Imperial College London

ProfessorMariaCharalambides

Faculty of EngineeringDepartment of Mechanical Engineering

Professor of the Mechanics of Materials
 
 
 
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Contact

 

+44 (0)20 7594 7246m.charalambides Website

 
 
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Location

 

516City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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97 results found

Djekic I, Mujčinović A, Nikolić A, Jambrak AR, Papademas P, Feyissa AH, Kansou K, Thomopoulos R, Briesen H, Kavallieratos NG, Athanassiou CG, Silva CLM, Sirbu A, Moisescu AM, Tomasevic I, Brodnjak UV, Charalambides M, Tonda Aet al., 2019, Cross-European initial survey on the use of mathematical models in food industry, Journal of Food Engineering, Vol: 261, Pages: 109-116, ISSN: 0260-8774

Mathematical modelling plays an important role in food engineering having various mathematical models tailored for different food topics. However, mathematical models are followed by limited information on their application in food companies. This paper aims to discuss the extent and the conditions surrounding the usage of mathematical models in the context of European food and drinks industry. It investigates the knowledge, nature and current use of modelling approaches in relation to the industry main characteristics. A total of 203 food companies from 12 European countries were included in this research.Results reveal that the country where the company operates, and size of the company, are more important predictors on the usage of mathematical models followed by the type of food sector. The more developed countries are positioned at the higher level of knowledge and use of available models. Similar pattern was observed at the micro level showing that small or medium sized companies exhibit lack of knowledge, resources and limiting usage of models.

Journal article

Wood J, Gauvin C, Young C, Taylor A, Balint D, Charalambides Met al., 2019, Reconstruction of historical temperature and relative humidity cycles within Knole House, Kent, Journal of Cultural Heritage, Vol: 39, Pages: 212-220, ISSN: 1296-2074

It is essential for the preservation of cultural heritage that the effects of climate change are investigated. With this in mind, the daily temperature and relative humidity (RH) cycles within the Brown Gallery at Knole House, Kent, have been reconstructed for the period 1605 – 2015 enabling the study of low-cycle environmental fatigue on a set of 17th century panel paintings. By establishing a relationship between the temperature in the Brown Gallery and the Hadley Centre Central England Temperature (HadCET) dataset over a sixteen year period (2000 – 2015), it is possible to use the full HadCET dataset to obtain the daily minimum and maximum temperatures in the Brown Gallery for the period 1878 – 2015. Using a Fourier series to fit the periodic data it is then possible to extrapolate back to 1605. Furthermore, correction factors derived using the HadCET average daily temperature in the period 1772 – 1877 and average monthly temperature in the period 1659 – 1771 are applied to the temperature data to increase the model accuracy. The daily minimum and maximum RH for the period 1605 – 2015 are obtained using the Brown Gallery maximum and minimum temperatures respectively, and assuming that the daily dew point temperature at Knole is calculated by subtracting a monthly-dependent constant from the daily minimum temperature at Knole, thus enabling the calculation of the daily actual water vapour pressure of air. Changes in RH are a result of the daily temperature cycle changing the saturation vapour pressure of air in the gallery. This data is valuable as it enables a study of the effects of low-cycle fatigue on the 17th century panel paintings housed in the Brown Gallery at Knole House, Kent due to these temperature and relative humidity cycles. Furthermore, the method presented offers a technique that can be utilised to replicate the internal environment for any unheated monument building so that the effects of past and future temper

Journal article

Charalambides M, Skamniotis C, Matthew E, 2019, Computer simulations of food oral processing to engineer teeth cleaning, Nature Communications, Vol: 10, Pages: 1-12, ISSN: 2041-1723

Oral biofilm accumulation in pets is a growing concern. It is desirable to address this problem via non-invasive teeth cleaning techniques, such as through friction between teeth and food during chewing. Therefore, pet food design tools are needed towards optimising cleaning efficacy. Developing such tools is challenging, as several parameters affecting teeth cleaning should be considered: the food’s complex mechanical response, the contacting surfaces topology as well as the wide range of masticatory and anatomical characteristics amongst breeds. We show that Finite Element (FE) models can efficiently account for all these parameters, through the simulation of food deformation and fracture during the first bite. This reduces the need for time consuming and costly in-vivo or in-vitro trials. Our in-silico model is validated through in-vitro tests, demonstrating that the initial oral processing stage can be engineered through computers with high fidelity.

