The engineering aspects of automated prepreg layup: History, present and future
Introduction
Future aircraft programs, such as the Boeing 787 and Airbus A350XWB, contain more than 50% by weight of advanced composite components. Consequently the rate and economy of composite manufacture needs to improve to meet the requirements of these and future build programs. Additional areas where advanced composites are of increasing interest are renewable energy and automotive, where advanced composites need to be cost effective in manufacture when compared to their metallic counter-parts. To achieve this automation is one way forward.
Automated Tape Laying (ATL) and Automated Fibre Placement (AFP) are the two main technologies that are employed today to manufacture advanced composite laminates from unidirectional prepregs. ATL is employed to deliver wide prepreg tape onto a surface whilst automatically removing the ply backing. Layup speed, tape temperature, speed and tape tension can be controlled during layup. AFP is similar to ATL but utilises a band of narrow prepreg slices, which are collimated on the head and then delivered together.
A review of ATL layup was published by Grimshaw [1], however this source covers only a single industrially relevant equipment supplier. Similarly, Evans [2] published a review of AFP systems only pertaining to a single industrial system. Short introductions to different aspects ATL and AFP are also given by Åström [3], Campbell [4] and Gutowski [5]. Recently, Sloan [6] has published an industrially focused overview of ATL and AFP.
Despite this, the authors are not aware of any other independent review of this important area of composite manufacture; indeed even the above reviews were never peer-reviewed publications. Further, while most components are manufactured from thermoset prepreg, most research in the field was directed at thermoplastic layup. Currently, an increasing amount of research is being conducted to improve existing thermoset layup processes, see Fig. 1. It shows the result of a literature search on Google Scholar for the number of archive publications for AFP and ATL. The results were summed over a 5-year period to provide meaningful trends. Filament winding with respect to composite manufacture (excluding process relevant to electrical components) is shown in an inset graph as a reference to illustrate the relative shortcomings in terms of scientific publications, and consequently understanding, for ATL and AFP.
With this in mind, this paper will review the historic development of ATL and AFP to highlight the development, and also present the current State-of-the-Art (SOA) for both processes. Lastly, current and future research opportunities are discussed. This work will mostly aim to identify the engineering aspects of thermoset prepreg layup, but thermoplastic prepreg is also discussed, where analogies are appropriate. Special emphasis is placed on the impact of current trends in areas such as structural tailoring and out-of-autoclave curing with respect to automated layup.
Section snippets
Early developments
Carbon fibres became commercially available from 1966 [7] onwards, and very early on it was realised that prepreg layup could be automated to improve the productivity and consistency of manual layup. ATL systems were conceived from the end of the 1960s onwards [8] and by the middle of the 1970s research systems were developed and in application use. The earliest known reference to an ATL is a patent assigned to Chitwood and Howeth [9] in 1971, describing a method of laminating composite tape
Early developments
Quite possibly the first published AFP system has already been introduced in Fig. 2. The 1974 Goldsworthy [10] patent described an ATL system but also highlighted the challenge of conforming a tape to a curved surface. To address this the layup head had the ability to slit down the wide tape into 3.2 mm slices and then deliver those at individual speeds by keeping the additional material on the head, Fig. 8. In reality this would have resulted in technical limitations during material layup and
Productivity
Currently, increases in productivity are the overriding goal for both ATL and AFP. Consequently, all potential improvements that are discussed in the following are governed by, or linked to potential productivity increases. Potential improvements are possible through improved software, machine layouts, materials and enhanced layup. The first two are discussed in this section, while the latter two are discussed in following sections.
Examples of software improvements can be found in Debout et al.
Summary
ATL and AFP are finding more wide-spread adoption in a number of industries due to potential reliability and economic improvements. ATL has been developed since the 1970s as an automated version of manual tape laying and offers high productivity and reliability for simple or low complexity components. It is in particular highly productive for large simple flat components, and able to handle high areal weight materials with few modifications. Future developments in renewable energy, for example
Acknowledgements
Studentship funding for D.H.-J.A. Lukaszewicz from Airbus Operations Ltd. is gratefully acknowledged. The authors would like to thank Dr. K. Hazra, Dr. J. Etches (University of Bristol) and M. Buckley (Airbus Operations Ltd.) for helpful discussions.
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