Showing papers in "Thin-walled Structures in 2017"

On the crashworthiness performance of thin-walled energy absorbers: Recent advances and future developments

Abstract: Over the past several decades, a noticeable amount of research efforts has been directed to minimising injuries and death to people inside a structure that is subjected to an impact loading. Thin-walled (TW) tubular components have been widely employed in energy absorbing structures to alleviate the detrimental effects of an impact loading during a collision event and thus enhance the crashworthiness performance of a structure. Comprehensive knowledge of the material properties and the structural behaviour of various TW components under various loading conditions is essential for designing an effective energy absorbing system. In this paper, based on a broad survey of the literature, a comprehensive overview of the recent developments in the area of crashworthiness performance of TW tubes is given with a special focus on the topics that emerged in the last ten years such as crashworthiness optimisation design and energy absorbing responses of unconventional TW components including multi-cells tubes, functionally graded thickness tubes and functionally graded foam filled tubes. Due to the huge number of studies that analysed and assessed the energy absorption behaviour of various TW components, this paper presents only a review of the crashworthiness behaviour of the components that can be used in vehicles structures including hollow and foam-filled TW tubes under lateral, axial, oblique and bending loading.

Dynamics, vibration and control of rotating composite beams and blades: A critical review

Abstract: Rotating composite beams and blades have a wide range of applications in various engineering structures such as wind turbines, industrial fans, and steam turbines. Therefore, proper understanding of such structures is of a great importance. As a result, the behavior of rotating composite beam structures has received a lot of attention. This paper presents a comprehensive review of scholarly articles about rotating composite beams as published in the past decades. The review addresses analytical, semi-analytical and numerical studies dealing with dynamical problems involving adaptive/smart/intelligent materials (e.g. piezoelectric materials, electrorheological fluids, shape memory alloys, etc.), damping and vibration control, advanced composite materials (e.g. functionally graded materials and nanocomposites), complicating effects and loadings (e.g. added mass, tapered beams, initial curve and twist, etc.), and experimental methods. Moreover, the influence of Vlasov or restrained warping, out-of-plane warping, transverse shear, arbitrary cross-sectional geometry, trapeze phenomena, swept tip, size-dependent effect, as well as other areas that have been considered in research, are reviewed in depth. The review concludes with a presentation of the remaining challenges and future research needs.

Natural frequencies of functionally graded plates with porosities via a simple four variable plate theory: An analytical approach

Abstract: In this paper, the free vibration analysis of rectangular plates composed of functionally graded materials with porosities is investigated based on a simple first-order shear deformation plate theory. The network of pores in assumed to be empty or filled by low pressure air and the material properties of the plate varies through the thickness. Using Hamilton's principle and utilizing the variational method, the governing equations of motion of FG plates with porosities are derived. Considering two boundary layer functions, the governing equations of the system are rewritten and decoupled. Finally, two decoupled equations are solved analytically for Levy-type boundary conditions so as to obtain the eigenfrequencies of the plate. The effects of porosity parameter, power law index, thickness-side ratio, aspect ratio, porosity distribution and boundary conditions on natural frequencies of the plate are investigated in detail.

Thermal and mechanical stability of functionally graded carbon nanotubes (FG CNT)-reinforced composite truncated conical shells surrounded by the elastic foundations

Abstract: The thermal and mechanical stability of a functionally graded composite truncated conical shell reinforced by carbon nanotube fibers and surrounded by the elastic foundations are studied in this paper. Distribution of reinforcements across the shell thickness is assumed to be uniform or functionally graded. The equilibrium and linearized stability equations for the shells are derived based on the classical shell theory. Using Galerkin method, the closed – form expression for determining the linear thermal and mechanical buckling load is obtained. The paper also analyzed and discussed the effects of semi-vertex angle, shell length, volume fraction of fibers, distribution pattern of fibers, temperature, elastic foundations on the linear thermal and mechanical buckling loads of the functionally graded carbon nanotube fibers-reinforced composite (FG CNTRC) truncated conical shell in thermal environment.

