Publications
208 results found
Fan W, Yang H, Mao S, et al., 2024, Numerical analysis of fracture in core-shell particle reinforced composites, Composites Science and Technology, Vol: 250, ISSN: 0266-3538
The fracture of core-shell particle reinforced composites was simulated using a crack phase field method. A pre-existing crack may grow along the interface between the core and shell (core debonding), may penetrate the shell and the core (trans-core fracture), or may grow in the matrix (brittle fracture in the matrix). Shell fracture behaviours resulting from the competition between the fracture resistance and fracture driving force are crucial to the crack growth mechanisms. It was found that moderate strength and low modulus of the shell favoured trans-core fracture, and low strength and high modulus of the shell favoured core debonding. The effects on the overall mechanical properties were also identified. The composite strength usually benefited from stiffer and tougher components, regardless of the crack growth mechanisms, while the composite toughness was more complex. The toughening effect was highly related to how the crack evolved, as core debonding improved the toughness at the significant cost of strength, and trans-core fracture contributed most to the toughness with a slight reduction in strength compared with the matrix. These findings indicate design strategies for epoxy composites which can achieve a balance of strength and toughness, enabling the design of safer lightweight composites for electric vehicles and transport applications.
Yang H, Liu Z, Xia Y, et al., 2024, Mechanical properties of hierarchical lattice via strain gradient homogenization approach, Composites Part B: Engineering, Vol: 271, ISSN: 1359-8368
With the advancement of 3D printing technology, manufacturing metamaterials with extreme mechanical properties is becoming more feasible. Hierarchical lattices appear to be ideal candidates for obtaining desirable lightweight, high specific stiffness, and enhanced specific energy absorption. They can be constructed by architected substructures at multiple length scales. The interpretations of the underlying deformation mechanisms are necessary in order to manipulate with expected mechanical properties. In this paper, experiments were conducted to examine the effective mechanical behaviors of hierarchical lattice metamaterials. A strain gradient homogenization approach has been employed to correlate their morphological characteristics to the resulting material properties. The effective stiffness tensors are identified including the fourth-, fifth-, and sixth-order stiffness tensors. The higher-order inertial parameters are determined by a fitting procedure. Comparisons between direct finite element analyses and the homogenized strain gradient continua have been made for hierarchical lattice materials under static and dynamic loads. It is found that strain gradient homogenization approach can be an accurate, efficient and reliable way of predicting the mechanical behaviors of hierarchical lattice. A systematic analysis of geometrical parameters was conducted to uncover the underlying mechanisms responsible for the enhancement of effective properties. This work offers a novel approach for designing the mechanical properties of hierarchical metamaterials through precise control of secondary structure types and distributions.
Charalambides M, Taylor A, Young C, et al., 2023, A numerical model for predicting the time for crack initiation in wood panel paintings under low-cycle environmentally induced fatigue, Journal of Cultural Heritage, Vol: 61, Pages: 23-31, ISSN: 1296-2074
Determining the storage and display conditions for historical panel (wood) paintings requires a balance between ensuring the painting's preservation whilst also considering the energy consumption associated with climate control. The latter has become very important due to the need to lower the carbon footprint of museums and historical houses. In order to address this need, we have developed numerical models based on finite element analysis to simulate the initiation of two types of potential damage in panel paintings, namely interfacial and channelling cracks in the oil paint layer, under cyclically varying relative humidity. These models are based on our case study at Knole House (National Trust), Kent. Using known data for the past environment in which the paintings within the Brown Gallery at Knole House have been exposed, the ambient RH variation was approximated by three cycles, i.e., annual, biannual, and monthly varying cycles. Four RH cases, one containing all three cycles and each of the other three cases containing just two of the three cycles, were applied as boundary conditions to simplified geometries of the panel paintings in an effort to investigate the effects of the frequency and the amplitude of the variation on the possibility of cracking in the painting. The models need several material parameters as input which are not all available. Therefore, the study also includes some parametric studies to determine possible variations in the crack initiation. According to the model predictions, the channelling crack initiates slightly earlier than the interfacial crack. The crack initiation time in an uncontrolled environment (containing all three RH cycles) predicted by the model is approximately 120 years which empirically is a realistic estimate. Furthermore, the annual RH cycle (high amplitude and low frequency) has the most significant effect on the crack initiation. By removing the annual variation from the RH cycle, the initiation of both channelli
Wang D, Hu H, Li S, et al., 2023, Sensing-triggered stiffness-tunable smart adhesives, Science Advances, Vol: 9, Pages: 1-12, ISSN: 2375-2548
Artificial dry adhesives have exhibited great potential in the field of robotics. However, there is still a wide gap between bioinspired adhesives and living tissues, especially regarding the surface adaptability and switching ability of attachment/detachment. Here, we propose a sensing-triggered stiffness-tunable smart adhesive material, combining the functions of muscle tissues and sensing nerves rather than traditional biomimetic adhesive strategy that only focuses on structural geometry. Authorized by real-time perception of the interface contact state, conformal contact, shape locking, and active releasing are achieved by adjusting the stiffness based on the magnetorheological effect. Because of the fast switching of the magnetic field, a millisecond-level attachment/detachment response is successfully achieved, breaking the bottleneck of adhesive materials for high-speed manipulation. The innovative design can be applied to any toe's surface structure, opening up a previously unknown avenue for the development of adhesive materials.
