Sebastiaan Haanappel

Forming of Thermoplastic Composites

Design and production guidelines for UD reinforced thermoplastic composites are highly desirable. Therefore, forming experiments and simulations with a realistic complex shaped product were conducted. Thermoforming experiments with quasi-isotropic UD carbon/PEEK and 8HS woven glass/PPS composites showed a clear difference in formability. Many wrinkles develop near doubly curved areas for the considered UD composites, whereas significant in-plane shear is observed for the woven composites. Forming prediction tools can be utilised to optimise the product design with respect to formability. A forming prediction methodology is shown, which encompasses finite element modelling in combination with material models that describe major deformation mechanisms. Characterisation methods were developed to describe inter-ply friction and in-plane shear. Forming simulations are able to indicate the critical areas for the UD composites, as is concluded from the comparison of wrinkling and in-plane shear distributions within the formed specimens. Forming experiments and predictions match qualitatively well and this tool can successfully be utilised in the product design phases.

Bending Characterization of UD Composites

This paper presents a novel test set-up to characterize the bending behavior of UD laminates in thermoforming conditions. An analysis of the set-up and the applied boundary conditions is presented. A validation of the test procedure using purely elastic specimens, shows its potential to accurately measure bending properties, to be used for numerical forming simulations. Preliminary bending tests were performed on UD carbon PEEK specimens, showing a visco-elastic response. The actual bending phenomenon is confirmed to be intimately related to the intra-ply shear deformation mechanism.

Formability of Fiber-reinforced thermoplastics in hot press forming process based on friction properties

High performance composites are used in commercial applications in a steadily growing degree. This increase of advanced materials is accomponied with the development of fully automated fabrication processes. It aims to drive down the time and costs of the production while ensuring a high quality of the product. This can achieved by considering the process of hot press forming with continuous fiber reinforced thermoplastics. The development of the process is, however, accompanied with a few difficulties, which require more research. For example, composite materials with different architectures, lay-ups, and constituents, show large differences in formability. This research examines the effect of friction on the formability of thermoplastic composites. Both experiments and simulations were conducted. Demonstrator products have been press-formed from laminates with different materials and architectures (UD-carbon PEEK, UD-carbon-PEI, 8hs-glass PPS, 5hs-carbon PEEK and UD-glass PPS), to investigate their effects on formability. Creating a doubly curved shape from a flat laminate requires at least three deformation mechanisms, namely in-plane shear, bending and inter-ply slippage This paper focuses on the sliding mechanism and the corresponding friction. In order to quantify the amount of sliding in the press-formed product, a dot pattern has been applied to both surfaces of the laminate. The slip between the outer plies can be analyzed by means of photogrammetry. Besides, the friction coefficient of each material is measured in a special designed friction test set-up. It can be seen that the composite formability is directly linked to its friction properties. FE simulations of the press-form process will be performed based on the measured material properties, to demonstrate the influence of the materials friction coefficient.

Forming simulation sensitivity study of the double-dome benchmark geometry

Simulations of manufacturing processes are of utmost importance in order to check on process feasibility of composites products already during the design phase. In order to benchmark the different software for (thermo)forming simulations of textiles and composites a benchmark geometry was agreed during previous Esaform conferences. Round 2 results have led to the insight that a stronger definition of the benchmark was needed, see [1]. The geometry, referred to as double-dome, combines doubly curved regions with steep walls and small radii. Therefore it may be considered critical with respect to forming behavior. As testing material a Twintex comingled glass/PP both as plain and twill weave woven fabric were chosen [2]. This paper addresses the simulation of the double-dome with the finite-element software Aniform. Shear angles, draw-in and the possible presence of wrinkles will be taken into account and compared to round 2 results of other participants. Additionally, a numerical sensitivity study of material and process parameters will be presented in order to identify major influences on the forming results. The paper concludes with a number of recommendations for further research as well as possible improvements for numerical modeling. [1] Sargent et.al., “Benchmark study of finite element models for simulation the thermostamping of woven-fabric reinforced composites”. Proceedings of the 13th Esaform Conference, Brescia 2010. [2] Cao et.al., “Characterisation of mechanical behaviour of woven fabrics: experimental methods and benchmark results”, Composites Part A: Applied Science and Manufacturing, 2008.

Stamp forming optimization for formability and crystallinity

The stamp forming process is well suited for high volume production of thermoplastic composite parts. The process can be characterized as highly non-isothermal as it involves local quench-cooling of a molten thermoplastic composite blank where it makes contact with colder tooling. The formability of the thermoplastic composite depends on the viscoelastic material behavior of the matrix material, which is sensitive to temperature and degree of crystallinity. An experimental study was performed to determine the effect of temperature and crystallinity on the storage modulus during cooling for a woven glass fiber polyamide-6 composite material. An increase of two decades in modulus was observed during crystallization. As this will significantly impede the blank formability, the onset of crystallization effectively governs the time available for forming. Besides the experimental work, a numerical model is developed to study the temperature and crystallinity throughout the stamp forming process. A process window can be determined by feeding the model with the experimentally obtained data on crystallization.

Thermoplastic composites manufacturing by thermoforming

Thermoforming or hot press forming is a fast manufacturing process of thermoplastic composite laminates. Good understanding of the forming mechanisms and the resulting process constraints on the product design are crucial for an efficient development process of such parts. The forming processes, the forming mechanisms involved, and their limitations are reviewed in this chapter. The currently applied fabric-reinforced composites are discussed first, followed by higher performance unidirectional reinforcements, which pose new challenges in forming technology. The chapter concludes with a brief outlook on the medium-term developments to improve the efficiency of the manufacturing and development processes.