Showing papers on "Flexural rigidity published in 2020"

Analysis of Residual Flexural Stiffness of Steel Fiber-Reinforced Concrete Beams with Steel Reinforcement

Abstract: This paper investigates the ability of steel fibers to enhance the short-term behavior and flexural performance of realistic steel fiber-reinforced concrete (SFRC) structural members with steel reinforcing bars and stirrups using nonlinear 3D finite element (FE) analysis. Test results of 17 large-scale beam specimens tested under monotonic flexural four-point loading from the literature are used as an experimental database to validate the developed nonlinear 3D FE analysis and to study the contributions of steel fibers on the initial stiffness, strength, deformation capacity, cracking behavior, and residual stress. The examined SFRC beams include various ratios of longitudinal reinforcement (0.3%, 0.6%, and 1.0%) and steel fiber volume fractions (from 0.3% to 1.5%). The proposed FE analysis employs the nonlinearities of the materials with new and established constitutive relationships for the SFRC under compression and tension based on experimental data. Especially for the tensional response of SFRC, an efficient smeared crack approach is proposed that utilizes the fracture properties of the material utilizing special stress versus crack width relations with tension softening for the post-cracking SFRC tensile response instead of stress-strain laws. The post-cracking tensile behavior of the SFRC near the reinforcing bars is modeled by a tension stiffening model that considers the SFRC fracture properties, the steel fiber interaction in cracked concrete, and the bond behavior of steel bars. The model validation is carried out comparing the computed key overall and local responses and responses measured in the tests. Extensive comparisons between numerical and experimental results reveal that a reliable and computationally-efficient model captures well the key aspects of the response, such as the SFRC tension softening, the tension stiffening effect, the bending moment-curvature envelope, and the favorable contribution of the steel fibers on the residual response. The results of this study reveal the favorable influence of steel fibers on the flexural behavior, the cracking performance, and the post-cracking residual stress.

The mechanics of built-up cold-formed steel members

Abstract: The paper presents methods of analysis of built-up sections in which the discrete locations of fasteners is accounted for explicitly, rather than by smearing their effect using continuous shear flexibility as in current approaches. By considering fasteners at discrete points, it is possible to analyse the effects of placing additional fasteners at the ends (end fastener groups), to account directly for actual support conditions and to determine the optimum locations of fasteners. The paper first outlines the linear analysis of beams in flexure and introduces the notion of the effective flexural rigidity to account for partial composite actions. Closed form solutions are provided for five load and end support cases to demonstrate the application of the analysis. Next, the paper describes the linear analysis of built-up sections in torsion, considering first uniform torsion followed by non-uniform torsion. Closed form solutions are obtained for the effective torsion rigidity (GJeff) of built-up sections featuring closed loops. A framework is also presented for determining the effective torsion rigidity (EIw,eff) of open built-up sections in non-uniform torsion. The paper concludes with the analysis of built-up sections subject to flexural buckling. A general variational buckling equation is derived followed by an energy-type method for calculating buckling loads for common end support conditions, including columns supported on flexible end tracks. Closed form solutions are presented for up to seven rows of fasteners longitudinally. While by nature approximate, the solutions are shown to be highly accurate. Comparisons are made between the presented closed form solutions and buckling load predictions obtained using current design provisions.

On Calculating the Bending Modulus of Lipid Bilayer Membranes from Buckling Simulations.

Abstract: The bending modulus is an important physical constant characterizing lipid membranes. Different methods have been devised for calculating the bending modulus from simulations, and one of them, named the buckling method, is nowadays widely used due to its simplicity and numerical stability. However, questions remain on the reproducibility, finite size effects, and interpretation of results on lipid mixtures. Here we explore the dependence of simulation results on the system size and the strain. We find that the dimensions of the box have a negligible impact on the results when the system size is beyond a certain threshold. We then calculate the bending rigidity for of a series of common single-component lipid bilayers (PC, PS, PE, PG, and SM), as well as a number of binary and ternary lipid mixtures. We find that bending moduli of lipid mixtures can be predicted from the weighted average of the moduli of the individual components, as long as the mixture is homogeneous. For phase-separated mixtures, the apparent elastic modulus is closer to the value of the softer component. Predictions of the bending modulus based on the area compressibility modulus are found to be generally unreliable.