López-Cabrera HR, Figueroa-López U, Taylor AC, et al., 2023, Dynamic fracture resistance under plane strain conditions of high-density polyethylene nanoclay composites, Polymers, Vol: 15, Pages: 1-15, ISSN: 2073-4360
Polymer nanoclay composites have received significant attention due to their substantially enhanced mechanical, thermal and barrier properties. However, the effect of these nanoclays on the dynamic fracture resistance of a polymer matrix during fast fracture events has not been documented. In this study, the effect of nanoclay addition on the rapid crack propagation (RCP) resistance of high-density polyethylene (HDPE) was investigated through the high-speed double torsion test. Results showed that the addition of 1, 3, and 5% of nanoclays improved the dynamic fracture resistance under the plane strain conditions (Gd1) of HDPE up to 65%. An increase in the storage and loss modulus, and a decrease in crystallinity and melt flow index with nanoclay content was also found. Although the presence of agglomerates can hinder the enhancement of Gd1 as it promotes agglomerate fracture and debonding, the increase in energy consumption through fibrillation and crazing promoted by the nanoclay prevails, suggesting that the nanoclay's toughening effect that has been extensively reported under quasi-static and impact tests, is also present under RCP conditions, and that the HDPE nanocomposites could be used in applications in which RCP must be prevented.
Fan W, Yang H, Taylor AC, 2023, Numerical analysis of fracture in interpenetrating phase composites based on crack phase field model, Composites Science and Technology, Vol: 232, Pages: 1-9, ISSN: 0266-3538
A numerical model based on crack phase field analysis is introduced to study the quasi-static fracture process in interpenetrating phase composites (IPCs). Materials were considered elastic solids, and the interface was assumed to be perfectly bonded. Tougher and stiffer tougheners lead to more fracture in the brittle phase, but less fracture in the toughening phase. Thus, the overall fracture performance results from competition between increasing breakage in the brittle phase and declining breakage in the toughening phase. The toughening mechanisms are discussed from both stress-strain and crack topology viewpoints. The toughening phase transfers the load from the crack tip to the whole domain until the maximum stress is reached, and impeded crack growth occurs afterwards. The load transferring and impediment effects made the brittle phase engage in fracture, and several crack propagation patterns were identified for the sacrificial fracture behaviour, namely, crack deflection, crack bridging, crack branching, microcracking and crack blocking. Moreover, fracture in three different microstructures (co-continuous, particle-reinforced, laminar) was compared, and the most effective toughening morphology depends on the tougheners and the loading states. This methodology enables optimum microstructures to be identified to achieve high toughness in aerospace and energy generation applications, increasing safety and reducing weight.
Panta J, Rider AN, Wang J, et al., 2023, High-performance carbon nanofiber reinforced epoxy-based nanocomposite adhesive materials modified with novel functionalization method and triblock copolymer, Composites Part B: Engineering, Vol: 249, Pages: 1-13, ISSN: 0961-9526
New high-performance epoxy-based nanocomposite adhesive materials were developed with the innovative use of nanomaterials to enhance the bond strength, especially at elevated temperatures. A tetraglycidyl diaminodiphenylmethane (TGDDM) based aerospace-grade epoxy adhesive (EA9396) with a high glass transition temperature and viscosity was combined with functionalized CNFs and a phase-separated poly(styrene)-poly(butadiene)-poly(methyl methacrylate) triblock copolymer (SBM). The ozone-treated CNFs (OZ-CNFs) were functionalized with polyethyleneimine dendrimer (PEI + OZ-CNFs), or a polyamine hardener (H + OZ-CNFs) and characterized using X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) to confirm the chemical reaction of the amine functional groups. Room temperature mechanical testing revealed the optimized nanocomposite adhesives containing SBM and CNFs functionalized with either PEI or hardener resulted in a 40% increase in lap shear strength (50 MPa) when compared to the unmodified epoxy (35.6 MPa). At the elevated temperature of 90 °C, a strength improvement of 35% and 38% (29.5 MPa and 30 MPa) was observed for the adhesives containing SBM and CNFs functionalized using PEI or hardener, respectively. SEM images showed that CNF functionalization provided a more uniform distribution in the adhesive and enhanced the SBM plastic deformation during crack propagation. The PEI and hardener functionalization also significantly reduced the level of CNF pull-out, which was believed to enhance the energy dissipation mechanisms responsible for the significant increase in the lap shear strength.
Carolan D, He S, Taylor AC, 2023, Advances in toughening strategies for structural adhesives, Advances in Structural Adhesive Bonding, Second Edition, Pages: 251-286, ISBN: 9780323984379
The toughness of structural adhesives is the critical property impacting the in-service performance of bonded joints. This chapter discusses various types of toughening, including physical and chemical modification to the base polymer, are discussed in detail, with a focus on the mechanisms that contribute to improved toughness. Toughening strategies, such as the incorporation of filler particles, and the creation of nanostructured materials, are also examined. These strategies include toughening with rubber or silica nano- and microparticles as well as using carbon systems such as nanotubes and graphene or derivatives. The chapter also explores the various models used to predict adhesive toughness, including those based on fracture mechanics. The chapter concludes by discussing current and future research efforts focused on developing green chemistry solutions for improving adhesive toughness, as well as addressing end-of-life considerations for structural adhesives. The focus of the research community is to create more sustainable, environmentally friendly adhesives that continue to meet the high performance demands of modern engineering applications.
