Imperial College London

Professor M Ahmer Wadee

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Professor of Nonlinear Mechanics
 
 
 
//

Contact

 

+44 (0)20 7594 6050a.wadee Website

 
 
//

Assistant

 

Ms Ruth Bello +44 (0)20 7594 6040

 
//

Location

 

421Skempton BuildingSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

198 results found

Köllner A, Gardner L, Wadee MA, 2023, A new approach to the stability design of Ramberg-Osgood material struts, Structures, Vol: 56, Pages: 1-11, ISSN: 2352-0124

An energy formulation employing total potential energy principles is presented to derivea governing equation for strength predictions of struts made from materials followingthe Ramberg–Osgood constitutive law such as stainless steel, cold-formed steel, andaluminium alloys. The formula is generic and applicable to arbitrary cross-sections and allstrut slendernesses for which flexural buckling is critical. Extensive comparisons againstexperimental data on square and rectangular hollow section struts as well as finite elementsimulations demonstrate the accuracy of the developed formula, while the effect of varyingmaterial parameters is examined through comprehensive parametric studies. Owing toits simplicity and its derivation based on mechanical principles, arbitrary configurationsof material parameters and cross-sections can be analysed, making the formula suitablefor use in design practice, representing effectively a non-iterative alternative to the widelyaccepted design load employing the tangent modulus. With the aid of the formula, newcolumn buckling design provisions are developed, which show excellent agreement withexperimental data and meet the reliability requirements specified within the structuralEurocodes.

Journal article

Behzadi-Sofiani B, Wadee MA, Gardner L, 2023, Major-axis buckling of pin-ended stainless steel equal-leg angle section members: FE modelling and design, Eurosteel 2023, Publisher: Ernst und Sohn, Pages: 2625-2630, ISSN: 2509-7075

The behaviour and design of cylindrically-pinned stainless steel equal-leg angle section members under compression and compression combined with strong-axis bending are investigated herein. Numerical models are developed by means of shell finite element modelling formulated within ABAQUS and validated against experimental data. A numerical parametric study is then presented considering both hot-rolled and cold-formed stainless steel equal-leg angle section columns alongside beam-columns with a wide range of cross-section and member geometries. Finally, new design proposals for pin-ended stainless steel equal-leg angle section members under compression and compression plus major-axis bending are developed and verified against the results of physical experiments and numerical simulations. The proposed design rules are shown to offer substantially more accurate and consistent resistance predictions compared to existing codified design rules.

Conference paper

Behzadi-Sofiani B, Wadee MA, Gardner L, 2023, Testing, FE modelling and design of pin-ended stainless steel equal-leg angle section columns and beam-columns, Journal of Constructional Steel Research, Vol: 208, Pages: 1-19, ISSN: 0143-974X

The behaviour and design of pin-ended stainless steel equal-leg angle section members under compression and compression plus minor-axis bending are investigated herein. The studied members are cylindrically pinned about the minor axis. An experimental investigation, including material testing, initial geometric imperfection measurements and physical tests on hot-rolled stainless steel equal-leg angle section members is first presented. Numerical models are developed and validated against the new experimental data. A numerical parametric study is then presented considering both hot-rolled and cold-formed stainless steel angle section columns alongside beam–columns with a wide range of slenderness values. Finally, new design proposals for pin-ended stainless steel equal-leg angle section members under compression and combined compression and minor-axis bending are developed and verified against the results of existing physical experiments, as well as the newly-generated test and numerical results. The proposed design rules are shown to offer substantially more accurate and consistent resistance predictions compared to existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor γM1 = 1.1, is verified following the EN 1990 procedure.

