Publications

The mechanical behaviour of confined carbyne chain, the thinnest structure in nature

Carbyne chains are the thinnest structures found in nature. The synthesis of long and stable inside carbon nanotubes has recently drawn renewed attention to this linear one-dimensional carbon allotrope. Carbyne’s mechanical properties have been predicted to exceed that of carbon nanotubes and graphene, making it a very suitable structural component for many nanoscale applications. While carbyne’s mechanical behavior under tensile loading is superlative, this linear chain of bonded carbon atoms readily buckles due to thermal fluctuations, showing minimal strength under compressive loading. Here we present a detailed study on the enhancement of carbyne’s mechanical properties under compression by confinement in small diameter carbon nanotubes. First, we develop a new classical empirical forcefield based on ab-initio calculations. Molecular dynamics (MD) simulations are then carried, using this forcefield, to determine the mechanical properties of carbyne under tensile loading, namely to assess their dependence on chain length and temperature. The bending rigidity of carbyne and its persistence length are also calculated. After the validation of the new forcefield, MD simulations are employed to study the mechanical behavior of carbyne under compressive loading, when confined inside (5,5), (6,6) (7,7) and (8,8) CNTs. It is found that the mechanical behavior of confined carbyne chains depends on the confinement radius and the chain length, and can be described in three stages. For “tight” confinements, carbyne chains assume a rigid rod behavior, allowing the calculation of compressive mechanical properties and attesting an effective “bracing” effect. For “looser” confinements, a spring like behavior arises which can be modeled by Hooke’s law. In the third stage confined carbyne chains tend to buckle into a large local bend, losing the elastic flexibility that characterizes the previous stage.

Editorial

This issue of Structures and Buildings presents five papers on different scientific topics, ranging from steel structures to reinforced-concrete structures.They shed light on technical problems usually dealt with in current engineering practice.

Influence of Shear Deformation on the Local and Global Buckling Behaviour of Composite Thin-Walled Members

This paper presents the formulation of a shear deformable Generalised Beam Theory (GBT) developed to perform accurate analyses of the buckling behaviour of laminated plate composite thin-walled members displaying arbitrary orthotropy. Besides including the "conventional" (non-shear) deformation modes, this GBT formulation also incorporates a set of (new) shear deformation modes. Following the derivation of the equilibrium equations and corresponding boundary conditions, their terms are physically interpreted, i.e., related to the member mechanical properties. Then, the derived shear-deformable orthotropic GBT is used to investigate the influence of shear deformation on the local and global buckling behaviour of lipped channel columns displaying cross-ply orthotropy. The GBT results are validated through a comparison with numerical values yielded by thin-shell FEM analyses.

Dynamic analysis of high-speed railway bridge decks using generalised beam theory

This paper presents the details and illustrates the application of a semi-analytical Generalised Beam Theory (GBT) formulation for the dynamic analysis of high-speed railway bridge decks. The technique, which takes into account the cross-section local/distortional and shear deformations, provides an original “doubly modal” representation of the dynamic response, which makes it possible to acquire deep insight into the mechanics of the deck structural behaviour. It is employed to investigate the dynamic behaviour of a real high-speed railway viaduct, comprising a series of identical 46m-span simply supported box girder pre-stressed concrete decks carrying two tracks – the augmented structural response associated with resonance due to the crossing of trains at high-speeds is captured. The results obtained provide evidence that local/distortional deformation can play a significant role on the dynamic response of such structures. Moreover, the GBT analyses, whose validity is confirmed through the comparison with values yielded by shell finite element analyses, are shown to provide accurate results while involving just a few degrees of freedom.

Advanced Computational Nanomechanics

Analysis of web crippling of litesteel beams strengthened with CFRP

GBT-based local and global vibration analysis of thin-walled members

This chapter discusses the use of numerical techniques to perform vibration analysis, the concepts of finite element analysis (FEA) and finite strip analysis (FSA). The literature review in the chapter has been organized according to the particular methodology employed: there are separate sub-sections dealing with investigations carried out by means of (1) the finite element method (mostly shell element discretizations), (2) the finite strip method and (iii) the generalized beam theory (GBT) – because the aim of the chapter is to present the fundamentals and illustrate the application of a GBT formulation to analyze the vibration behavior of thin-walled members, the last sub-section also includes a brief outline of its content. Silvestre and Camotim (2003) formulated, implemented, and validated an efficient beam finite element intended to perform GBT-based buckling analyses in the context of arbitrarily orthotropic thin-walled members. The most relevant steps involved in the formulation of this finite element, specialized for the vibration analysis of isotropic thin-walled members, are described briefly in the chapter.

