15 July 2005
15 July 2005
North Coast Tool & Mold and North Coast Composites have produced a prototype of a composite fan containment case that is undergoing testing for both existing and new engines built by jet engine manufacturer Williams Inc.
The tooling design and part development earned a “Turning Goals into Reality” (TIGR) award for The Companies of North Coast, presented to company president and CEO Richard Petrovich by NASA Glenn Research Center’s J. Victor Lebacqz for valuable contributions to jet engine containment concepts and aviation safety.
Once NASA’s and Williams’ testing is complete, North Coast Composites will fabricate the fan cases via resin transfer moulding (RTM) in production quantities.
Visitors to the recent SAMPE International conference and exhibition in Long Beach, California would have seen the prototype case on display.
Composite fan blade containment cases have been under investigation at NASA’s Glenn Research Center (GRC) since the mid-1990s. Under the Ultrasafe Program, GRC materials research engineer Gary Roberts led a project to develop new materials and demonstrate improved containment and damage tolerance, as well as commercial feasibility, for jet fan containment.
Roberts worked closely with composite material supplier A&P Technology, who came up with the concept of a braided carbon fibre/epoxy material for the fan case, under a NASA-sponsored Small Business Innovative Research (SBIR) program. The SBIR has been extended several times to the point where several engine manufacturers, including Williams, are now partners in the program, investigating specific product applications.
Mike Braley of A&P Technology says, “The advantage of braided material is that the fibre architecture is resistant to delamination and is inherently tougher, limiting crack propagation and giving the part more ductility.”
“It’s rare for an SBIR project to result in such a successful commercial product,” says Roberts. According to Petrovich, “This new tool design and manufacturing approach is a paradigm shift in fan containment technology. It was truly a humbling experience to receive this award at NASA’s headquarters in Washington, D.C. with representatives from NASA Glenn, Williams, and A&P Technology.”
Because the technology is geared toward several engine models, each with its own requirements, NASA’s goal was to simplify and streamline the part manufacturing process as much as possible, to encourage commercialization. “Design for manufacture” involved development of a robust reinforcement preform, easy to fabricate and handle, for cost-efficient out-of-press RTM moulding. The preform is made in an automated process developed by A&P Technology in which a braiding machine creates a triaxial carbon fibre braid that is flattened into a wide double-walled broadgood, with fibres at 0° and +/- 60°.
The flattened braid/tape is continuously wound around the circumference of a rotating “capstan” or mandrel in the exact shape of the particular manufacturer’s case. Because the capstan rotation draws the 12K T700 carbon tows directly from spools in the braiding machine, lengths of individual axial tows, which become the hoop fibres in the finished part, are drawn at slightly different rates so that the fibre length exactly matches the varying fan case profile diameter. The flattened braid, wound 10 to 20 times around the capstan, is wide enough to completely cover the width of the fan case and can be transferred from the capstan to a lightweight transport cylinder for shipping to North Coast Composites. Friction between the hoop and bias fibres is sufficient to prevent slippage and deformation during transport.
North Coast Tool & Mold contributed to the success of the project with its innovative RTM tooling. An inner mandrel, with the same shape and profile as the containment case, accepts the preform and is placed within an enclosing, multi-piece outer tool. The outer tool incorporates some simple yet important features that ensure consistent and repeatable tensioning of the flange detail for the fan case, critical for attaching the case to the engine via drilled holes and fasteners. North Coast designed a channel around the edge of the outer tool that captures and anchors the excess braid extending beyond the edges of the mandrel. Removable radial tooling segments are fitted into the channel and pinned in place, tensioning the preform by holding the material taut. The tool’s end plates cover the tooling segments during injection and cure. This minor yet innovative detail allows the creation of a flange of uniform and accurate thickness that can accept the required attachment holes.
Rich Petrovich explains, “The tooling detail creates a robust flange, while adding to the very high mechanical performance of the part. More importantly, it ensures that the flange tensioning is a mechanical process and thereby repeatable. We have minimized the variables in the flange layup process.”
The finished fan case weighs 15 lb less than the steel equivalent case, which is considered a huge reduction by engine manufacturers eager to reduce aircraft weight and improve fuel efficiency. Williams’ Jim Dorer, one of the company’s engineering specialists, says that a 40 percent weight savings may ultimately be possible over the metallic baseline.
NASA is proceeding with full-size ballistic testing on the case part. After an initial blade impact test, the composite fan case will be loaded to simulate an unbalanced engine shut-down blade-loss scenario, which will include cyclic fatigue testing and measurement of damage propagation.
Once NASA and Williams give the go-ahead, North Coast Composites will begin serial production of the fan cases in its Cleveland facility later this year.
“The project illustrates what can be done with well-designed and highly accurate RTM tooling,” concludes Petrovich. “Closed mould tooling can be readily justified by the resulting tight part tolerances, good surface finish and much lower reject rate. You’re really reducing your part cost and project risk in the end. We work very hard to exceed the expectations of our customers — they trust us with their projects and we perform accordingly.”
The image shows the finished Resin Transfer Moulded fan case showing variation in profile and flange detail.