Journal article

Bekele A, Wadee MA, Phillips ATM, 2023, Enhancing energy absorption through sequential instabilities in mechanical metamaterials, Royal Society Open Science, Vol: 10, Pages: 1-19, ISSN: 2054-5703

Structural components designed to absorb energy and shield a more valuable structure ideally require mechanical properties that combine a relatively high load-carrying capacity followed by a practically zero stiffness. This ensures that a specified energy quantity may be absorbed within a limited displacement and that any stress transfer to the valuable structure is minimized. Material damage has been historically mobilized to provide such properties but this obviously renders such components to be single-use. In contrast, mobilization of elastic instability can also provide the desired combination of properties but without necessarily damaging the material. This reveals an intriguing possibility of such components being potentially repairable and theoretically reusable with no significant loss in performance. A series of analytical, finite element and experimental studies are presented for a bespoke mechanical metamaterial arrangement that is designed to buckle sequentially and behave with the desired ‘high strength–low stiffness’ characteristic. It is found that the various axial and rotational stiffnesses associated with the geometric arrangement and its constituent connections may be tuned to provide the desired mechanical behaviour within the elastic range and delay the onset of significant damage thereby rendering the concept of harnessing instability to be feasible.

Journal article

Shen J, Lapira L, Wadee MA, Gardner L, Pirrera A, Groh RMJet al., 2023, Probing in-situ capacities of prestressed stayed columns: Towards a novel structural health monitoring technique, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 381, ISSN: 1364-503X

Prestressed stayed columns (PSCs) are structural systems whose compressive load-carrying capacity is enhanced through pre-tensioned cable stays. Much research has demonstrated that PSCs buckle subcritically when their prestressing levels maximise the critical buckling load of the theoretically perfect arrangement. Erosion of the pre-tensioned cables’ effectiveness (e.g. through creep or corrosion) can thus lead to sudden collapse. The present goal is to develop a structural health monitoring (SHM) technique for in-service PSCs that returns the current structural utilisation factor based on selected probing measurements. Hence, PSCs with different cable erosion and varying compression levels are probed in the pre-buckling range within the numerical setting through a nonlinear finite element model. In contrast with previous work, it is found presently that the initial lateral stiffness from probing a PSC provides a suitable health index for in-service structures. A machine learning based surrogate is trained on simulated data of the loading factor, cable erosion, and probing indices; it is then used as a predictive tool to return the current utilisation factor for PSCs alongside the level of cable erosion given probing measurements, showing excellent accuracy and thus provides confidence that an SHM technique based on probing is indeed feasible.

Journal article

Ni Y, Zhu S, Tong Z, Xu X, Zhou Z, Lim CW, Wadee MA, Yiatros Set al., 2023, Stability of composite cylindrical shells with non-classical hygrothermal-electro-elastic coupled loads, Journal of Engineering Mechanics, Vol: 149, Pages: 1-15, ISSN: 0733-9399

An accurate nonlinear hygrothermal-electro-elastic (HTEE) buckling analysis of piezoelectric fiber reinforced composite cylindrical shells subjected to the coupled loading effects of axial compression and hydrostatic pressure is established by considering non-uniform pre-buckling effect. Nonlinear governing equations are derived based on higher-order shear deformation theory and Novozhilov’s nonlinear shell theory. Accurate critical bucklingstresses and pressures, and explicit buckling modes for both axisymmetric and non-axisymmetric buckling are obtained by the Galerkin method. A comparison between the new prediction and existing results is presented and excellent agreement is reported. A comprehensive parametric study of geometric parameters, end conditions, distribution patterns and hygrothermal-electric multi-physical fields on the buckling behavior of HTEE composite cylindrical shell is also analyzed and discussed.

Journal article

Afkhami S, Amraei M, Poutiainen I, Gardner L, Piili H, Wadee MA, Salminen A, Björk Tet al., 2023, Data related to the manufacturing and mechanical performance of 3D-printed metal honeycombs, Data in Brief, Vol: 46, Pages: 1-15, ISSN: 2352-3409

The data available in this article include 3D mechanical designs used for the computer-aided fabrication of metal honeycombs produced by additive manufacturing and studied in [1]. In addition, the force-displacement data utilized to evaluate the mechanical performance of the metal used in this study are available via the digital image correlation technique. Further, the surface features obtained using 3D scanning microscopy of the fabricated parts are available as raw files and processed data. Finally, the impact test data are presented as high-frame-rate videos showing the time-displacement numerical values. This information has been provided in this data article to complement the related research, serve as a guide for future studies, and ensure the data's repeatability and reliability of the related research paper. The research article [1] investigates the mechanical performance and failure mechanism of additively manufactured metallic honeycombs under various scenarios, from quasi-static to dynamic loading. It also investigates the design optimization of these energy-absorbing hollow structures by comparing hollow structures made of three distinct novel cell designs (triangular, diamond-shaped, and diamond-shaped with curved walls) with traditional honeycombs made of hexagonal cells.

