner hub sector with the trunnion, inner and outer bushes of blade trunnion was generated taking into account the cyclic symmetry of the runner design based on the modern automated design engineering system. A diagram of application of external loads from the blade and lever to the given 3D model of the Kaplan runner hub segment was developed. The contact problem was formulated to determine the stress-strain state as well as the contact pressures at the inner and outer bronze bushes of the Kaplan runner blade trunnions in different operating conditions. The problem was formulated for the finite element method, taking into consideration the diagram of external load application and contact restraints to the given 3D model of the Kaplan runner hub sector in the software package for engineering calculations. Using calculation results, principal stress distribution diagrams and the distribution diagram for the contact pressure at the outer and inner bronze bushes of blade trunnions were obtained. Strength calculation results were processed using the data of principal stress distribution diagrams, and the contact pressure values at the inner and outer bronze bushes of blade trunnions were determined. A methodology for further use of the given analytical model in the evaluation of stress-strain state of Kaplan runners involving modern automated design engineering systems and software package for engineering calculations was developed. The comparison of stress-strain states of the blade trunnion bushes was performed for the old and new designs of the Kaplan runner seal.

Prediction of the Stress State of Pond Ash Road Embankments

Several full-scale in-tray stamp tests of the pond ash road embankment model were carried out under static and dynamic loading conditions. Based on the results, equations were proposed and verified for predicting the major principal stresses at different depths in the ash roadbed, depending on the degree of compaction, humidity, own weight of the technogenic soil, and type of the external load application.

Effects of external light load on posture and foot pressure distribution in young adults: A pilot study

Holding an asymmetrical load, like a rucksack, grocery bag, or travel backpack, is a part of daily and occupational activities. This pilot study was aimed to determine the effects of light, asymmetrical, handheld load on the body posture and foot pressure distribution in young adults. Thirteen young volunteers, free of any injury or pain, took part in the study (aged 21.53 ± 1.10 years). All the study participants were right-handed. An additional load (4 kg, placed in the rucksack) was asymmetrically held in each hand while standing still. For evaluation of the posture and foot pressure distribution, the investigation focused on three positions, i.e., no-load, a rucksack in the right hand, a rucksack in the left hand. We used a surface topography system and force platform for evaluation of the body posture and foot pressure examination, respectively. The results showed that the main changes in the body posture caused by an external load application manifested in trunk imbalance. The trunk was shifted in the direction opposite to the load. Trunk imbalance changes were statistically significant in all three comparisons (p = 0.001). Interestingly, changes in the rotations and lateral deviation seem to be dependent on the hand dominance. Greater foot pressure was observed on the loaded limb (p < 0.01), and highly significant differences were noted between the left and right foot in both loaded postures (p < 0.001). Further research needs to be done to check if lateralization is connected with body posture while holding an external load.

The Effect of Residual Stresses on Stress–Strain Curves Obtained via Profilometry‐Based Inverse Finite Element Method Indentation Plastometry

This paper concerns, the effect of (unknown) residual stresses in the near‐surface region of a sample on outcomes of an indentation plastometry technique for obtaining stress–strain relationships, using relatively large spherical indenters. The technique is based on the iterative finite element method (FEM) simulation of indentation, so as to infer the true stress–strain curve of the material (with the target outcome being the profile of the residual indent). It is expected that residual stress levels (if significant compared with the yield stress) are likely to influence the outcome, and indeed this forms the basis of many attempts to measure residual stresses in this way (using a known stress–strain curve). However, there are important issues of sensitivity here, which affect the reliability of such procedures and are also relevant to the accuracy of stress–strain curves inferred from measurements made on samples containing unknown levels of residual stress. Herein, both experimental work on samples with (equal‐biaxial) residual stresses created by the application of external loads and extensive FEM modeling to explore various scenarios are covered. The main conclusion is that the sensitivities involved are in general very low, particularly with relatively deep indentation. Inferred stress–strain curves are thus likely to be quite accurate, even in the presence of relatively high levels of residual stress. Conversely, the measurement of residual stress levels via this procedure is likely to be rather inaccurate, although the reliability is improved using shallow penetration (low ratio of depth‐to‐indenter radius). It is also noted that tensile residual stresses tend to influence outcomes more strongly than compressive ones.

