ETC High Performance Composites - A Composites Materials Primer

ETC High Performance Composites

1.0 Composite Technologies: An Overview

The word Composite literally means "made of several parts." Composite Material therefore is a material system composed of two or more elements. In the United States over the past twenty or so years, particularly in the aviation engineering and manufacturing communities, we have come to narrow the popular definition of composites to include only the non-metallic material format which consists of a fibrous reinforcement of either glass, carbon, or Kevlar® (other possibilities exist) encapsulated in a hardened (or cured) matrix of any one of several hundred resin systems (a.k.a., Organic Matrix Composites). These materials are characterized by their relative high strength-to-weight ratios when compared to more traditional metallic components.

Significant research, development and advancement has also been made in the areas of Metal Matrix and Ceramic Matrix Composites but, in this primer, we will concentrate on Organic Matrix Composites which have the most widespread use. Herein, we will provide a very general overview of composite engineering and manufacturing technologies, including a discussion as to the advantages and limitations of these materials.

2.0 History

Early craftsmen discovered, centuries ago, that the strength of wooden structures could be vastly improved by gluing (or laminating) thin pieces of wood together with the grain of each layer oriented at various angles to one another. Thus was born structures made of laminated composite material. The material elements have changed as manufacturing technologies have improved, but the principle is the same. By orienting filaments of reinforcement (in the early case, the wood fibers, or grain) in pre-determined or engineered directions, and by maintaining that alignment (via resin or glue), anticipated loads to be experienced by the structure can be more predictably and efficiently resolved. These techniques were widely used even in industrial-age applications as exemplified by structures made with woven cloth or paper embedded in resin ("phenolic") used as insulators and for other applications. The point is, composites have been with us for a long time. Even the earliest aircraft (Wright Brothers) had what were, essentially, composite skins which were made by stretching thin fabric over laminated wooden beams and tubular elements, and painting, or "doping" glue over the fabric to improve structural stiffness and strength.

It would fair to say that the aviation / aerospace industry (commercial and military), with it's near-obsessive need to develop structural components with high physical integrity, but which were also very light in weight, was the major impetus to the development of today's modern, advanced composite materials technologies.

The earliest modern composite reinforcing material was Fiberglass Reinforced Plastic (FRP), or more commonly, Fiberglass. As the name implies, filaments of glass are twisted into rovings, which are then either woven (not unlike cloth) into a fabric-like material, or placed as "unidirectional" or "random" direction fibers into a resin matrix, which is then cured, or hardened. It was found that parts properly engineered and molded of fiberglass (FRP) could frequently directly replace steel and/or aluminum components without any structural compromise, and at a substantial weight savings.

3.0 Structural Elements

A composite part has two primary elements: (1) the reinforcement, and (2) the matrix, or (for organic matrix composites) the resin system. It's important to understand that for the most part, the strength of a composite part is contributed by the reinforcement, not the matrix (although the matrix does it's necessary part in holding everything together, it distributes the load among the fibers via shear mechanisms, and provides nearly all "z" direction (through the thickness) and interlaminar (shear) integrity to laminates.

4.0 Reinforcements

While other reinforcement materials are available, typically three (3) families of reinforcements compose the bulk of modern composite technologies. These three reinforcement materials are: fiberglass, carbon-fibers, and aramid fibers (e.g., Kevlar®). Naturally, hybrids of two or more reinforcements may be used in a single component, if indicated by the intended application. Each one of these families of reinforcements is available in literally hundreds of different formats, that is, the reinforcement material is first formed into thin filaments and then processed into any number of physical forms, generally at weaving mills.

The designer must first determine which of the three "families" of reinforcement are indicated for a contemplated application. To simplistically characterize the primary contributions of each of these three materials to an engineered composite structure, you could attribute the following features accordingly:

Fiberglass = Economy
Carbon Fiber = Stiffness
Kevlar® = Impact Resistance

While all three of these reinforcement materials are of low density and high strength, relative to each other glass is generally the least efficient, from a strength / weight standpoint, but the most economical. Carbon has the highest modulus and strength (there are several different moduli available from less than 30 Msi to over 60 Msi). While carbon has a specific modulus (modulus divided by density) that is higher than steel, it also tends to be the most expensive reinforcement. Kevlar®, an organic aramid fiber developed by DuPont, provides good tensile properties and excellent impact resistance at moderate cost but it can complicate certain manufacturing processes and tends to be hygroscopic, which necessitates careful avoidance of moisture penetration into the laminate.

After the engineer has selected the family of reinforcement (or combination thereof), then the format of the material must be determined. Material is available as continuous tow or roving, as unidirectional tapes of parallel, continuous fibers ("uni"), woven into various patterns and styles such as satin and plain weaves, stitched or knitted (unwoven fabrics), or as a felt or mat (randomly oriented fibers). Each of these formats provide certain advantages and limitations to a composite structure, generally in terms of load resolution, molding process, anticipated environment, economy, and aesthetics.