Journal article

Behzadi-Sofiani B, Gardner L, Wadee MA, 2023, Behaviour, finite element modelling and design of cruciform section steel columns, Thin Walled Structures, Vol: 182, Pages: 1-13, ISSN: 0263-8231

A study into the mechanical behaviour and design of steel equal-leg cruciform section members subjected to axial compression is presented. Experimental data from the literature are used to validate shell finite element models developed within the commercial package ABAQUS for their load-deformation and ultimate behaviour. A numerical parametric study then considers cruciform section columns with a wide range of slenderness ratios and various common boundary conditions. The mechanical response and load-carrying capacity are shown to be dependent not only on the column slenderness but also on the torsional to flexural elastic buckling load ratio. Finally, a recently established design approach for steel angle section columns is extended to include cruciform section members, and verified against the available results. It is found through reliability analysis based on the EN 1990 procedure that the new proposal also offers substantially improved accuracy and consistency in strength predictions compared to the existing codified design rules for cruciform section columns; a recommended partial safety factor of 1.0 is determined.

Journal article

Lapira L, Gardner L, Wadee MA, 2023, Elastic local buckling formulae for thin-walled I-sections subjected to shear and direct stresses, Thin Walled Structures, Vol: 182, ISSN: 0263-8231

Formulae for calculating the elastic local buckling stresses of doubly-symmetric thin-walled I-section girders subjected to combined shear and direct stresses, accounting for the interaction between the plate elements are presented. The interaction between the plate elements (i.e. the flanges and web) is bounded by a theoretical lower-bound, where there is no interaction and the critical plate is considered to be simply-supported, and a theoretical upper-bound where interaction is strongest and the critical plate is considered to have rotationally fixed edges. The interaction is accounted for by introducing an interaction coefficient ζ that quantifies the relative level of fixity between the aforementioned lower and upper bounds. Expressions to calculate ζ are calibrated using results from finite element analyses generated in Abaqus. Doubly-symmetric I-sections of varying geometric proportions loaded in shear, major axis bending, compression and a full range of combinations thereof are considered. Using the developed formulae, the elastic local buckling stresses of the studied cross-sections are accurately predicted, typically within 5% of the values obtained from FE models; this is a significant improvement over the results determined in the traditional manner in which plate element interaction effects are ignored, where the full cross-section buckling stress is shown to be underestimated by as much as 40%.

Journal article

Köllner A, Wadee MA, 2022, A novel discrete coordinate approach to modelling nonlinear structural instability problems with material damage, European Journal of Mechanics A: Solids, Vol: 96, Pages: 1-13, ISSN: 0997-7538

A novel analytical framework for the inclusion of active damaging processes in a structural stability analysis is presented. The framework considers quasi-static deformation processes and is formulated in terms of a discrete coordinate approach, employing an extended total potential energy functional, where the current damage state is expressed in terms of geometric configuration and applied loading. Within the current framework, an efficient analysis of structures prone to buckling instability and material damage is enabled particularly where damage initiates once structural instability has been triggered. Composite panels with multiple damage mechanisms (e.g. delaminations, matrix cracks) are studied to showcase its application.