Generalised Beam Theory based Local and Global Dynamic Analysis of High-Speed Railway Bridge Decks

No abstract is provided for this article.

C13 – a new empirical force field to characterize the mechanical behavior of carbyne chains

An accurate prediction of the mechanical behavior of long carbyne chains depends on the suitable modeling of bond alternation in these chains. While first-principles methods are a good approach, less computationally demanding empirical potentials are preferable for large carbyne-containing systems. AIREBO and Reax empirical potentials have extensively and successfully been used for simulating the mechanical behavior of graphene and carbon nanotubes. However, it remains unclear if these potentials can be directly applied in the accurate mechanical modeling of carbon nanostructures with sp hybridization, without re-parameterization. Here, a new force-field for carbyne, designated as C13 potential, that takes bond alternation into account, is presented. This new empirical potential was parameterized from ab initio calculations. Molecular dynamics (MD) simulations using the developed force-field are then conducted to determine the mechanical properties of carbyne chains under tensile loading, namely to assess their dependence on chain length and temperature. The bending stiffness of carbyne and its persistence length are also calculated. The results obtained are validated through comparison with results available in the literature. Lastly, the C13 potential is employed to model, for the first time, the tensile and the compressive behaviors of the hybrid system composed of carbon nanotubes infilled with carbyne chains.

A neural network based closed-form solution for the distortional buckling of elliptical tubes

Following the Eurocode 3 philosophy, it is expected that the design of elliptical hollow section (EHS) tubes will be based on the slenderness concept, which requires the calculation of the EHS critical stress. The critical stress of an EHS tube under compression may be associated with local buckling, distortional buckling or flexural buckling. The complexity in deriving analytical expressions for distortional critical stress from classical shell theories, led us to apply Artificial Neural Networks (ANN). This paper presents closed-form expressions to calculate the distortional critical stress and half-wave length of EHS tubes under compression, using ANN. Almost 400 EHS geometries are used and based solely on three parameters: the outer EHS dimensions (A and B) and its thickness (t). Two architectures are shown to be successful. They are tested for several statistical parameters and proven to be very well behaved. Finally, some simple illustrative examples are shown and final remarks are drawn concerning the accuracy of the closed-formed formulas.

GBT-Based Analysis of The Distortional Post-Buckling Behaviour of Cold-Formed Steel Z-Section Columns and Beams

An investigation on the distortional post-buckling behaviour of cold-formed steel Z-section columns and beams is presented. All the results are obtained by means of geometrically non-linear analyses based on a recently developed GBT formulation, which incorporates (i) conventional (shear undeformable), (ii) shear (non-linear warping) and (iii) transverse extension deformation modes. The results presented, most of which are compared with shell-element FEM analyses (for validation purposes), include post-buckling equilibrium paths and the evolution, along those paths, of the member deformed configuration and relevant displacements or stresses. Taking advantage of the GBT unique modal features, these results are discussed in great detail and it is possible to unveil, explain and/or shed some new light on a number of interesting and scarcely known phenomena.

Computational simulation of γ-graphynes under monotonic and hysteretic loading

The modeling and simulation of γ-graphyne under monotonic and hysteretic in-plane loading is presented. An atomistic finite element model of γ-graphyne was developed. Four different loading tests were conducted, including unidirectional tests in armchair and zigzag direction, bidirectional test and shear test. From monotonic analyses, the mechanical properties (elastic and strength) were obtained and validated. Hysteretic (cyclic) tests were performed to obtain stress–strain curves and the variation of dissipated energy with the number of cycles. The failure modes and the load-carrying capacity are analyzed. The proposed model simulates correctly the γ-graphyne monotonic and hysteretic mechanical behavior.