Thermal buckling and postbuckling of functionally graded graphene-reinforced composite laminated plates resting on elastic foundations

Abstract: This paper presents the modeling and analysis for the thermal postbuckling of graphene-reinforced composite laminated plates resting on an elastic foundation and subjected to in-plane temperature variation A micromechanical model is used to estimate the temperature-dependent material properties of the graphene-reinforced composites (GRCs) Piece-wise functionally graded (FG) GRC layers along the thickness direction of a plate is considered in this study Employing the higher order shear deformation plate theory, the governing equations for FG-GRC plates are derived and the effects of plate-foundation interaction and temperature variation are included in the modeling A two-step perturbation technique is applied to obtain the buckling temperature and the thermal postbuckling load-deflection curves for perfect and imperfect FG-GRC laminated plates The results show that the buckling temperature as well as thermal postbuckling strength of the plates can be increased as a result of the functionally graded graphene reinforcement for the plates

Crushing response of square aluminium tubes filled with polyurethane foam and aluminium honeycomb

Abstract: Experimental investigations were conducted to study the axial crushing behaviour of square aluminium tubes with different configurations. Quasi-static compressive loads were applied to square hollow aluminium tubes, aluminium honeycomb-filled tubes, polyurethane foam-filled tubes and aluminium tubes filled with both polyurethane foam and aluminium honeycomb at constant velocities of 0.15 mm/s, 1.5 mm/s and 15 mm/s, respectively. The effects of honeycomb core, polyurethane foam, combined polyurethane foam and honeycomb on the axial crushing behaviour of square aluminium tubes were discussed. The influence of crushing velocity on these different tubular structures was also studied. Experimental results showed that the deformation mode was a progressive folding mode for square hollow aluminium tubes, while it was a splitting mode for square tubes filled with both polyurethane foam and aluminium honeycomb. The fold wavelengths of some typical cases were measured. The most crashworthy combination was found to be square aluminium tubes filled with both polyurethane foam and aluminium honeycomb, where the maximum increases of mean crushing force, energy absorption and specific energy absorption were up to 349%, 334% and 109% respectively, compared with those of hollow tubes.

Energy absorption mechanics for variable thickness thin-walled structures

Abstract: This paper introduces axial functionally graded thickness (AFGT) and lateral functionally graded thickness (LFGT) to thin-walled square structures separately, and then investigates their crashworthiness theoretically, numerically, experimentally under axial crushing load. The quasi-static axial crush experiments and the corresponding finite element models are first conducted to analysis the deformation mode and crushing force for uniform thickness (UT), AFGT and LFGT square tube under the same mass. Then, theoretical models predicting the mean crushing forces of AFGT and LFGT square tubes are established. The results show that both theoretical solutions and numerical results for FGT tubes agree well with the experimental results. Energy absorption characteristics between FGT and UT square tubes with same mass are compared based on the validated numerical models, which shows that AFGT square tube can effectively reduce the initial peak force compared to UT square tube while LFGT square tube remarkably surpasses the UT square tube in specific energy absorption ( SEA ) under axial crushing. Furthermore, parametric studies are performed to investigate the effects of gradient thickness variation on the energy absorption characteristics of AFGT and LFGT square tubes. The results again demonstrate that both AFGT and LFGT square tubes can improve the crashworthiness of thin-walled square tubes.

Analysis of bi-directional functionally graded plates by FEM and a new third-order shear deformation plate theory

Abstract: Modern structures and components may require advanced materials whose properties vary continuously not only in one specified direction, but also different other directions. In particular, the bi-directional functionally graded materials (2D-FGMs) introduced are expected to have more effective properties, consequently eliminating commonly awkward problems such as local stress concentrations and delamination. In this paper, buckling and bending behaviors of 2D-FGM plates, which are of great importance in the design and development of engineering applications, are numerically analyzed by a finite element model. The plate kinematics are described using a new third-order shear deformation plate theory (TSDT), without the need for special treatment of shear-locking effect and shear correction factors. The present TSDT theory based on rigorous kinematic of displacements, which is shown to be dominated over other preceding theories, is derived from an elasticity formulation, rather by the hypothesis of displacements. The materials are assumed to be graded in two directions and their effective properties are computed through the rule of mixture. The accuracy of the proposed approach assessed on numerical results is confirmed by comparing the obtained results with respect to reference published solutions. The effects of some numerical aspect ratios such as volume fraction, boundary conditions, thickness to length ratio, etc. on static deflections and critical buckling are numerically studied. The investigation of results confirms that such aforementioned aspect ratios have significant effects on the mechanical behaviors of plates.