Journal article

Afkhami S, Amraei M, Gardner L, Piili H, Wadee MA, Salminen A, Björk Tet al., 2022, Mechanical performance and design optimisation of metal honeycombs fabricated by laser powder bed fusion, Thin Walled Structures, Vol: 180, Pages: 1-17, ISSN: 0263-8231

Honeycomb structures have a wide range of applications, from medical implants to industrial components. In addition, honeycombs play a critical role when passive protection is required due to their low density and high energy absorption capabilities. With the transition of additive manufacturing from a rapid prototyping approach to a manufacturing process, this technology has recently offered designers and manufacturers the ability to fabricate and modify lattice structures such as honeycombs. The current study presents the application of laser powder bed fusion, a common additive manufacturing process for producing industrial metal components, for fabricating metal honeycombs. In addition, this study examines three modified designs that can only be practically fabricated using additive manufacturing and compares them with conventional honeycombs. For this purpose, quasi-static and dynamic compression tests are conducted to evaluate and compare the performance of the honeycomb structures. The results show that the structures produced by additive manufacturing have acceptable performance compared to conventional honeycomb structures, and laser powder bed fusion can be considered to be a reliable manufacturing method for honeycomb production. Furthermore, the honeycombs produced according to the modified designs generally outperformed their counterparts made from the typical hexagonal cells. Ultimately, the use of triangular cells as a design modification is proposed toproduce honeycombs with promising performance characteristics in all of their principal axes and under various pressure scenarios, from quasi-static to dynamic loading rates. Finally, this study also investigates the applicability of a newly developed maraging steel for additive manufacturing of honeycombs. Microstructural analysis and quasi-static tensile tests have confirmed the material properties for this purpose.

Journal article

Wadee MA, Bekele A, Phillips ATM, 2022, Harnessing instabilities within metamaterial structures, Publisher: CRC Press, Pages: 653-659

Conference paper

Behzadi-Sofiani B, Gardner L, Wadee MA, 2022, Numerical simulation and design of steel equal-leg angle section beams, Publisher: CRC Press, Pages: 937-943

Conference paper

Zhang W, Gardner L, Wadee MA, Chen KS, Zhao WYet al., 2022, On the uniform torsional rigidity of square concrete-filled steel tubular (CFST) sections, Structures, Vol: 43, Pages: 249-256, ISSN: 2352-0124

There are a number of scenarios in which structural members experience torsion, including under the direct application of torsional loading, when transverse loading is applied at an eccentricity to the shear centre and as a second order effect arising from lateral torsional instability. To date, the torsional rigidity of concrete-filled steel tubular (CFST) sections has yet to be fully explored; hence a study into the uniform torsional rigidity of square CFST sections is presented herein. First, the strain energy of square CFST sections is formulated, in which the longitudinal warping displacement is assumed to have an undetermined constant. The undetermined constant is then deduced by means of the principle of minimum strain energy, and thus an analytical expression for the uniform torsional rigidity of square CFST sections is obtained. The accuracy of the derived formula is verified against existing theoretical solutions for simplified scenarios, test data and the results of numerical simulations. Finally, the influence of the key parameters in the derived formula for the torsional rigidity of square CFST sections are analysed, and a simplified design formula is presented.

Journal article

Behzadi-Sofiani B, Gardner L, Wadee MA, 2022, Testing, numerical analysis and design of stainless steel equal-leg angle section beams, Structures, Vol: 37, Pages: 977-1001, ISSN: 2352-0124

The stability and design of stainless steel equal-leg angle section members subjected to uniaxial bending are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and physical tests on hot-rolled austenitic stainless steel equal-leg angle section beams is first presented. The test results are then used to validate shell finite element models developed within ABAQUS, which are in turn used to undertake numerical parametric studies that consider both hot-rolled and cold-formed equal-leg angle section beams in austenitic, duplex and ferritic stainless steel with a wide range of slenderness values. Recent studies have shown that for angles under major-axis bending, both lateral-torsional and local buckling can arise, while under minor-axis bending, lateral-torsional buckling and Brazier-type flattening can occur. When designing for major-axis bending according to Eurocode 3, both local and lateral-torsional buckling are considered; it is shown herein that equal-leg angle sections under major-axis bending can be designed using a normalised slenderness based on the minimum of the local and lateral-torsional elastic buckling moments, while also considering their ratio. Under minor-axis bending, however, in comparison with the current provisions in Eurocode 3 that only require cross-section checks, it is shown that both safer and more accurate resistance predictions can be achieved when account is taken for lateral-torsional buckling and Brazier-type flattening. New design proposals for stainless steel equal-leg angle section beams, covering both major- and minor-axis bending, are therefore developed. The proposed design rules offer substantially more accurate and consistent resistance predictions compared to existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor γM1 = 1.1 , is verified following the procedure provided in EN 1990.