5.0 The Matrix or Resin System

There are two different "families" of resins, thermoset and thermoplastic. For the purpose of this general overview, it should be sufficient to note that, as the name implies, a thermoset resin becomes rigid (or set) once cured, and that heat is often used to begin or accelerate that curing process. In addition, the polymerization (curing) process generally generates its own heat during the molecular cross-linking process that takes place (an exothermic reaction). Control of this thermal build-up is critical to prevent carbonizing (burning), shrink cracking, and/or other associated problems in the structure. A thermoplastic resin on the other hand, becomes pliable with the application (or re-application) of heat. There are fundamental differences in the chemistry that cause these distinctions and, although the High Performance Composites Team is experienced with and uses thermoplastic resins when indicated or required, this discussion will be centered around the more-commonly used thermoset resin systems.

A thermoset resin system is generally a two-part system consisting of the resin itself and a hardening agent, or catalyst. Among the thermoset resins, there is another divide between room temperature and elevated temperature cure systems. Resin formulators have hundreds of systems available to the molder. Many resins are difficult to categorize, but generally there are many types and strengths of epoxy resins (two-part, usually high temperature cure), polyester resins (low cost, room-temperature cure), and vinyl-ester resin systems (low cost, low viscosity, good water resistance, and processing ease). In addition, there are many specialized resins, such as bismalimide, which evidences good thermal endurance.

It should be noted that the reinforcement being used must be sized or surface finished, to be compatible with the intended resin system. The desired or indicated molding process also plays an important role in resin selection, as do environmental, thermal, and operational load conditions of the component being engineered.

6.0 The Molding Processes

6.1 Wet Lay-Up Molding

The oldest technique of composite molding technologies, the hand wet lay-up process generally involves use of an open mold, which is first prepared with waxes and/or release agents, and then the technician paints or sprays a film of surfacing gel coat which is allowed to partially cure before the introduction of layers of reinforcement. The reinforcement (glass, carbon, Kevlar®) can be applied dry (that is, without resin), and then saturated with the resin after laying it in the mold, or the reinforcement may be applied via a chopper or spray-up gun which cuts strings of dry continuous fibers and mixes them with a metered amount of resin as it "shoots" the mix onto the mold surface. After one or several layers, or plies, have been applied, the resin is forced to wet-out the reinforcement, and excess resin is removed, via hand working the still uncured material with serrated rollers.

Typical applications for this manufacturing process:

Low cost, low strength parts (e.g., sliding boards, bath tubs, cosmetic fairings).
Advantages of this process include:
- Requires only modestly-skilled workers, and is therefore rather inexpensive.
- Wet lay-up resources are available throughout the country, typically near marinas.
- Uses "soft" tooling (molds made very much like negatives of the parts themselves).
Limitations of this process include:
- Low strength (comparative to other processes).
- High (and less controlled) resin content, and concomitant high weight.
- Finished on one side only.
- Low production rates.

6.2 Bag Molding

Bag Molding is similar to the wet lay-up process in that an open mold is "prepped" and laminated. However, with the bag molding process, a film or rubber skin (a bag) is applied after rolling-out the preform and prior to curing. Then a vacuum (Vacuum Bagging) is applied to extract excess resin and to improve consolidation (an increase in the reinforcement / resin ratio caused by applying atmospheric pressure to the laminate during the curing process). This bagging can also be used during Autoclave Molding (or Pressure Bagging) to produce a higher differential pressure on the laminate, and increased consolidation, by applying high pressure to the outside of the bag and venting the inside to atmosphere.

Typical Applications:

Higher strength / lower weight (that open molding) parts.
Generally for non-critical use (Vacuum Bagging).
"Safety-of-Flight" items (Autoclave Molding).
Advantages of this process include:
- Can use "soft" tooling.
- Modest material and equipment investment (Vacuum Bagging).
- Highly consolidated, primary structure components (Autoclave Molding)
Limitations include:
- Finished on one-side only.
- Material waste (bagging film).
- Labor intensive.
- Can require significant investments (Autoclave Molding).

6.3 Compression Molding

A catalyzed resin system is pre-impregnated onto the reinforcement material via metered application (prepreg), partially cured and then frozen to prevent further polymerization (the resin cure process). This material is stored in freezers until ready to cut and lay-up into a preform. The material itself has a tack which serves to hold adjoining layers in the proper position to each other.

After the various pieces of material are oriented into a preform in accordance to the engineer's ply schedule, the preform is placed into a two piece, steel (generally), matched mold set which is mounted in a press (High Performance Composites has two general purpose presses, a 75 and a 200 Ton, as well as some dedicated, integral mold / presses). The general purpose presses are hydraulic, and have heated platens (the mold halves may also be heated). As the mold-halves close on the pre-form, consolidation pressures are created, and heat is applied which serves to begin and accelerate the curing process. (Prepregs are often used in vacuum bag and autoclave molding as well.)

Typical Applications:

Optimum-strength, primary structure composite parts (aircraft landing gears, military anti-ballistic helmets, etc.)
Advantages of this process include:
- High strength, lightest weight arising from optimum fiber volume.
- Precisely controllable variables of pressure and temperature.
- Part is finished on both sides.
Limitations include:
- High tooling investment required for each part.
- High capital equipment investment required.
- High operator skill levels required.
- High material costs (the prepreg).