Deformation behavior of polyurethane treated in argon plasma: impact of surface and subsurface properties

Separation into surface and subsurface is important when studying complex macromolecular compounds. Investigation of surface-modified materials under application of external load is another challenge. Polyurethane is a phase-separated polymer. It internal structure (stiff fibrils percolating the softer matrix) is covered by a soft nanolayer. Treatment in argon plasma leads to gradual etching and altering the surface and subsurface structures. In first 60 s of the treatment the outer soft layer decreases from 6 to 2 nm becoming stiffer; fibrilar structures are destroyed. After 120 s of the treatment the stiffness and thickness of the coating reaches equilibrium. Cracks were observed in the uniaxial stretched state on the surfaces after treatment < 60 s; the materials with longer treatment show no strain-induced defects.

Measurement of Residual Stresses Using Indentation Plastometry

This study concerns the potential use of the indentation plastometry technique to obtain information about residual stresses. This is a logical extension of the technique, which is based on iterative FEM simulation of the indentation procedure so as to infer the true stress-strain curve of the material, with the target outcome being the profile of the residual indent. Once this has been obtained (in the form of parameter values in a constitutive law), a logical extension of the procedure is to create an initial (residual) stress in the model and then converge on the value that gives the best agreement with experiment. In the present work, this has been done for some copper sheet samples, with different levels of residual stress created experimentally via the application of external loads. This has been done for a series of equal biaxial stress levels. The inferring of residual stress levels in this way, and comparison with expected levels, has not been reported previously. The main conclusion is that the sensitivity of the measured indent profile to the residual stress level is low. This is particularly the case for compressive stresses and for relatively deep indents, but it is also true for tensile stresses and shallower indents. The prospects for accurately inferring residual stress levels via this procedure are therefore poor. A positive aspect to this rather negative conclusion is that, when obtaining plasticity characteristics in this way, there need be little concern about the outcome being influenced by the presence of (unknown) residual stresses.

Brain Injuries in American Football: Understanding the Injury, Difficulty in Helmet Optimization, and Current Communication Practices – A Narrative Review

Background: Over 2.7 million people suffer traumatic brain injuries (TBIs) annually in the United States. TBI involves the application and generation of external forces and impulse loads respectively to the head whereby the brain moves relative to the skull. Despite numerous studies, further understanding of TBIs is necessary, requiring consistent attention. Objective: The purpose of this article is to investigate the history of American football helmets and provide an academic and practitioner review as it relates to TBIs. This study is a literature review that also considers perspectives from an autoethnographic frame. Method: An extensive literature review was performed to assess the history of TBI as it relates to American football. This article evaluates helmet design optimization and American football safety as well as an exploration into the sports’ education methods for players and staff alike. Results: Despite developing helmet designs that can better attenuate impact forces, reducing linear and rotational movement, the skull and brain move very differently relative to one another. Helmet designs and tools for measuring forces require further validation techniques to determine resultant forces and movement for the brain. Current biomechanics research lacks sufficient methodology for defining TBI thresholds, making helmet optimization difficult. Conclusion: According to past research, no helmet can eliminate all TBI risk; however, processes are in place lead by the National Football League (NFL) and NFL Players Association to educate players, coaches, and staff at all levels of competition of the protective capabilities of available helmet options.

Mechanisms of acute ankle syndesmosis ligament injuries in professional male rugby union players: a systematic visual video analysis

BackgroundThe mechanisms of acute ankle syndesmosis ligament injuries in professional rugby union are not well understood.AimTo describe the mechanisms of acute ankle syndesmosis ligament injuries in male professional rugby union...