This study aims to evaluate the effectiveness of Ultrasonic-assisted drilling (UAD) of Glass laminate aluminium reinforced epoxy (GLARE) at high cutting speeds (Spindle speeds: 3000–7500 rpm; feed rates 300–750 mm/min) by analysing the thrust force and hole quality metrics (surface roughness, hole size, and burr formations. The research also presents numerical modelling of FMLs under conventional and UAD regimes to predict thrust force using ABAQUS/SIMULIA. The thrust force and exit burrs were reduced by up to 40.83 % and 80 %, respectively. The surface roughness metrics (R_a and R_z) were slightly higher using UAD but remained within the desirable limits of surface roughness for machined aeronautical structures. The discrepancy between the simulation and experimental results was adequate and did not exceed 15 %. The current study shows that it is feasible to drill holes in GLARE using higher cutting parameters and maintain excellent hole quality, which means increased productivity and reduced costs.

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The research focuses on the simultaneous deformation of uncured metal-composite laminates under the press forming process of a double-curved dome part. The study is designed to evaluate the influence of material type, inter-ply friction and clamping force by the use of a finite element modelling method. The result shows that the main failure mode for aluminium-based hybrid materials is metal cracking while prepreg buckling dominates the failure of stainless steel-based hybrid materials. The increase of clamping force contributes to the deformation of fibre reinforced prepreg layer and decrease the risk of prepreg buckling, but the simultaneous increase of plastic deformation tends to induce the failure of metal cracking. The variation of friction coefficient at the metal-prepreg interface affects the inter-ply sliding displacement during the forming process. This work provides a numerical method on the material selection and process parameter optimisation where compatible deformation of the individual layers can be achieved through its own mechanisms.@en

This study focuses on the spring-back as a function of the degree of cure on single-curved metal-composite laminates. The manufacturing through a hot-pressing process involves different (curing) stages and can reduce the spring-back with the proper combination of forming and curing. The cure-dependent spring-back is measured and analysed as a function of material constituents, fibre directions, laminate layups, and the process parameters including temperature, holding time and pressure. The results demonstrate that the spring-back ratio after full-cured process is relatively small and mainly depends on the mechanical properties of the metal sheet in laminates. However, temperature and time have a significant effect on the spring-back of partially-cured laminates and the same type of fibre prepreg combined with two different metal sheets have similar trends of spring-back reduction. Moreover, the study found that the hybrid laminates with aluminium sheet delaminate at low pressure after full-cured, while the delamination disappears as the pressure increases. The characterisation on cure-dependency of the spring-back contributes to a better understanding of the deformability of the metal-composite laminates during the hot-pressing process and offers an opportunity to tune the spring-back of these laminates.

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The bias-extension test is one of the test methods to characterise the intra-ply shear behaviour of continuous fibre reinforced composites including fabrics and unidirectional (UD) materials. For the determination of the major mechanical properties of metals, often a uniaxial tensile test is used. Combination of these two methods for the shear deformation of hybrid metal-composite laminates is proposed comparing the method for cross-plied unidirectional prepregs and woven fabric prepregs. The effects of material constituent, shear rate, preheat temperature and normal pressure on the intra-ply shear behaviour are investigated. The results indicate that the material constituents and the frictional responses depending on processing parameters play a critical role in the shear characterisation of the hybrid laminate. The shear angle measurement at four typical strains demonstrates that the support of metal layers improves the shear deformability by delaying the onset of fibre wrinkling. This modified intra-ply shear test contributes to a better understanding of the process design for wet (uncured) hybrid metal-composite laminate manufacturing.

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Chopped Tape Thermoplastic Composites (CTTCs) offer high formability and performance for complex-shaped components in the aerospace and automotive industries. However, the mesoscopic discontinuity leads to spatial variabilities and correspondingly high scatter in the elastic properties of CTTCs due to the random orientations of chopped tapes and chopped tape-cavity edge interactions. Here we propose a new approach to investigate the effect of mould cavity edges on chopped tape orientation and hence the mechanical properties of CTTCs. Based on this approach, a Set Voronoi tessellation was implemented to represent the variability of local Young's Modulus and chopped tape-cavity edge interactions occurring during the manufacturing process. It was confirmed that the chopped tapes align along the edges, and progressively transition to a random orientation towards the middle of the specimen. The results were validated on moulded specimens and demonstrated the ability to deconvolute the edge interaction.