Crashworthiness and numerical simulation of hybrid aluminium-CFRP tubes under axial impact

Abstract: In the present study, the crashworthiness and the numerical simulation of circular hybrid aluminium-CFRP tubes are investigated. It can be shown that hybrids provide significant lightweight potential. The specific energy absorption is 37% higher compared to a pure aluminium structure. The post-crash analysis is done by computed tomography methods. The failure of the hybrid component shows a mixture of energy absorption mechanisms of its pure materials. Hybrid-specific energy absorption mechanisms compensate the limited primary energy absorption mechanisms of the CFRP and metal components. The simulation of axial-loaded composite structures is challenging the currently available simulation methods. In the present work, a multi shell model is used for the simulation of the hybrid structure. This approach enables an efficient design of CFRP-aluminium hybrid components. First approaches towards the simulation of hybrid specific failure modes are given.

Axial compressive behaviour of concrete-filled double-tube stub columns with stiffeners

Abstract: Concrete-filled double-tube (CFDT) columns, which are a new type of composite columns, have high fire resistance and the potential to be widely used in high-rise buildings. It is expected that the amount of steel used in a regular CFDT column will be relatively high due to the use of double tubes. To reduce the steel consumption, a thin-walled steel tube with longitudinal stiffeners may be adopted for the outer tube in a CFDT column. This paper studies the behaviour of concrete-filled stiffened double-tube (CFSDT) stub columns under axial compression. Tests on 12 CFSDT stub columns and two reference columns were carried out accordingly, and the test results confirm that the stiffened columns have high strength and good deformation capacity. A finite element model was developed to analyse the interaction between the steel tubes and concrete. Based on further parametric studies, a superposition model was proposed to predict the axial compressive strength of CFSDT stub columns.

Robust design criterion for axially loaded cylindrical shells - Simulation and Validation

Abstract: A currently used guideline for cylinder structures under axial compression is the NASA SP-8007 which is based on empirical data from the 1960s. This guideline provides knock-down factors (KDF) for the lower bound of the buckling load which depend on the cylinder radius-to-thickness ratio but neglect the influence of the cylinder length L. Experimental results indicated an influence of the cylinder length on the buckling load but a clear dependency could not be established because of the insufficient amount of available data. A comprehensive numerical investigation was performed in order to study the influence of length effect on the lower bound of the buckling load. The numerical analysis is based on the single boundary perturbation approach (SBPA) for cylindrical shells. The results verify that there is a significant influence of the cylinder length L on the lower bound of the buckling load. Semi-analytic knock-down factors for the stability failure of axially loaded cylindrical shells were determined which can be used for a simple and fast approximation of the lower bound of the buckling load. The corresponding SBPA thresholds were validated with a number of high fidelity buckling experiments and deliver much higher KDFs than currently used empirical guidelines.

Visco-nonlocal-refined Zigzag theories for dynamic buckling of laminated nanoplates using differential cubature-Bolotin methods

Abstract: Present analysis, deals with dynamic buckling of sandwich nano plate (SNP) subjected to harmonic compressive load based on nonlocal elasticity theory. The material properties of each layer of SNP are supposed to be viscoelastic based on Kelvin-Voigt model. In order to mathematical modeling of SNP, a novel formulation, refined Zigzag theory (RZT) is developed. Furthermore, the surrounding elastic medium is simulated by visco-orthotropic Pasternak foundation model in which damping, normal and transverse shear loads are taken into account. Using energy method and D′Alembert's principle, the size dependent governing motion equations are derived. In this study, the governing motion equations are solved numerically using new procedure namely differential cubature (DC) method in conjunction with Bolotin method. The effects of some remarkable parameters such as viscoelastic foundation, damping coefficient of viscoelastic plates, aspect ratio, amount of small scale effect, various boundary conditions, different values of fiber orientation of the face sheets, number of grid points and thickness-length ratio on the dynamic instability region (DIR) are investigated. The results show that considering viscoelastic property of system is essential to obtain real mechanical behavior and instability of systems. In addition, the surrounding elastic medium is an effective parameter on the DIR of SNP.

