Dr. Damith Mohotti is a Chartered Structural Engineer with over 20 years of professional experience. He is a Visiting Associate Professor at the University of Wollongong and a Senior Lecturer at UNSW Canberra. Throughout his career, he has actively participated in numerous research and consultation projects. Dr. Damith has established himself as one of Australia’s emerging scientists in critical infrastructure protection within a very small-time frame. He has engaged in many different research projects in this area and has won several project contributions awards. Being a member of the Eureka prize-winning team for safeguarding Australia in 2013 can be considered the highest achievement. Dr Damith is currently recognised as one of the leading researchers in advanced numerical simulations.
Fibrillar dry adhesives are widely used due to their effectiveness in air and vacuum conditions. ... more Fibrillar dry adhesives are widely used due to their effectiveness in air and vacuum conditions. However, their performance depends on various factors. Previous studies have proposed analytical methods to predict adhesion strength on micro-patterned surfaces. However, the method lacks interpretation on which parameters are critical. This research utilizes gradient-boosting machine learning (ML) algorithms to accurately predict adhesion strength. Additionally, explainable machine learning (XML) methods are employed to interpret the underlying reasoning behind the predictions. The analysis demonstrates that gradient boosting models achieve a high correlation coefficient (R > 0.95) in accurately predicting pull-off force on micro-patterned surfaces. The use of XML methods provides insights into the importance of features, their interactions, and their contributions to specific predictions. This novel, explainable, and data-driven approach holds potential for real-time applications, aiding in the identification of critical features that govern the performance of fibrillar adhesives. Furthermore, it improves end-users' confidence by offering human-comprehensible explanations and facilitates understanding among non-technical audiences.
The paper presents a study on the suitability of using a Polymer as a protective layer for critic... more The paper presents a study on the suitability of using a Polymer as a protective layer for critical structural elements prone to impact damage. Conventional damage mitigation methods include using an outer steel jacket with a void, protecting the inner concrete-steel composite column. Whilst it is usually found that the outer steel jacket can partially negate the impact load, the inner column still experiences a significant brunt of the impact load. In order to minimise the damage to the overall critical structural element, a concept of a Polyurethane Polymer-Filled-Concrete-Steel Double-Skin Tubular Column is proposed. Scaled-down versions of two polymerfilled double-skin column configurations were tested under low-velocity impact using a drop tower. The pre and post-damage columns were tested for their axial capacities. These polymer-filled columns were critically compared to their nominal samples without the Polyurethane fill. It was shown that the mid-span deflection and localised indentations were reduced by 17%, and the localised deformations (at the point of impact) were reduced by up to 70% overall. The columns with Polyurethane also showcased retention of 70% of axial capacity post-impact, whilst the nominal sample was only capable of retaining 48%. Finally, the intrinsic properties of Polyurethane were discussed along with a critical discussion on its ability and mechanism on how energy is dissipated by comparing mechanical and chemical properties.
Graphene Oxide (GO) has been emerged as one of the most trending graphene derivative nanomaterial... more Graphene Oxide (GO) has been emerged as one of the most trending graphene derivative nanomaterials in the construction industry. Because of the presence of oxygenated functionalities attached to the aromatic structure, GO can be easily dispersed under an aqueous solution when compared to other graphene-based derives. GO has an ability to transform conventional construction materials into stronger concrete composites since it can enhance the mechanical and durability properties of concrete. In this study, powder form of GO was added to the concrete mixes at dosages of 0.02%, 0.04%, 0.06% and 0.08% by weight of cement. A polycarboxylate (PCE) based superplasticiser was used to improve the dispersion of GO in the concrete mix. The abrasion resistance of GO reinforced concrete was tested to investigate its possible use in rigid pavement applications. The mechanical performance of GO concrete was evaluated in terms of compressive and indirect tensile strengths. The test results show that the addition of GO from 0% to 0.08% enhance the compressive strength, indirect tensile strength and abrasion resistance by 20.7%, 11.8% and 22.9% at 28-days, respectively, which is significantly higher than the control mix. At 56-days, results show 18.5%, 12.3% and 19.7% increments, respectively. The findings confirmed that the addition of GO can remarkably enhance the mechanical and durability properties of concrete. Therefore, it demonstrates promising outcomes of utilizing GO in different concrete applications due to its superior performance.
