Showing papers in "Thin-walled Structures in 2018"

Variational approach for wave dispersion in anisotropic doubly-curved nanoshells based on a new nonlocal strain gradient higher order shell theory

Abstract: In this paper, a variational approach for the wave dispersion in anisotropic doubly-curved nanoshells is presented. To study the doubly-curved nanoshell as a continuum model, a new size-dependent higher-order shear deformation theory is introduced. In order to capture the small scale effects, nonlocal strain gradient elasticity theory has been implemented. The present model incorporates two scale coefficients to examine the wave characteristics much accurately. Based on Hamilton’s principle, the governing equations of the doubly-curved nanoshells are obtained. These equations are solved via analytical approach. From the best knowledge of authors, it is the first time that present formulation is used to investigate the wave dispersion in anisotropic doubly-curved nanoshells. Also, it is the first time that small scale effects are considered in doubly-curved nanoshells made of anisotropic materials. Unlike the classical (scaling-free) model, the presented nonlocal strain gradient higher-order model shows a good calibration with the experimental frequencies and phase velocities. It is demonstrated that the material properties, nonlocal-strain gradient parameters and wave number have remarkable influences on wave frequencies and phase velocities. Presented results for wave dispersion can serve as benchmarks for future analysis of doubly-curved nanoshells.

Free vibration analysis of saturated porous FG circular plates integrated with piezoelectric actuators via differential quadrature method

Abstract: The free vibration analysis of a circular plate made up of a porous material integrated by piezoelectric actuator patches has been studied. The plate is assumed to be thin and its shear deformations have been neglected. The porous material properties vary through the plate thickness according to some given functions. Using Hamilton's variational principle and the classical plate theory (CPT) the governing motion equations have been obtained. Simple and clamped supports have been considered for the boundary conditions. The differential quadrature method (DQM) has been used for the discretizations required for numerical analysis. The effect of some parameters such as thickness ratio, porosity, piezoelectric actuators, variation of piezoelectric actuators-to-porous plate thickness ratio, pores distribution and pores compressibility on the natural frequency, radial and circumferential stresses has been illustrated. The results have been compared with the similar ones in the literature.

GBTul 2.0 − A second-generation code for the GBT-based buckling and vibration analysis of thin-walled members

Abstract: This paper presents the fundamentals and illustrates the application and potential of the recent 2.0 release of the software G BTul – a computer program developed by the authors and made available as freeware on the website of the Department of Civil Engineering of the University of Lisbon. The program is based on Generalised Beam Theory (GBT), a bar theory accounting for cross-section in-plane and out-of-plane (warping) deformation, and performs linear buckling and undamped free vibration analyses of prismatic thin-walled members. Its domain of application is much wider than that of the previous release (1.0β), making it possible to analyse single or multi-span members (i) with various support conditions, namely those due to discrete bracing systems, (ii) exhibiting arbitrary (open, closed or “mixed”) flat-walled cross-sections and (iii) acted by fairly general loadings, including concentrated and/or distributed transverse forces applied away from the member shear centre axis. After providing a brief overview on the GBT fundamentals, the program capabilities and innovative aspects are addressed, and its application is illustrated by means of a few relevant numerical examples. Moreover, the program Graphical User Interface is described and the procedures and/or options associated with its main commands are mentioned.

Crashworthiness design for bio-inspired multi-cell tubes with quadrilateral, hexagonal and octagonal sections

Abstract: Multi-cell tubes have been widely used in vehicle engineering for their excellent energy absorption capacity. In this paper, a group of bionic multi-cell tubes (BMCTs) with quadrilateral, hexagonal and octagonal sections were proposed. The BMCTs were constructed by filling the cylindrical tubes into different position of multi-cell tubes (MCTs), which was inspired by the microstructure of beetle forewings. The finite element (FE) models under axial impact loading were established and then validated by the Simplified Super Folding Element (SSFE) theory. The crashworthiness of different BMCTs and MCTs was compared, and the results showed that the sixth type of bionic multi-cell tube with octagonal section (O-BMCT-6) has the best crashing performance. Then, the multiobjective optimization design of O-BMCT-6 was conducted by using non-dominated sorting genetic algorithm II (NSGA-II) and radial basis function (RBF) metamodels. The optimal O-BMCT-6 showed superior crashworthiness and could be used as an energy absorber.