Buckling of spherical shells subjected to external pressure: A comparison of experimental and theoretical data

Abstract: This paper focuses on spherical shells under uniform external pressure. Ten laboratory scale models, each with a nominal diameter of 150 mm, were tested. Half of them were manufactured from a 0.4-mm stainless steel sheet, whereas the remaining five shells were manufactured from a 0.7-mm sheet. The geometry, wall thickness, buckling load, and final collapsed mode of each spherical shell were measured, as well as the material properties of the corresponding sheet. The buckling behaviors of these shells were demonstrated analytically and numerically according to experimental data. Analyses involved considering the average geometry, average wall thicknesses, and average elastic material properties. Numerical calculations entailed considering the true geometry, average wall thicknesses, and elastic-plastic modeling of true stress–strain curves. Moreover, the effects of purely elastic and elastic-perfectly plastic models on the buckling loads of spherical shells were examined numerically. The results of the experimental, analytical, and numerical investigations were compared in tables and figures.

Shear buckling and stress distribution in trapezoidal web corrugated steel beams

Abstract: Due to their lightweight and superior load carrying capacity, corrugated web steel beams (CWSBs) have gained popularity in the last few decades. CWSBs are known to fail at much higher loads compared to stiffened flat web beams. To understand their shear response, a series of three-point load tests were performed on five shear-critical trapezoidal corrugated web beams. The test results confirmed the existence of the three shear buckling modes of failure: local, global, and interactive. In addition, all tested beams were observed to have a residual strength that is about half of their ultimate load carrying capacity regardless of the shear buckling mode. Results of the nonlinear finite element analysis showed that the shear stress is at its maximum and uniformly distributed throughout the web until buckling, afterwards, it decreases and its distribution is uneven while the entire resistance is provided by the increased tensile stress. Furthermore, stocky corrugated webs were shown to reach shear yield strength. Comparison between existing analytical models for the estimation of shear strength against test data showed that EN-1993-1-5 is accurate and conservative enough for an economic design.

Seismic response of CFS shear walls sheathed with nailed gypsum panels: Experimental tests

Abstract: Among the several available building systems, constructions involving cold-formed steel (CFS) profiles represent an efficient and reliable solution. These systems are very suitable to be used in pre-fabricated modular constructions, thanks to their lightness and possibility to automate the building process. In a framework of the European project ELISSA (Energy Efficient LIghtweight-Sustainable-SAfe-Steel Construction), which was devoted to the development and demonstration of CFS modular systems, an experimental campaign aimed at investigating the seismic response of this system was carried out at University of Naples Federico II. Specifically, the studied system was a sheathing-braced CFS solution, in which the seismic resistant elements were made of CFS stud shear walls laterally braced with gypsum-based panels. The sheathing panels were attached to the CFS frame by means of ballistic nails, whereas clinching points were used for steel-to-steel connections. This paper shows the results of tests performed on shear walls and on the relevant ballistically nailed panel-to-steel connections. In particular, four full scale shear walls were tested, in which the influence of the aspect ratio, the type of loading and the effect of finishing was investigated.

Comparative study between XFEM and Hashin damage criterion applied to failure of composites

Abstract: This paper presents a comparative study between Hashin damage criterion and the eXtended Finite Element Method (XFEM) applied to the failure of fiber reinforced polymers (FRP). A brief literature review on failure criteria to predict the failure of FRP is firstly presented. Then, finite element models of square plates with different layer configurations, containing a circular hole with distinct radii and subjected to monotonic uniaxial tension are described within the framework of ABAQUS package. The models are validated by comparison between the numerical results and those of a benchmark model. Finally, the influence of (i) stacking sequence, (ii) hole radii and (iii) failure criteria (Hashin and XFEM) on the load vs. elongation paths, stresses distributions and collapse configurations of the plates is shown and discussed and some conclusions are drawn.

