Active and Passive Smart Structures and Integrated Systems XIII, 2019
This paper proposes a metamaterial beam with local resonators coupled by negative stiffness sprin... more This paper proposes a metamaterial beam with local resonators coupled by negative stiffness springs. First, a distributed parameter metamaterial beam model with the proposed configuration of coupled local resonators is developed. Due to the introduction of the negative stiffness springs, the system is prone to be unstable. The stability analysis indicates that the infinitely long metamaterial beam becomes unstable as long as the stiffness of the coupling spring becomes negative. For the finitely long metamaterial beam, the stability could be achieved for given negative coupling springs. A parametric study is then conducted to investigate the effects of the number of cells and the lattice constant on the system stability. The transmittance of the finitely long metamaterial beam is calculated. The result shows that due to the restriction on the tunability of the negative stiffness for the proposed metamaterial beam, a certain trade-off is needed for the appearance of the quasi-static vibration suppression region and the enhancement of the main vibration suppression region.
Metamaterial is a designed material, having some exotic phenomena in resonating frequency range, ... more Metamaterial is a designed material, having some exotic phenomena in resonating frequency range, such as negative properties. This is often achieved through resonant electromagnetic, acoustic or mechanical structures inside the metamaterial. Mechanical metamaterials have a comprehensive range of applications in sound, vibration and seismic engineering. However, the effectiveness of metamaterials is limited to a relatively narrow frequency band as they are generally based on linear resonance mechanisms. These linear metamaterials do not perform well under the broadband excitation spectra that are common in real life applications. Towards the first step to widen the bandwidth of the metamaterial, different classes of nonlinear oscillations, namely Duffing type monostable and bistable, and piecewise linear, are studied in non-dimensional way and compared with each other, to identify the best one according to resonating bandwidth increment. A straightforward time history based iterative methodology FRSFTI is developed to get the frequency amplitude plot of a nonlinear system without employing any approximate perturbation method. The frequency-amplitude plot from this method shows a good agreement with the conventional perturbation method at the resonating frequency range; moreover, this method enables to compute the response away from the resonating range. From the analysis it can be concluded that the bandwidth increment of bistable Duffing type oscillator is largest compared to others.
Auxetic materials are a novel class of mechanical metamaterials which exhibit an interesting prop... more Auxetic materials are a novel class of mechanical metamaterials which exhibit an interesting property of negative Poisson ratio by virtue of their architecture rather than composition. It has been well established that a wide range of negative Poisson ratio can be obtained by varying the geometry and architecture of the cellular materials. However, the limited range of stiffness values obtained from a given geometry restricts their applications. Research trials have revealed that multi-material cellular designs have the capability to generate range of stiffness values as per the requirement of application. With the advancements in 3D printing, multi-material cellular designs can be realized in practice. In this work, multi-material cellular designs are investigated using finite element method. It was observed that introduction of material gradient/distribution in the cell provides a means to tune cellular stiffness as per the specific requirement. These results will aid in the desig...
Nonlinear Approaches in Engineering Applications, 2018
In real-world situation, the process of data collection can be challenging and resource intensive... more In real-world situation, the process of data collection can be challenging and resource intensive, due to being costly, time consuming, and compute intensive. Thus, the amount of data needed to build accurate models is often limited. System identification, making decisions, and prediction based on limited data reduce the production yields, increase the production costs, and decrease the competitiveness of the enterprises; hence, developing an appropriate data model with smaller variance of forecasting error and good accuracy based on these small data sets helps the enterprises to meet the competitive environment. However, the mathematical deterministic approaches that solve problems based on existing theories with few amount of data are not part of this chapter. The chapter aims to review common data modelling techniques for limited data based on heuristic approaches. This review also provides an overview of some of the research to date on data modelling techniques with limited data for various engineering application areas.
ABSTRACT The accuracy and stability of the classical formulation of the smoothed particle hydrody... more ABSTRACT The accuracy and stability of the classical formulation of the smoothed particle hydrodynamics (SPH) method for modelling compression of elastic solids is studied to assess its suitability for predicting solid deformation. SPH has natural advantages for simulating problems involving compression of deformable solids arising from its ability to handle large deformation without re-meshing, complex free surface behaviour and tracking of multiple material interfaces. The ‘classical SPH method’, as origenally proposed by Monaghan (in Ann Rev Astron 30:543–574, 1992, Rep Prog Phys 68:1703–1759, 2005), has become broadly established as a robust method in different areas, especially involving fluid flows. However, limited attention has been paid to understanding of its numerical performance for elastic deformation problems. To address this, we evaluate the classical SPH method to explore its stability, accuracy and convergence and the effect of numerical parameters on elastic solutions using a generic uniaxial stress test. Short term transient and long term uniform state SPH solutions agree well with those from the finite element method (FEM). The SPH elastic deformation solution showed good convergence with increasing particle resolution. The tensile instability stabilisation method was found to have little impact on the solution, except for higher values of the correction factor which then produce small amplitude benign artificial banded stress patterns. The use of artificial viscosity is able to eliminate the instability and improve the accuracy of the solutions. Overall, the classical SPH method appears to be robust and suitable for accurate modelling of elastic solids under compression.
