Composite Materials for the Aerospace Industry

Leah Boehm

Israel Aircraft Industries

Advanced composite materials used in design of aeronautical structures, have in recent years made their way into the design of many primary structures. A large number of factors affect the complexity of composite materials design, including material combinations of matrix and fibers, number of layers, ply direction and inexpensive fabrication procedure.

Autoclave processing of high performance composite structures is the most widely applied fabrication method. While this procedure provides a good quality product, it is very expensive (high raw materials and autoclaves costs) and time consuming.

A further development trend in the field of high-performance composites is Resin Transfer Molding (RTM) manufacture. This method is capable of production of parts of geometrical complexity and accuracy impossible with other methods with a cost reduction. Some developments has taken place in process monitoring systems for RTM, usually based on sensors arrays in tooling that can monitor resin location and condition via measurements of changes in various electrical parameters as resin flow progresses and cure proceed. Extensive academic studies have also been carried out on modeling resin flow processes in RTM systems.

Another slightly different method developed recently is the Liquid Resin Infusion (LRI) which is less costly and less time consuming. The same resin is used for both technologies. Resin suitable for RTM and LRI are necessarily low viscosity fluids at the injection temperature. This requirements restricts addition of toughening modifiers, and produces a tendency for brittle matrices. A major advantage of the resin system selected, is the ability to gel the system at a relatively low temperature, and to achieve required thermal performance by an extended post cure at elevated temperature. The major difference expected between LRI and RTM processing is in fiber volume fraction.

Various methods are available, each with its own advantages and disadvantages, and with varying consequences on structural performance.

Various Aircraft parts fabricated from composite materials using different processes is shown In Figure 1.

Fig 1: Composite parts of an Aircraft

A great effort is invested in detecting damage in commercial aircraft. The development of an health monitoring system that measures strains at predetermined critical locations during the aircraft operation, is of high priority. A fiber optic based health monitoring system was developed for composite structures, using multiplexed Bragg gratings for strain and temperature sensing. All the aspects of the technology have been investigated, including the embedding process, the integration of the opto- mechanical interface, the suitable connectors. Also, a deep study was performed in order to evaluate the effect of the embedded optical fiber on the mechanical properties of the structure and on the stress and strain distributions of composite materials. Specimens with both uniform and variable strain fields were tested.

Excellent correlation between the Bragg sensors readings and the conventional electrical resistance strain gage data, was obtained. As can be seen in Fig. 2:

Fig.2

A health monitoring system is now being applied to a demonstrator comprising an Israel Aircraft Industries commercial aircraft aileron.