: Aircraft engineering ( . .)

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Materials

 

Composite materials have gained their acceptance among structural engi- neers during the last decades. The performance of a composite depends upon:

The composition, orientation, length and shape of the fibers;

 

The properties of the material used for the matrix (or resin);

 

The quality of the bond between the fibers and the matrix material. Composite materials consist of any of various fibrous reinforcements coupled with a compatible matrix to achieve superior structural performance. The most important contribution to material strength is that of fiber orientation. Fibers can be unidirectional, crossed ply, or random in their arrangement and, in any one direction, the mechanical properties will be proportional to the amount of fibers oriented in that direction. Reduced properties result from the shear strength of the weak matrix. In fact, both the strength and module of a composite in a ply are reduced considerably when the angle of the applied load deviates from the di- rection of the filaments in the composites. Fig. 2.1.1 shows how a unidirectional reinforced composite will have a far lower strength in transverse tension than one loaded exactly in the direction of the fibers. Therefore, it is evident that with increasingly random directionality of fibers, mechanical properties in any one direction are lowered. Thus, because of their low mechanical properties normal to the fiber direction, laminate composites will need to be strengthened or stif- fened by laying up plies (unidirectional tape or woven fabric) in different direc- tions. Such lamination will be necessary because stresses in a loaded component or panel can vary in both the X and Y direction.

Laminate properties of various combinations of plies oriented at different angles can be calculated through the use of computer programs to produce the best design. These computer aids are particularly helpful, because of the com- posite's non-isotropic properties, in calculating the various properties of any combination of oriented plies.

In this Chapter only those materials which are used on airframe structures will be discussed the data given is general and, while it may be used by the de- signer to do rough sizing, it is not appropriate for stress analysis or final sizing

use. No composite material design allowable data is given (usually this data is part of a company's proprietary data) in this chapter because numerous varieties exist and many improved products are available every year.

Material selection

 

Material selection plays a large part in final cost, not only because the raw material itself is expensive but also because the material selected often deter- mines downstream manufacturing costs. The material selection criteria are given below:

Cost

 

Available mechanical and environmental properties database

 

Suitability for use in proposed manufacturing processes

 

Structural performance

 

Ease of processing

 

Ease of handling

 

Supportability

 

Maximization of knowledge base

 

Available processing data

 

Immediate or near term commercial availability

 

There are many cases where more than one material can meet the struc- tural and/or weight requirements specified for a given part. Assume there is a choice between a unidirectional material form (which can be used on the auto- mated lay-up machine) and a broadgoods form (fabric or woven) of the same ma- terial. Clearly there is a difference in the costs of these raw material forms: un- idirectional prepreg generally less expensive because the material supplier has not gone through the added step of weaving the broadgoods fabric. At the same time, it may take more time to fabricate a laminate component from unidirection- al tape than form broadgoods. Therefore, there is a tradeoff between actual raw material costs and the downstream manufacturing costs which are predeter- mined in choosing a particular raw material.

Obviously, along with cost considerations, any design must carefully match the requisite properties with the candidate composite material. Once the optimum, or best available, material is chosen, the designer must be concerned

with any additional limitations that material selection might impose on the capa- bilities of the design. The common areas of concern include hot/wet properties, notched effect (if fasteners or small cutouts are used in design), in-service tem- perature, and impact resistance. Critical limitations that must be considered in composite design include the relatively low strength and stiffness in the out-of- plane direction and often poor shear properties. These factors must be con- sidered to prevent delaminating under compressive loading or inadequate out-of- plane load-carrying capabilities.

Various kinds of composite materials with temperature resistant matrices and high-performance reinforcements are currently available or are in advanced stages of development

Since the properties of composites depend critically on the processes used to make them, designers must work with prepreg and fiber producers to achieve desired results. The designers should be aware of the weaknesses of various fibers and construction methods and so design around them. The following are material specification requirements:

Fiber and fabric properties

 

Storage and retest requirements

 

Uncured prepreg properties

 

Cured or co-consolidated prepreg properties

 

Mechanical properties

 

Environmental testing

 

Processability trials

 

Chemical characterization

 

Non-destructive inspection (NDI)

 

In general, most of the major reinforcement systems have been well cha- racterized for many years, and performance improvements have occurred in rel- atively small increments. However, improvements in the matrix resins (both thermoset and thermoplastic) have allowed great strides in composite fabrication, producibility, performance, and stability.