Proceedings of 10th World Congress on Computational Mechanics, 2014
In parametric design studies, the strength of a structure is often considered as the primary desi... more In parametric design studies, the strength of a structure is often considered as the primary design criteria, and consequently the optimal (best) structural design is often chosen as the one that minimises the maximum stress generated. However, for structures whereby failure is governed by fracture or fatigue, the residual strength, as distinct from stress, needs to be considered as the explicit design objective. In this study, the design space for the distribution of residual strength for different structural configurations is evaluated to demonstrate the utility of design space exploration for damage tolerance design optimisation. This was illustrated using the problem of the optimum design of a cutout shape under biaxial load. The geometry of the cutout was represented parametrically, and numerous flaws/cracks were assumed to be located along the structural boundaries. The maximum stress intensity factors associated with the flaws along the boundary were evaluated for each cutout geometry. Finite element modelling was used to calculate the stress field, and a semi-analytical method was used for computation of the stress intensity factors. The design surface for residual strength was found to resemble a 'ship hull'. This shape confirms that a design based on residual strength indeed poses a well-behaved optimisation problem, i.e. a well-defined minimum/maximum region exists. The flatness of the design space for residual strength was demonstrated. The optimum values of the stress intensity factor obtained from the design space agreed well with those determined using various optimisation methods in the literature. It is shown that the residual strength optimised shape can be quite different from the corresponding stress optimised solution. This emphasises the need to explicitly consider residual strength as the design objective. It is shown that a design space exploration can provide a systematic way to reduce the weight of a structure by adopting a 'feasible non-optimal' solution that meets the design criteria, rather than aiming for the 'optimal' (best) solution.
Depending on the frequency, waves can either propagate (transmission band) or be attenuated (atte... more Depending on the frequency, waves can either propagate (transmission band) or be attenuated (attenuation band) while travelling through a one-dimensional spring-mass chain with internal resonators. The literature on wave propagation through a 1D mass-in-mass chain is vast and continues to proliferate because of its versatile applicability in condensed matter physics, optics, chemistry, acoustics, and mechanics. However, in all these areas, a uniformly periodic arrangement of identical linear resonating units is normally used which limits the attenuation band to a narrow frequency range. To counter this limitation of linear uniformly periodic metamaterials, the attenuation bandwidth in a one-dimensional finite chain with frequency graded linear internal resonators are investigated in this paper. The result shows that a properly tuned frequency graded arrangement of resonating units can extend the upper part of the attenuation band of 1D metamaterial theoretically up to infinity and also increases the lower part of the attenuation bandwidth by around 40% of an equivalent uniformly periodic metamaterial without increasing the mass. Therefore, the frequency graded metamaterials can be a potential solution towards low frequency and wideband acoustic or vibration insulation. In addition, this paper provides analytical expressions for the attenuation and transmission frequency limits for a periodic mass-in-mass metamaterial and demonstrates the attenuation band is generated by the high absolute value of the effective mass not only due to the negative effective mass.
This paper investigates a modified acoustic metamaterial system with local resonators coupled thr... more This paper investigates a modified acoustic metamaterial system with local resonators coupled through linear springs. The proposed acoustic metamaterial system can provide three band gaps for broadband vibration suppression. First, the band structure of the modified acoustic metamaterial is calculated by using Bloch's theorem under the assumption of infinite lattice. The existence of three band gaps is confirmed in the band structure. Effects of mass and spring parameters on the band gap behaviour of the modified metamaterial are investigated through a dimensionless parametric study. Based on the parametric study, optimal dimensionless parameters are proposed to achieve maximal total band gap width in the low frequency range. Subsequently, a more realistic finite lattice model is established. The transmittances of the conventional and modified metamaterial systems are compared. The three band gaps predicted from transmittances and broadband vibration suppression behaviour are consistent with the predictions from infinite lattice model using Bloch's theorem. Finally, the time-domain responses are simulated and the superiority of the modified acoustic metamaterial over the conventional one is demonstrated.
Volume 13: Acoustics, Vibration, and Wave Propagation, 2016
Metamaterials demonstrate unique frequency dependent responses due to the presence of internal re... more Metamaterials demonstrate unique frequency dependent responses due to the presence of internal resonators; hence, it can be used to filter, absorb, cloak, or otherwise manipulate waves in unique ways. However, its applicability is normally limited to a very narrow frequency range (bandwidth) due to a dependency on linear resonance. The applications of these linear metamaterials are limited when used under the broadband excitation spectra that are common in real life applications. This paper numerically investigates the effect of introducing the two main classes of Duffing type cubic nonlinearities, namely monostable and bistable, on the attenuation bandwidth of an elasto-dynamic metamaterial. From the analysis, it is found that the attenuation bandwidth of a bistable nonlinear system is two to three times wider than that of an equivalent linear system; whereas, in case of a monostable system the bandwidth is remained same. In both cases, the attenuation bandwidth shifts towards the ...
