Browsing by Author "Mallikarachchi, HMYC"

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item: Conference-Abstract
3D full field deformation measurement using digital image correlation
(Department of Civil Engineering, University of Moratuwa, 2021-11) Lowhikan, SS; Mallikarachchi, HMYC; Hettiarachchi, P
3D deformation and strain are crucial parameters in engineering design and construction. Precise 3D full-field measurement is useful in identifying the response of the structure under a given loading condition. Digital Image Correlation (DIC) is a non-contact optic-based technique that may prove to be ideal for full-field deformation applications. It has the potential to become an inexpensive, simple, and accurate solution for deformation measurement. All DIC measuring systems now available consist of expensive software packages and experimental facilities which are difficult to access. Therefore, a cost-effective method must be developed to use in the local context efficiently. This research mainly focuses on the development and validation of the cost-effective precise noncontact- based deformation measurement technique. In the proposed method, 3D full-field deformation of the deforming object is measured using two digital cameras and an image processing toolbox available in the MATLAB commercial package. Further, the proposed method efficiency is enhanced by developing it as a standalone application, which can be installed and used by any technician conveniently and utilized for various laboratory Civil Engineering applications. Enhancing the proposed DIC algorithm to improve resolution in 3D applications and extending to phase-changing materials and developing Graphical User Interface (GUI) and general guidelines to be used by a technician were defined as objectives of this research. The proposed measuring system consists of two digital cameras mounted on a rigid frame as it is targeted to capture the specimen and connected to a computer. A random speckle pattern must be applied on the specimen’s surface to track the deformation. The proposed image processing algorithm was developed in MATLAB by using a computer vision toolbox. Later Graphical User Interface was developed using MATLAB App Designer. The developed system was used to obtain the results and validated for rigid body motion tests of concrete cube and cylinder, compression test of concrete, uni-axial tensile test of a dog bone aluminium specimen and shrinkage cracks of mortar experiments. In conclusion, a cost-effective and reliable measurement system was developed by using DIC techniques and MATLAB computer vision toolbox, with its performance validated experimentally by assessment of measurements of the in-plane strain of materials. Even though it has some limitations, the developed algorithm and application can be effectively used for laboratory-scale Civil Engineering related experiments. Also, the application that was developed can be handled by technicians who do not have much knowledge nor understanding of programming languages. The Graphical User Interface that was developed is easy to use and saves considerable time. The performance of the system that has been developed can be assessed and improved for greater precision.
item: Conference-Full-text
3d full-field deformation measuring technique with optics-based measurements
(IEEE, 2018-05) Randil, OPC; Mallikarachchi, HMYC; Chathuranga, D
Full-field deformation measurements plays a vital role in designing, monitoring and retrofitting many engineering products ranging from civil engineering structure to aerospace applications. Optics-based measurements have gained a vast popularity over last two decades. This paper attempts to establish a method in measuring displacements in a 3D environment, using optics-based measurements, in a cost-effective manner. The study focuses on measuring displacements of a moving object with two stationary cameras. Images acquired from two cameras are processed through MATLAB software package and selected region of the object is reconstructed in a virtual 3D environment at each step. Digital image correlation technique is then adopted to measure displacement by comparing the coordinates of reconstructed objects in each step. It is shown that the displacements can be measured to an accuracy of 0.15 mm (in 20 mm) for the selected object giving an accuracy of 98%.