Nonlinear primary resonance of functionally graded porous cylindrical shells using the method of multiple scales

Abstract: An analytical method is proposed for the nonlinear primary resonance analysis of cylindrical shells made of functionally graded (FG) porous materials subjected to a uniformly distributed harmonic load including the damping effect. The Young's modulus, shear modulus and density of porous materials are assumed to vary through the thickness direction based on the assumption of a common mechanical feature of the open-cell foam. Three types of FG porous distributions, namely symmetric porosity distribution, non-symmetric porosity stiff or soft distribution and uniform porosity distribution are considered in this paper. Theoretical formulations are derived based on Donnell shell theory (DST) and accounting for von-Karman strain-displacement relation and damping effect. The first mode of deflection function that satisfies the boundary conditions is introduced into this nonlinear governing partial differential equation and then a Galerkin-based procedure is utilized to obtain a Duffing-type nonlinear ordinary differential equation with a cubic nonlinear term. Finally, the governing equation is solved analytically by conducting the method of multiple scales (MMS) which results in frequency-response curves of FG porous cylindrical shells in the presence of damping effect. The detailed parametric studies on porosity distribution, porosity coefficient, damping ratio, amplitude and frequency of the external harmonic excitation, aspect ratio and thickness ratio, shown that the distribution type of FG porous cylindrical shells significantly affects primary resonance behavior and the response presents a hardening-type nonlinearity, which provides a useful help for the design and optimize of FG porous shell-type devices working under external harmonic excitation.

NURBS-based isogeometric thermal postbuckling analysis of temperature dependent graphene reinforced composite laminated plates

Abstract: A thermal postbuckling analysis for composite laminated plates reinforced with graphene sheets is performed in this research. All of the thermomechanical properties of the composite media are assumed to be temperature dependent. Volume fraction of the graphene in each layer is assumed to be different which results in a piecewise functionally graded plate. Based on the third order shear deformation plate theory of Reddy, the total strain energy of the plate is obtained. Composite laminated plate is assumed to be under uniform temperature rise. Properties of the graphene reinforced composite media are estimated by means of a refined Haptin-Tsai approach which contains efficiency parameters to capture the size dependency of the constituents. Afterwards, a non-uniform rational B-spline (NURBS) based isogeometric finite element method is implemented to study the thermal postbuckling response of the graphene reinforced composite laminated plates. Thermally induced postbuckling curves of the composite plate reinforced by graphene are provided for different functionally graded patterns, aspect ratios, side to thickness ratios and boundary conditions. It is shown that, FG-X pattern of graphene reinforcement results in the highest critical buckling temperature and the lowest postbuckling deflection.

Postbuckling of functionally graded graphene-reinforced composite laminated cylindrical shells subjected to external pressure in thermal environments

Abstract: The current investigation deals with the buckling and postbuckling behaviors of graphene-reinforced composite (GRC) laminated cylindrical shells subjected to lateral or hydrostatic pressure under thermal environmental conditions. The piece-wise GRC layers are arranged in a functionally graded (FG) pattern along the thickness direction of the shells. The temperature dependent material properties of GRCs are estimated by the extended Halpin–Tsai micromechanical model with graphene efficiency parameters being calibrated against the GRC material properties from a molecular dynamics simulation study. We employ the Reddy’s higher order shear deformable shell theory in association with the von Karman geometric nonlinearity to model the shell buckling problem under different thermal environmental conditions. The buckling pressure and the postbuckling equilibrium path for the perfect and geometrically imperfect GRC laminated cylindrical shells are obtained by applying a singular perturbation technique along with a two-step perturbation approach. We observe that the piece-wise functionally graded distribution of graphene reinforcement can increase the buckling pressure and the postbuckling strength of the GRC laminated cylindrical shells subjected to external pressure.

Analytical behavior of CFDST stub columns with external stainless steel tubes under axial compression

Abstract: Concrete-filled double skin steel tubular (CFDST) stub columns with external stainless steel and internal carbon steel tubes can be considered as new types of composite members and expected to combine the advantages of all three kinds of materials. This paper presents non-linear finite element (FE) analysis and design of circular and square CFDST stub columns with external stainless steel under axial compression. FE models are developed, where non-linear material property of stainless steel is considered, and verified through comparisons with experiments in terms of failure modes, load-deformation histories and ultimate strength. Behaviors of stainless steel composite columns are compared with that of columns with both carbon steel tubes. Parametric studies are conducted to investigate the influence of the outer stainless steel tube strength, concrete strength, inner carbon steel tube strength and hollow ratio on structural behavior of axially loaded columns in terms of loading and interaction performance.