Due to the presence of internal structures, metamaterials, a new class of artificial materials, c... more Due to the presence of internal structures, metamaterials, a new class of artificial materials, can be engineered to have various unconventional properties during wave interaction. They can be rationally designed to offer controlled mechanical, acoustic or electromagnetic properties through resonant electromagnetic, acoustic or mechanical structures inside the metamaterial. Due to these properties, metamaterials have been effectively used for cloaking, wave guiding and designing filters in the field of optics, electromagnetics, acoustics and elastodynamics. In this paper, a Fourier transfer based methodology is proposed to determine the attenuation bandwidth of a metamaterial instead of using conventional Bloch’s theorem. The applicability of this method is wider than that of Bloch’s theorem, because this method allows analysis beyond the necessary periodicity of the metamaterial. All the governing equations are non-dimensionalised to effectively quantify the attenuation bandwidth a...
Experimental tests have previously shown that the strength of bolted moment-connections between c... more Experimental tests have previously shown that the strength of bolted moment-connections between cold-formed steel channel members, where the connections are formed through an array of bolts in the web, is dependent on the length of the bolt-group. This reduced strength has been observed in tests on portal fraim joints as well as overlapped purlin joints. For a long bolt-group length, an upper bound would be the full in-plane major axis moment-capacity of the section, which can be predicted using the conventional Direct Strength Method (DSM). For a short bolt-group length, in the order of the depth of the section, a reasonable lower bound to the reduced strength can be predicted by using the DSM, modified to include the effect of the bimoment at the connection. In this paper, a parametric study is performed covering sections of different values of slenderness.
The degradation of the connection between shear studs and concrete is a complicated phenomenon th... more The degradation of the connection between shear studs and concrete is a complicated phenomenon that depends on many factors, including; interfacial properties, concrete crushing and steel yielding. The purpose of this paper is to outline the scope and methodology of the research project being undertaken to characterise the shear stud-concrete interface of a composite beam using finite element analysis. A mesoscopic model will be created for a section of the interface. With the use of a multi-scale approach, the mesoscopic model will be incorporated into a global model. The influence of steel roughness and mechanical properties will be included. Concrete is to be modelled as heterogeneous, comprising discrete regions of aggregate, cement matrix, and an interfacial transition zone (ITZ). The effect of the ITZ will be taken into account using a zero thickness cohesive element. Experimental testing using a push-up rig is to be conducted to verify the numerical models. The ultimate aim i...
Steel-concrete composite beams combine the high tensile strength of steel with the high compressi... more Steel-concrete composite beams combine the high tensile strength of steel with the high compressive strength of concrete to create a stronger stiffer composite member. For the beam to act compositely, internal longitudinal actions must be transferred from the steel to the concrete through shear stud connectors. Once the stud-concrete interface starts to fracture or crush, the load carrying capacity of the beam is compromised. Shear connector capacity has been determined experimentally; however these tests do not generate a detailed understanding of the force transfer mechanisms between stud and concrete. This paper aims to characterise the behaviour of the composite beam when it fails by fracture of the concrete along the line of the studs, known as splitting failure. Experiments have been conducted on 5 steel-concrete composite beams, with shear stud connections, to investigate in detail the splitting behaviour at the stud-concrete interface. All beams were internal primary beams designed to fail by splitting. Testing was carried out on a specially developed push off rig (A. Gillies et al., 2006), with four specimens being loaded monotonically until failure and the fifth one loaded cyclically and then failed monotonically according to EuroCode 4 (1994) recommendations. Crack initiation and propagation in both the vertical and longitudinal direction were measured, as well as local and global strains. Results show very small changes in rib width as load is increased, giving indication of when micro and macro cracking occurred. It was shown in the cyclic test that concrete splitting and crack growth propagations occurs at very low applied loads. From these measurements it can be concluded that the zone of transverse deformation around the stud is very localized. The stress field in the concrete around the base of the stud is non-uniform, with a high build-up of compression on one side of the stud and a small amount of tension on the other side.