item: Conference-Abstract
Analysing the non-linear bending behaviour of ultra-thin woven composites at high curvatures
(Department of Civil Engineering, 2023-09-27) Tennakoon, TMH; Mallikarachchi, HMYC; Mallikarachchi, C; Hettiarachchi, P; Herath, S; Fernando, L
Deep space missions require self-deployable structures built of ultra-thin materials which can be carried to space in a limited space. Therefore, a growing demand for ultra-thin woven composites has been identified in space engineering applications. Understanding their mechanical behaviour is crucial for the effective optimisation of future structures because they experience extreme curvatures when in use in both folding and deploying mechanisms. It is more challenging to predict the overall mechanical behaviour of these composites due to their complicated geometry and nonlinear behaviour of its constituent parts. A common method to solve this problem is multiscale modelling, in which the system is simultaneously described by multiple models at varying scales. Micromechanical, mesomechanical and macromechanical scales are taken into consideration for woven fibre composites. Physical experiments revealed that ultra-thin woven fibre composites show a significant drop in bending stiffness at higher curvatures. The first objective of the study focuses on checking whether there is a thickness reduction of the plies at high curvatures which can be a possible reason for the reduced bending stiffness at high curvatures. As the second objective, it is expected to introduce air voids in resin to capture the non-linear bending response of woven fibre composites observed under high curvatures. Due to the deformation of the fibres and the weave structure at higher curvatures, woven fibre composites may exhibit variations in thickness. Resin matrices usually have lower stiffness compared to the reinforcing fibres. As a result, when the composite is subjected to higher curvatures, the fibres on the inner side of the curve may experience compression, leading to a reduction in thickness, while the resin matrix tends to flow and redistribute in response to the applied forces. The flow of resin can have both positive and negative effects on the thickness variation. On one hand, the ability of the resin to flow and redistribute can help accommodate the compression of the fibres and reduce the overall thickness reduction. According to the obtained results from the Dry Fibre model and the Resin model, except for the mid-section of the Dry Fibre model, there is no significant thickness variation at other locations compared to the original thickness in both the Resin model and Dry Fibre model. Although there is a thickness reduction at the mid-section of the dry fibre model, at high curvatures thickness starts to increase again which is against the experimentally observed bending behaviour. So, according to the obtained thickness variation results from the two models, there is no clear connection between bending stiffness reduction and thickness variation. To check the effect because of voids, Fibre Volume Fraction of the composite was used. The Fibre Volume Fraction is defined as the ratio of the volume of fibres present to the total volume of the layer. From the second objective, it was observed that there is a 6-8 % reduction in longitudinal stiffness, transverse stiffness, and shear stiffness with the void ratio. Variation in the Poisson’s ratio with the void ratio is low compared to other mechanical properties.
item: Conference-Abstract
Analysis of curved crease origami structures
(Department of Civil Engineering, 2023-09-27) Kuruppu, KALH; Mallikarachchi, HMYC; Mallikarachchi, C; Hettiarachchi, P; Herath, S; Fernando, L
Research on origami-based folding patterns has led to major technical developments from nanoscale metamaterial to large-scale deployable space structures. Deployable space structures such as solar sails and reflectors require them to be stored in a small volume while being able to deploy into a large configuration when in operation. The developability of origami facilitates the employment of deployability and self-actuation qualities in making these lightweight structures. In general, these structures are constructed with ultrathin materials and the quality of deployed surface increases the efficiency of the functionality of the structure. The curved crease origami structures consist of fewer creases than their equivalent straight crease counterparts. Lower number of creases leads to increase in efficiency as well as faster manufacturing rate. At present origami related research is mainly focused on predicting straight crease behaviour and the possible use of curved crease origami folding patterns requires more attention. This research is focused on studying the effect of membrane thickness on the folding behaviour of the curved-crease Miura Ori structures. Analytical equations for predicting the edge curve motion were first considered after a thorough literature review and an elliptical curved-crease Miura structure with a radii 40 mm and 69 mm made of 80 gsm copier paper was selected as a case study. The proposed numerical scheme for predicting folding and deployment behaviour discretises the curved crease into a series of straight line segments which are then replaced with a series of rotational springs. The equivalent rotational stiffness of a perforated straight crease was measured using a simple experimental setup which measures the force required to open a crease with crease angle opening. Same procedure was repeated for three different specimens and the mean rotational stiffness was used as an input to the rotational spring employed in the numerical model. The selected curved crease pattern was then simulated using the proposed numerical technique to obtain the deformed configuration under predefined loading conditions. The predicted shape was then validated against surface mesh obtained using a LiDAR scan of a physically constructed model under similar loading conditions. The experimentally validated numerical technique was then used to assess the changes in folding behaviour with changing membrane thicknesses. It is shown that the membrane thickness has a clear impact on the folding of the curved crease Miura Ori structure. Change in edge curve location leads to an overall change in displacement of the folded structure and hence the overall deployability of the structure changes with varying membrane thickness. This change of the edge curve coordinates gets accumulated when the base structure is tessellated to form the final deployable structure.