Journal article

Zhou J, Liu J, Zhang X, Yan Y, Jiang L, Mohagheghian I, Dear J, Charalambides Met al., 2019, Experimental and numerical investigation of high velocity soft impact loading on aircraft materials, Aerospace Science and Technology, Vol: 90, Pages: 44-58, ISSN: 1270-9638

Bird strike on aircraft remains a serious threat to flight safety. Experimental investigations employing real birds are associated with high cost and low reproducibility. Therefore, physical substitute materials are often used instead of real birds. This study investigates the soft impact loading on aluminium and laminated glass targets from ballistic gelatine and rubber projectiles. The two targets simulate strike on the aircrafts' fuselage and windshield respectively. The full field out of plane displacements of the targets were recorded for velocities 110 to 170 m s−1 using digital image correlation during gas gun experiments. A simulation model based on Smoothed Particle Hydrodynamics was developed and validated against the experimental data from all four projectile-target material combinations. It was shown that for the same momentum, a rubber projectile exerts a higher pressure on a target as compared to gelatine, even though the out of plane displacements and in-plane strains are similar. This led to fractures in the impacted laminated glass when rubber was used. The study offers new experimental data as well as efficient design modelling tools to mitigate damage imposed during bird strike. The models provide a way towards enabling the optimisation of real, large scale aircraft structures and components.

Journal article

Skamniotis CG, Elliott M, Charalambides MN, 2019, On modelling the constitutive and damage behaviour of highly non-linear bio-composites - Mesh sensitivity of the viscoplastic-damage law computations, International Journal of Plasticity, Vol: 114, Pages: 40-62, ISSN: 0749-6419

The large strain fracture of non-linear complex solids concerns a wide range of applications, such as material forming, food oral processing, surgical instrumental penetration as well as more recently, the design of biodegradable composites for packaging and bio-medical use. Although simulations are a powerful tool towards understanding and designing such processes, modelling ductile fracture in materials such as soft natural composites imposes a new challenge, particularly when the fracture patterns cannot be pre-defined. Here we bring to light new information on these aspects of benefit to the multidisciplinary community, by characterising and modelling the deformation and fracture of short cellulose fibre starch extruded composites. Hyperviscoelastic-Mullins damage laws show merits in modelling such complex systems. Yet they are inferior to a viscoplastic-damage law able to capture exactly their highly non-linear, rate dependent and pressure dependent pseudo-plastic stress-strain response. The viscoplastic-damage law also predicts fracture based on experimental toughness values without pre-specifying the crack path in a Finite Element (FE) model, displaying superiority over the conventional cohesive zone approach. Yet, despite using a toughness parameter to drive crack propagation, spurious mesh dependency is still observed while other previously unreported sources of error imposed by the finite element aspect ratio are also highlighted. The latter is rectified by developing a novel numerical strategy for calculating the characteristic element length used in the damage computations. Inherent mesh dependency suggests that non-local damage models may be essential to model this newly investigated class of natural composites.

Journal article

Wood JD, Gauvin C, Young CRT, Taylor AC, Balint DS, Charalambides MNet al., 2018, Cracking in paintings due to relative humidity cycles, 22nd European Conference on Fracture (ECF) - Loading and Environmental Effects on Structural Integrity, Publisher: Elsevier B.V., Pages: 379-384, ISSN: 2452-3216

A numerical study is performed using the finite element method to consider the effects of low-cycle fatigue, specifically induced through relative humidity cycles on paintings. It has been identified that there are two major crack types in paintings, these being (i) an interfacial crack (delamination) between paint and support and (ii) a through-thickness (channel) crack in the paint layer itself, arresting on the interface. Therefore a 2D plane strain model for each type of crack has been created, which both consist of an alkyd paint modelled using a visco-hyperelastic material model and a primed canvas which is assumed to behave in a linear elastic manner. To account for fatigue damage in both models, cohesive elements located along the interface or through the film thickness respectively, are used and the traction-separation law has been modified to incorporate a fatigue damage parameter. It is possible to expose the models to the same relative humidity cycles, which would typically be seen in museums, enabling the prediction of time to first crack and which crack type is more readily grown in the painting.

Conference paper

Butt S, Mohammed I, Raghavan V, Powell H, Osborne J, Charalambides Met al., 2018, Quantifying the differences in structure and mechanical response of confectionery products resulting from the baking and extrusion processes, Journal of Food Engineering, Vol: 238, Pages: 112-121, ISSN: 0260-8774

