Category: Postgraduate Research

Radhakrishnan, A., Maes, V.K., and Kratz, J.

Conventional oven-based curing of thermoset composites is an energy-intensive process. This arises from the inefficient heating of a large volume of air combined with tooling that is typically 10-40 times heavier than the composite part manufactured on the tool. This large thermal mass potentially leads to a larger cure gradient, i.e., spatial change in temperature within the composite parts, and manifests as distortion or residual stresses both causing part failure, higher scrappage, and increase cost. Cure gradients can further be made worse by the exothermic reaction causing thick regions to become local hot spots as the part cures. To avoid cure gradients, manufacturers generally apply slow heating rates to allow heating to even out and reduce exothermic peaks. These slow heating rates in turn increase cycle times and energy consumption. Thus, the manufacturer is caught between the two competing priorities of quality and production rate. To push production rates while maintaining part quality, smart tooling solutions are required.

Researchers at the Bristol Composites Institute (BCI) typical features such as corners and ramps to evaluate two innovative approaches improving part quality while reducing cycle times and energy consumption: 1) direct zonally heated tools and 2) additively manufactured (AM) metal tools (Figure 2).

 

 

 

 

 

 

 

 

Figure 1. Benefits of out-of-autoclave curing of a complex part using zonally heated tooling with direct heating compared to using an oven.

 

 

 

 

 

Figure 2. Benefits of using AM Tool instead of a solid tool for curing complex part

Heated tooling introduces heat directly to the tool surfaces or volume through heated fluid circulation or heating elements. While this process reduces energy consumption by 45% compared to traditional oven or autoclaves curing process, as illustrated in Figure 1, the cost of heated tooling can be high. However, the true potential lies beyond energy efficiency, but rather in the ability to tailor the temperature profile applied to different regions. By introducing zonal heating, 17% faster cure cycles can be achieved while reliably ensuring high quality by reaching moulding temperatures quicker and more spatially uniform by the cure profile at the thick and thin regions (Figure 1). This allows for greater throughput, which ultimately results in cost savings and increased production capacity using a smaller factory footprint. Therefore, while the initial investment may be higher, the long-term benefits of zonally heated tooling make it a promising option for industrial applications.

In the feasibility study funded by the University of Bristol EPSRC Impact Acceleration Award, we explored the application of cure-kinetic coupled numerical simulations to design cure cycles for single and dual-zone heated tooling. The numerical predictions of the thermal profile were successfully validated experimentally using embedded thermocouples in the manufacturing of complex parts. The independent zonal thermal control approach reduced the spatial gradients in temperature and degree of cure without worsening the exotherm. Further developments are underway in applying machine learning, in-situ sensors, and advanced thermal management for adaptive cure control to manage heating as well as cooling to reduce overall cycle time and energy.

Additive manufacturing is one such enabling route that was explored in our feasibility study with the University of Bath funded by CIMComp Future Composite Manufacturing Research Hub. The use of AM removes design restrictions placed on monolithic tooling manufactured via subtractive processes like machining and milling. In the study, we explored lattice-based metal tooling manufactured via powder bed fusion for efficient composite curing. Lattice structures have repeating unit cells, and selecting the appropriate unit cell can improve thermal and structural properties like heat transfer and stiffness. Our work investigated a series of flat tools with a range of parameters including lattice geometry, density, and face sheet thickness to assess AM capabilities in meeting tooling requirements such as dimensional tolerances, stiffness and heating rate and found gyroid lattices performed exceptionally well. This lattice architecture was then translated to produce a tool for manufacturing a complex geometry

Direct zonal heated tooling reduced cycle time by 17% and energy by 45% while improving part quality with a reduced cure gradient. Combining this approach with AM tooling resulted in an additional 20% reduction in cure cycle time and a 45% reduction in energy use. Compared to conventional solid tooling using oven curing, the direct heating and AM design saved around 35% in cure cycle time and 70% in energy use. Future work on these innovative tooling concepts can have a considerable impact, particularly in designing cost-effective and energy-efficient tooling for manufacturing high-quality composite parts.

For further reference:

Zonally Heated Tooling for Moulding Complex and Highly Tapered Composite Parts|Frontiers|2023

A Feasibility Study of Additively Manufactured Composite Tooling| IAM2022 Proceedings| 2023

 

 

 

 

Koptelov A., Belnoue J.P-H., Georgilas I., Hallett S. R., Ivanov D.S.

The complexity of composites manufacturing stems from the nature of composite precursors—the combination of loosely-joined fibre network and liquid viscous resin—often heterogenous and enhanced with tougheners or functional additives. is compliant, deforms irreversibly, exhibits almost negligible resistance to axial compressive stresses and has a multitude of flow/deformation mechanisms (i.e., the internal or percolation flow of resin, flow of fibrous suspensions, densification of reinforcement, relative movement of plies, and others) – Figure 1. This makes precursors prone to defects at all stages of the composites manufacturing process.

One of the fundamental processes, universal almost for the entire range of composites manufacturing methods, is consolidation, where a composite precursor undergoes compression to engage plies in contact, squeeze out volatiles, control fibre volume fraction and thickness, obtain near-net component shapes, etc. This is a quality-critical process – deformability of composite precursors defines their susceptibility to defects, their compliance with dimensional tolerances, and the occurrence of shape distortions.