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Autoclave curing is one of the most energy consuming processes in manufacturing carbon fibre reinforced polymers. In order to improve the energy efficiency, one needs to understand energy usage in an autoclave and factors that influence it. This work presents two thermodynamic based models for estimating energy consumption in an autoclave. The first model is an analytical approach based on simplified heat capacity equation. The second model combines the Multi-Relaxation-Time Lattice Boltzmann method (MRT LBM) with Fourier heat equation to simulate autoclave temperature flow and energy consumption. The output from the two models were compared to energy consumption data collected using a power meter. The estimated values from the MRT LBM method showed a better match with only 1% difference from the experimental value. Since the two models are parametric and scalable, a what-if analysis was carried out to investigate the influence of varying process parameters on autoclave energy consumption. Parameters including cure cycle, autoclave size and loading capacity.

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Shrinkages, distortions and high residual stresses in the thermoplastic composite parts are induced due to high processing temperature, anisotropy, and fiber–matrix shrinkage mismatch. In this paper the shrinkages have been investigated experimentally and modeled by thermo-mechanical constitutive equations for PolyPhenylene Sulfide (PPS) and the unidirectional Carbon Fiber (PPS/CF) composite prepreg. The thermal shrinkage and the crystallization shrinkage were retrieved from Thermal Mechanical Analysis and compared to a Pressure specific volume Temperature diagram. To describe the crystallization shrinkage in the cooling process accurately, the crystallization kinetics of PPS was evaluated using Differential Scanning Calorimetry. The temperature-dependent elastic modulus was measured by a shear rheometer to formulate a new constitutive model. The mathematical model for shrinkage was validated by a press consolidated [0]₁₂ laminate and unbalanced laminates in four lay-ups. The thermo-mechanical model results presented here provide significant rules for the thermomechanical and shrinkage predictions for the industrial applications of thermoplastic composite.

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Hot-pressing can be an attractive fabrication method that enables the forming of hybrid materials like thermoset based fibre metal laminates. However, the process simulation on press forming requires accurate material characterization and boundary conditions to facilitate part design for a defect-free component. In order to improve the overall predictive simulation quality, the inter-ply sliding at metal-prepreg interfaces which is one of the critical deformation mechanisms is considered. An inter-ply friction model has been established using an experimental friction test apparatus and the effect of slip rate, normal force and temperature is taken into consideration. To validate the proposed friction model, a comparative study between results obtained from the numerical model and the experimental ones is carried out. The research demonstrates that the inter-ply friction model can be a valuable building block for the finite element simulation of the hot-pressing process for thermoset based fibre metal laminates. @en

Discontinuous tape composites have considerable attention due to their high formability and tailorable structures. Despite their advantages, this discontinuity leads to complex structures and makes it difficult to predict their mechanical properties. On the other hand, they have high orientational and dimensional sensitivity, which causes spatial variability and complexity in the structure to predict the mechanical properties. This spatial variability is also related to the mould cavity. A constitutive model was improved to explain the relationship between DT orientations and the mould cavity. According to the modelling technique, a random DT distribution was generated by Random Sequential Adsorption then, the Set Voronoi Tessellation was implemented to obtain DT layers. Afterwards, the Classical Laminate Theory and Finite Element Method were applied to compare the virtual net-shaped DT specimens. The results of both methods showed high stiffness at the edges of the specimens. @en

GLARE laminates are multi-layered metal-composite materials created from bonding sheets of metallic alloys with carbon or glass fibre layers. The application of hybrid-conventional machining processes such as ultrasonic-assisted drilling (UAD) is becoming of great interest to the aerospace industry due to its capability in reducing the cutting forces and tool wear which are directly responsible for drilling-induced delamination. There is rich literature on the conventional drilling (CD) of GLARE, but no work reported using UAD process. This study will fill this gap and investigate the UAD of GLARE laminates using an indigenously developed UAD system. The influence of spindle speed and feed rate on thrust force and surface roughness metrics (R_a and R_z) were investigated under CD and UAD regimes. The quality of the borehole and damage mechanisms in the laminate constituents was examined using scanning electron microscopy (SEM). The contribution of the drilling parameters on the measured outputs was further evaluated using the analysis of variance (ANOVA) statistical analysis. It was found that UAD resulted in a significant reduction in thrust force by up to 65% while surface roughness metrics R_a and R_z were unaffected by the type of drilling process used. SEM analysis showed irregular and fuzzier surfaces in glass fibre layers in holes machined using UAD due to the longitudinal vibration of the tool.

