Fiber placement and fiber steering systems and corresponding...

Details Fiber placement and fiber steering systems and corresponding... Fiber placement and fiber steering systems and corresponding...

2001-11-14

2004-09-28

Follansbee, John (Department: 2121)

Data processing: generic control systems or specific application

Specific application, apparatus or process

Product assembly or manufacturing

C700S118000, C700S119000, C703S007000

Reexamination Certificate

active

06799081

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to composite fabrication. More specifically, the present invention relates to composite fabrication employing tow-optimized designs. Advantageously, corresponding system and software programs for generating a tow-optimized composite structure are also disclosed.
Historically, research efforts in connection with composite manufacturing technology have focused on performance, rather than cost, considerations. This trend changed in the 1990's when strict new cost guidelines were applied to emerging civil and military platforms, such as the Joint Strike Fighter.
The aerospace industry has responded to the low cost composites challenge by developing innovative manufacturing techniques, such as producing unitized parts with automated processes. The most significant technology promising reduced cost fabrication is the fiber placement process, which allows large, complex shaped composite structures to be produced faster, approximately 40% cheaper, and with greater quality than traditional approaches. Fiber placement has been used to manufacture military hardware such as the inlet duct of the Joint Strike Fighter (see the article by A. L. Velocci in
Aviation Week and Space Technology
(May 11, 1998. pp. 75-76) and the landing gear pod fairing of the C-17 transport (as discussed by V. P. McConnell in
High Performance Composites
(July/August, 1998. pp. 48-50)), as well as lighter aircraft for civil aviation (as mentioned in the report by E. H. Phillips in
Aviation Week and Space Technology
(Aug. 31, 1998. p. 39)).
FIG. 1
depicts the inlet duct 5 of the Boeing Joint Strike Fighter (X-32) which duct is fabricated in accordance with the known fiber placement process.
Fiber placement is a modern, automated method of manufacturing a composite structure. This manufacturing method has received significant attention recently due to well-documented success in producing complex composite structures in a cost-effective manner. What is not well documented is the fact that the capabilities of existing fiber placement hardware far exceed the capabilities of current design engineering tools, particularly with respect to the ability to fabricate structures exhibiting steered or curvilinear fiber paths.
Fiber placement is a unique process combining the differential material payout capability of filament winding and the compaction and cut-restart capabilities of automatic tape laying. In the fiber placement process, narrow (~0.125 in.) strips or “tows” of resin impregnated fiber are drawn under tension across a tool geometry by a computer controlled head. This head is capable of delivering up to approximately thirty adjacent tows simultaneously, allowing for high production rates. The narrow tows provide precise control over fiber orientation and, since each tow can be controlled independently, thickness tapers on complex geometry are readily produced. It will be appreciated that the control of fiber adds and cuts (the start and stop of individual tows) is controlled by a computer via a CAD interface.
FIG. 2
illustrates a plurality of feed paths employed in one layer of a composite structure in making a predetermined bend. From
FIG. 2
it will be apparent that the feed rate of each tow is also individually controlled, allowing the longer path, i.e., the outside tows, of a steered radius to feed faster than the shorter, i.e., inside, path tows. The ability to support differential tow feed rates combined with the ability to drop individual tows provides the opportunity to place fibers along a relatively tight radius with no degradation in component quality. Fiber steering is made possible by local compaction during placement of the fibers, with each of the impregnated tows having enough tack to overcome any sliding forces.
It should be mentioned that when tows are steered through a radius, the fibers on the outside of the radius are placed in tension and the fibers on the inside of the radius are placed in compression. However, since the fibers are inextensible, the fibers along the inside radius can buckle if the steering is severe. Industry quality assurance programs have demonstrated that using fiber placement technology, carbon/epoxy fiber path geometry can be tailored to a maximum steering radius of twenty inches with no loss in specimen quality. See the discussion by B. Mcilroy in the “Fiber Placement Benchmark and Technology Roadmap Guidelines (Final Report),” Air Force Research Laboratory contract F33615-95-2-5563 The Boeing Company, 1999). See also the article by R. Flory et al. entitled “Effect of Steering and Conformance Requirements on Automated Material Deposition Equipment.” (Charles Stark Draper Laboratory, Inc. technical capability document). Tighter steering radii are possible if the extent of the steering is not severe, e.g., if the arc radius extends less than forty-five degrees. In contrast, tape laying equipment, i.e., equipment performing another automated process utilizing single bands of material approximately six inches in width, is restricted to maximum steering radii in excess of twenty feet, or almost no steering.
It should also be mentioned that typical fiber placed parts might generate anywhere from 2% to 15% scrap, compared with 50% to 100% for conventional hand layup, as discussed by R. Aarns (The Boeing Company) during the Technical Contribution Award speech delivered at AIAA St. Louis Section Honors and Awards Banquet (20 May, 1999). This reduced material scrap rate directly equates to acquisition cost savings due to reduced material usage. Furthermore, the large unitized structures capable of being fabricated equate to life cycle cost savings due to reduced nonrecurring costs, hands-on labor, and part tracking. Finally, the automated process leads to increased accuracy (and, thus, improved quality) and reduced costs due to fewer processing errors and scrapped parts. Each of these advantages has propelled fiber placement into the spotlight. Thus, at the present moment, designs are being developed and produced throughout the aerospace industry, which designs are equivalent to conventional hand layup components, but at reduced cost.
In this development frenzy, a key advantage, i.e., the previously mentioned capability of fiber steering, is being largely overlooked. For example, fiber steering offers potential weight savings by overcoming the restriction of discrete linear fiber orientations commonly associated with traditional composites. More specifically, with conventional hand layup composites, one starts with tape or fabric plies of linear fiber orientation, and assembles these into desired stacks of laminate families, i.e., combinations of various orientations in a preferential stacking sequence. Within a given component, there are two predominant design conditions to consider: (1) overall laminate thickness required; and (2) the proper combination and stacking of various lamina orientations. To change either thickness or orientation requires a point discontinuity in the plies, which necessitates a ply termination at a boundary between adjacent regions of differing orientation. However, current analytical techniques focus on laminate optimization and not ply optimization, thus producing design concepts that are not optimized for either manufacturability or cost of production.
In order to create an efficient design for any component, the component design process must include a detailed consideration of the specific manufacturing processes involved. However, recently implemented, popular automated methods for producing composite structures have yet to develop and distribute the detailed process advantages and limitations in a format that is accessible to the design engineer. As such, there are many preliminary, and in some cases detailed, designs violate absolute requirements of the chosen manufacturing process. This absence of available information in the earliest stages of the design process necessitates redefinition of components, often several times. Furthermore, incorrect or incomplete

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