Dynamic crushing response of auxetic honeycombs under large deformation: Theoretical analysis and numerical simulation

Abstract: In order to comprehensively understand the dynamic response of auxetic honeycombs, theoretical analysis are conducted to predict the NPR effect and the crushing stress of the re-entrant hexagonal honeycomb. The honeycomb’s crushing stress is a function of the cell’s geometric parameters, crushing velocity and the mechanical property of the cell-wall material. Results show that the crushing stress enhances with the increasing crushing velocity. A dynamic sensitivity index is employed to quantitatively evaluate this enhancement. It is shown that small cell-wall angle, low relative density or high cell-wall length ratio of the honeycomb attribute high velocity-sensitivity to the crushing stress. The Poisson’s ratio of the re-entrant honeycomb is also expressed as a function of the cell’s geometric parameters. It is revealed that the NPR effect enhances with the increasing cell-wall angle and the decreasing cell-wall length ratio. All the theoretical predictions are verified by numerical simulations. Besides, an interesting phenomenon is noticed that the crushing velocity has significant influence on the honeycomb’s NPR effect at the early stage of crushing. However, this influence almost vanishes when the overall strain is larger than about 0.2. This present work is supposed to shed light on the design of the auxetic honeycomb.

Wave propagation in viscoelastic thin cylindrical nanoshell resting on a visco-Pasternak foundation based on nonlocal strain gradient theory

Abstract: Wave propagation in viscoelastic single walled carbon nanotubes is investigated by accounting for the simultaneous effects of the nonlocal constant and the material length scale parameter. To this end, thin shell theory is used to model the viscoelastic single walled carbon nanotubes, and the nonlocal strain gradient theory is used to account for the effects of the nonlocal constant and the material length scale parameter. The Kelvin–Voigt model is used to model the viscoelastic property, and the governing equations are derived through Hamilton’s principle. The viscoelastic single walled carbon nanotube medium is modeled as visco-Pasternak. The results demonstrate that viscoelastic single walled carbon nanotube rigidity is higher in the strain gradient theory and lower in the nonlocal theory in comparison to that in the classical theory. Also, the size effects, nanotube radius, circumferential wavenumber, nanotube damping coefficient, and foundation damping coefficient exert considerable effects on viscoelastic single walled carbon nanotube phase velocity.

Dynamic crushing behavior and energy absorption of graded lattice cylindrical structure under axial impact load

Abstract: Dynamic behavior of lattice cylindrical structures with triangular and hexagonal configurations subjected to constant velocity impact was studied theoretically and numerically. The dynamic plateau stress of lattice cylindrical shell was well predicted by analytical predictions based on the one-dimension shock theory. The uniform and density gradient lattice cylindrical structures were investigated using finite element models. It was found normalized plastic energy absorption was significantly affected by relative density for two kinds of lattice cylindrical shells. And the ratio of cell wall to skin thickness was found the vital factor determining the specific energy absorption and deformation modes of lattice sandwich cylindrical shell. By introducing density gradient along crushing direction, the results showed that, for lattice cylindrical shell, introducing positive density gradient can enhance energy absorption at the early stage in high velocity. For lattice sandwich cylindrical shell, introducing density gradient can efficiently reduce the peak crushing force but have little effect on the energy absorption.

Experimental and numerical behaviour of eccentrically loaded high strength concrete filled high strength square steel tube stub columns