Extrusion has potential advantages over baking in terms of throughput, asset cost and flexibility. However, it is challenging to achieve through extrusion the “light, crispy” texture of a more traditional baked confectionery. This study compares and contrasts for the first time confectionery products produced through these two processes, i.e. baking and extrusion. The microstructural differences are measured using imaging techniques, i.e. Scanning Electron Microscopy (SEM) and X-Ray Tomography (XRT) whereas mechanical characterisation is used to highlight differences in the resulting mechanical properties. Crucial information is presented which shows that the two technologies result in different mechanical properties and microstructures, even if the level of porosity in the two products is kept constant. In addition, confectionery products whether they are produced through baking or extrusion, have irregular geometries. The latter makes mechanical characterisation a real challenge. Therefore this study also presents rigorous methods for measuring true mechanical properties such that meaningful and valid comparisons may be made. The accuracy of the chosen methodologies is verified through experiments using flat and tubular extruded geometries as well as testing the products in various directions. It was concluded that the manufacturing method and, in the case of extrusion, the initial moisture content influences the microstructure and mechanics of confectionery products, both of which have an impact on consumer sensory perception.

Journal article

Skamniotis C, Patel Y, Elliott M, Charalambides Met al., 2018, Toughening and stiffening of starch food extrudates through the addition of cellulose fibres and minerals, Food Hydrocolloids, Vol: 84, Pages: 515-528, ISSN: 0268-005X

Pet food, one of the largest type of commercial packaged foods, continuously sets new challenges, amongst them the possibility to enhance palatability via adjusting product composition. This will optimise texture perception across consumer groups of diverse chewing capabilities, as well as improve food oral breakdown efficiency with further impact on metabolic health and nutrient bioavailability in the digestive process. Our aim is to pioneer new methods of controlling texture by answering longstanding questions such as the impact of nutrients on the mechanical properties of foods. The impact of cellulose fibres and minerals on the fracture toughness and stiffness properties of starch food extrudates is investigated for the first time through employing tensile tests and two fracture toughness tests namely Essential Work of Fracture (EWF) and cutting, on four different compositions. Fibres alone are found to increase stiffness (stiffening) and toughness (toughening) whereas minerals decrease stiffness (softening) with a minor influence on toughness. Interestingly, fibres and minerals combined maximise toughening at 28% compared to pure starch, due to the synergistic effect of fibre-matrix de-bonding and fibre breakage mechanisms at the crack tip. These new results indicate that texture can be significantly altered through the addition of minerals and short fibres. Such information is critical in the design of products that need to satisfy both nutritional and textural criteria.

Journal article

Mohagheghian I, Charalambides M, Wang Y, Jiang L, Zhang X, Yan Y, Kinloch A, Dear JPet al., 2018, Effect of the polymer interlayer on the high-velocity soft impact response of laminated glass plates, International Journal of Impact Engineering, Vol: 120, Pages: 150-170, ISSN: 0734-743X

The choice of the polymer interlayer is a key consideration for laminated aircraft windshields. Such windshields often employ chemically strengthened glasses and are required to withstand impact by birds, hail-stones and other foreign bodies. In the present study, windshields employing three different polymer interlayer materials were investigated under high-velocity impact by a soft projectile: Thermoplastic Polyurethane (TPU), Polyvinyl Butyral (PVB) and Ionoplast interlayer-SentryGlas® Plus (SGP). Parameters such as the polymer interlayer type and thickness, multi-layering the interlayer and the sensitivity of the behaviour of the windshield to the environmental temperature were studied. The performance was assessed through a series of laboratory-scale impact experiments (using a bird-substitute material) and modelled via finite element simulations (using a smoothed particle hydrodynamics approach). The experimental and numerical results were found to be in good agreement for the three polymer interlayers investigated. The polymer interlayer type was found to have the most significant effect on both the deformation and the failure of the laminated glass windows at room temperature, i.e. 25 °C. However, the influence of the polymer interlayer type became less pronounced at lower temperatures. The novel modelling that has been developed assists in the choice of the best polymer interlayer, including the multi-layering of interlayers, for complex windshield designs.

Journal article

Charalambides M, Li-Mayer J, Martinez M, Lambros Jet al., Determination of Mixed-Mode Cohesive Zone Failure Parameters Using Digital Volume Correlation and the Inverse Finite Element Method, 17th International Conference on Fracture and Damage Mechanics

Conference paper

Zhang R, Li-Mayer J, Charalambides M, 2018, Development of an image-based numerical model for predicting the microstructure-property relationship in Alumina Trihydrate (ATH) filled Poly (methyl methacrylate) (PMMA), International Journal of Fracture, Vol: 211, Pages: 125-148, ISSN: 0376-9429