Mohd Yasin SB, Terry J, Taylor A, 2023, Fracture and mechanical properties of impact toughened polypropylene composite: modification for automotive dashboard–airbag application, RSC Advances: an international journal to further the chemical sciences, Vol: 13, Pages: 27461-27475, ISSN: 2046-2069
Thermoplastic olefin (TPO) is the principal material for automotive instrument panels and is prone to fracture especially under heavy airbag deployment, which can prevent the airbag deploying properly. Thus, polyolefin elastomer (POE) was introduced to improve impact properties and fracture resistance. Fundamental methods to characterise TPO with the addition of POE are proposed. The influence of POE content on the mechanical properties was examined. With increasing POE content, the storage modulus and glass transition temperature values decreased, and the damping increased due to the POE increasing the polymer chain mobility. The tensile modulus, ultimate tensile strength and yield stress decreased with increasing POE content, while the ductility of the blends significantly increased. Furthermore, the POE reduced hardness and increased energy absorption during impact. In the fracture analysis, the addition of POE content increased the fracture resistance by increasing the crack energy and decreasing the resistance to crack initiation. Fractographic analysis showed how stretched microfibrils in the blends increase the fracture resistance. These results gave a significant indication of the utility of the elastomer in improving some mechanical properties and impact toughness of the interior automotive material to resist an undesired failure or over-fracture in airbag deployment.
Kopsidas S, Olowojoba G, Stone C, et al., 2022, Lightning strike damage resistance of carbon-fiber composites with nanocarbon-modified epoxy matrices, Journal of Applied Polymer Science, Vol: 139, Pages: 1-18, ISSN: 0021-8995
Carbon-fibre reinforced polymer (CFRP) composites are replacing metal alloys in aerospace structures, but they can vulnerable to lightning strike damage if not adequately protected due to the poor electrical conductivity of the polymeric matrix. In the present work, to improve the conductivity of the CFRP, two electrically-conductive epoxy formulations were developed via the addition of 0.5 wt% of graphene nanoplatelets (GNPs) and a hybrid of 0.5 wt% of GNPs/carbon nanotubes (CNTs) at an 8:2 mass ratio. Unidirectional CFRP laminates were manufactured using resin-infusion under flexible tooling (RIFT) and wet lay-up (WL) processes, and subjected to simulated lightning strike tests. The electrical performance of the RIFT plates was far superior to that of the WL plates, independent of matrix modification, due to their greater carbon-fibre volume fraction. The GNP-modified panel made using RIFT demonstrated an electrical conductivity value of 8 S/cm. After the lightning strike test, the CFRP panel remains largely unaffected as no perforation occurs. Damage is limited to matrix degradation within the top ply at the point of impact and localised charring of the surface. The GNP-modified panel showed a comparable level of resistance against lightning damage with the existing copper mesh technology, offering at the same time a 20 % reduction in the structural weight. This indicates a feasible route to improve the lightning strike damage resistance of carbon-fibre composites without the addition of extra weight, hence reducing fuel consumption but not safety.
He S, Carolan D, Fergusson A, et al., 2022, Investigating the transfer of toughness from rubber modified bulk epoxy polymers to syntactic foams, Composites Part B: Engineering, Vol: 245, Pages: 1-15, ISSN: 0961-9526
Syntactic foams are lightweight, high specific strength materials used in the aerospace and naval10 industries. Their utility is limited by their brittleness. The epoxy polymer matrix in an epoxy/hollow11 glass microsphere (GMS) syntactic foam was modified using carboxyl-terminated butadiene-acrylonitrile12 (CTBN) rubber with the aim to increase fracture toughness. The microstructure and fracture properties13 were investigated, and compared to CTBN modified bulk epoxy polymers. The formation of complexCTBN microstructures was responsible for the increase in fracture energy, from 193 J/m214 for theunmodified syntactic foam, to 296 J/m215 at 12 wt% CTBN modification. However, this increase is muchsmaller than for the CTBN modification of bulk epoxy polymers, where an increase from 101 J/m216 to1112 J/m217 was measured for the same CTBN concentration. There is little toughness transfer from the18 bulk epoxy polymers to the syntactic foams, attributable to small interstitial regions between the GMS,19 restricting plastic zone size. A statistical approach to the analytical modelling of fracture energy in the20 bulk epoxy polymers highlights the importance of considering the underlying distribution of rubber21 particle and void sizes. The increase in fracture energy achieved in this work can increase the overall22 usefulness of syntactic foams in structural applications.