The reinforcing fiber may have a negative thermal expansion coefficient along its axis, a property that makes possible the design of structures with zero

or very low linear and planar thermal expansion. Thus, the main support truss for the mirrors of the "Space Telescope" is made of a carbon/epoxy composite to meet extreme close tolerance requirements

It is worthwhile to note research on organic conducting polymers, which would have many airframe applications, such as to provide shielding on compo- site structures for sensitive control electronics from electro-magnetic interfe- rence (EMI). Another related application is lightning strike tolerance on air- frame structures.

Hybrid laminates

 

Hybrid systems, made by combining two or more types of fibers in a sin- gle laminate, can be tailored to meet specific performance requirements, and are an effective means of reducing the cost of composites. The unique performance of one of the reinforcing materials compared to the other can enable the compo- site to do a job that neither can do independently. While hybrid composites any offer the best choices for some design cases, designing with hybrids is somewhat complicated because most of their properties are not as easily characterized as are those of single fiber composites. Applications and data show that different fibers can be combined successfully in a structure in many ways. The fibers can be used in different layers or even in completely different parts of the same structural element. They also can be blended to form a hybrid tape or woven to form a hybrid fabric.

For hybrid constructions, directional response and failure parameters should be defined for each material. Care must be taken to provide reinforcement for all loading directions. Since carbon or graphite is conductive both thermally and electrically and has a slightly negative coefficient of thermal expansions, it is conceivable for designers to develop hybrid material geometries with structural responses totally different from the existing conventional metal materials.

As the cost/performance tradeoff becomes more critical, hybrids may be- come the material system of choice for more structural uses, making them mate- rials with a future. Nevertheless, the future of hybrids in the airframe industry appears uncertain and much still needs to be learned about this systems. Issuers peculiar to hybrid systems are described below:

(a) Material more tailored to specific needs than is available with a single fi- ber-matrix combination is very desirable.

(b)Different fibers have different:

 

Strains to failure

 

Module

 

Coefficients of thermal expansion

 

Coefficients of moisture expansion

 

(c) Thermally induced stresses exist in every hybrid lamina (below the lami- nate level)

Caused by different thermal expansion characteristics of constituents

 

Can be large enough to cause failure without mechanical load

 

(d) Effectively an infinite variety of hybrids is possible

 

Each new hybrid must have some minimum level of material property qualification

The wide variety of possible hybrids (just like the wide variety of poss- ible laminates) must be deliberately restricted on purely practical grounds.

There are typically three methods of hybridization:

 

Interplay Different reinforcements are stacked in separate layers with no mixing within the layers

Interplay Different reinforcements are commingled within a layer ei- ther by alternating strands or mixing chopped fibers

Selective placement the laminate is basically composed of one rein- forcement, but a different reinforcement is added in certain areas (such as cor- ners, ribs, etc.)

Hybrid reinforcements can be combined in almost all material forms in- cluding:

Prepregs

 

Fabric

 

Woven roving

 

Chopped fibers

 

Common hybrids include: (a) Carbon/Aramid

Can be combined without residual thermal stresses since coefficients of thermal expansion are very similar (b) Carbon/Glass

Increased impact strength

 

Improved fracture toughness

 

No galvanic corrosion

 

Reduced cost over all carbon fiber laminates

 

Organic matrices

 

The purpose of the matrix is to bind the reinforcement (fiber) together and to transfer load to and between fibers, and to protect the flaw- or notch-sensitive fibers from self-abrasion and externally induced scratches. The matrix also pro- tects the fibers from environmental moisture and chemical corrosion or oxida- tion, which can lead to embrittle-ment and premature failure. In addition, the matrix provides many essential functions from an engineering standpoint: the matrix keeps the reinforcing fibers in the proper orientation and position so that they can carry the intended loads, distributes the loads more or less evenly among the fibers, provides resistance to crack propagation and damage, and provides all of me interlaminar shear strength of the composite. The matrix gen- erally determines the overall service temperature limitations of the composite and may also control its environmental resistance. In summary, the matrix:

distributes loads through the laminate

 

protects fibers from abrasion and impact

 

determines:

 

compressive strength

 

transverse mechanical properties

 

interlaminar shear

 

service operating temperature

 

selection of fabrication process and tool design

 

contributes to fracture toughness

 

With any fiber, the material used for the matrix must be chemically com- patible with me fibers and should have complementary mechanical properties. Al- so, for practical reasons, the matrix material should be reasonably easy to process.

