Experimental and numerical validation of an inter-ply friction model for thermoset based fibre metal laminate under hot-pressing conditions

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.Aerospace Manufacturing Technologie

Identifying microstructural features in unidirectional composite tapes

Carbon fibre-reinforced polymer composites (CFRPs) outperform most structural engineering materials in specific stiffness and/or specific strength, especially in their unidirectional configuration. Unidirectional composites can be found as individual structural elements in cables or pin-loaded straps; they are however most commonly found in the form of tapes, representing a semi-finished product for subsequent processing to laminates by tape laying, winding or press moulding. The outstanding properties of such composites are affected by its microstructure. It influences the structural performance and fatigue life when architected into thin ply composites [1]. The microstructure is also affected by processing conditions, respectively recursively affects processability as observed in the deconsolidation [2] or intimate contact formation [3] during laser assisted tape laying. This work presents a novel approach to identify microstructural features. This is achieved by Voronoi tessellation-based evaluation of the fibre volume content on cross-sectional micrographs, with consideration of the matrix boundary. The method [4] is shown to be robust and is suitable to be automated and has the potential to be expanded into 3d imaging techniques [5]. It further has the potential to discriminate specific microstructural features and to relate them to processing behaviour. The method is experimentally validated on tape samples with characteristic processing history.Aerospace Manufacturing TechnologiesAerospace Structures & Computational Mechanic

Stochastic Modelling of Randomly Oriented Tapes Thermoplastic Composites in Net-Shaped Specimens

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.Aerospace Manufacturing Technologie

Comparative life cycle assessment of thermoplastic and thermosetting CFRP in aerospace applications

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.Aerospace Manufacturing Technologie

Method for the Microstructural Characterisation of Unidirectional Composite Tapes

The outstanding properties of carbon fibre-reinforced polymer composites are affected by the development of its microstructure during processing. This work presents a novel approach to identify microstructural features both along the tape thickness and through the thickness. Voronoi tessellation-based evaluation of the fibre volume content on cross-sectional micrographs, with consideration of the matrix boundary, is performed. The method is shown to be robust and is suitable to be automated. It has the potential to discriminate specific microstructural features and to relate them to processing behaviour removing the need for manufacturing trials.Aerospace Manufacturing TechnologiesAerospace Structures & Computational Mechanic

An inter-ply friction model for thermoset based fibre metal laminate in a hot-pressing process

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.Aerospace Manufacturing Technologie

Tailoring mechanical properties of randomly oriented tape (ROT) composites: An experimental study

Discontinuous fiber composites (DFC) have properties such as notch insensitivity, short processing times, and large shaping freedom [1-3]. However, the mechanical behavior of ROT composites is less predictable compared to continuous fiber composites due to the mesoscopic heterogeneity of the material [4]. Controlling the tape alignment is a compelling approach for tailoring the mechanical properties of the ROT composites, enabling better control and prediction of the material properties such as tensile strength and stiffness [5]. However, for a successful implementation of the alignment method in industry, a fast tape alignment method is needed. In this study, a quantitative assessment has been made between three alignment methods. The methods have been evaluated in respect of the level of alignment and the manufacturing process by means of a decision-making matrix. The level of alignment of each method has been determined using computer vision on the orientation of individual tapes. The final alignment method has been selected which uses vertical mesoscopic sieves with a high aspect ratio to rotate tapes in a preferred direction during deposition. With this alignment method, ROT have been aligned at -45˚, 0˚, and +45˚ with respect to the loading direction. With these alignment tools, CF/PEEK ROT have been deposited inside a cavity followed by a consolidation cycle at 45 bar and 385˚C. The effect of alignment was examined by comparing the mechanical response of samples with: ROT, being longitudinally aligned, and a laminate of [+45˚,-45˚]s aligned pseudo-layers. The results of tensile tests showed that by aligning tapes in the longitudinal direction, the average tensile stiffness and strength increased by 145% (from 32.5 to 79.7 GPa) and 96% (from 202 to 396 MPa) respectively compared to randomly orientated tapes. These results show the potential gain in material properties and the ease of implementation of the method.Aerospace Manufacturing Technologie

Toughening of epoxy systems with interpenetrating polymer network (IPN): A review