This paper presents the first-ever investigation of Menger fractal cubes' quasi-static compressio... more This paper presents the first-ever investigation of Menger fractal cubes' quasi-static compression and impact behaviour. Menger cubes with different void ratios were 3D printed using polylactic acid (PLA) with dimensions of 40 mm × 40 mm × 40 mm. Three different orders of Menger cubes with different void ratios were considered, namely M1 with a void ratio of 0.26, M2 with a void ratio of 0.45, and M3 with a void ratio of 0.60. Quasi-static Compression tests were conducted using a universal testing machine, while the drop hammer was used to observe the behaviour under impact loading. The fracture mechanism, energy efficiency and force-time histories were studied. With the structured nature of the void formation and predictability of the failure modes, the Menger geometry showed some promise compared to other alternatives, such as foams and honeycombs. With the increasing void ratio, the Menger geometries show force-displacement behaviour similar to hyperelastic materials such as rubber and polymers. The third order Menger cubes showed the highest energy absorption efficiency compared to the other two geometries in this study. The findings of the present work reveal the possibility of using additively manufactured Menger geometries as an energy-efficient system capable of reducing the transmitting force in applications such as crash barriers.
Windstorms and tornados can cause severe damages to different structure types regardless the mate... more Windstorms and tornados can cause severe damages to different structure types regardless the materials they have been made of. These damages are caused both by the extreme wind velocity and any flying debris within the wind field. Windborne debris is classified based on shape and aerodynamical properties into three types: compact, rod, and plate-like debris. According to both American and Australian Standards, the performance of any structural element is to be tested under the impact of rod-like debris. Plywood is used globally to create both structural and non-structural elements such as doors and window shutters, and thus, in this study, a numerical simulation of rod-like debris was conducted using LS-DYNA to study the effect of debris impact velocity on the deformation of a plywood plate. In this way, the effect of the plate thickness and modulus of elasticity on the deformation were also investigated. Taguchi Method was adopted to make the simulation more robust and reduce the required computational time and costs. The results showed that as the debris impact velocity increased, both the deformation of the plywood plate at penetration and the residual kinetic energy of the debris increased. However, for the same plate thickness and debris impact velocity, as the modulus of elasticity increased, the deformation of the plate at the point of penetration decreased.
Journal of Dynamic Behavior of Materials, Oct 2, 2020
Fabric-based soft armour is the most commonly employed means of personal protection under conditi... more Fabric-based soft armour is the most commonly employed means of personal protection under conditions where ballistic threats are prevalent. The fabric types most commonly used in soft armour are p-Aramids and Ultra High Molecular Weight Polyethylene (UHMWPE). Extensive research, both numerical and experimental, has been carried out to improve and optimise the performance of such materials. Numerical modelling has proven to be a useful tool in capturing the kinetics of highly dynamic events such as ballistic impact. However, the rate-sensitive properties of materials such as p-Aramids and UHMWPEs remain largely unaddressed in numerical models. Furthermore, approximating failure with element deletion based on critical stress levels is unable to account for the progressive damage that yarns undergo during an impact. This paper aims to propose a rate-sensitive constitutive model (RSCM) including rate-sensitive stress-strain response, failure and progressive damage for such materials. A hybrid modelling approach is also presented where different element formulations were used while retaining computational efficiency. The RSCM was implemented as a user-defined material subroutine in LS-DYNA. The results of numerical simulations are in close agreement with experimental results. Hence, the RSCM presented in this paper can be used to accurately model the response of high-performance Twaron fabrics made from p-Aramids subjected to ballistic impact.
h i g h l i g h t s PU resins were prepared from the rapid reaction between PKO-p and MDI in the ... more h i g h l i g h t s PU resins were prepared from the rapid reaction between PKO-p and MDI in the presence of PEG as the plasticizer. The uniaxial tensile characteristics under loading and unloading conditions and the cyclic softening behavior were examined. PUs are highly strain rate dependent and exhibits stress-strain non-linearity. The mechanical response of PUs can be described as hyper-viscoelastic.