Barbera D, Young C, Charalambides M, et al., 2022, A methodology for the use of alkyd paint in thermally aged easel painting reconstructions for mechanical testing, Journal of Cultural Heritage, Vol: 55, Pages: 237-244, ISSN: 1296-2074
For the preservation of painted cultural heritage on wooden substrates, it is important to understand the fracture mechanisms in the multilayer system of which they are constructed and how the environment plays a role in the composites’ physical properties. Past research has investigated the material response of each constituent layer but much more needs to be done to represent the heterogeneous composite structure of easel paintings. In recent years fracture mechanics concepts have been applied to glue and glue/chalk multilayers. However, few experiments have been conducted on multilayers that include oil paint, due to its very long, and impractical drying time, which can be a few years up to decades depending on the type of study. The paper presents a methodology for the use of thermally aged alkyd paint in easel painting reconstructions for mechanical testing, specifically as a substitute for naturally aged traditional linseed oil paint. Elastic and failure properties of the paint have been obtained from environmentally-controlled tensile tests on thin free-film samples. To obtain the characteristic properties of increased elastic modulus and reduced ductility, a thermal ageing protocol has been experimentally developed. The results are compared with data from the published literature, theoretical models and with 30-year-old samples of cold-pressed linseed oil lead white paint tested within this research work. The final methodology provides the research community with a viable way to produce samples that can be used to understand the behaviour of a (simplified) but complete multilayer system.
Zhang R, Mohammed IK, Taylor AC, et al., 2022, A microstructure image-based numerical model for predicting the fracture toughness of alumina trihydrate (ATH) filled poly(methyl methacrylate) (PMMA) composites, Composites Part B: Engineering, Vol: 232, Pages: 109632-109632, ISSN: 1359-8368
A novel finite element model is proposed here for predicting the fracture toughness using real microstructuralimages and accounting for several parameters that can affect the crack propagation such as filler content, particleshape, particle agglomeration and particle debonding. The damage energy prior to the catastrophic failure of thewhole microstructure is taken as the energy required for crack initiation, and the fracture toughness is calculatedusing the concept of a critical crack size. The predictions agree well with the measured values of the criticalenergy release rate at 20 ◦C as a function of both volume fraction and mean particle size. In addition, a para-metric study showed that an increase in interfacial cohesive energy leads to higher fracture energies at 60 ◦C. Theproposed methodology shows great potential and can be widely applied to other particulate composites, enablingindustry to cost-effectively develop tougher, hence safer and more durable, particulate composites
Guild FJ, Tsang WL, Taylor AC, 2022, Silica nano-particle filled polymers: Debonding and microstructure, Composites Science and Technology, Vol: 218, Pages: 1-12, ISSN: 0266-3538
The use of thermosetting polymer composites and adhesives in structural applications such as electric cars is limited by the poor toughness of the polymer due to its highly crosslinked structure. The addition of a small weight fraction of silica nano-particles to thermoset polymers is a highly effective means of improving the toughness and mechanical properties of the polymer, but not seen with micro-sized particles. This effect has been investigated using experiments and numerical analysis. The ‘plasticity’ seen in experimental results can be attributed to the debonding of the particles and a debonding criterion is found to be the initiation fracture energy to debond a silica particle. The microstructure has been examined to determine the dispersion of the nano-particles both quantitively and qualitatively, and some differences have been observed. These differences have been investigated using numerical simulations, and different values of strain energy are found in the particles for different small-scale microstructures. The results of these simulations can explain the effect of adding a small weight fraction of nano-sized particles to polymers due to small scale clustering. This new analysis can increase the confidence in the ability to achieve consistently high levels of fracture toughness and mechanical properties for these nano-particle composites.
Terry JS, Taylor AC, 2021, The properties and suitability of commercial bio-based epoxies for use in fiber-reinforced composites, Journal of Applied Polymer Science, Vol: 138, Pages: 1-12, ISSN: 0021-8995
Environmental concerns about fiber composites are leading manufacturers to consider bio‐based alternatives to petroleum‐derived epoxies. Such a substitution is hindered by a lack of information, so commercially available bio‐based epoxy systems have been compared, their mechanical properties measured, and fiber composites produced by vacuum infusion. Most high bio‐based content resins for infusion use conventional curing agents. Bio‐based content is generally added using Epicerol, but also other bio‐based precursors. A diglycidyl ether of bisphenol A system produced using Epicerol achieves 20 % bio‐based content, but achieves higher contents when Epicerol is used in diluents. Fully bio‐based monomers can be deleterious to the mechanical properties and glass transition temperature (Tg), so are used sparingly. The most‐promising systems (28 % to 43 % bio‐based) compare well to conventional epoxies, possessing good strength, stiffness, toughness, and a reasonable Tg. These partially bio‐based epoxies offer an immediate lower‐carbon alternative for vacuum‐infused composites in marine, sports equipment, and wind energy.