Particulate composites are found in a wide range of applications. Their heterogeneous microstructure affects their bulk behavior and structural performance, however tools for predicting this important structure-property relationship are still lacking. In this study, a numerical method that can provide predictions of the mechanical response of a particulate polymeric matrix composite as a function of volume fraction and particle mean diameter is presented. The work is derived for an alumina trihydrate filled poly(methyl methacrylate) but the methodology is generic and can be used for any particulate composite. Representative Volume elements are determined through images obtained from scanning electron microscopy. The model takes into account the possibility of failure through interface debonding as well as cracks through the matrix. The model predictions for the modulus and fracture strength of the composites are validated through independent experiments on the composite. The numerical results are also used to qualitatively explain the trends measured regarding the fracture toughness of the composites. Compared to other literature on particulate composites, our study is the first to report accurate stress–strain distributions as well as fracture predictions whilst all the necessary model parameters defining the failure criteria are all derived through independent experiments. This paves the way for a relatively simple methodology for determining structure-property relationships in composites design, enabling smarter material utilization and optimal mechanical properties.

Journal article

Kinloch AJ, Rizi K, Mohammed IK, Charalambides MN, Murdan K, Xu Ket al., 2018, A systematic approach to the formulation of anti-onychomycotic nail patches, European Journal of Pharmaceutics and Biopharmaceutics, Vol: 127, Pages: 355-365, ISSN: 0939-6411

Nail patches have a potential role as drug carriers for the topical treatment of nail diseases such as onychomycosis, a common condition. Our aim was therefore to develop a systematic and novel approach to the formulation of a simple drug-in-adhesive ungual patch. Twelve pressure-sensitive adhesives (PSAs), four backing membranes, two release liners and three drugs were screened for pharmaceutical and mechanical properties. From this initial screening, two PSAs, two drugs, one backing membrane and one release liner were selected for further investigation. Patches were prepared by solvent-casting and characterised. The patches had good uniformity of thickness and of drug content, and showed minimal drug crystallisation during six months of storage. Meanwhile, the drug stability in the patch upon storage and patch adhesion to the nail was influenced by thenature of the drug, the PSA and the backing membrane. The reported methodology paves the way for a systematic formulation of ungual nail patches to add to the armamentarium of nail medicines. Further, from this work, the best patch formulation has been identified.

Journal article

Skamniotis CG, Elliott M, Charalambides MN, 2017, On modeling the large strain fracture behaviour of soft viscous foods, Meeting of the Institute-of-Non-Newtonian-Fluid-Mechanics (INNFM), Publisher: AIP Publishing, ISSN: 1070-6631

Mastication is responsible for food breakdown with the aid of saliva in order to form a cohesive viscous mass, known as the bolus. This influences the rate at which the ingested food nutrients are later absorbed into the body, which needs to be controlled to aid in epidemic health problems such as obesity, diabetes, and dyspepsia. The aim of our work is to understand and improve food oral breakdown efficiency in both human and pet foods through developing multi-scale models of oral and gastric processing. The latter has been a challenging task and the available technology may be still immature, as foods usually exhibit a complex viscous, compliant, and tough mechanical behaviour. These are all addressed here through establishing a novel material model calibrated through experiments on starch-based food. It includes a new criterion for the onset of material stiffness degradation, a law for the evolution of degradation governed by the true material’s fracture toughness, and a constitutive stress-strain response, all three being a function of the stress state, i.e., compression, shear, and tension. The material model is used in a finite element analysis which reproduces accurately the food separation patterns under a large strain indentation test, which resembles the boundary conditions applied in chewing. The results lend weight to the new methodology as a powerful tool in understanding how different food structures breakdown and in optimising these structures via parametric analyses to satisfy specific chewing and digestion attributes.

Conference paper

Charalambides M, Skamniotis, Elliott M, On modelling the large indentation fracture behaviour of incompressible soft viscous food structures, Physics of Fluids, ISSN: 1070-6631

Journal article

Butt S, Charalambides M, Mohammed IK, Powell Het al., 2017, A Comparison of the Mechanical and Sensory Properties of Baked and Extruded Confectionery Products, 20th International ESAFORM Conference on Material Forming, Publisher: AIP Publishing, ISSN: 1551-7616

Traditional baking is the most common way of producing confectionery wafers, however over the past few decades, the extrusion process has become an increasingly important food manufacturing method and is commonly used in the manufacturing of breakfast cereals and filled snack products. This study aims to characterise products made via each of these manufacturing processes in order to understand the important parameters involved in the resulting texture of confectionery products such as wafers. Both of the named processes result in brittle, cellular foams comprising of cell walls and cell pores which may contain some of the confectionery filling. The mechanical response of the cell wall material and the geometry of the products influence the consumer perception and preference. X-Ray micro tomography (XRT) was used to generate geometry of the microstructure which was then fed to Finite Element (FE) for numerical analysis on both products. The FE models were used to determine properties such as solid modulus of the cell walls, Young’s modulus of the entire foam and to investigate and compare the microstructural damage of baked wafers and extruded products. A sensory analysis study was performed on both products by a qualified sensory panel. The results of this study were then used to draw links between the mechanical behaviour and sensory perception of a consumer. The extruded product was found to be made up of a stiffer solid material and had a higher compressive modulus and fracture stress when compared to the baked wafer. The sensory panel observed textural differences between the baked and extruded products which were also found in the differences of the mechanical properties of the two products.