Abstract: Using high strength materials in concrete filled steel tube (CFST) columns is expected to achieve better structural performance and fulfil the requirements of sustainable construction. To study the mechanical behaviour of eccentrically loaded high strength concrete filled high strength square steel tube (HCFHSST) stub columns, this paper describes 12 tests with different eccentricity ratios and steel ratios. The cubic strength of high strength concrete under investigation was 110.5 MPa, and the yield strength of the high strength steel was about 434 MPa. Curves of load-lateral deformation were presented, along with values of ductility index, and the minimum ductility index based on the steel ratio of columns was suggested. Finite element analysis (FEA) software ABAQUS was applied to simulate HCFHSSTs. The analytical results were in good agreement with the experimental ones. The load-lateral deformation curve was divided into four stages: elastic, elastic-plastic, plastic hardening and descending. The confinement effect of steel tube at various stages was analysed. The parametric studies were carried out to evaluate the influences of the eccentricity ratio, concrete compressive strength, steel yield strength and steel ratio on the strength reduction factor (SRF), concrete contribution ratio (CCR), P-M and P/Pu-M/Mu interaction curves of the HCFHSST members. The bending moments at balanced points of P/Pu-M/Mu curves calculated by the plastic stress distribution models (PSDM) and FEA models were compared. The ultimate bearing capacities obtained from the tests and the values calculated from the AISC 360, GB 50936 and CECS 28: 90 design codes were compared. Finally, the formulas were proposed to predict the P/Pu-M/Mu curves for the HCFHSST stub columns subjected to eccentric load. The proposed formulas’ predictions agreed well with the test results.

Behaviour of CFRP-confined concrete-filled circular steel tube stub columns under axial loading

Abstract: This paper presents an experimental and theoretical study of CFRP-confined concrete-filled circular steel tube (CFT) stub columns, which aims at investigating the effects of different numbers of CFRP layers and concrete strengths on the mechanical performance of CFRP-confined CFT stub columns. Based on continuum mechanics, a mechanical model of concentric cylinder consisting of CFRP, circular steel tube and concrete core under concentric loading was established and the corresponding elasto-plastic methods were obtained through a FORTRAN program. The influence of the number of CFRP layers on the ultimatecapacity, ductility and confinement effect of the steel tube on the core concrete wasdiscussed and identified. Finally,based on the limit equilibrium and elasto-plastic methods, a simplified formula for the ultimatecapacity of CFRP-confined CFT stub columns was proposed. Good agreement of the ultimate capacity was found between the elasto-plastic methods and the proposed formula, with the maximum discrepancy less than 12%.

Comparative analysis of crashworthiness of empty and foam-filled thin-walled tubes

Abstract: Quasi-static axial compression tests were conducted on two types of empty aluminum alloy tubes (circular and square) and five types of aluminum ex-situ foam filled tube structures (foam-filled single circular and square tubes, foam-filled double circular and square tubes, and corner-foam-filled square tube). The load-deformation characteristics, deformation mode and energy absorption ability of these structures were investigated. Several parameters related to their crashworthiness were compared, including the specific energy absorption, the energy-absorbing effectiveness factor, etc. The influence of physical dimension on the crashworthiness of these structures was explored. Dimensions of the inner tube were found to have significant influence on the structural crashworthiness of foam-filled double tubes. The averaged crush force, specific energy absorption, energy absorption per stroke and energy-absorbing effectiveness factor of thin-walled circular structures are higher than those of thin-walled square structures, respectively. Foam-filled single and double circular tube structures are recommended as crashworthy structures due to their high crush force efficiency and energy-absorbing efficiency.

Out-of-plane crashworthiness analysis of bio-inspired aluminum honeycomb patterned with horseshoe mesostructure

Abstract: Numerous composite structures with excellent integrative performance that can replicate the mechanical properties of biological materials have been created to fill gaps in material-property charts, and these bio-inspired structures have crucial implications in a wide range of engineering communities. In this paper, a series of novel bio-inspired aluminum honeycombs consisting of horseshoe mesostructure have been proposed on the basis of triangular honeycomb, square honeycomb, hexagonal honeycomb and kagome honeycomb to improve the energy absorption capacity. The three-dimensional finite element models of the bio-inspired horseshoe-shaped aluminum honeycombs are developed in order to explore the mechanical behaviors under the out-of-plane uniform compression. The simulation results are validated based on the compression experiments of regular hexagonal honeycombs. Besides, parametric investigations are carried out to understand the influences of the wave amplitude, wave number and cell-wall thickness on the out-of-plane crashworthiness. The numerical results demonstrate that adding the horseshoe mesostructure to the regular honeycombs can increase the plateau force greatly compared with the traditional honeycomb structure, leading to the higher specific energy absorption although increasing the initial peak force as well. Finally, a multi-objective optimization is carried out to seek for the optimal honeycombs with the maximum specific energy absorption together with the minimum initial peak force simultaneously.