Charalambides M, Zhang R, Taylor A, et al., 2021, A numerical investigation of interfacial and channelling crack growth rates under low-cycle fatigue in bi-layer materials relevant to cultural heritage, Journal of Cultural Heritage, Vol: 49, Pages: 70-78, ISSN: 1296-2074
In traditional and modern paintings on canvas or wood, two crack types have been identified, these are: (i) delamination between two of the many layers and (ii) channelling through the paint layer, terminating at the paint-substrate interface. One cause of this damage can be attributed to environment-induced low-cycle fatigue, specifically through relative humidity and temperature fluctuations. We present novel 2D as well as 3D finite element models that, for the first time, identify the time for each type of crack to initiate under a variety of realistic relative humidity (RH) cycles, as well as the corresponding crack growth rates. The focus is on modern paintings that have some layers executed in alkyd paint, found to be a vulnerable layer in a relatively short period of time. The paintings are idealised as a two-layer construction with a visco-hyperelastic alkyd paint layer on a linear elastic (acrylic) primed canvas substrate. Cracks, both interfacial and channelling, are represented using cohesive elements. To simulate the damage caused by a relative humidity cycle, a fatigue damage parameter was incorporated in the traction-separation law using a user-defined field. It was found that channelling cracks initiate slightly earlier than interfacial cracks for all the environmental conditions studied. Specifically, for an RH cycle of 35%–90%, channelling cracks initiate at 2.2 years and grow at an accelerating rate, while the interfacial crack initiates at 2.6 years and grows at a stable rate of approximately 0.1 mm/year. Narrower RH cycles lead to longer crack initiation times, e.g. the channelling crack initiates at 13.9 years under 40%–65% RH, and when the RH cycle was further narrowed to 45%–55%, the initiation time increased to 86 years. Our models are applicable to other painted or coated cultural heritage objects and can be used to inform preservation and environmental control strategies.
Sorce FS, Ngo S, Lowe C, et al., 2021, The effect of structure-property relationships on the formability of pigmented polyester coatings, Progress in Organic Coatings, Vol: 154, Pages: 1-12, ISSN: 0300-9440
Pre-painted metal sheet (PPM) is used in applications from domestic appliances to architectural cladding. The coating provides excellent aesthetics and corrosion protection, but must possess excellent formability to not fail due to the large strains applied during the folding and hemming processes used to produce components. Therefore it is important to understand how the coating formulation affects the free-film properties and formability of a coating system. Polyester coatings crosslinked with hexa(methoxymethyl)melamine (HMMM) with a Tg of ∼ 40 °C were used, pigmented with TiO2. The glass transition temperature was increased by increasing the crosslinker content (from 5 % to 30 %), decreasing the adipic acid content (from 24 % to 12 %) and decreasing the molecular weight (from Mn = 3300 g/mol to Mn = 1500 g/mol). The chemical structure of the resin had little effect on the formability of the coatings when the test temperature was normalised with respect to Tg. The formability measured using the Erichsen cupping and T-bend tests is related to the tensile properties of the free-films. The damage induced by the T-bend is greater than that by the Erichsen cupping test due to the higher applied deformation rates in T-bend tests. Increasing the apparent yield and fracture stress increases the likelihood of damage at lower deformation levels, whilst increasing the strain to failure decreases the likelihood of damage in both the T-bend and Erichsen cupping tests. The strength of these correlations reduces with an increase in T-bend level as the magnitudes of the strains applied are reduced. This work takes a holistic approach to correlate structure and tensile properties with formability in both the Erichsen cupping test and T-bend test for the first time, enabling industry to improve coating performance significantly but cost-effectively.
Mulakkal M, Castillo Castillo A, Taylor A, et al., 2021, Advancing mechanical recycling of multilayer plastics through finite element modelling and environmental policy, Resources, Conservation and Recycling, Vol: 166, ISSN: 0921-3449
Plastics are attracting negative publicity due to the scale of current pollution levels, yet they are irreplaceable in several applications such as food packaging, where different types of plastics are combined in laminate form to produce multilayered packaging (MLP) materials which extend the life of food items packaged within them. Increased plastic recycling is urgently needed, however for MLP it is particularly difficult. For the first time, this study combines engineering tools with environmental policy towards developing solutions for current single use plastic packaging. This study investigates recycling challenges for MLP and emerging melt-blending based mechanical recycling solutions as this is the main current method for material recovery of conventional plastics. Melt-blending of MLP with compatibilisers is explored, and the current lack of models addressing the influence of compatibilisers is identified. This gap in knowledge is addressed using novel engineering models based on the finite element (FE) micromechanical modelling technique to estimate the mechanical properties of recycled blends. Our model output is compared with experimental data available in literature and the good agreement highlights its predictive ability, providing a fast and cost-effective novel method for optimising recycled plastics. The policy aspect proposes the introduction of twenty policies based on mission-oriented innovation strategy to enable deployment of the recycling technologies studied whilst improving the viability of recycling of material currently not recycled. Implementation of these measures by the stakeholders will enable adoption of new MLP recycling techniques, create demand for recycled materials from MLP and incentivise MLP collection to mitigate pollution.