Vibration analysis of magneto-electro-elastic heterogeneous porous material plates resting on elastic foundations

Abstract: This paper proposes a four-variable shear deformation refined plate theory for free vibration analysis of embedded smart plates made of porous magneto-electro-elastic functionally graded (MEE-FG) materials Magneto-electro-elastic properties of FG plate are supposed to vary through the thickness direction and are estimated through the modified power-law rule in which the porosities with even and uneven type are approximated The governing differential equations and boundary conditions of embedded porous FG plate under magneto-electrical field are derived through Hamilton's principle based on a four-variable tangential-exponential refined theory which avoids the use of shear correction factors An analytical solution procedure is used to achieve the natural frequencies of embedded porous FG plate supposed to magneto-electrical field with various boundary condition Influences of several important parameters such as material graduation exponent, porosity volume fraction, magnetic potential, electric voltage, various boundary conditions, elastic foundation parameters and plate side-to-thickness ratio on natural frequencies of embedded porous MEE-FG plate are investigated and discussed in detail It is concluded that these parameters play significant roles on the dynamic behavior of porous MEE-FG plates resting on elastic foundation Presented numerical results can serve as benchmarks for future analyses of MEE-FG plates with porosity phases

Free vibration of joined conical-conical shells

Abstract: Free vibration analysis of a joined shell system composed of two conical shells is analysed in this research It is assumed that the system of joined shell is made from a linearly elastic isotropic homogeneous material Both shells are unified in thickness To capture the through-the-thickness shear deformations and rotary inertias, first order theory of shells is accompanied with the Donnell type of kinematic assumptions to establish the general motion equations and the associated boundary and continuity conditions with the aid of Hamilton's principle The resulted system of equations are discreted using the semi-analytical generalised differential quadrature (GDQ) method Considering various types of boundary conditions for the shell ends and intersection continuity conditions, an eigenvalue problem is established to examine the vibration frequencies as well as the associated mode shapes After proving the efficiency and validity of the present method for the case of thin isotropic homogeneous joined shells, some parametric studies are carried out for the system of combined moderately thick conical-conical

Large amplitude vibration of FG-CNT reinforced composite annular plates with integrated piezoelectric layers on elastic foundation

Abstract: The aim of the present study is to investigate the applications of piezoelectric layers and carbon nanotubes (CNTs) in enhancing nonlinear vibration behaviors of functionally graded carbon nanotube reinforced composite (FG-CNTRC) annular plates. A nonlinear formulation is developed with regard to the first-order shear deformation theory, von Karman geometrical nonlinearity in conjunction with the Hamilton principle. A Pasternak elastic foundation is assumed to be in contact with the annular plate during deformation. The distribution of electric potential across the thickness of piezoelectric layers is simulated via a combination of sinusoidal and linear functions. Both closed and open circuit electrical boundary conditions are taken into account for the bottom and top surfaces of piezoelectric layers. Generalized differential quadrature method is utilized to discretize the nonlinear equations of motion, Maxwell equation and boundary conditions, and then direct iterative method is used to solve the nonlinear system of equations. An extensive parametric study is directed to provide an insight into effects of the thickness of piezoelectric layers, piezoelectric materials, volume fraction and distribution of CNTs, geometrical parameters, elastic foundation coefficient, and mechanical/electrical boundary conditions on the nonlinear dynamic responses of the annular plate. It is found that both distribution and volume fraction of CNTs have a remarkable effect on nonlinear natural frequencies of FG-CNTRC annular plates. It is also revealed that the type of piezoelectric materials, thickness of piezoelectric layers and electrical boundary condition play a pivotal role in improving dynamic responses of FG-CNTRC annular plates. It is also found that presence of elastic foundation increases hardening responses of FG-CNTRC annular plates.