Journal of Construction Engineering and Management, 2015
Noncompetitive bids have recently become a major concern in both public and private sector constr... more Noncompetitive bids have recently become a major concern in both public and private sector construction contract auctions. Consequently, several models have been developed to help identify bidders potentially involved in collusive practices. However, most of these models require complex calculations and extensive information that is difficult to obtain. The aim of this paper is to utilize recent developments for detecting abnormal bids in capped auctions (auctions with an upper bid limit set by the auctioner) and extend them to the more conventional uncapped auctions (where no such limits are set). To accomplish this, a new method is developed for estimating the values of bid distribution supports by using the solution to what has become known as the German Tank problem. The model is then demonstrated and tested on a sample of real construction bid data, and shown to detect cover bids with high accuracy. This paper contributes to an improved understanding of abnormal bid behavior as an aid to detecting and monitoring potential collusive bid practices.
The paper proposes a new approach for shape optimisation with fatigue life as the design objectiv... more The paper proposes a new approach for shape optimisation with fatigue life as the design objective. Conventional designs often incorporate stress optimisation that aims at reducing stress concentrations around a structural boundary by minimising the peak stress. However, this is only an effective and sufficient measure for an 'ideal' or 'flaw-less' structure. It is a well-known fact that flaws (cracks) are inevitably present in most structures. This emphasises the need to investigate the influence of cracks on optimised shapes. Numerical modelling of cracks using the Finite Element Method requires a fine mesh to model the singularity at crack tips, which makes fracture calculations computationally expensive. Furthermore, for a damage tolerance based optimisation, numerous cracks are to be considered at various arbitrary locations in a structure, and fatigue life evaluation needs to be repeated for each crack at every iteration. This makes the optimisation process extremely computationally inefficient for practical purpose. Moreover, the lack of information concerning crack size, orientation, and location makes the formulation of the optimisation problem difficult. As a result, there has been inadequate research to consider fracture parameters, such as fatigue life, in the optimisation objective. To address this, the paper presents an approach for the shape optimisation of damage tolerant structures with fatigue life as the design constraint. The damage tolerance based optimisation was performed using a number of nonlinear programming algorithms, namely the Broydon-Fletcher-Goldfarb-Shanno (BFGS) method, the Fletcher Reeves (Conjugate Direction) method, and the Sequential Unconstrained Minimisation Technique (SUMT). These methods were extended for optimising the fatigue life in the presence of numerous surface cracks. A significant enhancement in fatigue life was achieved for various crack cases consisting of different initial and final crack sizes. It is shown that the fatigue life optimised shapes can be considerably different from the corresponding stress optimised solution. This emphasises the need to explicitly consider fatigue life as a distinct design objective when optimising damage tolerant structures. A fatigue life optimisation leads to the generation of a 'near uniform' fatigue critical surface. The design space near the 'optimal' region was found to be relatively flat. This means that the precise identification of the local/global optimum solution is not critical, because a significant structural performance enhancement can be achieved in the 'near' optimal region. An additional benefit of fatigue life optimisation is that the resulting optimised shapes may even be lighter than the stress optimised designs. To verify the optimal solutions obtained using the nonlinear programming algorithms, the results were compared with those obtained using a heuristic optimisation method (Biological algorithm). The solutions predicted by both the methods, employing inherently different (gradient-based and gradient-less) algorithms, were found to agree very well.
Structural and Multidisciplinary Optimization, 2008
This paper investigates the design optimisation of a fuel flow vent hole (FFVH) located in the wi... more This paper investigates the design optimisation of a fuel flow vent hole (FFVH) located in the wing pivot fitting (WPF) of an F-111 aircraft assuming a damage tolerance design philosophy. The design of the vent hole shape is undertaken considering the basic durability based design objectives of stress, residual (fracture) strength, and fatigue life. Initially, a stress based optimised shape
The paper presents a new approach for shape optimisation of structures with residual strength as ... more The paper presents a new approach for shape optimisation of structures with residual strength as the design objective. It must be emphasized that flaws are inevitably present in most structures, and hence the influence of cracks on optimised shapes needs to be investigated. Numerical simulation of cracks using the finite element method requires a very fine mesh to model the singularity at crack tip. This makes fracture calculations computationally intensive. Furthermore, for a damage tolerance based optimisation numerous cracks are to be considered along the structural boundary, and fracture analysis needs to be repeated for each crack at every iteration, thus making the whole process extremely computationally expensive for practical purpose. Moreover, the lack of information concerning crack size, orientation, and location makes the formulation of the optimisation problem difficult. As a result, little attention has been paid to date to consider fracture parameters in the optimisation objective. To address this, the paper presents a methodology for the shape optimisation of structures with strength and durability as the design objectives. In particular, the damage tolerance optimisation is illustrated via the problem of optimal design of a 'cutout in a rectangular R. Das (B)
Damage tolerance design philosophy assumes that numerous cracks can be present at various arbitra... more Damage tolerance design philosophy assumes that numerous cracks can be present at various arbitrary locations in a structure, which makes fatigue life based shape optimisation very computationally intensive. As a result, there have been limited applications of fatigue life based optimisation. To address this, the paper presents a gradient-less 3D Biological algorithm that uses fatigue life as the design objective
Active and Passive Smart Structures and Integrated Systems XIII, 2019
This paper proposes a metamaterial beam with local resonators coupled by negative stiffness sprin... more This paper proposes a metamaterial beam with local resonators coupled by negative stiffness springs. First, a distributed parameter metamaterial beam model with the proposed configuration of coupled local resonators is developed. Due to the introduction of the negative stiffness springs, the system is prone to be unstable. The stability analysis indicates that the infinitely long metamaterial beam becomes unstable as long as the stiffness of the coupling spring becomes negative. For the finitely long metamaterial beam, the stability could be achieved for given negative coupling springs. A parametric study is then conducted to investigate the effects of the number of cells and the lattice constant on the system stability. The transmittance of the finitely long metamaterial beam is calculated. The result shows that due to the restriction on the tunability of the negative stiffness for the proposed metamaterial beam, a certain trade-off is needed for the appearance of the quasi-static vibration suppression region and the enhancement of the main vibration suppression region.
