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The Aerotiss® O3S – One Side Straight Stitching – technology consists of assembling a dry composite preform by means of carbon threads (fig.1). One of the advantages of such figa technology is a quicker manufacturing process that integrates mechanical functions and cuts down on weight. Technological demonstration and mechanical characterization show the technology’s potential: its strength capability is twice that of bonding.
(Published on June-July 2005 – JEC Magazine #18)
BY JÉRÔME BERTRAND, ENGINEER, HEAD OF RE-ENTRY VEHICLES ARCHITECTURE & ENGINEERING
BRUNO DESMARS, R&D ENGINEER, MATERIAL CHARACTERISATION, NON DESTRUCTIVE INSPECTION DPT EADS
EADS SPACE Transportation (EADS-ST), has been working with the EADS Common Research Centre (EADS-CRC-F) on the multidirectional Resin Transfer Moulding (RTM) / Aerotiss® O3S technology  &  both for space applications (e.g. launchers and missiles) and for aviation applications (e.g. parts with fittings). The key drivers are mass savings, cost effectiveness and quicker manufacturing, thanks to fewer parts, less assembly and less inspection.
The EADS-ST technology roadmap consists of: - demonstrating the technological feasibility of complex parts with integration of mechanical functions; - improving the existing sizing rules to better predict Aerotiss® O3S behaviour under combined loads.
The objective was to design and manufacture a complex RTM part (fig.3) with good material health and fittings designed to sustain loads up to 400 kN (span 1000mm – width 750mm – height 100mm – total mass 11kg) using industrial tools and facilities. The preform was manufactured using the qualified G1151 carbon fabric and the Aerotiss® O3S stitching technology on a thick Z spar (fig.2). Compared to the parts already manufactured by EADS CRC-F, this spoiler demonstrator was innovative in terms of thickness variations (from 1.9mm in the skin up to 25mm in the fittings). It was also a more complex Aerotiss® O3S design than a U or T-shaped sample. The sides were also included in the manufacturing process. The ultimate objective is to obtain a “one-shot” spoiler including both windward and leeward skins.
Justification – sizing & tests
To master the design of stitched parts with Aerotiss® O3S, tests under combined loads at an elementary scale were performed. Tests included studying the stitching density and orientation (table 1). T-shaped samples (fig.4) were manufactured in G1151/ RTM6 with Aerotiss® O3S threads in T800H-12k. The thickness of the samples is close to 10mm in web and flange. The samples are tested in ARCAN configuration  (fig.5), which introduces a combination of tension and shearing loads, whose ratio is selected by the angle of loading. All the samples are instrumented so that experimental results can be compared to Finite Element Methods to define failure criterion and design rules.
The results of the tests are all available. A preliminary analytical exploitation was performed, presented in fig.6. The failure occurs in the Aerotiss® O3S joint at high levels of stresses: up to 80 MPa in tension for 90° stitching and up to 80MPa in shearing for 45° stitching. Moreover, for combined stitching in S4, the strength is about 65 MPa in tension and 70 MPa in shearing, which illustrates the high mechanical strength of such a technology compared to bonding (limited to 30-40 MPa). The combination of the normal and tangential yarn strength in each direction weighted by the number of lines in each direction gives a good forecast of the experimental level of failure for combined stitching orientation (fig.7). Test exploitation includes FEM analysis and will allow defining design rules to evaluate the most suitable design for Aerotiss® O3S (density, stitching orientation) versus the loads of the targeted full-scale application. Design methodology will be completed using at least the characterization of out-ofplane loads, thickness effects, temperature and ageing influence, and full-scale validation for the intended application.
EADS-ST’s stitching unit has 10 axes and can stitch a complex preform up to 50mm thick automatically. The long-range size of robot cell (length=5.5m, diameter=2.6m) can stitch a large preform of full 3D shape. It was modified using an automatic program machine to control various stitching parameters in order to manage and guarantee the stitching process. The robot environment and stitching devices were designed with CATIA CAD tools (fig.8) in order to simulate the mechanical movement and thus avoid any machine/preform collision during the stitching pattern manufacturing. The numerical programme was managed by the Direct Numerical Control database.
A simulation of the injection process is required at the very beginning of the study to assist the mould design, i.e. placement of injection line and vents and to define the injection procedure (resin parameters: injection temperature, quantity of resin and choice of injection machine, time required for injection with respect to the injection time range of the resin). LCM flow software was used by EADS CRC-F (fig.9). The spoiler demonstrator combined many geometrical difficulties such as: complex shape with spar, fittings, sides, windward skin, and different thicknesses. These problems, including possible non-impregnated areas and exothermic effects, were solved before injection. EADS CRC-F tools were used for the injection and cure of the demonstrator. The injection machine is designed for industrial RTM applications and can inject a big resin volume (volume only limited by the gel time of the injected resin). Injection and curing in autoclave was an appropriate solution to reduce the size and cost of the demonstrator injection mould.
EADS-ST has all the necessary tools to perform non-destructive inspections after manufacturing on both prototype and massproduced parts. Both ultrasonic and tomography techniques are used. Early design takes into account US inspection constraints such as parallel faces in the stitching area. Tomography is a specific tool using X-rays to perform a virtual cut of a part. In the framework of the spoiler demonstrator, tomography was very useful and the good condition of the part (stitching area) was demonstrated as shown in figure 10.
EADS-ST has a solid background in the field of composite materials: composite motor cases and high-pressure vessels, composite launchers thermal protections and re-entry thermal protections, launchers fairings, satellites central tubes, antenna reflectors, with various technologies (filament winding, weaving, laying-up and stitching) as well as autoclave or non-autoclave curing processes.
Through the successful manufacture of a complex aeronautic part (spoiler), EADS-ST validated the design, the process and the inspections with Aerotiss® O3S. The good material condition obtained in the spoiler illustrates the technological mastering of such a technology which makes it possible to manufacture very complex parts in one shot. Moreover, mass savings up to 30% (compared to metallic solutions) were demonstrated in the field of space and military applications on complex shape parts. Improvement in the sizing rules dedicated to Aerotiss® O3S has been also initiated by testing T-shaped samples under combined loads. Available results already demonstrate the high mechanical potential of such a technology compared to bonding. Looking towards the future, EADS-ST has the experienced teams, skills and tools required to design, manufacture and control structures with Aerotiss® O3S.