Epoxy resins are widely used for different commercial applications, particularly in the aerospace industry as matrix carbon fibre reinforced polymers composite. This is due to their excellent properties, i.e., ease of processing, low cost, superior mechanical, thermal and electrical properties. However, a pure epoxy system possesses some inherent shortcomings, such as brittleness and low elongation after cure, limiting performance of the composite. Several approaches to toughen epoxy systems have been explored, of which formation of the interpenetrating polymer network (IPN) has gained increasing attention. This methodology usually results in better mechanical properties (e.g., fracture toughness) of the modified epoxy system. Ideally, IPNs result in a synergistic combination of desirable properties of two different polymers, i.e., improved toughness comes from the toughener while thermosets are responsible for high service temperature. Three main parameters influence the mechanical response of IPN toughened systems: (i) the chemical structure of the constituents, (ii) the toughener content and finally and (iii) the type and scale of the resulting morphology. Various synthesis routes exist for the creation of IPN giving different means of control of the IPN structure and also offering different processing routes for making composites. The aim of this review is to provide an overview of the current state-of-the-art on toughening of epoxy matrix system through formation of IPN structure, either by using thermoplastics or thermosets. Moreover, the potential of IPN based epoxy systems is explored for the formation of composites particularly for aerospace applications.Aerospace Manufacturing Technologie

Effect of dwell stage in the cure cycle on toughening of epoxy using thermoplastic multilayers

Epoxies with high cross-linking densities are brittle and hence have a low fracture toughness. However, different methods are known to increase fracture toughness. Numerous approaches are known to incorporate a second phase into the epoxy matrix, such as rubber, inorganic nanoparticles or thermoplastics, referred to as bulk resin modification. These tougheners usually form specific morphologies during the curing phase of epoxy, resulting in improved fracture toughness of the system. Unfortunately, for some tougheners, the addition of second phase into the epoxy system also results in a reduction in overall modulus and limitation in end-use temperature of the system. In the case of thermoplastic tougheners, the second phase is created by diffusion and dissolution, followed by reaction induced phase separation, leading to a morphology in the micrometer range. However, the influence of the curing history beyond phase separation, using two dwell cure cycles with varying dwell time/degree of cure, on the interphase dimension and final morphology for PEI having a contrasting phase behaviour (UCST), is not well understood. The research presented in this work aims to understand the interphase formation, to later attain the desired droplet size and interphase morphology for improved material toughness. This aim is achieved by analyzing the influence of dwell time by considering two main cases for each selected 1st dwell temperature (120-180˚C): (i) wait until the onset of phase separation (OPS) before increasing the temperature to 200°C (second dwell), (ii) wait until 80% degree of cure (80% DOC) before the second dwell. At all processing temperatures, a distinct gradient morphology (Fig. 1a ) was clearly observed for both cases (OPS and 80% DOC). The SEM micrographs revealed the formation of a larger interphase region (71 μm) for the OPS case as compared to the 80% DOC case (56 μm). Figure 1b shows the interphase thickness as a function of 1st dwell temperature for both OPS and 80% DOC cases. It can be seen that the interphase thickness increased with 1st dwell temperature for both cases, until 160˚C after which it slightly decreased for a 1st dwell temperature of 180˚C. This work highlights, i) the importance of the curing process beyond phase separation to control interphase dimension and final morphology and, ii) the influence of both these parameters on the toughness enhancement.Aerospace Manufacturing Technologie

Nanoengineering of fibre surface for carbon fibre-carbon nanotube hierarchical composites

We aim to enhance the carbon fibre (CF)-matrix interface by synthesizing carbon nanotubes (CNTs) on the surface of the CF, creating a hierarchical composite. A 12 nm thick aluminium oxide film applied by atomic layer deposition (ALD) provides protection of the CF from deterioration during CNT growth in a chemical vapour deposition (CVD) process. However, the adhesion of alumina to CF, grown in classical water/trimethylaluminium ALD is severely diminishing during CNT growth, as detected by interface shear strength (IFSS) measurements. In our approach to improve the CF-alumina adhesion, we employed a pre-treatment of the CF with ozone and entirely replaced water with ozone in the ALD process, to promote the covalent bonding of the alumina to the CF surface. The current results show a new perspective in achieving the CNT synthesis on the CF while successfully mitigating its detrimental effects on the fibre mechanical properties.Aerospace Manufacturing Technologie