item: Conference-Abstract
Characterising the self-opening behaviour of single creased Kapton polyimide films
(Department of Civil Engineering, University of Moratuwa, 2021-11) Navaratnarajah, S; Mallikarachchi, HMYC; Hettiarachchi, P
Use of thin folded membranes for deployable structures is becoming increasingly popular especially in aerospace applications such as a deployable solar arrays, sun shields, and solar sails. The folding and compaction process of thin membranes, which introduces permanent, nonrecoverable, localized plastic deformation, changes the geometric shape and material properties. Therefore, precise prediction of folding and deployment behaviour is essential for the mission's success as incorrect folding, storage, and deployment could result in damaging the membrane or not achieving the expected deployed configuration. Virtual simulation is a feasible solution in comparison with physical testing which requires reduced gravity, friction, and air-drag-free environment in design optimization of these structures. However, accurate idealisation schemes will significantly reduce number of elements meaning lower the degrees of freedom and hence reduce the computational cost. Therefore, a proper understanding of the mechanics of creased membrane structures is the key, in formulating such idealisations. The underlying mechanics in the deployment of creased membrane structures from the folded state to the deployed state involves two phases. First, the structure self-opens from the fully folded state to the stress-free stable state. It then requires an external force to deploy from stress-free state to fully deployed state. This can be referred to as forced opening. The focus of the previous studies was limited to the characterisation of crease behaviour during forced opening but not the self-opening which is also crucial in the design of gossamer structures. In this research, an attempt has been made to characterise the crease mechanics of single creased thin Kapton polyimide membranes during their self-opening behaviour using two different experimental approaches. One experimental study investigates the rotational motion of a panel in a single creased membrane immediately after creasing where angle, angular velocity, and angular acceleration variation were obtained to develop the moment-rotation response. The second experimental study evaluates the moment-rotation response of crease during quasi-static folding of the single creased membrane once it achieved the stress-free stable state. It has been found that the moment-rotation response during the self-opening behaviour shows a linear trend for all thicknesses considered. The effect of membrane thickness and width on the crease rotational stiffness was also investigated in the experimental study. Accordingly, crease stiffness increases with increasing thickness and is independent of the width of the specimen. A simple analytical study was performed to predict the rotational stiffness of the crease which shows a good qualitative agreement with physical experiment results. Finally, the results from the self-opening study were combined with the forced opening study which was done by previous researchers. Based on the results it is reasonable to assume the fold-line stiffness to be linear during the whole deployment with a constant crease stiffness. This value can be easily incorporated as spring stiffness in the finite element model by idealising the crease region as a rotational spring to reduce the computational cost.
item: Conference-Full-text
A Comparative Study to Assess the Sustainability of Bamboo Reinforced Concrete over Conventional Steel Reinforced Concrete
(2022-08-23) Vitharana, TVDVK; Bandaranayake, SS; Jayasinghe, MTR; Herath, HMST; Mallikarachchi, HMYC
Amongst various sustainable building materials, bamboo reinforced concrete has come into concern as a potential substitute for steel-reinforced concrete. This has drawn attention due to some major drawbacks of steel in reinforced concrete including elevated cost, high energy consumption, increased emission of carbon, etc. Bamboo is a rapidly renewable material that has displayed outstanding mechanical properties which are adequate to replace steel in reinforced concrete. In the present context, in addition to its structural performance, it is of vital importance to evaluate the sustainability of bamboo reinforced concrete over conventional reinforced concrete. In this work, the structural capacity of replacing steel with bamboo in different structural elements was analytically determined. Subsequently, a comprehensive work study-based analysis was executed to compare the sustainability of a bamboo reinforced concrete beam element with a conventional steel-reinforced concrete beam element in terms of their embodied carbon and cost. Results of this study justify the possibility of promoting bamboo as a potentially sustainable alternative to steel in reinforced concrete as bamboo reinforced concrete displays drastically reduced sustainability indices for embodied carbon, and cost while maintaining adequate structural capacities.