Zhang R, Stannard A, Street G, et al., 2021, Towards optimisation of rolling process of potato dough: Effect of processing on the microstructure and the mechanical properties, Journal of Food Engineering, Vol: 291, ISSN: 0260-8774
The quality of potato chips is highly dependent on the mechanical properties of the dough sheet produced prior to frying. It has been well established that poor mechanical properties result in fragile dough sheets and associated high product wastage. However, the effect of the rolling process on the mechanical properties of the dough is unknown so the optimum rolling process can only be obtained via a trial and error approach. This work reports for the first time the effects of dry flake size and rolling parameters on the mechanical performance of potato dough sheets. The laboratory scale rolling setup used a 10 cm roller diameter with a 0.2 mm gap height. Furthermore, an experimental method was developed enabling rigorous tensile testing of fragile potato dough sheets. The mechanical performance of the potato dough sheets was anisotropic, as the Young's modulus and strength were 35% and 57% higher across the rolling direction than those along the rolling direction, respectively. The formability, i.e. the ability to form a coherent sheet of the potato dough is improved by using smaller dry flakes (<500 μm). However, further decrease in the flakes size had no effect on the mechanical behaviour of potato dough sheets, i.e. flakes with diameter smaller than 212 μm showed similar tensile response to flakes smaller than 500 μm. Rolling the dough increases the coherence and the strength of the potato dough sheets, but also introduces defects orientated across the rolling direction which decrease the strength if the dough is rolled too many times. For example, sheets rolled for seven passes showed over 100% improvement in failure stress comparing to sheets rolled for five passes, but when the sheets were rolled for the eighth pass, the failure stress dropped by 17%. Due to the viscoelasticity of the dough, both the tensile modulus and strength of the sheets are higher when tested at higher strain rate. In addition, at higher strain rate, the defects in the shee
Sorce FS, Ngo S, Lowe C, et al., 2021, Quantification and analysis of coating surface strains in T-bend tests, International Journal of Advanced Manufacturing Technology, Vol: 113, Pages: 1-18, ISSN: 0178-0026
Pre-painted sheet metal (e.g. coil coated with polyester-melamine) undergoes large deformations when formed into architectural cladding or white goods. The coatings provide protection and superior aesthetics, so must withstand failure by cracking or delamination during forming. The T-bend test is an industry standard test used to qualitatively compare the formability of coatings and mimics the conditions experienced during hemming processes. The failure of coatings during forming is strain governed, so understanding the surface strains in the T-bend test is of great interest to manufacturers. For the first time, the maximum surface strains experienced during the T-bend test have been predicted using finite element modelling (FEM) and verified experimentally using digital image correlation. The experimental shapes of the deformed blank are compared with the FEM results for further verification. In addition, a novel analytical model is proposed to determine the maximum surface strains. It is shown that strains of up to ~ 225% are applied during a 0T test (bent around a zero thickness spacer) reducing to ~ 23% at 4T (bent around a four times sheet thickness spacer). The finite element model, experimental data and new analytical model show excellent agreement and indicate that behaviour is independent of the substrate thickness or material used. Understanding the strain behaviour quantifies the formerly qualitative T-bend. This will improve the efficacy of the test, allowing metal formers and coating developers to better understand the performance requirements, to reduce waste and to develop better coatings.
Cheong Z, Sorce FS, Ngo S, et al., 2021, The effect of substrate material properties on the failure behaviour of coatings in the Erichsen cupping test, Progress in Organic Coatings, Vol: 151, Pages: 1-13, ISSN: 0300-9440
Pre-painted sheet metal produced by coil coating is subjected to large deformations during manufacture of white goods and architectural cladding. The thermosetting polyester coatings must resist failure by cracking, and their formability can be assessed qualitatively using the industry-standard Erichsen cupping test. However, this only provides strains much smaller than the coatings can withstand, and hence does not discriminate between coating behaviour. Finite element (FE) modelling has been used to show that the applied strain governs the failure of coil coatings during forming, and to demonstrate how increased surface strains can be achieved by altering key parameters to make the Erichsen cupping test discriminating and quantitative. The surface strains are increased by increasing the coefficient of friction between the indenter and the substrate, and by increasing the thickness of the substrate. A parametric study on substrate properties showed that a smaller strain hardening exponent (i.e. more plastic behaviour) gave higher surface strains. There was no variation in the surface strains over a temperature range of -60 °C to 60 °C. Understanding how the test conditions and substrate properties influence the surface strains improves the efficacy of the Erichsen cupping test. The surface strains applied to a coating can be varied by changing the substrate properties, which allows for greater differentiation between coatings and for the coating failure strains to be determined quantitatively. This provides a data-driven approach to develop and formulate better coatings using a single, efficient and easy test.
Kopsidas S, Olowojoba G, Kinloch A, et al., 2021, Examining the effect of graphene nanoplatelets on the corrosion resistance of epoxy coatings, International Journal of Adhesion and Adhesives, Vol: 104, Pages: 1-12, ISSN: 0143-7496
Graphene due to its two-dimensional structure, large surface area and high impermeability is regarded as an excellent functional filler for the development of anti-corrosive coatings by creating a natural barrier to the diffusion of electrolytes. Epoxy polymers are widely used as protective coatings, and in the present study, commercially-available graphene nanoplatelets (GNPs) were dispersed into an epoxy resin using three-roll milling (3RM). The GNP-modified epoxy was coated onto mild steel substrates, and cured. The coated panels were immersed into a corrosive environment of 3.5 wt% NaCl aqueous solution for 4–5 days. The adhesion of the coatings to the substrate was then measured using a cross-cut test. The addition of higher loadings of GNPs resulted in a deteriorating corrosion performance, with the 1.5 wt% and 3 wt% coatings exhibiting 53% and 91% damage, respectively, after the cross-cut tests. The unmodified epoxy and low GNP content coatings (≤0.5 wt%) demonstrated 0% damage. This shows that the corrosion behaviour of GNP/epoxy coatings is not dominated by barrier effects but by electrochemical factors. The addition of GNPs is only effective at low loadings, as higher contents result in electrically-conductive coatings that facilitate the conduction of corrosion currents.