Conference paper

Skamniotis CG, Charalambides MN, Elliott M, 2017, Chewing as a forming application: A viscoplastic damage law in modelling food oral breakdown, 20th International ESAFORM Conference on Material Forming, Publisher: AIP Publishing, ISSN: 1551-7616

The first bite mechanical response of a food item resembles compressive forming processes, where a tool is pressed into a workpiece. The present study addresses ongoing interests in the deformations and damage of food products, particularly during the first bite, in relation to their mechanical properties. Uniaxial tension, compression and shear tests on a starch based food reveal stress-strain response and fracture strains strongly dependent on strain rate and stress triaxiality, while damage mechanisms are identified in the form of stress softening. A pressure dependent viscoplastic constitutive law reproduces the behavior with the aid of ABAQUS subroutines, while a ductile damage initiation and evolution framework based on fracture toughness data enables accurate predictions of the product breakdown. The material model is implemented in a Finite Element (FE) chewing model based on digital pet teeth geometry where the first bite of molar teeth against a food item is simulated. The FE force displacement results match the experimental data obtained by a physical replicate of the bite model, lending weight to the approach as a powerful tool in understanding of food breakdown and product development.

Conference paper

Chong HM, Mohammed IK, Linter B, Allen R, Charalambides MNet al., 2017, Mechanical and microstructural changes of cheese cracker dough during baking, LWT- Food Science and Technology, Vol: 86, Pages: 148-158, ISSN: 0023-6438

Baked food snacks constitute an important market as a popular consumer product. The mechanical properties of cheese cracker dough at different stages of baking have been investigated as they can relate to the product's texture. The change in mechanical properties during baking was measured whilst the corresponding changes in microstructure were recorded using cryo-SEM at several interrupted baking conditions. The initial modulus of the dough increased with baking time due to starch melting or gelatinisation, melting of fat globules and evaporation of water. Simultaneously gas cells were found to begin forming. The data derived from the uniaxial compression, tension and shear experiments showed that the dough exhibited a rate dependent behaviour at all stages of baking with a power law index of approximately 0.2. Rheometric tests under dynamic heating conditions were also performed and it was found that the modulus decreased significantly, from 150 kPa to 10 kPa, with the initial rise in temperature. This study provides useful data for understanding the evolution of microstructure and rheology during the baking process and its impact on the texture of the final product.

Journal article

Vandenberghe E, Charalambides MN, Mohammed IK, De Ketelaere B, De Baerdemaeker J, Claes Jet al., 2017, Determination of a critical stress and distance criterion for crack propagation in cutting models of cheese, Journal of Food Engineering, Vol: 208, Pages: 1-10, ISSN: 0260-8774

A critical stress at a critical distance crack propagation criterion is a good way to model the fracture in cheese. This physical criterion states that the crack-tip node debonds when the stress at a specified distance ahead of the crack tip on the assumed crack path reaches a critical value. Although this criterion is already used in other research domains, no consistent information exists on how the critical stress and distance should be determined.A repeatable method for the determination of this criterion which combines experimental and numerical single edge notched bending tests was acquired. This criterion was validated with wire cutting experiments of cheese. The experimental and numerical results showed the same trend with a clear wire indentation and steady state cutting phase. The determination of a critical stress and distance criterion as proposed in this research is a good approach to model fracture and cutting of cheese.

Journal article

Skamniotis C, Kamaludin MA, Elliott M, Charalambides Met al., 2017, A novel essential work of fracture experimental methodology for highly dissipative materials, Polymer, Vol: 117, Pages: 167-182, ISSN: 0032-3861

Determining fracture toughness for soft, highly dissipative, solids has been a challenge for several decades. Amongst the limited experimental options for such materials is the essential work of fracture (EWF) method. However, EWF data are known to be strongly influenced by specimen size and test speed. In contrast to time-consuming imaging techniques that have been suggested to address such issues, a simple and reproducible method is proposed. The method accounts for diffuse dissipation in the specimen while ensuring consistent strain rates by scaling both the sample size and testing speed with ligament length. We compare this new method to current practice for two polymers: a starch based food and a polyethylene (PE) tape. Our new method gives a size independent and more conservative fracture toughness. It provides key-data, essential in numerical models of the evolution of structure breakdown in soft solids as seen for example during oral processing of foods.