The development of high strength and high thermal resistance is frequent- ly accompanied by complex cure procedures or britdeness in thermosets. Over- coming these obstacles has proven the key to developing viable composite ma- trices, with processing/fabrication constraints of fiber wet-out, prepreg shelf life, tack and drape, cure shrinkage, etc., adding to the complexity.

The organic matrices commonly used are broadly divided into the catego- ries of ther-moset and thermoplastic; organic matrices commonly used on air- frame structures are given below:

THERMOSET THERMOPLASTIC

 

Expoxy Polyethylene

 

Polyester Polystyrene

 

Phenolics Polypropylene

 

Bismaleimide (BMI) Polyemeretherketone (PEEK)

 

Polyimides Polyetherimide (PEI)

 

Polyethersulfone (PES)

 

Polyphenylene Sulfide

 

Polyamide-imide (PAI)

 

The relative advantages of thermosets and thermoplastics include: THERMOSET MATRICES THERMOPLASTIC MATRICES

(Characteristics)

 

Undergo chemical change when cured

 

Non-reacting, no cure required

 

Processing is irreversible

 

Post-formable, can be reprocessed

 

Low viscosity/high flow

 

High viscosity/low flow

 

Long (2 hours) cure

 

Short processing times possible

 

Tacky prepreg

 

Boardy prepreg

(Advantages)

 

Relatively low processing temperature

 

Superior toughness to thermosets

 

Good fiber wetting

 

Reusable scrap

 

Formable into complex shapes

 

Rejected parts reformable

 

Low viscosity

 

Rapid (low cost) processing

 

Infinite shelf life without refrigeration

 

High delamination resistance (Disadvantages)

 

Long processing time

 

Less chemical solvent resistance than thermosets

 

Restricted storage

 

Requires very high processing temperatures

 

Requires refrigeration

 

Outgassing contamination

 

Limited processing experience available

 

Less of a database compared to thermoset

 

Compared to thermoplastics, thermoset matrices offer lower melt viscosi- ties, lower processing temperatures and pressures, are more easily prepregged and are lower cost. On the other hand, thermoplastic matrices offer indefinite shelf life, faster processing cycles, simple fabrication, and generally do not re- quire controlled-environment storage or post curing.

Thermoset matrices

 

The most prominent matrices are epoxy, polyimides, polyester and phe- nolics.

Thermoset matrix systems have been dominating the composite industry because of their reactive nature. These matrices allow ready impregnation of fi- bers, their malleability permits manufacture of complex forms, and they provide a means of achieving high-strength, high-stiffness crosslinked networks in a cured part.

Epoxy

 

Epoxy systems are the major composite material for low-temperature ap- plications [usually under 200F (93 C)] and generally provide outstanding chemical resistance, superior adhesion to fibers, superior dimensional stability, good hot/wet performance, and high dielectric properties. Epoxy can be formu- lated to a wide range of viscosities for different fabrication processes and cure schedules. They are free from void-forming volatiles, have long shelf lives, pro- vide relatively low cure shrinkage, and are available in many thoroughly- characterized standard prepreg forms. They also have good chemical stability and flow properties, and exhibit excellent adherence and water resistance, low shrinkage during cure, freedom from gas formation, and stability under envi- ronmental extremes. In addition, on other very important advantage is the wealth of database information available.

The epoxy family is the most widely used matrix system in the advanced composites field. Because it is generally limited to service temperatures, this re- stricts use in many aerospace applications, where higher service temperatures are required.

The baseline system of epoxy used in the majority of applications in- cludes:

Superior mechanical properties

 

250F curing: 65F to 180F (-53 to 82 C) service temperatures

 

350F curing: - 65F to 250F (-53 to 121 C) short-term or 200F (93C) long-term service temperature

Epoxy matrices, the workhorse of the advanced composites industry today, are suitable for use with glass, carbon/graphite, aramid, boron, and other rein- forcements and hybrids. Yet greater demands can be met by conventional epox- ies are being made for today's parts, so a wide variety of epoxies are being devel- oped to handle the ever-increasing requirements for speed of fabrication, tough- ness, and higher service temperatures.