Sorce F, Ngo S, Lowe C, et al., 2020, The effect of varying molecular weight on the performance of HMMM-crosslinked polyester coatings, Progress in Organic Coatings, Vol: 149, ISSN: 0300-9440
Thermosetting polyester coatings crosslinked with hexa(methoxymethyl)melamine (HMMM) are ubiquitous for the pre-painted metal sheet used in white goods and architectural cladding. The coatings are typically 20 μm thick and must have superior resistance to cracking during the forming process to maintain their excellent aesthetics and corrosion resistance. Hence, understanding their structure-property relationships is key to design durable coatings with good formability. The thermo-mechanical properties of clear and TiO2-pigmented polyester-HMMM free-films with varying number average molecular weight (MW) from Mn =1500 g/mol to 3300 g/mol and a constant crosslinker content of 20 % have been determined, and this work provides a fundamental investigation into the effects of varying the MW for the first time. Increasing the MW decreases the glass transition temperature (Tg) as the crosslink density reduces due to fewer functional chain ends. The Young’s modulus and yield stress decrease with an increase in MW at low temperatures, and the strain to failure increases around Tg. The TiO2 pigment increases the stiffness of the coatings and reduces the strain to failure around Tg, but has a toughening effect at higher temperatures. All the coatings show comparable behaviour in the Erichsen cupping test; however, increasing the MW reduces the damage during the T-bend test as the coatings are able to withstand higher applied strains. Thus controlling the MW allows a balance of properties to be achieved, as increasing the MW reduces the Tg and modulus while increasing the strain to failure and formability of the coating. Such an understanding of the structure-property relationships allows for better formulating and targeted coating design, reducing cost and increasing performance in the coil coating industry.
He S, Carolan D, Fergusson A, et al., 2020, Mechanical and fracture properties of epoxy syntactic foams modified with milled carbon fibre
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. Syntactic foams are lightweight but brittle materials typically used as the core for sandwich composite panels. Foams comprising of ∼60 vol% hollow glass microspheres (GMS) in an epoxy matrix were modified by the addition of milled carbon fibre (MCF). Weight ratios of up to 30% MCF:GMS were used. The tensile modulus of the foams increased from 3.36 GPa up to 4.82 GPa with the addition of 30% weight ratio of MCF. The tensile failure strength of the syntactic foam decreased with low loadings of MCF, which is attributed to low load transfer capacity among the fibres due to poor fibre population. The tensile failure strength recovers when more MCF particles are added. The fracture energy of the syntactic foam showed an increase of 217%, from 182 J/m2 to 396 J/m2, due to the addition of 30% weight ratio of MCF. Toughening mechanisms were identified as crack deflection, debonding and subsequent plastic void growth, and fibre pull-out.
Deng X, Kinloch AJ, Pimenta S, et al., 2020, Toughening epoxy composites using nano- And microcellulose modifiers
The fracture properties and toughening mechanisms of cellulose- and cellulose-rubber hybrid-modified epoxy polymers and glass-fibre (GF) composites are investigated. The cellulose modifiers used are microcrystalline cellulose (MCC) and cellulose nanocrystals (CNC), and the rubber modifiers are carboxyl-terminated butadiene-acrylonitrile (CTBN) and core-shell rubber (CSR). The toughening mechanisms of the MCC-epoxy and CNC-epoxy were identified to be crack deflection, shear band yielding, particle rupture or pull-out and debonding of the cellulose particles, which was followed by plastic void growth. An additive toughening effect is observed for the hybrid polymers. Analytical modelling of the fracture energies showed that the particle pull-out toughening contribution is negligible for CNC-epoxy, and the particle debonding and rupture toughening contributions are negligible for MCC-epoxy. The GF composites were manufactured using the wet-layup process. Cellulose modifiers did not increase the composite propagation fracture energy (GC,prop) but slight increases in GC,prop occurred for the CNC hybrids. Increases in the fibre-matrix adhesion reduced the fibre toughening mechanisms in the composites that were modified with only MCC or CNC. The crack tip deformation zone is smaller than the MCC particles, reducing their toughening ability in the GF composites.
He S, Carolan D, Fergusson A, et al., 2020, Mechanical and fracture properties of epoxy syntactic foams modified with milled carbon fibre
Syntactic foams are lightweight but brittle materials typically used as the core for sandwich composite panels. Foams comprising of ∼60 vol% hollow glass microspheres (GMS) in an epoxy matrix were modified by the addition of milled carbon fibre (MCF). Weight ratios of up to 30% MCF:GMS were used. The tensile modulus of the foams increased from 3.36 GPa up to 4.82 GPa with the addition of 30% weight ratio of MCF. The tensile failure strength of the syntactic foam decreased with low loadings of MCF, which is attributed to low load transfer capacity among the fibres due to poor fibre population. The tensile failure strength recovers when more MCF particles are added. The fracture energy of the syntactic foam showed an increase of 217%, from 182 J/m2 to 396 J/m2, due to the addition of 30% weight ratio of MCF. Toughening mechanisms were identified as crack deflection, debonding and subsequent plastic void growth, and fibre pull-out.