Journal article

Mohagheghian I, Wang Y, Zhou J, Yu L, Guo X, Yan Y, Charalambides MN, Dear JPet al., 2017, Deformation and damage mechanisms of laminated glass windows subjected to high velocity soft impact, International Journal of Solids and Structures, Vol: 109, Pages: 46-62, ISSN: 1879-2146

Bird strike can cause serious risks to the safety of air travel. In this paper, the aim is to improve design by determining deformation and damage mechanisms of laminated glass windows when subjected to high velocity soft impacts. To achieve this, laboratory-scale impact experiments using bird substitute materials were performed in the velocity range of 100–180 m s−1. An important step forward is that high-speed 3D Digital Image Correlation (DIC) has effectively been employed to extract the full-field deformation and strain on the back surface of the specimens during impact. The finite element simulations were performed in Abaqus/explicit using Eulerian approach and were able to represent successfully the experiments.For the laminated glass structures investigated, the damage inflicted is strongly sensitive to the nose shape of the projectile and most deleterious is a flat-fronted projectile. Two threshold velocities for impact damage have been identified associated with firstly the front-facing and secondly the rear-facing glass layer breaking. The order of the glass layers significantly influences the impact performance. The findings from this research study have led to a deeper and better-quantified understanding of soft impact damage on laminated glass windows and can lead to more effective design of aircraft windshields.

Journal article

P Mohammed MA, Wanigasooriya L, Chakerabarti-Bell S, Charalambides MNet al., 2016, Extrusion of unleavened bread dough: experiments and simulations, Journal of Rheology, Vol: 61, ISSN: 0148-6055

An experimental and numerical study on ram extrusion of bread dough was conducted in order to develop predictive models for the pressures involved, as well as the deformation of the extruded dough. Such studies are needed as high pressures can potentially lead to significant degassing, tearing and shearing of the dough and hence poor bread quality; the latter limits the use of extrusion processes which would otherwise be a cost – effective forming process. A laboratory extrusion rig was designed, with dies of varying angles and exit radii. Rate dependent behaviour was observed from tests conducted at different extrusion speeds, and higher extrusion pressure was reported for dies with smaller exit radius or larger die angle. A simulation of extrusion was performed to predict the extrusion pressure as well as the extrudate swell, as a function of die geometry and extrusion rate. A continuum approach was taken in the constitutive model of dough which is a starch filled system in a protein matrix. A nonlinear viscoelastic model combined for the first time with the Mullins model for filled rubbers is found to capture the continuum behaviour well. A Coulomb friction law combined with a maximum shear stress limit was used to describe the contact definition between the extrusion barrel and the dough. Higher die angles and higher extrusion speeds require higher shear stress limit values for the model and the experiments to agree. A possible reason for this is that the shear stress limit depends on maximum strain as well as strain rate imposed during the extrusion process. Static zones were observed both experimentally and numerically. The onset of the static zones was predicted well but quantifying the geometry of the latter needs further studies.

Journal article

Charalambides M, P Mohammed MA, Wanigasooriya L, 2016, Experimental and Numerical Investigation of Ram Extrusion of Bread Dough, ESAFORM2106, Publisher: AIP Publishing, Pages: 180004-180004, ISSN: 0094-243X

An experimental and numerical study on ram extrusion of bread dough was conducted. A laboratory ram extrusion rig was designed and manufactured, where dies with different angles and exit radii were employed. Rate dependent behaviour was observed from tests conducted at different extrusion speeds, and higher extrusion pressure was reported for dies with decreasing exit radius. A finite element simulation of extrusion was performed using the adaptive meshing technique in Abaqus. Simulations using a frictionless contact between the billet and die wall showed that the model underestimates the response at high entry angles. On the other hand, when the coefficient of friction value was set to 0.09 asmeasured from friction experiments, the dough response was overestimated, i.e. the model extrusion pressure was much higher than the experimentally measured values. When a critical shear stress limit, maxW, was used, the accuracy of themodel predictions improved. The results showed that higher die angles require higher maxWvalues for the model and theexperiments to agree.