Isogeometric vibration analysis of functionally graded nanoplates with the consideration of nonlocal and surface effects

Abstract: Presented in this paper is a size-dependent analysis of the surface stress and nonlocal influences on the free vibration characteristics of rectangular and circular nanoplates. Nanoplates are assumed to be made of functionally graded materials (FGMs) with two distinct surface and bulk phases. The nonlocal and surface effects are captured by the Eringen and the Gurtin-Murdoch surface elasticity theories, respectively. The Mori-Tanaka distribution scheme is also used for obtaining material properties of nanoplate. In addition to the conventional procedure of deriving the formulation, a novel matrix-vector form of the governing differential equations of motion is presented. This form has the capability of being used directly in the finite element method or isogeometric analysis. To show the effects of surface parameters and small scale influences on the vibrational behavior of rectangular and circular FGM nanoplates with various boundary conditions, several case studies are presented.

Structural behavior of UHPC filled steel tube columns under axial loading

Abstract: Steel tube filled with ultra-high performance concrete (UHPCFST) is an innovative and efficient structural form. To promote its application, a comprehensive experimental program was conducted to investigate the structural behavior of UHPCFST columns subjected to axial compression. The key issue is to clarify the differences in mechanical behavior between UHPCFSTs and CFSTs, and to evaluate whether the current design guidelines related to CFSTs are applicable to UHPCFSTs. To address this, the compression characteristics of UHPCFSTs were analyzed, including failure mode, load versus deformation relationship, axial compressive strength and strain development. The test results showed that the steel tube and UHPC worked well together, but the enhancement effect of the steel tube on the core UHPC strength was not as significant as that of ordinary concrete. Moreover, an experimental database of UHPCFSTs including this study was established, and the experimental results were compared with the predictions by various design codes. Based on the regression analysis of the database results, a simplified model for predicting the ultimate strength of UHPCFSTs was developed. It was concluded that the proposed model could accurately predict the axial compressive strength of UHPCFSTs with circular and square cross-sections, and was also applicable to UHPCFSTs with high-strength steel (HSS).

Experimental study on the composite action in sheathed and bare built-up cold-formed steel columns

Abstract: This paper reports on experiments addressing the buckling and collapse behavior of common built-up cold-formed steel (CFS) columns. The built-up column consists of two individual CFS lipped channels placed back-to-back and connected at the web using two self-drilling screw fasteners at specified spacing along the column length. The experiments aim to quantify ultimate strength, composite action, member end fixity, and buckling interactions and collapse behavior for common built-up CFS members. The testing also explicitly explores the effect of sheathing, as typically employed in cold-formed steel framing, on the response. The experiments provide benchmarks for design that include specific considerations for both thin-walled buckling and fastener behavior. A total of 17 monotonic, concentric compression tests with a column length of 1.83 m (6 ft) are completed with an array of position transducers monitoring displacements at key locations. Tests are conducted with the built-up member seated in CFS tracks. Results indicate a large range of deformation behavior, with local-global interaction and flexural-torsional modes common in many of the unsheathed specimens. Columns sheathed with oriented strand board on both flanges behave as braced against global buckling in the plane of the wall, and local buckling induced failures prevail. The end condition for the tested built-up members seated in track is determined to be semi-rigid, but generally closer to fixed than pinned.

Testing, simulation and design of cold-formed stainless steel CHS columns

Abstract: Stainless steel tubular members are employed in a range of load-bearing applications due to their strength, durability and aesthetic appeal. From the limited existing test data on stainless steel circular hollow sections (CHS) columns it has been observed that the current Eurocode 3 provisions can be unconservative in their capacity predictions. A comprehensive experimental programme has therefore been undertaken to provide benchmark data to validate numerical models and underpin the development of revised buckling curves; in total 17 austenitic, 9 duplex and 11 ferritic stainless steel CHS column buckling tests and 10 stub column tests have been carried out. Five different cross-section sizes (covering class 1 to class 4 sections) and a wide range of member slendernesses have been examined. The experiments were initially replicated using finite element (FE) simulations; the validated FE models were then used to generate 450 additional column buckling data points. On the basis of the experimental and numerical results, new design recommendations have been made for cold-formed stainless steel CHS columns and statistically validated according to EN 1990 [1] .