Experimental investigation and a novel direct strength method for cold-formed built-up I-section columns

Abstract: This paper aims at investigating the structural response and predicting the ultimate strength of the cold-formed built-up I-section columns affected by local, distortional, global and in particular by the local-distortional (LD) interactive and local-distortional-global (LDG) interactive buckling modes. For this purpose, a total of 18 single C-section columns and 18 built-up I-section columns were tested under uniaxial compression load, respectively. The cross-sectional dimension, the thickness and the length of the tested members were varied in the test so as to cover a wide range of local, distortional and overall slenderness. It was shown in the test that noticeable LD interaction was observed for a built-up column with short length as well as LDG interaction for a built-up column with intermediate length. Due to the clear evidence obtained in the test that LD and LDG interactions cause substantial ultimate strength erosion in cold-formed built-up I-section column, a novel direct strength based method was proposed in this paper to quantify such an erosion effect. The validity of the proposed method was then verified by comparing the results obtained from the proposed method with the test results in this paper as well as several other test results in the literature. The comparison results proved that the proposed method can be used successfully in estimating the ultimate strength of cold-formed built-up I-section column affected by pure buckling mode as well as interactive buckling mode.

Crashworthiness design for trapezoid origami crash boxes

Abstract: A thin-walled tube referred to as trapezoid origami crash box is recognized as an energy absorption device. The surface of this tube is prefolded in the light of a developable origami pattern that is delicately designed to introduce a special structure on the surface of a square tube. This special structure, known as trapezoid folded lobe, is employed as a type of geometric imperfection to lower the peak force and as a mode inducer to trigger the complete diamond mode. The quasi-static numerical simulations reveal that the complete diamond mode is successfully triggered. Moreover, geometric and compliance analysis suggest that three key parameters, the number of module M, dihedral angle θ and area ratio ω , could greatly affect the collapse behavior. Based on those analysis, an optimal trapezoid origami crash box is designed to compare with the conventional square and octagonal tubes of identical weight. Furthermore, a series of diamond origami crash boxes are analyzed to compare with the trapezoid origami crash boxes. The comparative results show that the trapezoid origami crash box is the most desirable in terms of energy absorption.

Size effect of circular concrete-filled steel tubular short columns subjected to axial compression

Abstract: In this paper, thirty-six short columns with different diameters (150 mm ≤ d ≤ 460 mm) and steel ratios (4.0% ≤ α ≤ 10.0%) were tested to failure to investigate the size effect of circular concrete-filled steel tubular short columns subjected to axial compression. Size effects on the peak axial stress, peak axial strain, composite elastic modulus, and ductility coefficient were studied. The experimental results showed that the peak axial stress, peak axial strain and ductility coefficient of the specimens tended to decrease with the increase in the column diameter. The values of the composite elastic modulus remained almost constant when the diameter of the specimens increased, indicating that size effect on the composite elastic modulus was not obvious. Meanwhile, size effect on the peak axial stress was influenced by the steel ratio in the range of 4–10%. Furthermore, the size effect tended to decrease as the steel ratio increased. By comparing with the current codes, a reduction coefficient was introduced to consider the size effect of concrete core. Based on the reduction coefficient, the size effect of the concrete core inside the steel tube is found to be weaker compared with that of the unconfined concrete columns because of the confinement effect.

Analysis of FG-CNT reinforced composite conical panel subjected to moving load using Ritz method

Abstract: Forced vibration response of a conical panel subjected to the action of a moving load is investigated in the current research Panel is made from a carbon nanotube (CNT) reinforced composite where the CNTs as reinforcements are distributed either uniformly or functionally graded across the panel thickness Panel is formulated using the first order shear deformation shell theory and the Donnell kinematic assumptions It is subjected to a moving load whose path and velocity are both arbitrary The properties of the composite media are estimated according to a refined rule of mixtures approach The governing equations of motion of the shell are obtained according to the Ritz method where the shape functions are obtained according to the Gram-Schmidt process The developed equations with the aid of Ritz method are transformed into time-dependent ordinary differential equations whose solution is traced in time by means of the Newmark time marching scheme Numerical results are provided to explore the influences of semi-vertex and opening angles of the cone, geometrical parameters and also CNT characteristics of the shell It is shown that, dynamic deflection of the shell decreases significantly with the introduction of FG-X pattern of CNTs Furthermore, enrichment of the matrix with more CNTs alleviates the dynamic deflection of the conical shell