Flexural Behavior of Double-Skin Steel Tube Beams Filled with Fiber-Reinforced Cementitious Composite and Strengthened with CFRP Sheets.

Abstract: The concrete-filled double skin steel tube (CFDST) is a more viable option compared to a concrete-filled steel tube (CFST) due to consisting a hollow section, while degradation is enhanced simply by using carbon fiber-reinforced polymer (CFRP). Hence, the stabilization of a concrete's ductile strength needs high- performance fiber-reinforced cementitious conmposite. This study investigates the behavior of high-performance fiber-reinforced cementitious composite-filled double-skin steel tube (HPCFDST) beams strengthened longitudinally with various layers, lengths, and configurtion of CFRP sheets. The findings showed that, with increased CFRP layers, the moment capacity and flexural stiffness values of the retrofitted HPCFDST beams have significantly improved. For an instant, the moment capacity of HPCFDST beams improved by approximately 28.5% and 32.6% when they were wrapped partially along 100% with two and three layers, respectively, compared to the control beam. Moreover, the moment capacity of the HPCFDST beam using two partial layers of CFRP along 75% of its sufficient length was closed to the findings of the beam with two full CFRP layers. For energy absorption, the results showed a vast disparity. Only the two layers with a 100% full length and partial wrapping showed increasing performance over the control. Furthermore, the typical failure mode of HPCFDST beams was observed to be local buckling at the top surface near the point of loading and CFRP rapture at the bottom of effect length.

Experimental analysis of manufacturing parameters’ effect on the flexural properties of wood-PLA composite parts built through FFF

Abstract: This paper aims to determine the flexural stiffness and strength of a composite made of a polylactic acid reinforced with wood particles, named commercially as Timberfill, manufactured through fused filament fabrication (FFF). The influence of four factors (layer height, nozzle diameter, fill density, and printing velocity) is studied through an L27Taguchi orthogonal array. The response variables used as output results for an analysis of variance are obtained from a set of four-point bending tests. Results show that the layer height is the most influential parameter on flexural strength, followed by nozzle diameter and infill density, whereas the printing velocity has no significant influence. Ultimately, an optimal parameter set that maximizes the material’s flexural strength is found by combining a 0.2-mm layer height, 0.7-mm nozzle diameter, 75% fill density, and 35-mm/s velocity. The highest flexural resistance achieved experimentally is 47.26 MPa. The statistical results are supported with microscopic photographs of fracture sections, and validated by comparing them with previous studies performed on non-reinforced PLA material, proving that the introduction of wood fibers in PLA matrix reduces the resistance of raw PLA by hindering the cohesion between filaments and generating voids inside it. Lastly, five solid Timberfill specimens manufactured by injection molding were also tested to compare their strength with the additive manufactured samples. Results prove that treating the wood-PLA through additive manufacturing results in an improvement of its resistance and elastic properties, being the Young’s module almost 25% lower than the injected material.

Flexural strengthening of reinforced concrete beams with NSM-CFRP bars using mechanical interlocking

Abstract: Flexural strengthening of reinforced concrete (RC) beams using near-surface-mounted (NSM) technique has become an attractive alternative for rehabilitation using fiber reinforced polymer (FRP) materials. Previous studies have recommended using available anchoring techniques to overcome premature bonding failure. In this study, mechanical interlocking grooves were utilized to delay or prevent debonding failure. The first part of the study aimed to investigate the bond characteristics for NSM-CFRP bars by conducting several pullout tests on No. 6, No.10, and No.13 CFRP bars. Results indicate that mechanical interlocking grooves can significantly enhance the bonding capacity and prevent or delay premature bonding failure. In the second part, the proposed NSM CFRP strengthening technique was used to strengthen nine RC beams. In addition to longitudinal grooves, the proposed technique consisted of 6 mm wide lateral grooves (or mechanical interlocking) placed at 76 mm on center along the entire length of the strengthened beams. Steel reinforcement ratios of 0.7% and 0.4% were selected. All beam specimens were tested under four-point bending until failure. Results showed that strengthening was more effective for specimens with a lower steel reinforcement ratio. Percentages of enhancement in flexural strength were between 34-68% and 60–128% for specimens with 0.7% and 0.4% steel reinforcement ratios, respectively. Finally, a simple empirical model was created for the experimental results. Theoretical results showed reasonable agreement with the experimental results. However, the maximum load carrying capacity and flexural stiffness were overestimated for beams with a total reinforcement ratio (steel plus CFRP) larger than 1.1%.