Journal article

Long Y, Zeng L, Gardner L, Wadee MAet al., 2022, A new model for calculating the elastic local buckling stress of steel plates in square CFST columns, Thin Walled Structures, Vol: 171, ISSN: 0263-8231

Previous studies into the elastic local buckling response of the steel plates in concrete-filled steel tubular (CFST) sections have considered the effect of concrete infill on the plate boundary conditions and on the buckling mode shape (i.e. one-way buckling only), but have not considered the influence of hoop stresses. This is addressed in the current paper, where a new model is proposed in which the unloaded plate edges are assumed to be elastically restrained and account is taken of the influence of hoop stresses. Tensile hoop stresses are shown to delay the occurrence of local buckling in the steel plates of square and rectangular CFST sections, though their beneficial influence reduces with increasing rotational edge restraint. The model is verified against existing experimental results, revealing better predictions of elastic local buckling stresses compared to previous approaches. The influence of the key parameters — tensile hoop stresses, varying boundary conditions and differing width-to-thickness (b/t) ratios is subsequently explored using the developed model, where it is shown that the b/t ratio remains the dominant factor in defining the elastic local buckling stress of steel plates in CFST sections. Finally, an alternative equivalent thickness approach to allow for the influence of the hoop stresses is proposed.

Journal article

Behzadi-Sofiani B, Gardner L, Wadee MA, 2022, Testing, simulation and design of steel equal-leg angle section beams, Thin Walled Structures, Vol: 171, Pages: 1-20, ISSN: 0263-8231

The stability and design of steel equal-leg angle section members subjected to uniaxial bending are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and physical tests on hot-rolled steel equal-leg angle section beams is first presented. The test results, in combination with existing experimental data on steel equal-leg angle section beams collected from the literature, are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Next, a numerical parametric study is presented considering both hot-rolled and cold-formed steel angle section beams with a wide range of slenderness values. In major-axis bending, both lateral-torsionaland local buckling were observed, with the former characterised by lateral deflection and twist of the cross-section along the member length but no cross-section deformation and the latter by relative twist and transverse bending of the outstands. In minor-axis bending, lateral-torsional buckling and Brazier flattening were observed, with the latter characterised by splaying of the outstands. Note that, for equal-leg angles under minor-axis bending, lateral-torsional buckling is dominant when the cross-section tips are in compression, while Brazier flattening is more influential when the cross-section tips are in tension. When designing for major-axis bending according to Eurocode 3, both local and lateral-torsional buckling are considered; it is shown herein that equal-leg angle section beams under major-axis bending can be designed using a normalised slenderness based on the minimum of the local and lateral-torsional elastic buckling moments, while also considering the ratio of the local tothe lateral-torsional elastic buckling moments. For minor-axis bending, Eurocode 3 only requires cross-section checks; this is found to result in unsafe predictions in some cases. It is shown that both safer and more acc

Journal article

Shen J, Groh RMJ, Wadee MA, Schenk M, Pirrera Aet al., 2022, Probing the stability landscape of prestressed stayed columns susceptible to mode interaction, Engineering Structures, Vol: 251, Pages: 1-16, ISSN: 0141-0296

Prestressed stayed columns are structural systems where the compressive load-carrying capacity is enhanced through pre-tensioned external cable stays. Recent theoretical studies using analytical and nonlinear finite element models have shown that, under certain configurations, this enhancement leads to a sequence of closely spaced bifurcation points beyond the critical one. This undesirable characteristic can give rise to dangerously unstable interactive post-buckling behaviour including ‘mode jumping’ and ‘snaking’ phenomena. Even though these highly nonlinear behaviours can be readily modelled using numerical methods, they cannot be verified robustly using traditional quasi-static testing techniques based on force or displacement control at a single point. The current work explores a novel testing concept for potential experimental implementation, from the theoretical and numerical point of view. The concept allows the stability landscape of prestressed stayed columns to be ascertained by controlling the shape of the structure at multiple points. By controlling the mode shape of the structure, it is possible to traverse limit points, path-follow otherwise unstable equilibria, pinpoint bifurcation points and branch-switch between different post-critical segments of the equilibrium manifold. To explore the feasibility of the new testing method, we have created a virtual instantiation of the experiment in the commercial finite element package Abaqus, coupled to a control algorithm that coordinates the movements of the different control points. A number of different stability phenomena that have previously been identified analytically and numerically are reproduced successfully in the virtual test environment. Moreover, a noise sensitivity study is conducted to assess the robustness of the experimental technique proposed herein. The present work lays the foundation for physically assessing the stability landscape of prestressed stayed columns i