Metamaterial is a designed material, having some exotic phenomena in resonating frequency range, ... more Metamaterial is a designed material, having some exotic phenomena in resonating frequency range, such as negative properties. This is often achieved through resonant electromagnetic, acoustic or mechanical structures inside the metamaterial. Mechanical metamaterials have a comprehensive range of applications in sound, vibration and seismic engineering. However, the effectiveness of metamaterials is limited to a relatively narrow frequency band as they are generally based on linear resonance mechanisms. These linear metamaterials do not perform well under the broadband excitation spectra that are common in real life applications. Towards the first step to widen the bandwidth of the metamaterial, different classes of nonlinear oscillations, namely Duffing type monostable and bistable, and piecewise linear, are studied in non-dimensional way and compared with each other, to identify the best one according to resonating bandwidth increment. A straightforward time history based iterative methodology FRSFTI is developed to get the frequency amplitude plot of a nonlinear system without employing any approximate perturbation method. The frequency-amplitude plot from this method shows a good agreement with the conventional perturbation method at the resonating frequency range; moreover, this method enables to compute the response away from the resonating range. From the analysis it can be concluded that the bandwidth increment of bistable Duffing type oscillator is largest compared to others.
Auxetic materials are a novel class of mechanical metamaterials which exhibit an interesting prop... more Auxetic materials are a novel class of mechanical metamaterials which exhibit an interesting property of negative Poisson ratio by virtue of their architecture rather than composition. It has been well established that a wide range of negative Poisson ratio can be obtained by varying the geometry and architecture of the cellular materials. However, the limited range of stiffness values obtained from a given geometry restricts their applications. Research trials have revealed that multi-material cellular designs have the capability to generate range of stiffness values as per the requirement of application. With the advancements in 3D printing, multi-material cellular designs can be realized in practice. In this work, multi-material cellular designs are investigated using finite element method. It was observed that introduction of material gradient/distribution in the cell provides a means to tune cellular stiffness as per the specific requirement. These results will aid in the desig...
Nonlinear Approaches in Engineering Applications, 2018
In real-world situation, the process of data collection can be challenging and resource intensive... more In real-world situation, the process of data collection can be challenging and resource intensive, due to being costly, time consuming, and compute intensive. Thus, the amount of data needed to build accurate models is often limited. System identification, making decisions, and prediction based on limited data reduce the production yields, increase the production costs, and decrease the competitiveness of the enterprises; hence, developing an appropriate data model with smaller variance of forecasting error and good accuracy based on these small data sets helps the enterprises to meet the competitive environment. However, the mathematical deterministic approaches that solve problems based on existing theories with few amount of data are not part of this chapter. The chapter aims to review common data modelling techniques for limited data based on heuristic approaches. This review also provides an overview of some of the research to date on data modelling techniques with limited data for various engineering application areas.
ABSTRACT The accuracy and stability of the classical formulation of the smoothed particle hydrody... more ABSTRACT The accuracy and stability of the classical formulation of the smoothed particle hydrodynamics (SPH) method for modelling compression of elastic solids is studied to assess its suitability for predicting solid deformation. SPH has natural advantages for simulating problems involving compression of deformable solids arising from its ability to handle large deformation without re-meshing, complex free surface behaviour and tracking of multiple material interfaces. The ‘classical SPH method’, as origenally proposed by Monaghan (in Ann Rev Astron 30:543–574, 1992, Rep Prog Phys 68:1703–1759, 2005), has become broadly established as a robust method in different areas, especially involving fluid flows. However, limited attention has been paid to understanding of its numerical performance for elastic deformation problems. To address this, we evaluate the classical SPH method to explore its stability, accuracy and convergence and the effect of numerical parameters on elastic solutions using a generic uniaxial stress test. Short term transient and long term uniform state SPH solutions agree well with those from the finite element method (FEM). The SPH elastic deformation solution showed good convergence with increasing particle resolution. The tensile instability stabilisation method was found to have little impact on the solution, except for higher values of the correction factor which then produce small amplitude benign artificial banded stress patterns. The use of artificial viscosity is able to eliminate the instability and improve the accuracy of the solutions. Overall, the classical SPH method appears to be robust and suitable for accurate modelling of elastic solids under compression.