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This work quantifies and compares the environmental impact of a thermoset CFRP aircraft skin panel to that of a thermoplastic CFRP panel. This comparison is done using a cradle to gate life cycle assessment including impacts from raw material extraction, manufacturing and end of life. In addition, a hotspot analysis was performed to identify processes with the highest contribution to environmental impacts of the panels. The results show that the thermoplastic panel performed better in three endpoint damage categories including damage to human health, ecosystem and resources. The results also identify carbon fibre production, electricity usage for curing/consolidation and prepreging as the major contributors to the life cycle impacts of both panels. This provides decision makers with insights on where to focus on for future improvement actions aimed at reducing the environmental footprint of CFRP structures. @en

Forming process with pre-stacked and uncured thermoset fibre metal laminate offers improved deformability compared to full-cured laminate especially for the production of complex structural components. This work investigated the friction behaviour at the metal-prepreg interface of glass fibre reinforced aluminium laminate through an inter-ply friction test. The influence of sliding velocity, normal force, fibre orientation and resin viscosity coupled with temperature on static and kinetic friction coefficients were studied. The kinetic friction behavior in the transition region between mixed and hydrodynamic lubrication, showed a good agreement with the Stribeck-curve theory. While for the static friction, a modified Coulomb friction model was found to fit the experimental results. These models were translated into a phenomenological inter-ply friction model which was incorporated into Abaqus/Explicit as a user-defined friction subroutine for verification. The findings contribute to the development of the forming process with fibre metal laminates.

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During the manufacturing of fibre-reinforced laminates, undesired phenomena are produced, among which the residual stresses need investigation and analysis. The authors have previously presented the research done on the predictive modelling and measurement of the distortions happening during the manufacturing of fibre metal laminates (FML). In this paper, primary measurements of residual stresses are presented. First, the mechanisms governing the development of residual stresses in polymeric laminates are described. Then, the measurement techniques applicable to composite and hybrid materials are reviewed. The hole-drilling method is used to estimate the stress levels in the aluminium sheet of the FML. The results give a new understanding of the stress state in the FML. Comparisons made with the model predictions show the consistency of the results and indeed the need for further improvements for the model validations in terms of the residual stress values in the aluminium sheets.

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The correct prediction of a composite parts’ final performance is of paramount importance during the initial design phase of the manufacturing process. To this end the correct evaluation of the most effective process parameters and their influence on the parts performance is key for the success of the manufacturing process. Our aim with this paper is to provide methodologies for the prediction of the temperature field in thermoplastic composites during thermoforming and to propose a strategy for process parameter selection. We measured the temperature variations over the different thermoforming stages and compared these values with analytical and finite element results. Our results show the accuracy of the predictions and the importance of the correct laminate temperature with respect to the prediction of the parts’ spring-in angle. We discuss the essential features needed for accurate predictions of the temperature fields over the whole thermoforming process at an early design stage and the potential of a Machine Learning procedure based on Artificial Neural Network to aim for the optimum range of process parameters for a desired part performance outcome. In conclusion, we provide essential guidelines for blank temperature predictions, and the benefit of a machine learning-based tool over traditional approaches.

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The paper addresses the role played by the cure stage of a vacuum assisted resin transfer molding process in residual stresses generation. The Airstone 780E epoxy resin and Hardener 785H system broadly used in the wind turbine blade industry has been used in this study. The viscous–elastic properties of the resin have been characterized and implemented in a thermo-mechanical FE model. The model has been validated against manufactured [0/90]4 asymmetric laminates. Analysis of residual stresses generation highlighted that compressive stresses generation occurs when the cure is shrinkage dominated and tensile stresses when expansion dominated in the 0° plies. The finding points out that 10% reduction in warpage and 33% reduction in process time can be obtained by selecting cure cycle parameters that allow tensile stresses development during the cure process in the 0° plies.@en