Journal article

Behzadi Sofiani B, Gardner L, Wadee MA, 2021, Stability and design of fixed-ended stainless steel equal-leg angle section compression members, Engineering Structures, Vol: 249, ISSN: 0141-0296

The stability and design of fixed-ended stainless steel equal-leg angle section members subjected to axial compression are studied herein. An experimental investigation, comprising material testing, initial geometric imperfection measurements and tests on hot-rolled austenitic stainless steel equal-leg angle section columns is first presented. The test results, in combination with existing experimental data on stainless steel equal-leg angle section columns collected from the literature, are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Validation is performed by means of comparisons between the test and numerical results, considering ultimate loads, failure modes and the load–deformation responses, all of which are shown to be generally in good agreement. A numerical parametric study is then presented considering angle section columns in the three main families of stainless steel (austenitic, ferritic and duplex) with a wide range of slenderness values. The behaviour and normalised load-carrying capacity of the studied members is shown to be dependent on not only the column slenderness, but also the ratio of the elastic torsional-flexural buckling load to the elastic minor-axis flexural buckling load. Finally, a design approach recently proposed for carbon steel angle section columns is extended for application to stainless steel, and verified against the experimental and numerical results. The proposed approach offers substantially improved accuracy and consistency in strength predictions compared to the existing codified design rules. The reliability of the new design provisions, with a recommended partial safety factor ϒM1 = 1.1, is verified following the EN 1990 procedure.

Journal article

Bai L, Wadee MA, Köllner A, Yang Jet al., 2021, Variational modelling of local-global mode interaction in long rectangular hollow section struts with Ramberg-Osgood type material nonlinearity, International Journal of Mechanical Sciences, Vol: 209, ISSN: 0020-7403

A variational model describing the nonlinear mode interaction in thin-walled box-section struts under pure axial compression made from a nonlinear material obeying the Ramberg–Osgood law is presented. The formulation combines continuous displacement functions and generalized coordinates, leading to the derivation of a system of differential and integral equations that describe the static equilibrium response of the strut. Solving the system of equations using numerical continuation techniques reveals the strongly unstable post-buckling response arising from combined geometrical and material nonlinearities during the interactive buckling of the global and local buckling modes—the resulting behaviour being more unstable with decreasing material hardening. A finite element (FE) model is also devised and reveals very similar post-buckling behaviour as highlighted in the variational model. The results compare very well in terms of the mechanical destabilization and the post-buckling deformation, which verifies the analytical model.

Journal article

Lapira L, Wadee MA, Gardner L, 2021, Nonlinear analytical modelling of flat and hyperbolic paraboloidal panels under shear, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 477, Pages: 1-19, ISSN: 1364-5021

The hyperbolic paraboloid (hypar) form has been widely used in long-span roof structures and the subject of much research under out-of-plane loading. However, the behaviour of hypars under in-plane loading has been less keenly studied and there is no suitable guidance for their design in current codes of practice. A nonlinear analytical model treating the hypar as a deliberate imperfection applied to a flat plate is presented. A Rayleigh-Ritz formulation using appropriate shape functions is developed and the resulting equations are solved using numerical continuation techniques. The results are verified with nonlinear finite element models, showing good correlation across a range of thicknesses and degrees of initial curvature. Key moda lcontributions that influence the behaviour of the hypar are identified, providing insight into the nonlinear behaviour of hypars subject to in-plane shear. The main differences in behaviour between the flat plate and the hypar panel are shown to be most prevalent in the early stages of loading, where the influence of the initial geometry is at its greatest.