6.4 Resin Transfer Molding

Although the RTM process has been around for over forty years, it has gained new respectability in recent years as composite development resources have improved the processes to achieve good physical properties by optimizing fiber volume. In this process, dry material is placed into a two-piece, matched mold, which is then closed on the dry material. Resin is then pumped into the mold, saturating the dry reinforcement and filling all air voids. The resin cures either via time (room temperature cure) or by the application of heat, then the part is removed from the mold. There are many variations of this process, frequently proprietary to individual molders. A common variation is known as VARTM (Vacuum-Assisted Resin Transfer Molding) in which an open mold (one piece) is used to hold the dry material, and a bag is then sealed over the mold. Resin is then pumped into the envelope with the assistance of a vacuum, which is used to provide consolidation and remove excess resin. Another variation is High Performance Composites' proprietary process known as MMM (Massive Monolithic Molding) which has enabled the previously impossible molding of very large, single cure parts (some very large, chambered structures have been successfully molded that weight over 1,000 pounds).

Typical Applications:

Production-type, modest-strength parts such as bicycle wheels.
Advantages of this process include:
- Parts can be finished on both sides.
- Rapid production achievable.
Limitations include:
- Higher skill levels required for both tool designers and composite molders.
- Possibility of fiber drift, or motion during resin injection.
- Tooling and equipment investments can be prohibitive.

6.5 Sandwich Construction

Rather than a molding process, this is a composite-part configuration that is playing an increasingly important part in evolving composites technologies. Essentially, thin skins of engineered composite material covers a core of a lightweight, shear-carrying element of either honeycomb or structural foam. These sandwiches tend to offer the highest physical properties at the lowest possible weight.

Typical Applications of this process:

Gossamer structures of aircraft structural integrity, as in bulkheads, wings, propellers.
Advantages of this process include:
- Extremely efficient weight-strength ratios achievable.
- Sandwich panels can be molded with modest tooling as flat stock, then bonded / assembled.
Limitations of this process include:
- Surfaces must be finished to protect against moisture ingress.
- Careful control of processes required to assure good skin / core adhesion.
- Out-of-plane forces may damage thin skins more easily
- New failure modes (e.g., face sheet crippling) are introduce requiring more careful analysis.

7.0 Composite's "Rules-of-Thumb"

As the above generally illustrates, composite materials technologies are a diverse combination of science, craft and art. Structures engineered from advanced technology composite materials can provide significant contributions to a system in terms of reduced weight, high stiffness and high strength. Further, other specific factors, such as zero dimensional growth or shrink under thermal variations (i.e., zero CTE) can be achieved by the materials engineer. Environmental considerations may also be addressed in the design of a new composite structure. A composite component will not corrode (however, when designing a carbon fiber structure in contact with metal, care must be taken to avoid galvanic corrosion of the metal). Military advantages in terms of reduced radar signature and sound dampening can be achieved.

Generally, a properly engineered composite structure will not fail catastrophically; that is, failure behavior is more benign than with a metallic part. With metals, a "stress riser" (crack or other imperfection) will be the nucleation point for propagation and ultimate failure, particularly under fatigue loads. With an engineered, oriented composite structure, a crack will not propagate in the same manner; the fibers act as "crack arrestors" and the loads find alternate resolution paths through adjoining fibers.

An engineer experienced with composite materials can also achieve a substantial "parts reduction" when compared to metals design. The design freedom afforded by monolithic composite forming technologies cannot be duplicated with metals (which generally require mechanical fastening or welding).

On the other hand, composite materials are not the answer to all technical challenges. These materials contain some intrinsic disadvantages. They tend to be more expensive than metal and, since every new application presents the engineer with new load / environmental considerations, successful prototype development and testing tends to require greater non-recurring investments. However, if metals cannot solve a design problem or if an all-metal design has reached its limit of performance improvement, composite materials offers the "next step" in the engineering design and improvement process.

8.0 Summary

Successful engineering development of challenging composite materials projects requires a careful mix of technical disciplines. The materials / structural engineer must be intimately familiar with the nature and properties of the reinforcements, resin systems, molding processes, and the intended application. That engineer, however, is but one member of the highly skilled, High Performance Composites technical team, and works hand-in-hand with our trained and skilled analyst who computer-models and predicts behavior with our modern Finite Element Analysis techniques using our customized linear / non-linear analytical packages. Once a candidate design has been identified, and while the structural engineers continue to delineate every individual ply in a complex structure, completely different, but interdependent technical disciplines are brought into project play as our experienced composite tooling and quality assurance engineers define the manufacturing processes and design the necessary precision tooling and ancillary fixturing required to predictably and repeatedly mold the engineered structure.

High Performance Composites, a business unit of the Environmental Tectonics Corporation (ETC), is one of the few companies in the United States that specializes in the development, and subsequent manufacture of complex and challenging composite material structures. Core members of our team consist of experienced specialists with the required mix of technical disciplines, and have been making pioneering contributions to the technology since the mid 1950's.

125 James Way • Southampton, PA 18966 • 215-355-9100