Numerical study of filament suspensions at finite inertia

Abstract: We present a numerical study on the rheology of semi-dilute and concentrated filament suspensions of different bending stiffness and Reynolds number, with the immersed boundary method used to couple the fluid and solid. The filaments are considered as one-dimensional inextensible slender bodies with fixed aspect ratio, obeying the Euler–Bernoulli beam equation. To understand the global suspension behaviour we relate it to the filament microstructure, deformation and elastic energy and examine the stress budget to quantify the effect of the elastic contribution. At fixed volume fraction, the viscosity of the suspension reduces when decreasing the bending rigidity and grows when increasing the Reynolds number. The change in the relative viscosity is stronger at finite inertia, although still in the laminar flow regime, as considered here. Moreover, we find the first normal stress difference to be positive as in polymeric fluids, and to increase with the Reynolds number; its value has a peak for an intermediate value of the filament bending stiffness. The peak value is found to be proportional to the Reynolds number, moving towards more rigid suspensions at larger inertia. Moreover, the viscosity increases when increasing the filament volume fraction, and the rate of increase of the filament stress with the bending rigidity is stronger at higher Reynolds numbers and reduces with the volume fraction. We show that this behaviour is associated with the formation of a more ordered structure in the flow, where filaments tend to be more aligned and move as a compact aggregate, thus reducing the filament–filament interactions despite their volume fraction increases.

Bending: from thin interfaces to molecular films in microemulsions

Abstract: Surfactant film rigidity is a ubiquitous general concept that is quantified into two different units. We show here how to convert the bending rigidity from reduced units of a virtual infinitely thin film (not made of molecules) into the chemical unit (kJ.mol−1) of a realistic film of monomolecular thickness. In most cases, molecular lengths are not negligible versus curvature radius. Two bending constants for the elasticity of thin-shelled solids can be defined, as introduced by Gauss, whereas only one physical bending constant taking into account that the film cannot be torn has been introduced in the 1990s by Hyde and Ninham. The explicit conversion depends on the topology and is different in the quasi-planar approximation, as well as the ‘direct’ oil in water (o/w) or ‘reverse’ water in oil (w/o) case of spherical or cylindrical micelles. We show some examples for classical and nonclassical micelles and microemulsions of different compositions.

Mechanical performance of concrete-filled square steel tube stiffened with PBL subjected to eccentric compressive loads: Experimental study and numerical simulation

Abstract: This paper describes an experimental study on the flexural and eccentric compressive behavior of concrete-filled square steel tube (CFSST) beam-columns stiffened with PBL stiffeners. Three types of CFSST beam-columns were fabricated and tested under the flexural moment and eccentric compressive load. Among the three types of columns, the two types of columns were strengthened with the PBL and steel plate stiffeners; the other type of columns was not strengthened. In addition, extensive finite element analyses (FEA) were carried out to evaluate the stress distribution. Flexural test results show that failure modes of specimens without stiffeners were different from specimens with the PBL or steel plate stiffeners. The results also demonstrate the distribution of cracks in the concrete was affected by stiffeners and the columns with the PBL stiffeners have the most uniformed crack distributions. The capacity improved by the PBL stiffeners was insignificant. However, the flexural stiffness of the specimen stiffened with the PBL stiffeners was about 20% larger than the unstiffened specimen. Eccentric compressive load test results show the capacity of specimen with the PBL stiffeners was 6.95% larger than the unstiffened specimen. The eccentric compressive capacity decreased with the increase of the eccentricity ratio or slenderness ratio. FEA results show that stiffeners can effectively improve the mechanical behavior of CFSST specimens under bending and eccentric compressive loads.