Journal article

Behzadi Sofiani B, Gardner L, Wadee MA, Dinis P, Camotim Det al., 2021, Behaviour and design of fixed-ended steel equal-leg angle section columns, Sheffield, UK, Eurosteel 2020

Conference paper

Kyvelou P, Slack H, Wadee MA, Buchanan C, Gardner Let al., 2021, Material testing and analysis of WAAM stainless steel, Sheffield, UK, Eurosteel 2020

Conference paper

Behzadi-Sofiani B, Gardner L, Wadee MA, 2021, Fixed-ended stainless steel equal-leg angle section columns - behaviour and design, 8th international conference on coupled instabilities in metal structures (CIMS 2020)

Conference paper

Behzadi-Sofiani B, Gardner L, Wadee MA, Dinis P, Camotim Det al., 2021, Behaviour and design of fixed-ended steel equal-leg angle section columns, Journal of Constructional Steel Research, Vol: 182, ISSN: 0143-974X

The mechanical behaviour and design of fixed-ended steel equal-leg angle section members subjected to axial compression are addressed in this study. First, the critical buckling behaviour is described. Experimental data on steel equal-leg angle section columns collected from the literature are then used for the validation of numerical (shell finite element) models, developed within the commercial package ABAQUS. Validation is performed by means of comparisons between test and numerical results, considering ultimate loads and failure modes, all of which are shown to be generally in good agreement. A numerical parametric study is then presented considering steel angle section columns with a wide range of slenderness values. The behaviour and load-carrying capacity of the columns is shown to be dependent on, not only the column slenderness, but also the ratio of the elastic torsional-flexural buckling load to the elastic minor-axis flexural buckling load. Finally, the collected experimental and generated numerical results are used to develop a new design approach, suitable for incorporation into future revisions of Eurocode 3, for fixed-ended steel equal-leg angle section columns, reflecting the observations made. The proposed approach offers improved accuracy and consistency in strength predictions compared to the existing codified design rules. The reliability of the new design approach, with a recommended partial safety factor γM1 = 1.0, is verified following the EN 1990 procedure.

Journal article

Wu K, Wadee MA, Gardner L, 2021, Prestressed stayed beam-columns: sensitivity to prestressing levels, pre-cambering and imperfections, Engineering Structures, Vol: 226, ISSN: 0141-0296

The behaviour and structural performance of imperfect beam-columns with crossarms and externally prestressed cable stays are studied numerically, where the combination of bending and compression is assumed to be derived from the system self-weight acting orthogonally to the applied axial load. Both doubly-symmetric and mono-symmetric systems are studied. Sensitivity of the structural response to varying prestressing levels, pre-cambering and initial imperfections is investigated. Different initial imperfection levels and combinations are considered to facilitate the exploration of interactive buckling. The optimum prestressing force in terms of ultimate resistance and two structural efficiency indicators is also studied. It is found that relatively small crossarm lengths, stay diameters and crossarm length ratios should be avoided. Moreover, mono-symmetric cases are more sensitive to the level of pre-cambering than their doubly-symmetric counterparts. Considering both load-carrying capacity and structural efficiency, doubly-symmetric cases perform best with zero pre-cambering, but mono-symmetric cases perform best with upward pre-cambering. As for the true optimum prestressing levels, these are recommended to be significantly above the linearly obtained optimum to maximize the structural efficiency.

Journal article

Kyvelou P, Slack H, Daskalaki Mountanou D, Wadee MA, Britton T, Buchanan C, Gardner Let al., 2020, Mechanical and microstructural testing of wire and arc additively manufactured sheet material, Materials and Design, Vol: 192, ISSN: 0264-1275

