Название: Aircraft engineering (Морозова М. А.)

Жанр: Авиационные технологии и управление

Просмотров: 1511


Ceramic matrix composites

 

Ceramics in general are characterized by high melting points, high com- pressive strength, good strength retention at high temperatures, and excellent re- sistance to oxidation. As the aerospace industry seeks to build stronger, lighter weight, and more fuel-efficient aircraft, designers are turning to a whole host of new materials to fulfill these requirements. Ceramics have been used for the braking systems of commercial and military aircraft, including the F-16, Space Shuttle, B747-400, A320, etc. Some companies have produced preprototype air- craft engine parts, missile guidance fins, and prototype hypersonic fuselage skins made of ceramic composites. Ceramic composites also offer excellent oxidation resistance. In addition, they are made from preforms to net or near-net shape, thus requiring little post-machining. Before ceramic composites can be success- fully produced for applications, aerospace engineers must learn how to design with these materials. Ceramics cannot be joined with conventional fasteners, or machined as easily as metals. Therefore, engineers have turned to large furnaces

in which one-piece shapes can be made to near-net or net shape, eliminating the need for joining and the need for much post-machining. For example, a high tem- perature integral component of a Hermes leading section was produced from C/SiC, and an engine rotor was produced by briading SiC fibers into 3-D pre- forms for subsequent infiltration of the SiC-matrix by chemical vapor deposi- tion. The braided fiber/ceramic radome used on Patriot missiles is also manu- factured in one step.

The challenge here will be developing furnaces big enough to handle large parts and improving processes to create the optimum ceramic composites. On a per pound basis, it will be expensive, but the payoff will be better on a systems basis. If ceramic parts that operate at high temperatures are lighter weight and are more efficient than other counterparts, then the production costs will be paid back in the long run.

The dimensional stability shown by ceramics at high temperatures may prove beneficial for aerospace applications. Their dimensional stabiltiy is ever better than that of metal metrix composites, and in the form of reinforced glass ceram- ics they are very tough. They can be manufactured much like graphite or car- bon/epoxy composites. Dimensional stability makes glass ceramic composites particularly applicable for structures used in spacebased optical systems, where accurate operation depends on virtually no change in dimensions under varying temperatures. Another use is in low observables because ceramic composites have lower radar detectability than other composites.

There are a number of manufacturing processes for producing ceramic composites, including:

• chemical vapor deposition

 

• viscous glass consolidation

 

• polymer conversion

 

• powder and hot press techniques

 

• and gas-liquid metal reaction.

 

The weaknesses of ceramics include relatively low tensile strength, poor impact resistance, and poor thermal-shock resistance. The addition of a high- strength fiber to a relatively weak ceramic does not always result in composite

with a tensile strength greater than that of the ceramic alone. That is, at stress le- vels sufficient to rupture the ceramic, the elongation of the matrix is insufficient to transfer a significant amount of the load to the reinforcement, and the com- posite will fail unless the volume percentage of the fiber is extraordinarily high.

Therefore ceramic matrices are usually chosen for their ability to be processed without cracking. This requires a coefficient of thermal expansion that is close to that of the reinforcement. Approach to improving ceramics is to toughen the ceramic using whiskers or chopped fibers to reinforce the matrix. The aim is to retain the thermal strength, hardness and wear resistance which makes ceramics so desirable, while greatly increasing toughness, making failure more predictable and improving producibility.

A number of important technological barriers need to be overcome before advanced ceramics can achieve their full potential. Matrix brittleness, shrinkage associated with sintering, reactivity, and the generation of internal stresses due to thermal mismatch have proven to be only a few of the problems associated with the development of ceramic matrix composites. Difficulties also lie in the areas of production costs, reliability in service, and reproducibility in manufacture. Over all, overcoming brittleness has been the major stumbling block in the devel- opment of new ceramic products and applications. With the demand for high- temperature performance products, the future of advanced ceramics is growing and solutions to these challanges are being pursued.

 

REINFORCEMENTS (FIBERS)

 

While composite materials owe their unique balance of properties to the combination of matrix and reinforcement, it is the reinforcement system that is primarily responsible for such structural properties as strength and stiffness. The reinforcement dominates the field in terms of volume, properties, and design versatility. Almost all fibers in use in airframe structures today are solid and have a circular cross section or nearly so. Generally, the smaller the diameter the greater is the strength of the fiber. Other potential shapes such as polygonal, hexagonal, rectangular, irregular, and unusual shapes, are under development with possibilities for improved fiber strengths. Hollow fibers have been devel-

oped, are commercially available, and show promise for improved mechanical properties of composites, especially in compressive strength.

Composite reinforcement fibers are more expensive than current aluminum materials and represent a high percentage of the recurring cost in composite com- ponents. The following list gives a fiber cost comparison:

FIBER $/lb. Aluminum (for comparison)          1 — 5

Fiberglass         3 — 5

 

Kevlar-49®     10 — 20

 

Quartz  120

 

Because fiber materials have borrowed some of the terminology of the textile industry, the following terms are defined:

• Filament – the basic structural fibrous element. It is continuous, or at least very long campared to its average diameter, which is usually 5 – 10 micro-

ns.

 

• Yarn – A small, continuous bundle of filaments, generally fewer than

 

10,000.

 

The filaments are lightly stranded together so they can be handled as a single unit and may be twisted to enhance bundle integrity.