Nondeterministic optimization of tapered sandwich column for crashworthiness

Abstract: Foam-filled thin-walled structures signify a class of a promising energy absorber for improving the crashworthiness and safety of vehicles. Although the conventional deterministic optimization has been extensively applied to crashworthiness design of foam-filled thin-walled structures, the optimal solution could become infeasible when uncertainties of design variables and noise factors present in real world. To address this issue, a reliability based design optimization (RBDO) is adopted to consider the uncertainties of design variables and noise factors in crashworthiness optimization for the foam-filled bitubal tapered structure in this paper. Moreover, to comprehensively investigate the differences between deterministic and reliability based design optimization, single objective and multiple objective RBDO are established by integrating Kriging approximation with Monte Carlo Simulation (MCS). Since the optimal results of deterministic design usually converge at the constraint boundary, the solutions of RBDO often need to compromise some objective performance to satisfy the predetermined reliability levels. Furthermore, a comparative study on different Pareto fronts yielded from the deterministic optimization and RBDO under different reliability levels is conducted here. Besides, a grey relational analysis is carried out to determine the most satisfactory solution from the Pareto-set. The results demonstrate that the optimized foam-filled bitubal tapered columns are capable to considerably improve capacity of energy absorption with an increased reliability, potentially being a structural configuration for energy absorber.

Experimental investigation of local-flexural interactive buckling of cold-formed steel channel columns

Abstract: This paper presents the results of a comprehensive experimental programme aimed at studying the interaction of local and overall flexural buckling in cold-formed steel (CFS) plain and lipped channels under axial compression. The results were further used to verify the accuracy of the current design procedures in Eurocode 3, as well as to evaluate the effectiveness of a previously proposed optimisation methodology. A total of 36 axial compression tests on CFS channels with three different lengths (1 m, 1.5 m and 2 m) and four different cross-sections were conducted under a concentrically applied load and pin-ended boundary conditions. The initial geometric imperfections of the specimens were measured using a specially designed set-up with laser displacement transducers. Material tests were also carried out to determine the tensile properties of the flat parts of the cross-sections, as well as the cold-worked corner regions. A comparison between the experimental results and the Eurocode 3 predictions showed that the effective width approach combined with the P–M interaction equation proposed in Eurocode 3 to take into account the shift of the effective centroid consistently provided safe results. However, the Eurocode 3 procedures were also quite conservative in predicting the capacity pertaining to local-global interaction buckling, especially for plain channels. Furthermore, the experimental data confirmed the results of an optimisation study and demonstrated that the optimised CFS columns exhibited a capacity which was up to 26% higher than the standard channel with the same amount of material taken as a starting point.

Finite element analysis and design of cold-formed steel built-up closed section columns with web stiffeners

Abstract: This paper presents a numerical investigation and design of cold-formed steel built-up closed section columns with web stiffeners A finite element model (FEM), considering the initial geometric imperfections and nonlinear material properties, was developed to simulate the structural behaviour of fixed-ended built-up closed section compression members The comparison between the numerical results and the available test results show that this FEM can provide good predictions for both the ultimate strength and the failure modes of the test specimens The verified FEM was used to conduct an extensive parametric study for the investigation on the structural behaviour of cold-formed steel built-up closed sections with web stiffeners The parametric study was designed to investigate the effect of web stiffeners as well as to evaluate the current design method The column strengths obtained from the finite element analysis and the test results were compared with the design strengths calculated using the direct strength method in the North American Specification and the Australian/New Zealand Standard for cold-formed steel structures Design curves modified from the current direct strength method are proposed for flexural, local and distortional buckling The reliability analysis was used to assess the current design rules and the modified design curves It is shown that the modified direct strength method is generally conservative and reliable for the design of cold-formed steel built-up closed section compression members

Nonlinear thermal torsional post-buckling of carbon nanotube-reinforced composite cylindrical shell with piezoelectric actuator layers surrounded by elastic medium

Abstract: The paper focuses on thermal torsional post buckling of functionally graded carbon nanotube reinforced composite cylindrical shells with sur-bonding piezoelectric layers and embedded in an elastic medium. The distribution of reinforcements through the thickness of the shells is considered to be uniform and functionally graded. The basic equations using geometrically nonlinearity in von Karman-Donnell sense within the classical thin shell theory are established. The torsional post-buckling behavior of the piezoelectric functionally graded carbon nanotubes reinforced composite (FG-CNTRC) shells is analyzed with using the Airy's stress function and the Galerkin's method, in which a three-term approximate solution of the deflection of the shell is assumed. Effects of thermal environment, CNT volume fraction, piezoelectric layers (the thickness and the constant voltage), distribution type of the reinforcement, dimensional parameters and Winkler and Pasternak foundation are investigated in this paper. Numerical and graphical results show that the carbon nanotube volume fraction and the piezoelectric layers as well as the elastic foundation and the thermal loads have significantly influenced on the torsional postbuckling behavior of nanocomposite shells.