item:
Deployment dynamics of ultrathin composite booms with Tape-Spring hinges
Mallikarachchi, HMYC; Pellegrino, S
An experimental and numerical study of the dynamic deployment of stored strain energy deployable booms with tape-spring hinges made of woven carbon fiber composite is presented. The deployment consists of three phases: deployment, one or more attempts to latch, and a small amplitude vibration. Twelve nominally identical deployment experiments show that the deployment and vibration phases are repeatable, whereas considerable scatter is observed during latching. A high-fidelity finite clement shell model of the complete boom is used to carry out complete dynamic simulations with the Abaqus/Explicit finite element software. These analyses provide detailed time histories of deformation and stress distribution. By varying the end conditions at the root of the boom and the viscous pressure loading on the surface of the hinge region, the analyses provide 1) an envelope of responses that bound the complete set of experimental observations and 2) responses that closely approximate actual experiments. The presented approach is fully general and can provide high-fidelity simulations for any kind of stored-energy deployable structure.
item: Article-Abstract
Design of ultrathin composite self-deployable booms
Mallikarachchi, HMYC; Pellegrino, S
Recently developed analysis techniques for thin shells that can be folded elastically and are able to self-deploy are used to develop an iterative design approach for this type of structure. The proposed approach considers a series of potential designs and then evaluates, for each trial design, key performance parameters through a complete simulation of its folding and deployment behavior. This design approach is applied to a boom concept consisting of a thin-walled tube in which two tape-spring hinges are made by cutting diametrically opposite slots; the geometry of the slots is fully defined by the length, width, and end diameter of the cuts. A design for a two-hinge, 1-m-long, lightweight self-deployable boom that can be wrapped around a small spacecraft is developed; the hinge geometry is chosen such that there is no damage during folding/deployment of the boom, and also the boom becomes latched at the first attempt. The chosen boom design is successfully validated experimentally.
item: Conference-Abstract
Design-informed structural optimisation
(Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, 2022-12) Amarasinghe, IT; Herath, HMST; Mallikarachchi, HMYC; Mallikarachchi, C
Structural optimisation has become an important tool in the field of Civil Engineering, but there is limited research done on structural optimisation of specific structures and components, especially for large construction machinery. By optimising construction machinery components, it is possible to reduce the material usage and decrease the cost of the machine, without compromising its strength. This research study looks at a case study of optimising a wheel loader arm. Initially, the critical load calculation and the static force analysis of the wheel loader arm were conducted and the forces and reactions acting on the arm were obtained. Then a finite element analysis was conducted by assigning the relevant loads and boundary conditions and the results obtained deemed that the stresses and displacements of the arm were within the acceptable limits. The Solid Isotropic Microstructure with Penalisation (SIMP) for intermediate densities method is used for the topology optimisation process considering the minimum compliance as the objective function and the volume fraction as the constraint. Using the Abaqus FEA software, topology optimisation models were obtained for different volume fractions and the most optimum geometry comparing maximum von Mises stress, displacement, and mass with the original design. After the completion of the topology optimisation process, Computer-Aided Design models are generated by exporting the mesh into SOLIDWORKS. Subsequently, shape optimisation is conducted considering the different manufacturing constraints. The final optimised model has a 20.3% reduction of mass compared to the original structure, while stresses, displacement and strains are kept within the allowable limits in accordance with codes of practice. This case study demonstrates on how structural optimisation can be integrated into the designs of different structures and components. By using a similar method, it is possible to optimise different components of the wheel loader arm and other construction machinery components. These optimisations will reduce the weight and material usage of these components, which can help reduce the overall cost of the machines significantly.