Unmodified epoxies are brittle. When subjected to impact from a flying stone, an occasional bump, or a dropped wrench, etc., they can be damaged in- ternally and suffer loss of laminate compressive strength. Epoxies have been

modified or improved to increase their damage resistance. The result is tough- ened epoxies.

Epoxies have a tendency to absorb moisture; this absorbed moisture can lead to decrease mechanical properties especially at elevated temperatures. The presence of water decrees the glass transition temperature of the epoxy matrix, hence the term wet Tg This effect must be considered in design.

The following environmental hazards have detrimental effects on epoxy matrices:

Moisture

 

Temperature

 

Ultraviolet light

 

Hydraulic fluid

 

Fuel

 

Cleaning agents

 

POLYIMIDES (HIGHER SERVICE TEMPERATURE MATRICES) Polyimides are thermo-oxidatively stable and retain a high degree of their me- chanical properties at temperatures far beyond the degradation temperature of many polymers, often above 600F (320C). Several types with superior ele- vated temperature resistance are listed below:

Bismaleimides: good to 450F (230C), relatively easy to process

 

Condensation types: good to 600F (320C), very difficult to process

 

Addition types: good to 500 600F (260 320C), improved pro- cessability compared to condensation types

(a) BMI (Bismaleimides), a special polyimide system, operates around a

 

350F to 450F (177 to 230C) upper limit. BMI offer good mechanical strength and stiffness, but are generally brittle and may have cure-shrinkage. Other BMIs have significant improvements in toughening which greatly enhances their useful- ness. When good hot/wet performance or thermal stability beyond epoxy limits is desired, BMI matrices may be the matrix of choice. BMI characteristics are summarized below:

BMIs provide increased thermal stability compared to epoxies, with comparable processability

The major problem with BMIs has been increased brittleness over epox- ies with reduced damage resistance and toughness

BMI systems with improved toughness are available at the sacrifice mechanical properties

(b) PMR-15 (Polymerization of Monomeric Reactants) is a thermoset addi- tion polyimide which offers higher continuous service temperatures. Originally developed by NASA. Thermo-oxidative stability, relatively low cost, and availa- bility in a variety of forms make PMR-15 one of the candidates for airframe in- dustrial applications where performance from 500 to 600F (260 320C) is the key material selection criterion. PMR-15 processing is complicated, requir- ing application of near 600 psi (4.1 Mpa). A heated tool is often necessary to achieve faster heatup rates than are possible with conventional tooling in an au- toclave. The room temperature properties of PMR-15 are similar to those of

350F (177C) epoxy, but, unlike epoxy, properties do not decrease significant- ly until temperatures over 500F (260C) are reached, even after exposure to moisture. NASA has developed LARC-160, a "PMR" system which provides a significant improvement in processability over the PMR-15 matrix, with only a small loss in elevated temperature properties. However, both NASA's PMR-15 and LARC-160 matrices are still in progress under their continuing develop- ment programs.

(c) Polyester

 

Polyesters matrices can be cured at room temperature and atmospheric pressure, or at a temperature up to 350F (177C) and under higher pressure. These matrices offer a balance of low cost and ease of handling, along with good mechanical and electrical properties, good chemical resistance properties (especially to acids), and dimensional stability. Polyester combined with fiber- glass fibers becomes a very good radar-transparent structural material and po- lyester is also a relatively inexpensive matrix that offers a compromise between strength and impact resistance for use in aircraft radomes. Low mold-pressure requirements helped promote the manufacture of large polyester composite struc- tures, and this was further helped along by their relatively quick cure characte- ristics.

Vinyl esters are a subfamily of polyesters, derived from epoxy-matrix backbones, which provide higher tensile elongation, toughness, heat resistance, and chemical resistance than conventional unsaturated polyesters.

(d) Phenolics

 

Phenolics are the oldest of the thermoset matrices, and have excellent in- sulating properties, resistance to moisture, and good electrical properties (ex- cept arc resistance). The chemical resistance of phenolics is good, except to strong acids and alkalis. Phenolics are available as compression-molding com- pounds, and injection-molding compounds. This material is very useful in mili- tary and high-performance aerospace applications where radiation-hardness, di- mensional stability at high loads and temperatures, and ability to ablate may be critical to component survival.

 

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