Large-amplitude vibration of sigmoid functionally graded thin plates with porosities

Abstract: This research focuses on the large-amplitude vibration of sigmoid functionally graded material (S-FGM) thin plates with porosities. Porosities in S-FGM plates can happen due to technical issues during the preparation of S-FGMs. Two types of porosity distribution, i.e., even and uneven distribution, are taken into account. The material properties of S-FGM plates with porosities change smoothly along the thickness direction based on the sigmoid distribution law, which is described by modified piecewise functions. The geometrical nonlinearity is considered by applying the von Karman non-linear plate theory. The nonlinear governing equation of S-FGM plates with porosities is derived using the D′Alembert's principle. By applying the Galerkin method with the first three modes, the governing equation is discretized to three ordinary differential equations. Then, the method of harmonic balance is used to solve these discretized equations. Analytical results are verified numerically with the adaptive step-size fourth-order Runge-Kutta method. The stability of the steady-state response is examined by means of the perturbation technique. Furthermore, the maximum amplitudes of each mode during the vibration period are obtained and shown in the neighborhood of the fundamental mode. Study demonstrates that the S-FGM plates with porosities possess hardening spring characteristics in nonlinear frequency response. Moreover, a complex multi-solution phenomenon occurs in the present dynamic system which is rooted from the nonlinear mode interaction. Finally, investigation is made on the effects of porosity along with other key parameters on large-amplitude vibration response of porous S-FGM plates.

Statistical analysis of the tensile strength of GFRP, CFRP and hybrid composites

Abstract: Glass/epoxy, carbon/epoxy and hybrid (glass-carbon/epoxy) composites are subjected to quasi-static to high strain rate (542 s −1 ) studies to obtain the tensile strength. The two-parameter Weibull distribution is employed to measure the variability of the tensile strength of cross ply laminates. The Weibull parameters are obtained using the linear curve fit method. The theoretical tensile strength values are determined for the GFRP, CFRP and hybrid composites. The experimental and theoretical strength values are in good agreement. Using Hitachi scanning electron microscope instrument, the failure mechanisms are investigated in the fracture surfaces of glass/epoxy, carbon/epoxy and hybrid specimens, and discussed.

Dynamics of FG-CNT reinforced composite cylindrical panel subjected to moving load

Abstract: Present study deals with the dynamic response of a functionally graded carbon nanotube reinforced composite (FG-CNTRC) cylindrical panel subjected to moving load on the panel surface. Panel is formulated within the framework of first order shear deformation shell theory. Formulation is restricted to be geometrically linear. Distribution of CNTs across the panel thickness is considered to be uniform or functionally graded. Effective properties of the composite media are estimated using a refined rule of mixtures approach with introduction of efficiency parameters. The matrix representation of dynamic equations is obtained according to the Ritz method whose orthogonal shape functions are obtained according to the Gram-Schmidt process. The resulting dynamic equations are traced in time following the Newmark time marching scheme. Parametric studies are given to explore the characteristics of CNTs as reinforcements and influences of boundary conditions. It is shown that, increasing the volume fraction of CNT as reinforcements decreases the dynamic response of the panel. Furthermore, in comparison to other patterns of CNT dispersion, in FG-X pattern of CNT distribution, panel becomes more stiff and dynamic deflection decreases.

Material properties of cold-formed high strength steel at elevated temperatures

Abstract: This paper presents the material properties of cold-formed high strength steel at elevated temperatures. Material properties at elevated temperatures have a crucial role in fire resistance design of steel structures. The fire resistances of steel structures in the existing international standards are mainly based on experimental data of hot-rolled mild steel. However, investigation of high strength steel at elevated temperatures is limited. Therefore, a test program has been carried out to investigate the material properties of cold-formed high strength steel at elevated temperatures. The coupon specimens were extracted from cold-formed high strength steel square and rectangular hollow sections with nominal yield stresses of 700 and 900 MPa at ambient temperature. The coupon tests were carried out through both steady and transient state test methods for temperatures up to 1000 °C. Material properties including thermal elongation, elastic modulus, yield stress, ultimate strength, ultimate strain and fracture strain were obtained from the tests. The test results were compared with the design values in the European, American, Australian and British standards. The comparison results revealed the necessity of proposing specified design rules for material properties of cold-formed high strength steel at elevated temperatures. New design curves to determine the deterioration of material properties of cold-formed high strength steel at elevated temperatures are proposed. It is shown that the proposed design curves are suitable for high strength steel materials with nominal yield stresses ranged from 690 to 960 MPa at ambient temperature.