Experiment Investigation on Stress Characteristics of Grouting Microsteel Pipe Piles with Cement-Soil Wall

Abstract: Microsteel pipe piles are widely used in excavation support owing to their excellent bearing capacity, flexural rigidity, construction speed, and adaptability. In this study based on a project in soil-rock layers in Qingdao, China, a new type of support combining microsteel pipe piles mounted in cement-soil piles and anchors was adopted and studied. The inner force change laws and bearing capacity of microsteel pipe piles were discussed through in situ stress measurement and laboratory antibending tests on three microsteel pipe piles. It was found that the bending moment of the piles gradually increased with the foundation excavation and maximized at the pile head. The bending moment was larger at the upper and smaller at the lower part along the depth direction, indicating it is reasonable to design and calculate microsteel pipe piles by the pile-anchor system. The variation law of the foundation pit displacement with the excavation depth was monitored. The horizontal displacement of the foundation pit is the maximum at the base top, which is 6.5 mm. The flexural strength of the miniature steel tube pile after grouting is 40% higher than that of the miniature steel tube pile without grouting. It was indicated that the grouting microsteel pipe piles could be implanted in cement-soil piles, which improved the flexural rigidity of cement-soil piles and limited the deformation of foundation pits. This study provides reference for the design and construction of soil-rock foundation pit supporting projects. At the same time, it provides reference for the application of miniature steel pipe piles in other fields.

Pressure distribution over the leaflets and effect of bending stiffness on fluid─structure interaction of the aortic valve

Abstract: We describe a three-dimensional (3-D) fluid–structure interaction (FSI) simulation study of the aortic valve, where the flow is driven by a specified transient pressure drop along the aorta tube. The thickness of the leaflets is varied from 0.05 mm to 0.8 mm so that the normalized bending rigidity by the systolic pressure gradient covers a wide range from to 0.6. The non-uniform pressure distribution over the leaflets and the transient valve force are calculated, including the ‘water hammer’ effect during rapid closure. With low bending rigidity, the valve functions normally and produces physiological characteristics of healthy valves. However, exceedingly low rigidity leads to flapping motion of the leaflets and, in some cases (e.g. the normalized bending rigidity around 0.001), may reduce the performance index. As the leaflets become much stiffer, the valve is more difficult to open and slower to close, which leads to higher resistance and a reduced flow rate. Therefore, our results suggest that there is an optimal range of bending rigidity for the valve, roughly between 0.003 and 0.04 in normalized term. We further develop a one-dimensional unsteady flow model based on the momentum and mass conservation equations to replace the 3-D flow in the FSI simulation. The new flow model incorporates pressure loss across the valve as well as the leaflet motion. Comparison with the 3-D results shows that the reduced flow model is able to produce a reasonable 3-D deformation sequence of the leaflets, opening area and flow rate, especially in the cases of low bending rigidity.

Arc-welding based additive manufacturing for body reinforcement in automotive engineering

Abstract: Arc-welding based additive manufacturing is a cost-efficient, productive technology which has been shown to be capable of producing high-integrity components. It is suggested that in automotive engineering, this manufacturing process can be used to reinforce body components by generating stiffening elements. Benefits of this method could be more flexural rigidity with comparatively lower material volume. In the current study, wire arc additive manufacturing (WAAM) with an advanced short arc welding process with low heat input was chosen. The first objective of the work was to check possibility of generating a gusset plate on zinc-coated car body parts by additive manufacturing, for reinforcing of formed thin steel sheets. The second aim was to increase flexural rigidity of the sheets by depositing weld metal as a grid. Bending tests of the sheets indicated an increased flexural rigidity compared with the parent material. This production method and the results of this study are related to automotive engineering but could be employed for other applications. The aim is to demonstrate how these goals could be approached, what difficulties and limitations exist, and where further research work could be initiated.