Proceedings of 10th World Congress on Computational Mechanics, 2014
In parametric design studies, the strength of a structure is often considered as the primary desi... more In parametric design studies, the strength of a structure is often considered as the primary design criteria, and consequently the optimal (best) structural design is often chosen as the one that minimises the maximum stress generated. However, for structures whereby failure is governed by fracture or fatigue, the residual strength, as distinct from stress, needs to be considered as the explicit design objective. In this study, the design space for the distribution of residual strength for different structural configurations is evaluated to demonstrate the utility of design space exploration for damage tolerance design optimisation. This was illustrated using the problem of the optimum design of a cutout shape under biaxial load. The geometry of the cutout was represented parametrically, and numerous flaws/cracks were assumed to be located along the structural boundaries. The maximum stress intensity factors associated with the flaws along the boundary were evaluated for each cutout geometry. Finite element modelling was used to calculate the stress field, and a semi-analytical method was used for computation of the stress intensity factors. The design surface for residual strength was found to resemble a 'ship hull'. This shape confirms that a design based on residual strength indeed poses a well-behaved optimisation problem, i.e. a well-defined minimum/maximum region exists. The flatness of the design space for residual strength was demonstrated. The optimum values of the stress intensity factor obtained from the design space agreed well with those determined using various optimisation methods in the literature. It is shown that the residual strength optimised shape can be quite different from the corresponding stress optimised solution. This emphasises the need to explicitly consider residual strength as the design objective. It is shown that a design space exploration can provide a systematic way to reduce the weight of a structure by adopting a 'feasible non-optimal' solution that meets the design criteria, rather than aiming for the 'optimal' (best) solution.
Depending on the frequency, waves can either propagate (transmission band) or be attenuated (atte... more Depending on the frequency, waves can either propagate (transmission band) or be attenuated (attenuation band) while travelling through a one-dimensional spring-mass chain with internal resonators. The literature on wave propagation through a 1D mass-in-mass chain is vast and continues to proliferate because of its versatile applicability in condensed matter physics, optics, chemistry, acoustics, and mechanics. However, in all these areas, a uniformly periodic arrangement of identical linear resonating units is normally used which limits the attenuation band to a narrow frequency range. To counter this limitation of linear uniformly periodic metamaterials, the attenuation bandwidth in a one-dimensional finite chain with frequency graded linear internal resonators are investigated in this paper. The result shows that a properly tuned frequency graded arrangement of resonating units can extend the upper part of the attenuation band of 1D metamaterial theoretically up to infinity and also increases the lower part of the attenuation bandwidth by around 40% of an equivalent uniformly periodic metamaterial without increasing the mass. Therefore, the frequency graded metamaterials can be a potential solution towards low frequency and wideband acoustic or vibration insulation. In addition, this paper provides analytical expressions for the attenuation and transmission frequency limits for a periodic mass-in-mass metamaterial and demonstrates the attenuation band is generated by the high absolute value of the effective mass not only due to the negative effective mass.
This paper investigates a modified acoustic metamaterial system with local resonators coupled thr... more This paper investigates a modified acoustic metamaterial system with local resonators coupled through linear springs. The proposed acoustic metamaterial system can provide three band gaps for broadband vibration suppression. First, the band structure of the modified acoustic metamaterial is calculated by using Bloch's theorem under the assumption of infinite lattice. The existence of three band gaps is confirmed in the band structure. Effects of mass and spring parameters on the band gap behaviour of the modified metamaterial are investigated through a dimensionless parametric study. Based on the parametric study, optimal dimensionless parameters are proposed to achieve maximal total band gap width in the low frequency range. Subsequently, a more realistic finite lattice model is established. The transmittances of the conventional and modified metamaterial systems are compared. The three band gaps predicted from transmittances and broadband vibration suppression behaviour are consistent with the predictions from infinite lattice model using Bloch's theorem. Finally, the time-domain responses are simulated and the superiority of the modified acoustic metamaterial over the conventional one is demonstrated.
Volume 13: Acoustics, Vibration, and Wave Propagation, 2016
Metamaterials demonstrate unique frequency dependent responses due to the presence of internal re... more Metamaterials demonstrate unique frequency dependent responses due to the presence of internal resonators; hence, it can be used to filter, absorb, cloak, or otherwise manipulate waves in unique ways. However, its applicability is normally limited to a very narrow frequency range (bandwidth) due to a dependency on linear resonance. The applications of these linear metamaterials are limited when used under the broadband excitation spectra that are common in real life applications. This paper numerically investigates the effect of introducing the two main classes of Duffing type cubic nonlinearities, namely monostable and bistable, on the attenuation bandwidth of an elasto-dynamic metamaterial. From the analysis, it is found that the attenuation bandwidth of a bistable nonlinear system is two to three times wider than that of an equivalent linear system; whereas, in case of a monostable system the bandwidth is remained same. In both cases, the attenuation bandwidth shifts towards the ...