Conference paper

Kinloch AJ, mohammed IK, Charalambides MN, 2016, Modelling the Peeling Behavior of Soft Adhesives, 21st European Conference on Fracture, ECF21, 20-24 June 2016, Catania, Italy, Publisher: Elsevier, Pages: 326-333

Peel tests were performed on pharmaceutical drug patches which consisted of a polyester backing membrane supporting an acrylic pressure-sensitive adhesive (PSA) (without and with an anti-fungal drug present) adhered to a polyethylene substrate. Interfacial separation of the PSA from the polyethylene substrate was observed in most cases. Finite element (FE) peeling simulations were conducted which characterized the backing-membrane as an elasto-plastic power-law material, the PSA as a viscoelastic material and the interfacial properties with a cohesive zone model (CZM). The mechanical response of the backing membrane and the PSA were measured from tensile experiments while the rate-dependent cohesive zone parameters, i.e. the fracture energy and maximum stress, were measured directly from poker-chip probe tack tests. The numerical results from the CZM/FE simulations and the experimental values of the peel forces as a function of the peel angle, peel speed and PSA thickness were found to be in good agreement. Two different anti-fungal drugs were added to the PSA and the influence of the drug was investigated using contact angle measurements, tensile tests, dynamic mechanical analysis and peel tests.

Conference paper

Williams JG, Atkins AG, Charalambides MN, Lucas PWet al., 2016, Cutting science in biology and engineering, Interface Focus, Vol: 6, ISSN: 2042-8901

On 26–27 October 2015, the Theo Murphy international scientific meetingon ‘Cutting science in biology and engineering’ was held at the KavliRoyal Society Centre, Chicheley Hall, Buckinghamshire, UK. The meetingwas organized by Professor Gordon Williams FREng FRS, Professor TonyAtkins FREng, Professor Peter Lucas and Dr Maria Charalambides and itwas enabled through the Royal Society scientific programme. It connectedscientists from diverse backgrounds and disciplines including Biology andMechanical Engineering from around the world.

Journal article

Skamniotis, Patel Y, Charalambides MN, Elliott Met al., 2016, Fracture investigation in starch based foods, Interface Focus, Vol: 6, ISSN: 2042-8901

The study of oral processing and specifically cutting of the food piece during mastication can lead towards optimisation of products for humans or animals. Food materials are complex bio-composites with highly nonlinear constitutive response. Their fracture properties have not been largely investigated as yet while the need for models capable of predicting food breakdown increases. In this study, the blade cutting and the essential work of fracture (EWF) methodologies assessed the fracture behaviour of starch based pet-food. Tensile tests revealed rate dependent stiffness and stress softening effects, attributed to viscoplasticity and micro-cracking, respectively. Cutting data were collected for 5, 10 and 30 mm/s sample feed rates, whereas the EWF tests were conducted at 1.7, 3.3 and 8.3 mm/s crosshead speeds corresponding to average crack speeds of 4, 7 and 15 mm/s respectively. A reasonable agreement was achieved between cutting and EWF, reporting 1.26, 1.78, 1.76 kJ/m² and 1.52, 1.37, 1.45 kJ/m² values, respectively, for the corresponding crack speeds. These toughness data were used in a novel numerical model simulating the ‘first’ bite mastication process. A viscoplastic material model is adopted for the food piece, combined with a damage law which enabled predicting fracture patterns in the product.

Journal article

Mohammed IK, Charalambides MN, Kinloch AJ, 2016, Modeling the effect of rate and geometry on peeling and tack of pressure-sensitive adhesives, Journal of Non-Newtonian Fluid Mechanics, Vol: 233, Pages: 85-94, ISSN: 1873-2631

A model is developed for predicting separation along interfaces of pressure sensitive adhesives. Many authors have used the cohesive zone approach to solve such problems but the parameter calibration of such models remains uncertain. This study reports a novel method for determining such parameters. In addition, it provides crucial evidence for the suitability of the cohesive zone model approach in modelling interface fractures.Peel tests were performed at various rates using specimens which consisted of a polyester backing membrane supporting an acrylic pressure-sensitive adhesive (PSA) adhered to a polyethylene substrate. Interfacial separation of the PSA from the polyethylene substrate was observed. Finite element (FE) peeling simulations were conducted which modeled the backing-membrane as an elasto-plastic power-law material, the adhesive as a viscoelastic material and the interfacial properties with a cohesive zone model (CZM). The material properties of the backing membrane and the pressure-sensitive adhesive were measured from tensile and stress relaxation experiments. The rate-dependent CZM parameters were measured directly from poker-chip probe-tack tests which were performed at pull-off speeds which corresponded to the rates employed for the peel tests. The effect of the PSA thickness and test rate on both tack and peel was investigated experimentally, as well as modeled numerically. Good agreement was found between the experimentally measured and numerically predicted peel forces for different peel angles, speeds and PSA thicknesses. In addition, it was proven that the rate dependence observed in the peel and probe-tack data was dominated by the rate dependence of the interface properties, i.e. the time dependence of the two CZM parameters of maximum stress and fracture energy, rather than the time-dependent bulk viscoelasticity of the PSA peel arm.