item: Conference-Abstract
Effect of crease curvature on bending stiffness in curved crease origami
(Department of Civil Engineering, University of Moratuwa, 2024) Weerasekara, WMHGLCB; Mallikarachchi, HMYC; Pasindu, HR; Damruwan, H; Weerasinghe, P; Fernando, L; Rajapakse, C
Deployable systems play a vital role in design optimisation of spacecrafts. Limited payload capacity in launch vehicles often conflicts with the demand for large-area systems required for better performance in the operational phase. The art of origami enables the storage of a large surface structure in a compact volume by utilising plastically deformed fold lines known as creases. Origami can be categorised into two types as straight crease origami and curved crease origami based on the geometry of the crease. Straight crease origami demonstrates rigid foldability while curved crease origami facilitates smooth and efficient folding by incorporating panel bending. The use of curved creases in deployable membranes reduces the required number of creases compared to straight creases ultimately leading to less production time and better quality deployed surface. The bending stiffness of the crease and the external force required to fold the membrane are key aspects that influence the packaging efficiency. The main objective of this research is to assess the effect of crease curvature on the bending stiffness of curved creases. The study encompassed quasi-static folding and unfolding experiments of curved-crease specimens with different crease radii made of 80 gsm printer paper to understand crease behaviour. Furthermore, numerical models were developed to simulate the crease behaviour of curved creases and validated against experiments in order to develop prediction tools for future designs. The resistive force resulting in bending a curved crease specimen was measured experimentally for rectangular specimens with overall dimensions of 60 mm × 100 mm while varying crease radii from 35 mm to 75 mm. Additionally, the force-displacement response was recorded for flat sheets with dimensions of 50 mm × 60 mm and 100 mm × 60 mm to compare the bending stiffness of curved creases to that of a flat sheet having similar overall dimensions. The results demonstrate that the initial bending stiffness of curved-crease specimens with a low crease radius is lower but becomes higher at the maximum folded state compared to specimens with a high crease radius. Additionally, it was observed that the contribution of the unrestrained half of the specimen significantly increases the bending stiffness of a curved crease specimen during the folding motion for lower crease radius, whereas the contribution remains almost constant for higher crease radius. Numerical models of curved creases, developed in the commercial software Abaqus using a segmented curved-crease modelling approach incorporating straight crease characteristics, accurately estimate experimental reaction forces, indicating that the characteristics of straight creases can be used in modelling curved creases.
item: Conference-Full-text
Estimation of fatigue life of steel masts using finite element modelling
(Department of Civil Engineering, University of Moratuwa, 2015-10) Kariyawasam, KKGKD; Mallikarachchi, HMYC; Hettiarachchi, MTP
Fatigue is an important design consideration for tall steel structures. Accurate prediction of fatigue endurance is essential to design the elements subjected to wind and earthquake induced fatigue. The design guidelines given in codes of practices are applicable only to simple shapes and laboratory experimental verification is costly. Therefore, simulation using finite element software is becoming popular. This paper demonstrates successful coupling of Abaqus/FEA and fe-safe software in predicting the uniaxial and multiaxial fatigue behaviour of steel specimens. The simulated results were verified against experimental results available in literature. The verified simulation technique was used to examine the fatigue life of a64 m tall steel mastlocated on top of a 285m tall tower. Sensitivity of different fatigue inducing properties such as fatigue analysis method, surface finish and plate thickness on fatigue endurance was studied.