• Tow – A large bundle of continuous filaments, not twisted. The number of filaments in a bundle is usually 3000, 6000, or 12000 (3K, 6K or 12K tow).

12K tow is the cheapest, 3K tow is the most expensive (see list above). The smaller tow sizes are normally used in weaving, winding, and braiding applica- tions while large tow sizes are used in unidirectional tapes. Very thin tapes are also made from low filament-count tow for satellite applications.

• Fabric – Fabric is a planar textile structure produced by interlacing yarns, fibers, or filaments.

The function of the reinforcement (continuous fibers) includes:

 

• principal load-carrying member of the composite

 

• responsible for tensile, compressive, flexural strength and stiffness of the composite

• determines electrical properties

• thermal coefficient of expansion Reiforcement fibers are generally one of three types:

• Organic fibers — Organic fibers, like organic matrices, offer high strength and light weight, e.g., glass, aramid, PE, graphite carbon, etc.

• Ceramic fibers — Ceramic fibers can withstand high temperatures and insulate against heat, e.g., quartz, silicon carbide, alumina, etc.

• Metallic fibers — Metallic fibers permit composite to conduct or dissi- pate heat and electricity

Discontinuous reinforcement fiber materials provide:

 

• Cost effective production methods

 

• High dimensional accuracy

 

• Rapid cycle times

 

• High tooling cost

 

• Lower properties than continuous fibers

 

• Possibility for complex shapes

 

• Faster manufacturing methods

 

FIBERGLASS

 

The most widely used fiber is unquestionably fiberglass, which has gained acceptance because of its low cost, light weight, high strength, and non-metallic characteristics. Fiberglass composites have been widely used for aircraft parts that do not have to carry heavy loads or operate under great stress. They are used principally for fuselage interior parts such as window surrounds and sto- rage compartments, as well as for wing fairings and wing fixed trailing edge panels. Fiberglass is extensively used in primary structures of sport and utility aircraft as well as helicopter rotor blades.

The two most common grades of fiberglass are «E» (for electrical board) and «S» (high strength for structural use). E-glass provides a high strength-to- weight ratio, good fatigue resistance, outstanding dielectric properties, retention of 50\% tensile strength to 600°F (320°C), and excellent chemical, corrosion, and environmental resistance. While E-glass has proved highly successful in aircraft secondary structures, some applications require higher properties. To fill these demands, S-glass was developed, which offers up to 25\% higher compres-

sive strength, 40\% higher tensile strength, 20\% higher modulus, and 4\% lower density. This glass also has higher resistance to strong acids than E-glass, and more costly.

The use of other glass types such as A-glass, C-glass and even D-glass has been limited because they are lower strength and not suitable for structural pur- poses.

Designing with fiberglass is much simpler than designing with some other composite systems because of the large volume of empirical data collected over the years and the availability of standard systems with well-documented proper- ties from many manufacturers. Hollow glass fibers used in certain applications have demonstrated improved structural efficiency where stiffness and compres- sive strength are the governing criteria. The transverse compressive strength of a hollow fiber is lower than that of a solid fiber. Hollow fibers are quite difficult to handle as they break easily and may absorb moisture.

KEVLAR®      (Aramid fiber-Dupont product)

 

Kevlar fiber has  been used  for  structural applications since the early

 

1970s. Combining extremely high toughness and energy-absorbing capacity (very good projectile and ballistic protection characteristics has led to use in bullet- proof vests), tensile strength, and stiffness with low density (the lowest in re- cently developed advanced composite materials), Kevlar fiber offers very high specific tensile properties. Low compressive strength is one of the weaknesses of Kevlar. But where the highest compressive strength is needed, hybrids of Kev- lar and carbon fibers are generally used.

Kevlar have very high specific tensile strength. This provides the basis for the claim that Kevlar, on a pound-for-pound basis, is five times as strong as steel illustrates tensile stress/strain curves for tensile loading. Like most other com- posite materials, Kevlar has a classically brittle response with a tensile strength a little greater than 200 ksi (1.38 Gpa) and tensile modulus of 11 Msi (76 Gpa) for a typical unidirectional composite. When Kevlar is under compression, the behavior is quite different from the tensile response. At a compressive load about 20\% of the ultimate tensile load, a deviation from linearity occurs. This is an inherent characteristic of the Kevlar 49 fiber representing an internal buck-

ling of the filament. This unusual characteristic of Kevlar 49 fiber has made fail-safe designs possible because fiber continuity is not lost in a compressive or tensile failure. It has also limited the use of the fiber on major structural appli- cations.

When tensile and compressive loadings are combined in flexural bending, instead of the brittle failure encountered with glass and carbon fibers, the bending failure of Kevlar 49 is similar to what is observed with metals. This helps to ex- plain the outstanding toughness and impact resistance of composites reinforced with Kevlar.Another area in which Kevlar excells is vibration damping the de- cay of free vibrations for various materials, Kevlar is less prone to flutter and sonic fatigue problems than most other materials, Kevlar fibers also offer good fatigue, and chemical resistance, and retain their excellent tensile properties to relatively high temperatures. Because of their high specific properties and fewer handling problems, these fibers have replaced glass fibers in many applications. However, relatively low compressive strength has kept them out of many aircraft primary structures. New methods for machining Kevlar are also needed because the fibers are too tough to cut with conventional tools.