The Development and Manufacture of an Advanced Composite

Aircraft Equipment Rack and Workstation ©

Abstract

The redesign, replacement and modification of aircraft metallic secondary structures and support equipment with lighter composite materials can increase mission effectiveness by providing greater payload and/or extended flight time. Conversion and upgrading of older aircraft, which may be necessary for various reasons, requires creative alternatives and careful analysis directed not only to the weight reduction of such structures, but toward versatility and life-cycle cost reduction as well.

Since the 1970's, advanced composites have met and often exceeded expectations as primary and secondary structure on new aircraft. Although spearheaded by the military, composite conversion was often an arduous process because of the initial high cost of materials, limited design expertise and misconceptions from designers who were slow to be "converted."

Cost is now a principle driver in the selection of composites. Their superior performance and life-cycle effectiveness are well established. As raw material costs have come down and innovations in processing expand, composites deemed "exotic" a decade ago will find uses in a myriad of applications in existing aircraft.

This paper briefly reviews the design and manufacture of a carbon/epoxy equipment rack certified to an FAA Part 25 aircraft. It was prepared for those who may not have a comprehensive understanding of advanced composites. It is not intended as a technical treatise on the matter, but rather a general overview of the design and processes related to a specific application.

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Key Words: Advanced Composites, Carbon-Epoxy, Lightweight, Equipment Rack, Communications Rack, Secondary Structure, Workstation.

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Introduction

The composite equipment rack developed by the author provides a weight savings of 60 percent over the aluminum rack that it will replace. This expands the aircraft mission effectiveness by allowing for more fuel, increased payload, extended flight time or additional crew members.

Advantages in using advanced composites are listed below¹:

• Typically composites can provide a weight savings of between 25 and 50 percent over an aluminum structure.

• Specific tensile strength (ratio of material strength to density) is four to six times greater than aluminum or steel.

• Specific modulus (ratio of material stiffness to density) is three to five times greater than aluminum or steel.

• Composites are more versatile than metals and can be tailored to meet performance needs and complex designs.

• Excellent structural damping properties can be designed into composites.

• Fatigue and fracture resistance are superior, approaching 60 percent of ultimate strength (considerably higher than aluminum and steel).

There are some disadvantages in the use of composites. While the perception is often worse than the reality, some lingering concerns are: higher material costs, special handling, manufacturing and inspection procedures and unique, sometimes proprietary processes. However, if weight reduction is a necessity, advanced composites often offer the only suitable alternative.

Composite Rack Design Considerations

Carbon fiber reinforced composites have a lower density than aluminum. Consequently, a simple switch from aluminum to composite with no design change will result in a reduction of part weight. When composites began to show promise for saving weight over metal, there was little expertise in design. The simplest approach was to copy the original metal design and replace it with the lighter "black" carbon fiber composite, thus "black aluminum."

Robinson ² clarifies two misconceptions often attributed to composite applications by asserting that: 1.) Automation of composite fabrication is not often cost effective and, 2.) In spite of perceived higher materials and labor costs, "smart design" of composite structures can, indeed, be a cost safer over aluminum counterparts. Two brief excerpts are worth review here:

". . . a recent industry survey has shown that even for more cost-conscious applications, most companies still suffer from the so-called "black aluminum" syndrome, the mindless conversion of aluminum structures and design principles without consideration for the unique qualities and manufacturing technology of the composite materials."

"As most composite specialists now know, smart design (typically achieved by concurrent engineering) is by far the biggest cost saver in composite part production.

To take full advantage of the anisotropic nature of composite materials, it is desirable and practical to design specifically for the calculated loads of the composite structure. By controlling the number and orientation of fabric plies in the composite, loads are supported in the most efficient way possible. It is rarely true that the thickness of aluminum used in any given design is required for a composite replacement. Since metals are isotropic, there is always some material that is not being worked as hard as the rest. Metal components are often over built or oversimplified in applications where weight is not paramount. However, with careful design of carbon fiber reinforced components, it is possible to reduce this unworked material to an absolute minimum.

In this particular case, the current aluminum rack consists of 6061T6 aluminum square tubing with tabs centered on the tube that serve to carry cross members (Figure 1). The cross members, usually aluminum channels, support the equipment. While this design is acceptable for most stationary ground based racks, severe weight penalties are incurred when placing such a structure in an aircraft. Additionally, the tubes were joined by aluminum corner castings that added considerable weight to the overall rack.