Journal article

Hagan EWS, Charalambides MN, Young CRT, Learner TJSet al., 2015, The effects of strain rate and temperature on commercial acrylic artist paints aged one year to decades, Applied Physics A - Materials Science & Processing, Vol: 121, Pages: 823-835, ISSN: 1432-0630

Acrylic artist paints are viscoelastic composites containing a high molecular weight copolymer, pigment and a variety of additives. The glass transition temperature of the latex binder is typically slightly below ambient conditions, giving mechanical properties that are strongly dependent on strain rate and temperature. In previous work, the viscoelastic behaviour of custom-formulated latex artist paints was reported for films with known volume fractions of pigment using data from uniaxial tensile tests at different strain rates and temperatures. Secant Young’s modulus and failure strain master curves were constructed for each film through time-temperature superposition, allowing predictions beyond the experimental timescale at a selected reference temperature. A similar analysis is now presented for a small set of commercial artist paints tested at ages of 1 and 27 years. Experimental shift factor values are reported with fits to the Arrhenius, WLF and Vogel Fulcher equations, along with a comparison with published data for acrylic polymers. The tensile results highlight a spectrum of properties that acrylic paints may exhibit—brittle glass to hyperelastic—depending on the conditions during deformation. Strong similarities are shown between products from different manufacturers, and the findings suggest a high degree of stability with age. A method for predicting failure as a function of strain rate and temperature is also presented, and the methodology gives a framework for investigating other artist materials and the factors influencing their mechanical properties.

Journal article

Arora H, Tarleton E, Li-Mayer J, Charalambides M, Lewis Det al., 2015, Modelling the damage and deformation process in a plastic bonded explosive microstructure under tension using the finite element method, Computational Materials Science, Vol: 110, Pages: 91-101, ISSN: 0927-0256

Modelling the deformation and failure processes occurring in polymer bonded explosives (PBX)and other energetic materials is of great importance for processing methods and lifetime storagepurposes. Crystal debonding is undesirable since this can lead to contamination and a reductionin mechanical properties. An insensitive high explosive (PBX-1) was the focus of the study.This binary particulate composite consists of (TATB) filler particles encapsulated in a polymericbinder (KELF800). The particle/matrix interface was characterised with a bi-linear cohesive law,the filler was treated as elastic and the matrix as visco-hyperelastic. Material parameters weredetermined experimentally for the binder and the cohesive parameters were obtained previouslyfrom Williamson et al. (2014) and Gee et al. (2007) for the interface. Once calibrated, the materiallaws were implemented in a finite element model to allow the macroscopic response of thecomposite to be simulated. A finite element mesh was generated using a SEM image to identifythe filler particles which are represented as a set of 2D polygons. Simulated microstructureswere also generated with the same size distribution and volume fraction only with the idealisedassumption that the particles are a set of circles in 2D and spheres in 3D. The various modelresults were compared and a number of other variables were examined for their influence on theglobal deformation behaviour such as strain rate, cohesive parameters and contrast between fillerand matrix modulus. The overwhelming outcome is that the geometry of the particles plays acrucial role in determining the onset of failure and the severity of fracture in relation to whetherit is a purely local or global failure. The model was validated against a set of uniaxial tensiletests on PBX-1 and it was found that it predicted the initial modulus and failure stress and strainwell.Keywords: Particulate composites, High volume fraction, Finite Element Analysis,Micromechanics, Fract

Journal article

Zhang R, 2015, Microstructure-property relationships in alumina trihydrate filled poly (methyl methacrylate) composite materials, 2015 Global Conference on Polymer and Composite Materials (PCM 2015), Publisher: IOP Publishing: Conference Series, ISSN: 1757-899X

The mechanical properties (Young's modulus and fracture toughness) of composite made from a poly (methyl methacrylate) (PMMA) matrix filled with alumina trihydrate(ATH) are reported. The experiments were performed using flexural tests and single edge notched bend (SENB) tests. The composites samples were tested at a range of filler volume fractions (34.7%, 39.4% and 44.4%) and mean filler diameters (8 pm, 15 pm and 25 pm). The data of Young's modulus agreed well with the results of Lielens model and finite element analysis (FEA) model.

Conference paper

Forte AE, D'Amico F, Charalambides MN, Dini D, Williams JGet al., 2015, Modelling and experimental characterisation of the rate dependent fracture properties of gelatine gels, FOOD HYDROCOLLOIDS, Vol: 46, Pages: 180-190, ISSN: 0268-005X

Journal article

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