There is a rising demand for tall structures to fulfil the occupancy requirements in congested me... more There is a rising demand for tall structures to fulfil the occupancy requirements in congested metropolitan areas. On the other hand there is a considerable demand for monumental structures as tourist attractions in many different parts of the world. Wind behaviour is a key design parameter for such buildings and need to be assessed accurately in the preliminary and secondary design stages. As most of the design codes have their own limitations in providing necessary guidelines for the wind designs such as height limits of the buildings, the existing practice is to conduct wind tunnel tests to determine the wind induced loads on the buildings. However, the costs of wind tunnel tests are comparatively high and conducting wind tunnel tests at preliminary design stage is uneconomical. The shape of the building normally changes a few times during the preliminary stages and this will add to the testing costs. The rapid growth of Computation Fluid Dynamic (CFD) techniques over the last few decades enables Engineers to simulate the wind behaviour around moving objects such as aeroplanes and automobiles. Therefore, use of such methodology to predict wind loads on buildings, especially at the preliminary design stages is essential. This paper discusses a case study that carried out on a typical 208 m tall building with a rectangular geometry. A comparison of results obtained from CFD simulations to the predictions given by the Australian Wind Design Standards (AS1170.2) is presented. In addition the limitations given in different wind design codes are discussed
Over the past two decades, an upsurge in using Computational Fluid Dynamics (CFD) for wind design... more Over the past two decades, an upsurge in using Computational Fluid Dynamics (CFD) for wind design on tall buildings has been observed. An extensive amount of work has been performed, where validation has been at the forefront of most of these studies. Challenges associated with CFD and different methodologies used in the analysis haven't been well articulated within the scope of tall building design. Also, there is a lack of critical best practice guidelines in using CFD for wind analysis of structures which can be readily adopted by practitioners and researchers alike. To this end, this paper presents a comprehensive technical review of the application of Computational Fluid Dynamics (CFD) on tall buildings and structures. A thorough discussion of CFD and its concerning design challenges specific to tall building design, such as turbulence modelling, inflow turbulence and domain & mesh configurations, are discussed in detail. Furthermore, a comprehensive literature of CFD studies on tall buildings is presented, covering all important topics from basic bluff body aerodynamics to complex Fluid-Structure Interaction (FSI) studies. Where applicable, the literature is critically evaluated (impartially) in this paper and compared with supportive arguments from the author's extensive experience. Finally, the manuscript concludes with potential upcoming numerical methods such as the Lattice-Boltzmann Method (LBM) and the use of Artificial Intelligence (AI) to design tall buildings.
Fibrillar dry adhesives are widely used due to their effectiveness in air and vacuum conditions. ... more Fibrillar dry adhesives are widely used due to their effectiveness in air and vacuum conditions. However, their performance depends on various factors. Previous studies have proposed analytical methods to predict adhesion strength on micro-patterned surfaces. However, the method lacks interpretation on which parameters are critical. This research utilizes gradient-boosting machine learning (ML) algorithms to accurately predict adhesion strength. Additionally, explainable machine learning (XML) methods are employed to interpret the underlying reasoning behind the predictions. The analysis demonstrates that gradient boosting models achieve a high correlation coefficient (R > 0.95) in accurately predicting pull-off force on micro-patterned surfaces. The use of XML methods provides insights into the importance of features, their interactions, and their contributions to specific predictions. This novel, explainable, and data-driven approach holds potential for real-time applications, aiding in the identification of critical features that govern the performance of fibrillar adhesives. Furthermore, it improves end-users' confidence by offering human-comprehensible explanations and facilitates understanding among non-technical audiences.
The paper presents a study on the suitability of using a Polymer as a protective layer for critic... more The paper presents a study on the suitability of using a Polymer as a protective layer for critical structural elements prone to impact damage. Conventional damage mitigation methods include using an outer steel jacket with a void, protecting the inner concrete-steel composite column. Whilst it is usually found that the outer steel jacket can partially negate the impact load, the inner column still experiences a significant brunt of the impact load. In order to minimise the damage to the overall critical structural element, a concept of a Polyurethane Polymer-Filled-Concrete-Steel Double-Skin Tubular Column is proposed. Scaled-down versions of two polymerfilled double-skin column configurations were tested under low-velocity impact using a drop tower. The pre and post-damage columns were tested for their axial capacities. These polymer-filled columns were critically compared to their nominal samples without the Polyurethane fill. It was shown that the mid-span deflection and localised indentations were reduced by 17%, and the localised deformations (at the point of impact) were reduced by up to 70% overall. The columns with Polyurethane also showcased retention of 70% of axial capacity post-impact, whilst the nominal sample was only capable of retaining 48%. Finally, the intrinsic properties of Polyurethane were discussed along with a critical discussion on its ability and mechanism on how energy is dissipated by comparing mechanical and chemical properties.