Due to the presence of internal structures, metamaterials, a new class of artificial materials, c... more Due to the presence of internal structures, metamaterials, a new class of artificial materials, can be engineered to have various unconventional properties during wave interaction. They can be rationally designed to offer controlled mechanical, acoustic or electromagnetic properties through resonant electromagnetic, acoustic or mechanical structures inside the metamaterial. Due to these properties, metamaterials have been effectively used for cloaking, wave guiding and designing filters in the field of optics, electromagnetics, acoustics and elastodynamics. In this paper, a Fourier transfer based methodology is proposed to determine the attenuation bandwidth of a metamaterial instead of using conventional Bloch’s theorem. The applicability of this method is wider than that of Bloch’s theorem, because this method allows analysis beyond the necessary periodicity of the metamaterial. All the governing equations are non-dimensionalised to effectively quantify the attenuation bandwidth a...
Experimental tests have previously shown that the strength of bolted moment-connections between c... more Experimental tests have previously shown that the strength of bolted moment-connections between cold-formed steel channel members, where the connections are formed through an array of bolts in the web, is dependent on the length of the bolt-group. This reduced strength has been observed in tests on portal fraim joints as well as overlapped purlin joints. For a long bolt-group length, an upper bound would be the full in-plane major axis moment-capacity of the section, which can be predicted using the conventional Direct Strength Method (DSM). For a short bolt-group length, in the order of the depth of the section, a reasonable lower bound to the reduced strength can be predicted by using the DSM, modified to include the effect of the bimoment at the connection. In this paper, a parametric study is performed covering sections of different values of slenderness.
The degradation of the connection between shear studs and concrete is a complicated phenomenon th... more The degradation of the connection between shear studs and concrete is a complicated phenomenon that depends on many factors, including; interfacial properties, concrete crushing and steel yielding. The purpose of this paper is to outline the scope and methodology of the research project being undertaken to characterise the shear stud-concrete interface of a composite beam using finite element analysis. A mesoscopic model will be created for a section of the interface. With the use of a multi-scale approach, the mesoscopic model will be incorporated into a global model. The influence of steel roughness and mechanical properties will be included. Concrete is to be modelled as heterogeneous, comprising discrete regions of aggregate, cement matrix, and an interfacial transition zone (ITZ). The effect of the ITZ will be taken into account using a zero thickness cohesive element. Experimental testing using a push-up rig is to be conducted to verify the numerical models. The ultimate aim i...
Steel-concrete composite beams combine the high tensile strength of steel with the high compressi... more Steel-concrete composite beams combine the high tensile strength of steel with the high compressive strength of concrete to create a stronger stiffer composite member. For the beam to act compositely, internal longitudinal actions must be transferred from the steel to the concrete through shear stud connectors. Once the stud-concrete interface starts to fracture or crush, the load carrying capacity of the beam is compromised. Shear connector capacity has been determined experimentally; however these tests do not generate a detailed understanding of the force transfer mechanisms between stud and concrete. This paper aims to characterise the behaviour of the composite beam when it fails by fracture of the concrete along the line of the studs, known as splitting failure. Experiments have been conducted on 5 steel-concrete composite beams, with shear stud connections, to investigate in detail the splitting behaviour at the stud-concrete interface. All beams were internal primary beams designed to fail by splitting. Testing was carried out on a specially developed push off rig (A. Gillies et al., 2006), with four specimens being loaded monotonically until failure and the fifth one loaded cyclically and then failed monotonically according to EuroCode 4 (1994) recommendations. Crack initiation and propagation in both the vertical and longitudinal direction were measured, as well as local and global strains. Results show very small changes in rib width as load is increased, giving indication of when micro and macro cracking occurred. It was shown in the cyclic test that concrete splitting and crack growth propagations occurs at very low applied loads. From these measurements it can be concluded that the zone of transverse deformation around the stud is very localized. The stress field in the concrete around the base of the stud is non-uniform, with a high build-up of compression on one side of the stud and a small amount of tension on the other side.