Fibre metal laminates (FMLs) are multilayered metal composite materials currently used in aeronautical structures, especially where fatigue and impact resistance are required. FMLs are produced in large panels and often require assembly using the drilling process for riveting purposes. Hole making is a critical machining process in the joining and assembly of aeronautical components, which has to meet stringent tolerance requirements. This paper reports a systematic analysis of hole integrity when drilling an FML known as GLARE®. In particular, the primary objective is to investigate the impact of three different drill coatings (TiAlN, TiN and AlTiN/TiAlN), against several important hole parameters: thrust force, hole size, circularity, cylindricity and perpendicularity. The results show that TiAlN-coated drills produced the highest thrust force, while TiN-coated drills produced holes with the lowest deviation between the hole diameter measured at the entry and the exit and that the drill coating was the most influential parameter for the resulting hole size. TiAlN-coated drills resulted in the highest circularity at the upper part of the hole, while hole cylindricity tended to be best when using AlTiN/TiAlN- and TiN-coated drills. The ANOVA analysis shows that the drill coating and the spindle speed had a significant influence on hole size and circularity, while drill coating was the only influential parameter on hole cylindricity, and spindle speed was the only contributing parameter on hole perpendicularity. Finally, scanning electron microscopy analyses showed two distinct hole wall surface damage phenomenon due to broken fibres and evacuated metallic chips.

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Although aerospace traditionally has always had a multidisciplinary approach to engineering and design, the increasing complexity of aircraft and spacecraft and the rapid digitization within the aerospace industry has led to a large number of related engineering and scientific disciplines such as electrical engineers, computer scientist to work much more directly within the aerospace domain than before. Next to that there is a shortage of highly trained engineers worldwide to meet demand. As a result, there is a clear need to provide basic knowledge to non-aerospace engineers working in the field and to motivate and attract more people to engineering and aerospace in particular. This paper details how the creation of a Massive Open Online Course (MOOC) at an introductory level in Aerospace Structures and Materials provides an efficient and fit-for-purpose tool to achieve both aims. The paper will discuss the course design, the course set up, the course evaluation and how the course fits within the online learning philosophy of Delft University of Technology. It will use learning analytics to analyze our learners and their needs.@en

An innovative deicing system for aircraft leading edges has been developed which integrates heater elements into fibre metal laminates. Such an electrical system can lead to weight reductions and more efficient performances compared to conventional bleed air systems. However, the combination of thermal and mechanical loadings also raises new questions on the durability of such a structure, in particular due to the repeated heating to elevated temperature. The linear viscoelastic creep behaviour, including the effects of temperature and ageing, is therefore investigated for manufactured FM906 glass-fibre epoxy composite as used in heated GLARE. A master curve is derived based on the time–temperature and time–age superposition. The effect of physical ageing during loading is included in a long-term creep prediction.

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The manufacturing of Fibre Metal Laminates, consisting of alternating layers of pre-preg fibre and metal is complex, and defects can be introduced during manufacturing. The automation of the manufacturing steps is vital for future Glare production, and specific defects have to be considered. Among them, gaps between pre-preg plies represent a potential risk for the mechanical performances of the laminate. In this paper/research, non-destructive testing based on ultrasonic inspections are performed on Glare with pre-preg gaps in order to extend and improve the current state of the art to different pre-preg gap widths, depths, and lay-ups. Firstly, gaps were introduced in Glare specimens. Then, a conventional C-scan ultrasonic inspection is performed. In order to overcome the current limitations of a top or planar view of the laminates, the evaluation of the depth of the gaps in the laminates is performed by means of phased array ultrasonic testing. Finally, images of the laminate cross-section have been collected in order to provide a meaningful evidence of the ultrasonic-based analysis. Results from different Glare and gap widths and depths show the accuracy of the proposed investigation which is able to provide a detailed assessment of the gap occurrences also for very thin laminates.@en

Press forming of Fibre Reinforced Thermoplastics (FRTP) is a widely used manufacturing process. However, in order to boost innovation in FRTP production, new manufacturing strategies have to be implemented. In this context, the multistep forming process represents a promising concept for achieving a higher level of performances by means of customized fibre orientations, fibre types, fibre architecture, and thicknesses, but also higher product functionalities through the combination of different polymers in the same product. In order to improve the functional efficiency of FRTP components, this paper investigates the optimization of a multistep forming process of glass fibre fabric combined with polyetherimide (GF/PEI). The deformation mechanisms encompassing the multistep forming process is here analysed, along with the understanding of the effects of process parameters (e.g. temperature and pressure) over part quality. In particular, the feasibility of the reduction of the cycle time of the process is evaluated by means of active cooling. In addition, in order to foster future industrial application of multistep forming processes, robust and reliable process simulations are presented aiming at reducing development times and improving the overall cost-effectiveness. @en