Graphene Oxide (GO) has been emerged as one of the most trending graphene derivative nanomaterial... more Graphene Oxide (GO) has been emerged as one of the most trending graphene derivative nanomaterials in the construction industry. Because of the presence of oxygenated functionalities attached to the aromatic structure, GO can be easily dispersed under an aqueous solution when compared to other graphene-based derives. GO has an ability to transform conventional construction materials into stronger concrete composites since it can enhance the mechanical and durability properties of concrete. In this study, powder form of GO was added to the concrete mixes at dosages of 0.02%, 0.04%, 0.06% and 0.08% by weight of cement. A polycarboxylate (PCE) based superplasticiser was used to improve the dispersion of GO in the concrete mix. The abrasion resistance of GO reinforced concrete was tested to investigate its possible use in rigid pavement applications. The mechanical performance of GO concrete was evaluated in terms of compressive and indirect tensile strengths. The test results show that the addition of GO from 0% to 0.08% enhance the compressive strength, indirect tensile strength and abrasion resistance by 20.7%, 11.8% and 22.9% at 28-days, respectively, which is significantly higher than the control mix. At 56-days, results show 18.5%, 12.3% and 19.7% increments, respectively. The findings confirmed that the addition of GO can remarkably enhance the mechanical and durability properties of concrete. Therefore, it demonstrates promising outcomes of utilizing GO in different concrete applications due to its superior performance.
This paper presents the first-ever investigation of Menger fractal cubes' quasi-static compressio... more This paper presents the first-ever investigation of Menger fractal cubes' quasi-static compression and impact behaviour. Menger cubes with different void ratios were 3D printed using polylactic acid (PLA) with dimensions of 40 mm × 40 mm × 40 mm. Three different orders of Menger cubes with different void ratios were considered, namely M1 with a void ratio of 0.26, M2 with a void ratio of 0.45, and M3 with a void ratio of 0.60. Quasi-static Compression tests were conducted using a universal testing machine, while the drop hammer was used to observe the behaviour under impact loading. The fracture mechanism, energy efficiency and force-time histories were studied. With the structured nature of the void formation and predictability of the failure modes, the Menger geometry showed some promise compared to other alternatives, such as foams and honeycombs. With the increasing void ratio, the Menger geometries show force-displacement behaviour similar to hyperelastic materials such as rubber and polymers. The third order Menger cubes showed the highest energy absorption efficiency compared to the other two geometries in this study. The findings of the present work reveal the possibility of using additively manufactured Menger geometries as an energy-efficient system capable of reducing the transmitting force in applications such as crash barriers.
Windstorms and tornados can cause severe damages to different structure types regardless the mate... more Windstorms and tornados can cause severe damages to different structure types regardless the materials they have been made of. These damages are caused both by the extreme wind velocity and any flying debris within the wind field. Windborne debris is classified based on shape and aerodynamical properties into three types: compact, rod, and plate-like debris. According to both American and Australian Standards, the performance of any structural element is to be tested under the impact of rod-like debris. Plywood is used globally to create both structural and non-structural elements such as doors and window shutters, and thus, in this study, a numerical simulation of rod-like debris was conducted using LS-DYNA to study the effect of debris impact velocity on the deformation of a plywood plate. In this way, the effect of the plate thickness and modulus of elasticity on the deformation were also investigated. Taguchi Method was adopted to make the simulation more robust and reduce the required computational time and costs. The results showed that as the debris impact velocity increased, both the deformation of the plywood plate at penetration and the residual kinetic energy of the debris increased. However, for the same plate thickness and debris impact velocity, as the modulus of elasticity increased, the deformation of the plate at the point of penetration decreased.