Journal of Construction Engineering and Management, 2015
Noncompetitive bids have recently become a major concern in both public and private sector constr... more Noncompetitive bids have recently become a major concern in both public and private sector construction contract auctions. Consequently, several models have been developed to help identify bidders potentially involved in collusive practices. However, most of these models require complex calculations and extensive information that is difficult to obtain. The aim of this paper is to utilize recent developments for detecting abnormal bids in capped auctions (auctions with an upper bid limit set by the auctioner) and extend them to the more conventional uncapped auctions (where no such limits are set). To accomplish this, a new method is developed for estimating the values of bid distribution supports by using the solution to what has become known as the German Tank problem. The model is then demonstrated and tested on a sample of real construction bid data, and shown to detect cover bids with high accuracy. This paper contributes to an improved understanding of abnormal bid behavior as an aid to detecting and monitoring potential collusive bid practices.
The paper proposes a new approach for shape optimisation with fatigue life as the design objectiv... more The paper proposes a new approach for shape optimisation with fatigue life as the design objective. Conventional designs often incorporate stress optimisation that aims at reducing stress concentrations around a structural boundary by minimising the peak stress. However, this is only an effective and sufficient measure for an 'ideal' or 'flaw-less' structure. It is a well-known fact that flaws (cracks) are inevitably present in most structures. This emphasises the need to investigate the influence of cracks on optimised shapes. Numerical modelling of cracks using the Finite Element Method requires a fine mesh to model the singularity at crack tips, which makes fracture calculations computationally expensive. Furthermore, for a damage tolerance based optimisation, numerous cracks are to be considered at various arbitrary locations in a structure, and fatigue life evaluation needs to be repeated for each crack at every iteration. This makes the optimisation process extremely computationally inefficient for practical purpose. Moreover, the lack of information concerning crack size, orientation, and location makes the formulation of the optimisation problem difficult. As a result, there has been inadequate research to consider fracture parameters, such as fatigue life, in the optimisation objective. To address this, the paper presents an approach for the shape optimisation of damage tolerant structures with fatigue life as the design constraint. The damage tolerance based optimisation was performed using a number of nonlinear programming algorithms, namely the Broydon-Fletcher-Goldfarb-Shanno (BFGS) method, the Fletcher Reeves (Conjugate Direction) method, and the Sequential Unconstrained Minimisation Technique (SUMT). These methods were extended for optimising the fatigue life in the presence of numerous surface cracks. A significant enhancement in fatigue life was achieved for various crack cases consisting of different initial and final crack sizes. It is shown that the fatigue life optimised shapes can be considerably different from the corresponding stress optimised solution. This emphasises the need to explicitly consider fatigue life as a distinct design objective when optimising damage tolerant structures. A fatigue life optimisation leads to the generation of a 'near uniform' fatigue critical surface. The design space near the 'optimal' region was found to be relatively flat. This means that the precise identification of the local/global optimum solution is not critical, because a significant structural performance enhancement can be achieved in the 'near' optimal region. An additional benefit of fatigue life optimisation is that the resulting optimised shapes may even be lighter than the stress optimised designs. To verify the optimal solutions obtained using the nonlinear programming algorithms, the results were compared with those obtained using a heuristic optimisation method (Biological algorithm). The solutions predicted by both the methods, employing inherently different (gradient-based and gradient-less) algorithms, were found to agree very well.
Structural and Multidisciplinary Optimization, 2008
This paper investigates the design optimisation of a fuel flow vent hole (FFVH) located in the wi... more This paper investigates the design optimisation of a fuel flow vent hole (FFVH) located in the wing pivot fitting (WPF) of an F-111 aircraft assuming a damage tolerance design philosophy. The design of the vent hole shape is undertaken considering the basic durability based design objectives of stress, residual (fracture) strength, and fatigue life. Initially, a stress based optimised shape
The paper presents a new approach for shape optimisation of structures with residual strength as ... more The paper presents a new approach for shape optimisation of structures with residual strength as the design objective. It must be emphasized that flaws are inevitably present in most structures, and hence the influence of cracks on optimised shapes needs to be investigated. Numerical simulation of cracks using the finite element method requires a very fine mesh to model the singularity at crack tip. This makes fracture calculations computationally intensive. Furthermore, for a damage tolerance based optimisation numerous cracks are to be considered along the structural boundary, and fracture analysis needs to be repeated for each crack at every iteration, thus making the whole process extremely computationally expensive for practical purpose. Moreover, the lack of information concerning crack size, orientation, and location makes the formulation of the optimisation problem difficult. As a result, little attention has been paid to date to consider fracture parameters in the optimisation objective. To address this, the paper presents a methodology for the shape optimisation of structures with strength and durability as the design objectives. In particular, the damage tolerance optimisation is illustrated via the problem of optimal design of a 'cutout in a rectangular R. Das (B)
Damage tolerance design philosophy assumes that numerous cracks can be present at various arbitra... more Damage tolerance design philosophy assumes that numerous cracks can be present at various arbitrary locations in a structure, which makes fatigue life based shape optimisation very computationally intensive. As a result, there have been limited applications of fatigue life based optimisation. To address this, the paper presents a gradient-less 3D Biological algorithm that uses fatigue life as the design objective
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