Figure 1 - Aluminum (1.02 lb/ft) Vs. Carbon/Epoxy (0.39 lb/ft) Cross Section

Maximum weight reduction of the composite rack was attained by integrating the following principles and concepts:

1. The lower density of the carbon/epoxy laminate rendered a lighter part (.040 lb/in3 vs. .100 lb/in3).

2. Higher specific strength and specific stiffness of the laminate over aluminum allowed for a thinner wall section.

3. Replacing the square tube with a triangular tube reduced the area, thus reducing material usage and weight.

4. Eliminating the aluminum castings in favor of composite corner gussets (Figure 2) contributed not only to reduced weight but provided a superior load bearing structure.

Figure 2 - Rack Profile Showing Bonded Gussets

Material Selection

The composite material used in the rack consists of a carbon fiber woven fabric and unidirectional fiber tapes (reinforcement) which are impregnated with an epoxy resin (matrix). The combination of reinforcement and uncured resin is called a prepreg.

The designer has a vast selection of materials to choose from. The most common reinforcements and resins systems are listed below:

 In addition, the reinforcement can be supplied in many forms:

• Woven fabrics of different styles to provide drapability for highly contoured parts.

• Unidirectional fabrics and tapes that provide superior stiffness in one plane.

• Knit fabrics with fiber orientations in three or four axes.

• Hybrid fabrics that contain two or more different reinforcements for synergistic properties.

The matrix can also be supplied in different forms for specific applications. The selection process is complex and beyond the scope of this report however the customer's requirements and the structural and physical requirements of the composite part are all factors.

For this application, a woven carbon fabric was combined with a unidirectional carbon fiber tape. The matrix selected was a flame retardant 250^o F curing epoxy resin. This combination was chosen to meet the weight, flammability, bearing strength targets of the rack.

Composite Process Overview

Advanced composite processing grew out of the fiber reinforced plastics industry (fiberglass). The simplest fabrication method (which is still used) is to pour a catalyzed resin onto a layer of fabric pre-formed in a mold of the desired part shape. Additional layers of fabric are applied and the excess resin is manually pressed out of the fabric. The laminate is left to cure at room temperature.

The application of a sealed vacuum bag over the laminate aids in better forming the part to the mold contour. Removal of excess resin and air bubbles is also improved.

Liquid resin systems gave way to preimpregnated fabrics. These materials provide better resin control and more precise fiber placement. Most prepregs require oven processing to cure. This, in turn, provides superior process control.

Advanced composites fully matured when autoclaves became the standard method of processing. Both heat and pressure are applied at a controlled rate for precise polymerization of the resin. Consistent fiber-to-resin ratio and minimized void content are a result of this process.

Since autoclave processing is very costly, numerous alternative processing methods have been developed. Compression molding, resin transfer molding (RTM), pultrusion and filament winding are the predominant processes used today. All achieve the same objectives as the autoclave, often for far less cost.

Composite Rack Processing

Like other manufacturing processes, the volume of parts required often determines the best method of fabrication. The first generation of rack components were fabricated using vacuum bag/oven cure processing. As production demands increased, compression molding was adapted. A brief review of process alternatives is listed below:

Low Volume & Prototypes

• Likely Process: Hand lay-up, vacuum bag/oven cure processing.

• Advantages:  Low cost, rapid turn around.

• Disadvantages: Labor intensive with a relative high skill level, higher void content in laminate, possible higher scrap rate due to operator error. Secondary operations needed.

Moderate Volume, Sustained Production

• Likely Process: Compression Molding.

• Advantages: Simpler processing geared for higher production rates. Extended tool life. Lower void count and scrap rate. Two sided tooling surfaces.

• Disadvantages: High tooling costs and lengthened cycle time due to metallic tooling heat-up and cool down.

High Volume Production

• Likely Process: Resin Transfer Molding With "Near-Net-Shape" 3D Perform

• Advantages: Low cost tooling, fast cycle times and lower long term raw material cost. Operator skill level is not critical. High quality parts with negligible scrap rate.

• Disadvantages: Higher development and qualification cost. 3D preforms have limited database. Proprietary processes often involved.