Energy absorption analysis of a novel foam-filled corrugated composite tube under axial and oblique loadings

Abstract: Composite thin-walled structures are of much interest in differnet applications as well as energy absorption devices for their great crashworthiness and light weight. In this paper, a new corrugated composite cylindrical tube has been introduced in order to improve crashworthiness along with a stable crushing. In cylindrical composite tubes, the effects of corrugations regarding charactristics of energy absorption have underwent quasi-static axial and oblique loading investigations. For this reason, composite cylindrical tubes with different corrugation geometries were analyzed using finite element explicit code and the effects of corrugations on crush force effiency and specific energy absorption were comperhensively studied. The finite element model has been validated by experimental quasi-static compression tests. An efficient analytical solution for SEA during axial loading has been also derived and compared with FEM solution. Furthermore, a comparison of empty and foam-filled corrugated composite tubes has been done. Based on the obtained results, generating corrugated surfaces on tubes improved the crush force efficiency significantly in both axial and oblique crushings. Performing a parametric study on geometrical corrugation parameters of tubes has been indicated that the energy absorption of these structures depends strongly on the corrugation parameters. Furthermore the absorbed energy has been increased by using foams in both axial and oblique crushing. SEA increases by increasing the foam density while the CFE decreases.

Artificial neural networks and intelligent finite elements in non-linear structural mechanics

Abstract: In recent years, artificial neural networks were included in the prediction of deformations of structural elements, such as pipes or tensile specimens. Following this method, classical mechanical calculations were replaced by a set of matrix multiplications by means of artificial intelligence. This was also continued in finite element approaches, wherein constitutive equations were substituted by an artificial neural network (ANN). However, little is known about predicting complex non-linear structural deformations with artificial intelligence. The aim of the present study is to make ANN accessible to complicated structural deformations. Here, shock-wave loaded plates are chosen, which lead to a boundary value problem taking geometrical and physical non-linearities into account. A wide range of strain-rates and highly dynamic deformations are covered in this type of deformation. One ANN is proposed for the entire structural model and another ANN is developed for replacing viscoplastic constitutive equations, integrated into a finite element code, leading to an intelligent finite element. All calculated results are verified by experiments with a shock tube and short-time measurement techniques.

Shear resistance and post-buckling behavior of corrugated panels in steel plate shear walls

Abstract: Steel corrugated shear wall (SCSW) is an alternative to traditional shear walls with flat plates. However, shear resistance behavior and design of the infilled corrugated panels in SCSWs has not been well studies. This paper focuses on the shear resistance of sinusoidally corrugated panels in SCSWs under monotonic lateral shear force, via finite element analyses (FEA) considering both geometric nonlinearity and material elasto-plasticity. Firstly the effects of initial imperfections and geometric dimensions on shear resistance of corrugated panels are explored. Then based on extensive FEA, the maximum and the post-buckling strengths are investigated, and fitting equations to predict the shear resistant behavior of corrugated panels are proposed by introducing the normalized height-to-thickness ratio. It is found that, the maximum shear resistance of corrugated panels has a consistent relationship to the normalized height-to-thickness ratio, however variation of the post-buckling resistance is complex and geometric parameters have to be properly chosen to avoid significant strength drop after buckling. The equations proposed agree with the FEA results, and can be utilized in design of corrugated panels in SCSWs.

Lateral force resisting systems in lightweight steel frames : recent research advances

Abstract: Lightweight Steel Frames (LSF) made by framing thin gauge cold-formed steel (CFS) into different structural elements such as walls, trusses and joists are commonplace in Australia and many parts of the world. The great progress in the knowledge of CFS structures achieved in the past two decades, together with the modern design and fabrication methods supported by progressively improved specifications, have equipped the industry of the lightweight steel construction with tools and confidence to play an important part in the future of building construction. Despite the ever-increasing demand on the use of cold formed steel (CFS) framing into more complex and taller structures, the lateral load resistance capacity of lightweight steel frames has proven to be a major hindrance and a major concern. This paper reviews and summarises the research developments made in the area of lateral load resistance capacity of lightweight steel frames (LSF) as published in leading journals and codes’ provisions in the area. Research advances in conventional systems such as shear walls clad with face sheathings and LSF strap-braced wall systems in addition to other less conventional systems such as special bolted moment frames are reviewed here, and the solutions for improving the lateral performance of these systems are classified.