Journal of Dynamic Behavior of Materials, Oct 2, 2020
Fabric-based soft armour is the most commonly employed means of personal protection under conditi... more Fabric-based soft armour is the most commonly employed means of personal protection under conditions where ballistic threats are prevalent. The fabric types most commonly used in soft armour are p-Aramids and Ultra High Molecular Weight Polyethylene (UHMWPE). Extensive research, both numerical and experimental, has been carried out to improve and optimise the performance of such materials. Numerical modelling has proven to be a useful tool in capturing the kinetics of highly dynamic events such as ballistic impact. However, the rate-sensitive properties of materials such as p-Aramids and UHMWPEs remain largely unaddressed in numerical models. Furthermore, approximating failure with element deletion based on critical stress levels is unable to account for the progressive damage that yarns undergo during an impact. This paper aims to propose a rate-sensitive constitutive model (RSCM) including rate-sensitive stress-strain response, failure and progressive damage for such materials. A hybrid modelling approach is also presented where different element formulations were used while retaining computational efficiency. The RSCM was implemented as a user-defined material subroutine in LS-DYNA. The results of numerical simulations are in close agreement with experimental results. Hence, the RSCM presented in this paper can be used to accurately model the response of high-performance Twaron fabrics made from p-Aramids subjected to ballistic impact.
h i g h l i g h t s PU resins were prepared from the rapid reaction between PKO-p and MDI in the ... more h i g h l i g h t s PU resins were prepared from the rapid reaction between PKO-p and MDI in the presence of PEG as the plasticizer. The uniaxial tensile characteristics under loading and unloading conditions and the cyclic softening behavior were examined. PUs are highly strain rate dependent and exhibits stress-strain non-linearity. The mechanical response of PUs can be described as hyper-viscoelastic.
There is a rising demand for tall structures to fulfil the occupancy requirements in congested me... more There is a rising demand for tall structures to fulfil the occupancy requirements in congested metropolitan areas. On the other hand there is a considerable demand for monumental structures as tourist attractions in many different parts of the world. Wind behaviour is a key design parameter for such buildings and need to be assessed accurately in the preliminary and secondary design stages. As most of the design codes have their own limitations in providing necessary guidelines for the wind designs such as height limits of the buildings, the existing practice is to conduct wind tunnel tests to determine the wind induced loads on the buildings. However, the costs of wind tunnel tests are comparatively high and conducting wind tunnel tests at preliminary design stage is uneconomical. The shape of the building normally changes a few times during the preliminary stages and this will add to the testing costs. The rapid growth of Computation Fluid Dynamic (CFD) techniques over the last few decades enables Engineers to simulate the wind behaviour around moving objects such as aeroplanes and automobiles. Therefore, use of such methodology to predict wind loads on buildings, especially at the preliminary design stages is essential. This paper discusses a case study that carried out on a typical 208 m tall building with a rectangular geometry. A comparison of results obtained from CFD simulations to the predictions given by the Australian Wind Design Standards (AS1170.2) is presented. In addition the limitations given in different wind design codes are discussed
Over the past two decades, an upsurge in using Computational Fluid Dynamics (CFD) for wind design... more Over the past two decades, an upsurge in using Computational Fluid Dynamics (CFD) for wind design on tall buildings has been observed. An extensive amount of work has been performed, where validation has been at the forefront of most of these studies. Challenges associated with CFD and different methodologies used in the analysis haven't been well articulated within the scope of tall building design. Also, there is a lack of critical best practice guidelines in using CFD for wind analysis of structures which can be readily adopted by practitioners and researchers alike. To this end, this paper presents a comprehensive technical review of the application of Computational Fluid Dynamics (CFD) on tall buildings and structures. A thorough discussion of CFD and its concerning design challenges specific to tall building design, such as turbulence modelling, inflow turbulence and domain & mesh configurations, are discussed in detail. Furthermore, a comprehensive literature of CFD studies on tall buildings is presented, covering all important topics from basic bluff body aerodynamics to complex Fluid-Structure Interaction (FSI) studies. Where applicable, the literature is critically evaluated (impartially) in this paper and compared with supportive arguments from the author's extensive experience. Finally, the manuscript concludes with potential upcoming numerical methods such as the Lattice-Boltzmann Method (LBM) and the use of Artificial Intelligence (AI) to design tall buildings.
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Papers by Damith Mohotti