Different forms of deformation mechanisms take place at different structural scales and often occur in parallel. It is essential to have a comprehensive understanding of all these processes to predict the evolution of precursors throughout all stages of composite processing and assess the final architecture/ properties of the composite structure. Each of these mechanisms can be described by material models with various formulations involving large number of material parameters that cannot be determined from direct experiments.

A potentially dangerous trap is that available experimental data are often limited as material testing is both complicated and time consuming. The information obtained in these tests may appear to be deficient and may not reveal all the underlying processes. In this case, property identification may provide a seemingly good fit irrespective of which mechanisms is presumed to happen. However, it does not mean that such model represents the physical reality, and it can often fail to adequately represent a wider set of experimental data. This sets a fundamental dilemma, as the material behaviour (i.e., the model selected) needs to be decided prior to conducting the tests, which introduces a strong subjective element. There is, therefore, a need for a new testing methodology that is capable to identify the deformation mechanisms as well as the relevant material properties. This methodology should be able to check different hypotheses on the deformation mechanisms and autonomously design a testing program based on the measured behaviour of the material.

We have recently developed such adaptive framework [1,2]. It presents a system for autonomous characterisation of materials subjected to an application of pressure and temperature. The system comprises conventional testing machine with heat plates and an integrated “digital brain”, which allows to make decisions on loading path in real time in a “conversation” with material and without human intervention – Figure 2.

Figure 2. Autonomous testing setup

The framework automatically identifies the characteristic flow processes and the properties associated with the correspondent deformation mode, such as viscosity. As a result of the process, the framework creates a reliable digital twin of the material representative over large range of processing parameters. The fully functioning prototype has been successfully tested for various material systems including toughened prepregs and dry fabrics. These curves are rather different from conventional loading programmes and show the complexity of testing needed to identify the underlying deformation mechanisms.

The digitally-driven framework does not just test the material, it defines which flow mode is happening within it and is capable of sensing fine characteristics of material state. The material properties come as a by-product of such examination. This methodology leads to reduced number of experiments while making sure that the obtained data is representative and sufficiently captures all the main features of the material behaviour.

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1. Koptelov A., Belnoue J.P-H., Georgilas I., Hallett S. R., Ivanov, D.S. Revising testing of composite precursors – a new framework for data capture in complex multi-material systems, Composites Part A, 152 (2022) 106697.
2. Koptelov A., Belnoue J.P-H, Georgilas I., Hallett S.R., Ivanov D.S. Adaptive real-time characterisation of composite precursors in manufacturing, Frontiers in Materials, 214.

Bohao Zhang and Byung Chul Kim*

The continuous tow shearing (CTS) process is an advanced automated fibre placement technique with the capability of steering unidirectional prepreg tapes using in-plane shear deformation, without generating tape buckling, gaps and overlaps which can be commonly seen in conventional automated fibre placement process. However, the inherent fibre misalignment within the tape can induce fibre waviness during the CTS process, which is affected by processing parameters such as temperature, shear strain rate and fibre tension. It is of importance to characterise the shear response of the prepreg tapes subjected to large shear deformation by considering the operation processing parameters used in the CTS process.

Two commonly used test methods to characterise the in-plane shear deformation of composite materials, i.e., the picture frame and bias extension tests, are not suitable for characterising unidirectional prepreg tapes. Alternatively, the off-axis tension test could be used, but specimens can only be sheared to small shear angles. Therefore, in this work, a bespoke test fixture was designed to shear unidirectional prepreg tapes at various shear strain rates and fibre tensions (see Fig. 1 which shows the working mechanism of the test fixture) and investigate the effect of shearing conditions. Digital image correlation (DIC) was used to obtain full-field strains of the specimens and to investigate the fibre realignment during shearing.

 

Figure. 1. The bespoke in-plane shear test fixture used for shear deformation of a specimen. The image shows the specimen before (a) and after (b) shear deformation.

 

The experimental results (see Fig. 2) showed that temperature is critical to the fibre realignment during shearing, as the viscosity of the resin matrix significantly influence the level of fibre re-arrangement during the shearing process, determining the shear resistance of the tape material. Thus, the local shear angle measured by DIC became closer to the global shear angle as the temperature increased. (The blue line in Fig. 2(b) is the ideal condition where the local shear angle is the same as the global shear angle, reflecting a perfect shearing.) The shear rate effect was almost negligible when the temperature was sufficiently high due to the reduction of the resin viscosity. High fibre tension allowed fibres to maintain the straightness during shearing. For the CTS process, an optimal processing temperature should be firstly determined. However, its impact on the tackiness of the resin for deposition and adhesion between the prepreg tape and the backing paper should be considered. A high fibre tension is preferable, but it requires a more robust structure of the deposition system.
Please refer to the original paper for more details. (https://doi.org/10.1016/j.compositesa.2022.107168)

Figure. 2. Effect of temperature: (a) average material shear force vs. shear angle and (b) local shear angle vs. global shear angle for IM7/8552 specimens.