Bending characteristics of top-hat structures through tailor rolled blank (TRB) process

Abstract: Tailor rolled blank top-hat (TRBTH) structure, as a relatively new thin-walled configuration, was proposed in this study to better balance the crashworthiness and lightweight requirements, which was featured in a thicker wall thickness in the critical load-bearing areas and a thinner wall thickness in the other regions. To capture the non-uniform material properties and thickness variation of the TRBTH structure accurately, a more realistic finite element (FE) modeling technique was first developed and validated experimentally. It was found that the corresponding FE simulation results agreed well with the testing results. Second, the bending characteristics of TRBTH and uniform thickness top-hat (UTTH) structure with the equal mass were compared through the typical three-point and four-point bending conditions. It was revealed that the TRBTH had a higher bending resistance, which allowed absorbing more transverse crushing energy. Third, the detailed numerical analyses were performed for the bending deformation to explore the crushing advantages of the TRBTH structure more comprehensively. It was divulged that the TRBTH structures engaged more material to participate in deformation than the UTTH counterpart under the transverse loading. Finally, a parametric study was conducted to investigate the effects of the thickness distribution and length of thick zone on the three-point and four-point bending characteristics; and it was found that these structural parameters could affect the crashworthiness of TRBTH structure significantly in a form of different deformation patterns. For this reason, the bending characteristics can be further improved by optimizing the TRB parameters, making it more suitable for being an effective energy absorber.

A novel multi-cell tubal structure with circular corners for crashworthiness

Abstract: Multi-cell structures have proven to own excellent energy absorbing capability and lightweight effect in the automotive and aerospace industries. The cross-sectional configuration of the multi-cell structure has a significant effect on crashworthiness. Unlike existing multi-cell tubes, a new type of five-cell profile with four circular elements at the corners (C5C) was proposed in this study. To investigate the crashworthiness of the new C5C tube, finite element (FE) models were first established by using the nonlinear finite element code LS-DYNA and validated with experimental results. Following that, the comparison of the C5C tube and other multi-cell tubes with the same mass was conducted to quantify the relative merits of the C5C tube. Then, a detailed study was performed to analyze the effect of the corner-cell size and wall thickness. Finally, the optimization design was carried out to seek the optimal structure. The results showed that the new multi-cell structure can absorb much more crash energy than other four types of tubes. Moreover, the energy absorption of this new multi-cell tube C5C was affected by the corner-cell size and wall thickness significantly. A proper corner-cell size and slightly thicker internal ribs were recommended. In addition, the multi-objective particle swarm optimization (MOPSO) algorithm and radial basis function (RBF) surrogate model can optimize the structure effectively. The outcomes of the present study will facilitate the design of multi-cell structures with better crashworthiness.

Effect of 3D random pitting defects on the collapse pressure of pipe — Part I: Experiment

Abstract: This paper is aimed at assessing the effects of local random external pitting defects on the collapse pressure of pipe under external pressure. In this two-part paper series, the buckling performance of locally random corroded pipe was experimentally and numerically studied. The experimental program described in Part I involves seamless carbon steel tubes with D/t = 17 and with different size of pitting defects. The random pitting defects were introduced into the outside surface of pipe using 6% FeCl3 solution and then the collapse pressure of such locally random corroded pipe was obtained experimentally. The profile of pipe was examined in details using Hexagon's new RA7320 Portable Arm Coordinate Measuring Machine (PACMM) and the measured data were analyzed meticulously. It indicated that either the out-of-roundness imperfection shape n = 2 or n = 3 fits best with the measure data. The characteristics of random pitting corrosion were statistically analyzed and the mass loss of pipe due to pitting corrosion was also determined after buckling test, the analysis results indicate that both lognormal and generalized extreme value (GEV) distribution are adequate to depict the distribution of pitting depth and pitting diameter-to-depth ratio (DDR). The corrosion morphology was observed using scanning electron microscope (SEM), which indicates that the shape of pitting defect is cylindrical or semi-ellipsoid. Finally, the relationship between collapse pressure and geometry defects was analyzed based on the experiment results.