Deng X, Kinloch AJ, Pimenta S, et al., 2020, Toughening epoxy composites using nano- And microcellulose modifiers
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. The fracture properties and toughening mechanisms of cellulose- and cellulose-rubber hybrid-modified epoxy polymers and glass-fibre (GF) composites are investigated. The cellulose modifiers used are microcrystalline cellulose (MCC) and cellulose nanocrystals (CNC), and the rubber modifiers are carboxyl-terminated butadiene-acrylonitrile (CTBN) and core-shell rubber (CSR). The toughening mechanisms of the MCC-epoxy and CNC-epoxy were identified to be crack deflection, shear band yielding, particle rupture or pull-out and debonding of the cellulose particles, which was followed by plastic void growth. An additive toughening effect is observed for the hybrid polymers. Analytical modelling of the fracture energies showed that the particle pull-out toughening contribution is negligible for CNC-epoxy, and the particle debonding and rupture toughening contributions are negligible for MCC-epoxy. The GF composites were manufactured using the wet-layup process. Cellulose modifiers did not increase the composite propagation fracture energy (GC,prop) but slight increases in GC,prop occurred for the CNC hybrids. Increases in the fibre-matrix adhesion reduced the fibre toughening mechanisms in the composites that were modified with only MCC or CNC. The crack tip deformation zone is smaller than the MCC particles, reducing their toughening ability in the GF composites.
He S, Carolan D, Fergusson A, et al., 2020, Mechanical and fracture properties of epoxy syntactic foams modified with milled carbon fibre
© CCM 2020 - 18th European Conference on Composite Materials. All rights reserved. Syntactic foams are lightweight but brittle materials typically used as the core for sandwich composite panels. Foams comprising of ∼60 vol% hollow glass microspheres (GMS) in an epoxy matrix were modified by the addition of milled carbon fibre (MCF). Weight ratios of up to 30% MCF:GMS were used. The tensile modulus of the foams increased from 3.36 GPa up to 4.82 GPa with the addition of 30% weight ratio of MCF. The tensile failure strength of the syntactic foam decreased with low loadings of MCF, which is attributed to low load transfer capacity among the fibres due to poor fibre population. The tensile failure strength recovers when more MCF particles are added. The fracture energy of the syntactic foam showed an increase of 217%, from 182 J/m2 to 396 J/m2, due to the addition of 30% weight ratio of MCF. Toughening mechanisms were identified as crack deflection, debonding and subsequent plastic void growth, and fibre pull-out.
Sorce F, Lowe C, Ngo S, et al., 2019, The effect of HMMM crosslinker Ccntent on the thermal-mechanical properties of polyester coil coatings, Progress in Organic Coatings, Vol: 137, ISSN: 0300-9440
The thermosetting polyester-based coatings crosslinked with hexa(methylmethoxy)melamine (HMMM) used for coil coating sheet metal experience large deformations when formed into architectural cladding and white goods. Cracking of the 20-μm-thick coatings must not occur during forming, to prevent corrosion of the steel substrate, so the relationship between the composition and the thermal-mechanical properties is critical to develop highly formable and durable coatings, and to choose suitable forming conditions. Free films of coatings with 5 % to 30 % crosslinker content have been analysed. Dynamic mechanical analysis (DMA) showed that the glass transition temperature (Tg) and crosslink density increase with crosslinker content. Differential scanning calorimetry (DSC) has been used to measure the Tg from the thermal response, based solely on the chemical structure, and agrees well with the DMA.Tensile tests were performed at temperatures as a function of DSC Tg (Tg - 40 °C to Tg + 50 °C). There was little variation in Young’s modulus and strain to failure in the glassy region where the intermolecular forces dominate, but in the rubbery region governed by the covalent bonds a lower crosslinker content gave lower values. This indicates that the failure mechanism undergoes a transition with increasing temperature from being controlled by the brittle fracture stress to the yield stress. The addition of TiO2 pigment increased the modulus and apparent yield stress at low temperatures in the glassy region, and increased the strain to failure and failure stress in the rubbery region. Failure envelopes, normalising the tensile data with the DSC Tg and the crosslink density, show the dependence on crosslinker content and pigmentation. This allows the behaviour of coatings to be predicted from their structure, and enhanced coatings to be developed based on the required mechanical properties.
Tsang WL, Taylor AC, 2019, Fracture and toughening mechanisms of silica- and core–shell rubber-toughened epoxy at ambient and low temperature, Journal of Materials Science, Vol: 54, Pages: 13938-13958, ISSN: 0022-2461
The highly cross-linked thermosetting polymers used as adhesives and as the matrices of fibre composites for the construction of lightweight vehicles are very brittle, and finding effective toughening solutions for such engineering applications is a long-standing problem. An anhydride-cured thermosetting epoxy polymer has been modified by the addition of different wt% of silica nanoparticles, core–shell rubber particles and hybrids with equal wt% of both. The fracture energy was measured at ambient and low temperature (− 40 °C and − 80 °C) to understand the brittle fracture behaviour. The fracture and toughening mechanisms were identified by scanning electron microscopy of the fracture surfaces. Analytical models were used to predict the modulus and fracture energy; the predictions agreed very well with the measured values. Toughening using silica nanoparticles is especially efficient at low particle contents. This shows how epoxies can be toughened successfully for use in industrial and transport applications.
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