Rack Assembly methods

The triangular tubular sections (figure 1) are trimmed to fit the final configuration of the rack. The tubes and mating gussets are placed in an assembly bond fixture and permanently bonded with a structural adhesive. Secondary cross members and additional hardware are bolted into place allowing for modifications in the field. Standard assembly methods are used with the following exception: Cutting or drilling into the composite structure requires sealing the exposed area before fastener installation to prevent moisture absorption. Methods for this operation are well established.

Bolted gussets have been tested and are a suitable alternative for rack assemblies that are too large to fit through aircraft doors or require major modification after installation.

All hardware, enclosure panels and attachment points are installed before final inspection.

Electrical Bonding

Although carbon/epoxy laminates are conductive, the structure does not provide for electrical bonding in itself. Braiding and termination studs must be provided as required by the equipment configurations. These items are provided as specified by the customer.

Quality Assurance

Beyond standard QA practices, special procedures must be followed in composite fabrication. Raw materials have a limited shelf life and provisions must be in place for special handling, storage and processing of parts. Traceability of materials is maintained from receipt of the prepreg and adhesive until the final rack assembly is shipped. Material, handling and processing logs are carefully maintained. Critical lay-up and bonding operations are performed in a temperature and humidity controlled environment. Fabricators and inspectors are trained to detect anomalies based on accepted practices in the industry.

Test and Certification

In this case an FAA approved test plan³ was developed by an on-staff Designated Engineering Representative to substantiate the composite rack design. All load conditions were tested to failure. A list of conditions tested follows:

90^o Bonded Corner

Flange Bending

Edge Tear Out Perpendicular

Edge Tear Out Parallel

Insert Tear Out Perpendicular

Insert Tear Out Parallel

133^o Joint

Hat - Bottom Flange Bending

Hat - Edge Tear Out Perpendicular

Hat - Edge Tear Out Parallel

The final report⁴ concludes "All margins of safety are positive for all hardware by calculation or by inspection. This installation is designed to meet FAR requirements of 25.301, 25.305, 25.361, 25.615, 25.571, 25.878, and 25.625 using MIL-HDBK 5E and Composite Joint Testing . . . for allowables."

Maintenance and Repair

Substantial data on composite repair of military and commercial aircraft has been documented. The primary area of concern has been exterior aerodynamic surfaces subject to impact damage and environmental exposure.

Although the composite rack will not be subjected to harsh environments, provisions must be in place for their repair and maintenance. Complete repair manuals are provided with each installation.

The projected life of the composite rack is equal to that of a comparable metallic rack. As metallic structures must be protected from corrosion, composite structures must be protected from moisture absorption. The primary method of protection is to seal modified or damaged surfaces with industry standard sealants and adhesives. Convenient Semco cartridge type dispensers are often specified in MIL-Spec and industry repair manuals.

Conclusions

Advanced composite technology is as critical to modern day flight as the most advanced flight control systems and propulsion systems available. Military aircraft provide the test bed and leading edge for all advanced systems and, in time, that technology trickles down to an array of military and commercial products.

While the state-of-art aircraft and technology get the headlines, there are far more aircraft performing critical missions every day. Weight reduction of these aircraft, most of which were built before "stealth" was a household word, becomes more critical as these aircraft age and their missions become more complex and varied.

Modification to primary structures on aircraft is a costly endeavor for weight reduction alone. Yet careful weight analysis of what goes into the aircraft coupled with a paradigm shift from accepted standards can provide cost-effective solutions.

As the advanced composites industry has matured, costs have come down substantially. As fewer new aircraft are being built, modifications to existing aircraft have increased significantly.

The certification of a composite equipment rack to replace a heavy metallic rack is not unique, but simply a trend to extend mission effectiveness. Further examination of installed and removable components for conversion to advanced composites on aircraft with specialized missions will not only reduce long term cost but increase flight safety as well.

References

1. Excerpted from "General Use Considerations," Bryan R. Noton, Composites - Volume I, Engineered Materials Handbook, ASM International.

2. M. J. Robinson, A Qualitative Analysis of Some of the Issues Affecting the Cost of Composite Structures, 23rd Annual SAMPE Technical Conference, October 1991.

3. John G. Smith, Structures DER, Composite Joint Testing for Equipment Racks, Doc. No. 17636001, July 1995.

4. John G. Smith, Structures DER, Structural Substantiation for Next Generation Racks and Workstations, Doc. No. 17638003, August 1995.

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