Web-crippling of GFRP pultruded profiles. Part 1: Experimental study

There is evidence that glass fibre reinforced polymer (GFRP) pultruded profiles are particularly susceptible to transverse compressive loads, owing to the much lower mechanical properties in the direction transverse to the pultrusion axis. Although very relevant, the understanding about the web-crippling phenomenon in GFRP pultruded profiles is still very limited, as attested by the lack of information available in design codes and guidelines. This paper reports an experimental study about the web-crippling phenomenon in GFRP pultruded profiles with I-section. The experimental programme included comprehensive material characterization tests (tension, compression, flexure and shear), and full-scale web-crippling tests on four different I-profiles, with heights ranging from 100mm to 400mm, thus covering the vast majority of structural profiles currently available in the market. In the web-crippling experiments, two load configurations were tested: interior two flanges (ITF); and end two flanges (ETF). In addition, tests were performed with three different bearing lengths: 15mm, 50mm, and 100mm. The experimental results confirmed the susceptibility of GFRP pultruded profiles to transverse compressive loads, outlining the influence of both the load configuration and the bearing length on the web-crippling phenomenon in terms of strength, stiffness, and failure modes.

Fracture toughness-based models for web-crippling of pultruded GFRP profiles

This paper presents a numerical study on the web-crippling failure of pultruded glass fibre reinforced polymer (GFRP) profiles, under external-two-flange (ETF) and internal-two-flange (ITF) configurations. The numerical study is validated through experimental tests recently conducted by the authors on four I-section profiles and one U-section profile, obtained from four different suppliers. These web-crippling tests were simulated through shell finite element (FE) models where the transverse compressive fracture toughness of each GFRP material was implemented as a damage evolution parameter. In order to quantify the influence of material damage and instability effects on the web-crippling failure, three different analyses were implemented, accounting for (i) material damage, (ii) instability, and (iii) both effects. The results obtained through the analysis accounting for both material damage and instability effects showed a good agreement with experimental results, in terms of stiffness, failure modes and ultimate loads; moreover, the transverse compressive strain distributions obtained from the numerical models also agreed well with experimental results. Finally, the transverse compressive stresses were found to influence web-crippling the most, regarding both damage initiation and ultimate load stages.

Reliability-Based Code Calibration of Pultruded Glass Fibre-Reinforced Polymer I-Section Beams Under End-Two-Flange and Interior-Two-Flange Web-Crippling Loading Cases

Confinement-based direct design of circular steel tube confined concrete (STCC) short columns

A new design model to estimate the axial compressive resistance of circular steel tube confined concrete (STCC) short columns is proposed in this paper. These STCC columns differ from standard concrete-filled steel tubular (CFST) columns because their concrete core carries all the compressive stresses (the steel tube is not compressed). The design model considers the confinement effect of the outer steel tube on the concrete core. The experimental test results of axially-loaded STCC short columns with normal, high and ultra-high strength concrete were first collected from the literature. The test resistances of the columns were then compared to the predictions obtained by EC4 (2004), ANSI/AISC (2010), AIJ (2001), ACI 318R (2014) and CISC (2007), as well as analytical design models presented in the literature. This comparative study indicated that most design codes are conservative, while most analytical design models are unconservative. Additionally, they do not keep the same accuracy level over the whole slenderness range of circular steel tubes. Then, the test resistances were used to propose and calibrate a new formula to evaluate the lateral confining pressure (f rp ) provided by the steel tube to the concrete core of different grades. The f rp formula is used to propose a confinement-based direct resistance for axially-loaded circular STCC short columns. The results of the proposed design model are much better than those obtained from the available design predictions, regardless of the slenderness ratio of the steel tube and steel/concrete grades, which can also be used with stainless STCC short columns.

Reliability-based code calibration of local buckling strength design of pultruded GFRP I- and H-section columns

This paper presents and discusses a reliability-based code calibration (RBCC) procedure for the determination of optimal partial safety factors to be employed in the semi-probabilistic design rules of pultruded Glass Fibre-Reinforced Polymer (pGFRP) columns, susceptible to local buckling, considered in the technical specification CEN/TS 19101: 2022, Design of Fibre-Polymer Composite Structures. First, the description of the two distinct resistance models for local buckling of I- and H-section columns adopted in CEN/TS 19101:2022 is presented. Next, attention is turned to the detailed presentation of the procedure to perform a RBCC, including all the relevant data, with emphasis to all the probabilistic models adopted. Next, the results obtained by applying the RBCC procedure are presented and discussed. Lastly, the paper closes with the presentation of relevant concluding remarks and recommends the partial safety factors for I- and H-section pGFRP columns susceptible to local buckling triggered by the web and flanges.

Editorial

No abstract is provided for this article.