item: Thesis-Full-text
Estimation of Fatigue Life of Steel Masts using Finite Element Modelling
(2016-02-10) Danushka, KKGK; Mallikarachchi, HMYC
Estimation of Fatigue Life of Steel Masts using Finite Element Modelling Fatigue is an important design consideration for tall steel structures. Accurate prediction of fatigue endurance is essential to design the elements subjected to wind and earthquake induced fatigue. The design guidelines given in codes of practices are applicable only to simple shapes and laboratory experimental verification is costly. Therefore, simulation using finite element software is becoming popular. An attempt is made to couple Abaqus finite element analysis software and fe-safe software to estimate the fatigue life of a structure. First, the accuracy of the techniques and idealizations used in simulation are validated by simulating experiments available in the literature. Standard Uni-Axial fatigue experiments which were conducted at several strain amplitudes showed a closer relationship to simulation results. Moreover sensitivity of fatigue life to surface finish and stress strain dataset importing method in fe-safe software were evaluated. It was found that the surface finish is a highly sensitive parameter and it should be estimated accurately. Elastic plastic block method gave good results while elastic block method with neuter’s rule results were poor. This indicates the importance of using elastic plastic block method for low cycle fatigue especially when stress redistribution is high. Simulation result of multi-axial fatigue experiment showed similar results to results obtained from physical experiments. The verified technique was the applied to estimate the fatigue life of a 64 m tall steel mast with an opening located at the top of a 285 m tall concrete tower. The sensitivity of the plate thickness and shape of the opening of the mast were studied. It was found that small increase in plate thickness rapidly increases the fatigue endurance. This shows the importance of using stiffeners in fatigue prone areas. Comparison of the shape of the opening showed that square shape would have higher endurance than a circular shape of same opening area. However only monolithic sections were studied here and effects on welds and bolted connections are beyond the scope of this research
item: Article-Abstract
Failure criterion for two-ply plain-weave CFRP laminates
(2015-08-20) Mallikarachchi, HMYC; Pellegrino, S
We present an experimentally based failure criterion for symmetric two-ply plain-weave laminates of carbon fiber reinforced plastic. The criterion is formulated in terms of six force and moment stress resultants and consists of a set of three inequalities, related to in-plane, bending, and combined in-plane and bending types of failure. All failure parameters in the criterion are measured directly from five sets of tests. The new criterion is validated against an extensive data set of failure test results that use novel sample configurations to impose different combinations of stress resultants. It is found that the proposed criterion is safe for all test conditions and yet avoids excessive conservatism.
item: Conference-Abstract
Finite element simulation of thin folded membranes
Dharmadasa, BY; Mallikarachchi, HMYC
Deployable gossamer structures use thin filmed membranes folded into different patterns. Predicting the membrane behaviour and the stress propagation in these structures is complex due to non-linear characteristics at fold-lines. This paper investigates some of the current idealization methods (perfect hinge, perfect weld) and proposes a novel method, where connectors with rotational elasticity are defined. Furthermore a thorough investigation on key factors affecting the output is carried out. The results have proved that the novel simulation approach proposed is more accurate and careful selection of key factors could make the simulation efficient.
item: Conference-Abstract
Folding patterns for ultra-thin deployable membranes
Liyanage, PM; Mallikarachchi, HMYC
A deployable structure should mainly be adequately compact and should fit into any remaining space of the launch vehicle. The main factors which will determine these are the folding pattern, ease of deployment and stresses in the fold lines. Two folding patterns are selected based on extensive literature review to investigate the possibility of using those techniques for a large solar sail mission. It is expensive as well as extremely time consuming to perform experimental investigation under reduced gravity environment for this type of large membranes. Thus developing simulating techniques are quite important. Two models are simulated using Abaqus/Explicit commercial finite element software. Quasistatic conditions and numerical accuracy are verified by comparing strain energy together with kinetic energy and artificial strain energy. It is shown that spiral folding pattern requires less energy for deployment and hence that is preferred.