Crashworthiness analysis of octagonal multi-cell tube with functionally graded thickness under multiple loading angles

Abstract: This paper provides a study on a novel octagonal multi-cell tube with functionally graded thickness (FGT) under multiple loading angles. First, comparative analysis on the FGT tube and the counterpart tube with uniform thickness(UT) under multiple loading angles reveal that the energy absorption is more superior for the FGT tube when the loading angle exceeds the lower bound of the transition range of the UT tube. Second, parametric study on the FGT tube indicates that thickness gradient exponent and thickness range have significant effect on its crashworthiness. Third, multiobjective optimizations of the FGT tube are conducted, aiming to maximize specific energy absorption(SEA) and minimize initial force(IPF) under multiple loading angles, based upon the Non-dominated sorting genetic algorithm(NAGA-Ⅱ) and RBF technique. The optimized FGT tube demonstrates better crashworthiness than the UT tube in all design cases. These findings can provide valuable guidelines for the design of multi-cell tube with functionally graded thickness under multiple loading angles.

Nonlinear dynamic analysis and vibration of eccentrically stiffened S-FGM elliptical cylindrical shells surrounded on elastic foundations in thermal environments

Abstract: Elliptical cylindrical shell is one of shells with special shape. Up to date, there is no publication on vibration and dynamic of functionally graded elliptical cylindrical shells. Therefore, the purpose of the present study is to investigate the nonlinear dynamic response and vibration of imperfect eccentrically stiffness functionally graded elliptical cylindrical shells on elastic foundations using both the classical shell theory (CST) and Airy stress functions method with motion equations using Volmir's assumption. The material properties are assumed to be temperature - dependent and graded in the thickness direction according to a Sigmoid power law distribution (S-FGM). The S-FGM elliptical cylindrical shell with metal-ceramic-metal layers are reinforced by outside metal stiffeners. Both the S-FGM elliptical shell and metal stiffeners are assumed to be in thermal environment and both of them are deformed under temperature simultaneously. Two cases of thermal loading (uniform temperature rise and temperature variation through thickness) are considered. The nonlinear motion equations are solved by Galerkin method and Runge-Kutta method (nonlinear dynamic response, natural frequencies). The effects of geometrical parameters, material properties, elastic foundations Winkler and Pasternak, the nonlinear dynamic analysis and nonlinear vibration of the elliptical cylindrical shells are studied. The some obtained results are validated by comparing with those in the literature.

In–plane dynamic crushing behavior and energy absorption of honeycombs with a novel type of multi-cells

Abstract: In order to pursue better crashworthiness and higher energy absorption efficiency, a new type of multi-cell honeycomb (quadri-arc) was designed and followed by a series of numerical studies on in-plane dynamic crushing behavior and energy absorption property under different impact loading. Meanwhile, simulations of a regular circular single-cell honeycomb were also conducted as comparisons. Three distinct deformation modes were identified from the observation of deformation profiles: quasi-static, transition and dynamic, respectively. Simulations indicate that deformation modes are not only influenced by impact velocity but also sensitive to relative density of the honeycomb, based on which a deformation map was summarized. Furthermore, the plateau stress and energy absorption of the quadri-arc honeycomb as well as the circular honeycomb were explored and compared, during which effects of impact velocity and relative density of the honeycomb were discussed in detail. The results show that a much higher plateau stress and better energy absorption efficiency for the quadri-arc honeycomb are predicted compared to the circular honeycomb, especially in the quasi-static case. The investigation suggests that design of the quadri-arc multi-cell will enhance the crashworthiness and energy absorption capacity of honeycombs.