Wire and arc additive manufacturing (WAAM) is a method of 3D printing that enables large elements to be built, with reasonable printing times and costs. There are, however, uncertainties relating to the structural performance of WAAM material, including the basic mechanical properties, the degree of anisotropy, the influence of the as-built geometry and the variability in response. Towards addressing this knowledge gap, a comprehensive series of tensile tests on WAAM stainless steel was conducted; the results are presented herein. As-built and machined coupons were tested to investigate the influence of the geometrical irregularity on the stress-strain characteristics, while material anisotropy was explored by testing coupons produced at different angles to the printing orientation. Non-contact measurement techniques were employed to determine the geometric properties and deformation fields of the specimens, while sophisticated analysis methods were used for post processing the test data. The material response revealed a significant degree of anisotropy, explained by the existence of a strong crystallographic texture, uncovered by means of electron backscatter diffraction. Finally, the effective mechanical properties of the as-built material were shown to be strongly dependent on the geometric variability; simple geometric measures were therefore developed to characterise the key aspects of the observed behaviour.

Journal article

Wadee MA, Phillips ATM, Bekele A, 2020, Effects of disruptive inclusions in sandwich core lattices to enhance energy absorbency and structural isolation performance, Frontiers in Materials, Vol: 7, ISSN: 2296-8016

The energy absorption and structural isolation performance of axially-compressed sandwich structures constructed with stiff face plates separated with an auxetic lattice core metamaterial is studied. Advances in additive manufacturing increasingly allow bespoke, carefully designed, structures to be included within the core lattice to enhance mechanical performance. Currently, the internal structure of the lattice core is deliberately disrupted geometrically to engineer suitable post-buckling behaviour under quasi-static loading. The desirable properties of a high fundamental stiffness and a practically zero underlying stiffness in the post-buckling range ensure that energy may be absorbed within a limited displacement and that any transfer of strain to an attached structure is minimized as far as is feasible. It is demonstrated that such disruptions can be arranged to enhance the panel performance. The concept may be extended to promote cellular buckling where the internal lattice buckles with densification occurring at defined locations and in sequence to absorb energy while maintaining a low underlying mechanical stiffness.

Journal article

Wu K, Wadee MA, Gardner L, 2020, Interactive buckling in prestressed stayed beam-columns, International Journal of Mechanical Sciences, Vol: 174, ISSN: 0020-7403

The instability of beam-columns with crossarms and externally prestressed cable stays is studied analytically, where the combination of bending and compression is assumed to be derived from the system self-weight acting orthogonally to the applied axial load. A nonlinear analytical model for prestressed stayed beam-columns with doubly-symmetric and mono-symmetric configurations, based on the Rayleigh–Ritz method, is presented that captures modal interactions for perfect geometries explicitly for the first time. It is demonstrated that the theoretical compressive strength enhancements under certain configurations can only be obtained at the expense of triggering a sequence of destabilizing bifurcations. This can give rise to severely unstable interactive post-buckling behaviour including the so-called ‘mode jumping’ phenomenon. Moreover, for mono-symmetric stayed beam-columns, it is shown that the varying levels of prestress within the stays can lead to different buckling modes which all have their own characteristic post-buckling responses. The analytical model is verified using a nonlinear finite element model formulated within the commercial code ABAQUS and excellent comparisons are obtained.

Journal article

Wadee MA, Hadjipantelis N, Bazzano JB, Gardner L, Lozano-Galant JAet al., 2020, Stability of steel struts with externally anchored prestressed cables, Journal of Constructional Steel Research, Vol: 164, ISSN: 0143-974X

Externally anchored prestressed cables can be employed to enhance the stability of steel truss compression elements significantly. To demonstrate this concept, a system comprising a tubular strut subjected to an external compressive load and a prestressed cable anchored independently of the strut is studied. Energy methods are utilized to define the elastic stability of the perfect and imperfect systems, after which the first yield and rigid-plastic responses are explored. The influence of the key controlling parameters, including the length of the strut, the axial stiffness of the cable and the initial prestressing force, on the elastic stability, the inelastic response and the ultimate strength of the system is demonstrated using analytical and finite element (FE) models. To illustrate the application of the studied structural concept, FE modelling is employed to simulate the structural response of a prestressed hangar roof truss. A nearly two-fold enhancement in the load-carrying capacity of the truss structure is shown to be achieved owing to the addition of the prestressed cable.

Journal article

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00172848&limit=30&person=true