item: Conference-Abstract
Full-field deformation measurement system for early age concrete cracking
(Department of Civil Engineering, University of Moratuwa, 2024) Silva, KGNSS; Gamage, ES; Mallikarachchi, HMYC; Pasindu, HR; Damruwan, H; Weerasinghe, P; Fernando, l; Rajapakse, C
Identification of the risk of early age shrinkage cracks in fresh concrete is essential as they can lead structures to unserviceable conditions. These cracks occur when the shrinkage induced stresses exceed its tensile strength capacity of fresh concrete. In general, the risk of early age shrinkage cracks is evaluated by inspecting the tensile strain development of fresh concrete. Hence measurement of strain distribution of early age concrete is essential. However, measuring displacements and strains in fresh concrete is challenging due to its semi-solid state, which prevents the direct use of contact-based deformation measuring tools. There were several techniques used to measure the deformations of early age concrete. Some studies have used strain gauges attached to the moulds while some measured deformations by attaching strain gauges to posts cast into the concrete. However, deformations of moulds are not the same as that of concrete and semi-solid state of fresh concrete leads to relative movements between posts and accuracy can be compromised. Recent research has processed images, taken at specific time intervals, to calculate the deformation of preidentified targets. This overcame the challenges faced with contact-based methods but suffered from poor resolution and required image corrections. With recent rapid advancements in computer processing units and camera technology, avenues are now available to use machine vision techniques such as Digital Image Correlation (DIC). This study aims to develop an in house DIC based measurement system for evaluating full-field deformation of early-age concrete. An in-house 3D digital image correlation-based system was utilized for this purpose with a direct tensile apparatus, with modifications made to enhance accuracy. Great care was taken during the experiment procedure to distribute the speckle pattern on the semi-solid concrete surface. The modified DIC system was capable of generating full-field deformations and strain distributions of fresh concrete which is after 1 hour from casting. The suitability of the modified system was evaluated through experimental analysis. Physical measurements were obtained using a vernier calliper and the results depict that the developed technique can measure strains with an accuracy of over 94%. The precision of the system can be enhanced by optimizing the distribution of the speckle pattern. It is intended to utilise the developed DIC system to investigate the tensile properties of early-age concrete for different mix proportions.
item: Conference-Abstract
Homogenisation of ultra-thin woven fibre composite structures under high curvatures
(Department of Civil Engineering, Faculty of Engineering, University of Moratuwa, 2022-12) Weerasinghe, WUD; Herath, HMST; Mallikarachchi, HMYC; Mallikarachchi, C
Utilising fibre-based textile structure as the composite reinforcement results in making composite more tailorable and effective for applications where various types of loads are anticipated to be supported by the structure. Owing to that, growing demand for ultra-thin woven composites can be identified in weight-sensitive applications, especially in space engineering applications such as self-deployable structures. Both complex geometry and nonlinear behaviour of constituents of these composites make it more difficult to forecast the overall mechanical behaviour. Multiscale modeling approach can be identified as a popular strategy to overcome this issue where several models at various scales are utilised simultaneously to describe the system. For woven fibre composites, considered scales are micromechanical, meso-mechanical and macro-mechanical scales. Physical experiments revealed in the literature that these ultra-thin structures have experienced a significant drop in bending stiffness when subjected to extreme curvatures. This is a numerical study in meso-mechanical scale on the homogenised response of two-ply plain woven carbon fibre composites considering the inter-connection behaviour between contact surfaces within the ply and in between two plies. With dry fiber geometry and resin pocket introduced geometry, two distinct models were used in the analysis. To estimate the behaviour under higher curvatures, the study has been advanced further into the non-linear regime. Due to the severe curvatures that these ultra-thin woven composites are subjected to, slippage behaviour between yarns and between plies may take place, which would result in a drop in bending stiffness at those higher curvatures. Surface based cohesive constraints were defined to simulate the slipping behaviour using linear elastic traction-separation stiffness values. The dry fibre model captured the bending stiffness and Poisson’s ratio with good accuracy while the resin model captured the shear response better than the dry fibre model. Axial stiffness predictions remained almost the same for both cases. It is demonstrated that the proposed models can accurately predict the nonlinear flexural behaviour based on the experimental findings. Both models have shown their ability to accurately capture the bending stiffness reduction up to a curvature value of 0.14mm-1 and the stiffness reduction was overpredicted beyond that point. Reason behind the overprediction can be the allowance of relative moment between plies which can lead to the loss of compatibility between two plies and hence, each lamina contributes separately to the second moment of area instead of full thickness. Further development of resin model to capture both in-plane and out of plane properties in a single model can be recommended for future studies and also the effect of relative movement of plies to the second moment of area of laminate is recommended for further studies.
item: Conference-Abstract
Image based displacement measuring technique for in-plane loading
Diluxshan, A; Mallikarachchi, HMYC
Displacement/ strain measurement plays a significant role in understanding structural and material behaviour. In the local context, commonly used contact based systems have a major limitation when it comes to measuring deformation of a surface. A measuring probe can record the displacement/ strain only at a single location and hence require a larger number of pre-defined measuring points in order to calculate the deformation of the surface. Furthermore, the comparative stiffness between the measuring probe and the object under consideration has to be significantly larger. A non-contact optic based system is an ideal candidate for overcoming the problems encountered in such systems. This research attempts to utilize image processing techniques in order to develop a low cost non-contact based deformation measuring technique to measure the in-plane deformation of an object. A set of pre-defined measuring points (targets) were marked on a timber block and a series of images with a commonly available camera was taken while applying a load in the mid-span. The images were then processed using the image processing tools available in MATLAB software to track the motion of predefined targets. The calculated displacements were validated against the physical measurements taken during the bending test.
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Image based non-contact deformation measuring technique
(2015-05-27) Mallikarachchi, HMYC; Perera, GAU
Deformation and strain measurements play a vital role in civil engineering. The current practice is to use dial gauges to measure deformation over a large area and strain gauges when interested in a localised point of a structure. Both techniques use a contact based approach which may damage or introduce a significant change in structural response depending on material and structural properties. Digital Image Correlation (DIC) is a non-contact imaged based technique where the deformation of a specimen is determined by calculating changes in two images taken before and after applying a certain load. An automated software programme is developed to capture a given target and calculate the deformation using difference between two photographs taken during an experiment. Software outcome was verified against a dial gauge with 0.01 mm minimum count.
item: Conference-Abstract
Influence of number of plies on flexural behaviour of ultra-thin woven composites
(Department of Civil Engineering, University of Moratuwa, 2024) Wanasinghe, WMCS; Mallikarachchi, HMYC; Pasindu, HR; Damruwan, H; Weerasinghe, P; Fernando, L; Rajapakse, C
Deep space missions necessitate the development of weight-sensitive structures capable of supporting multiple operational configurations. The high strength-to-weight ratio and flexible material properties of woven fibre composites make them ideal for aerospace structures. Epoxy laminates reinforced with carbon fibre are often used in primary and secondary aircraft structures. Woven fibre composites have greater advantages compared to unidirectional fibre lamina. The number of fibres in a yarn and the type of weave determine the material properties. The high curvature experienced by space structures during folding and deployment points out the critical importance of understanding their bending behaviour for optimising future designs. Experimental studies show a significant reduction in the bending stiffness of ultra-thin woven composites when subjected to high curvatures according to the literature. The in-plane properties of woven laminates have been estimated through the development of numerous micromechanical analytical methods. Although in-plane properties can be accurately determined through Classical Lamination Theory (CLT) for thin woven composites, the flexural properties tend to over-predict by 200 – 400%. This paper introduces micro-mechanical models to capture the impact of ply count on the mechanical characteristics of thin woven fibre composites. Accordingly, this study expresses a geometric model with solid elements that simulate the effect of the two waviness with increasing plies in a Representative Unit Cell (RUC) to analyse the bending stiffness reduction near failure. The study focuses on a plain-woven carbon fibre composite having fibres arranged in an in-phase configuration. A finite element pre-processor, TexGen software, is used to generate the representative unit cell geometry. This generated RUC model is then imported into the commercial finite element software Abaqus/Standard to simulate the mechanical behaviour. The numerical results are validated with experimental results obtained from the literature. In addition, the study aims to investigate the behaviour of results derived from CLT under varying numbers of plies. The reduction in bending stiffness between CLT predictions and finite element (FE) simulations across all ply configurations varies from approximately 500% for single-ply laminates to 90% for three-ply laminates while 150% for two-ply laminates, highlighting the efficacy of the CLT approach in accurately predicting bending properties with increasing number of plies. One reason for this deviation is that CLT does not account for the inherent waviness of woven fibre composites, which generally exhibit a sinusoidal wave pattern. This waviness is reduced with an increasing number of plies, leading to a closer alignment between CLT and FE results.