Composites materials for aerospace applications

The range of materials can be classified into the categories: Metals, Polymers, Ceramics and inorganic glasses and composites. Metals lose their strength at elevated temperatures. High-Polymeric materials in general can withstand still lower temperatures. Ceramics outstrip metals and polymers in their favourable melting points, ability to withstand high temperatures, strength and thermal expansion properties, but due to their brittleness they are often unsatisfactory as structural materials. 

This lead to the exploration of composites. One may define a composite as material as a materials system which consists of a mixture or combination of two or more micro constituents mutually insoluble and differing in form and/or material composition. Examples of composites are steel reinforced concrete (metals + ceramics), vinyl-coated steel (metals + polymers), fibre reinforced plastics (ceramics + polymers).

Emergence of strong and stiff reinforcements like carbon fibre along with advances in polymer research to produce high performance resins as matrix materials have helped meet the challenges posed by the complex designs of modern aircraft. The large scale use of advanced composites in current programmes of development of military fighter aircraft, small and big civil transport aircraft, helicopters, satellites, launch vehicles and missiles all around the world is perhaps the most glowing example of the utilization of potential of such composite materials.

2.  The aerospace structures and features

Important requirements of an aerospace structure and their effect on the design of the structure are presented in table 1.

Table 1.  Features of aircraft structure.

Requirement	Applicability	Effect
·    Light-weight	All Aerospace Programmes	§ Semi-monocoque construction * Thin-walled-box or stiffened structures § Use of low density materials: * Wood  * Al-alloys  * Composites § High strength/weight, High stiffness/weight
·    High reliability	All space programmes	§ Strict quality control § Extensive testing for reliable data § Certification: Proof of design
·    Passenger safety	Passenger vehicles	§ Use of fire retardant materials § Extensive testing: Crashworthiness
·    Durability-Fatigue and corrosion     Degradation: Vacuum Radiation Thermal	Aircraft Spacecraft	§ Extensive fatigue analysis/testing * Al-alloys do not have a fatigue limit § Corrosion prevention schemes § Issues of damage and safe-life, life extension § Extensive testing for required environment § Thin materials with high integrity
·    Aerodynamic performance	Aircraft Reusable spacecraft	§ Highly complex loading § Thin flexible wings and control surfaces * Deformed shape-Aero elasticity  * Dynamics § Complex contoured shapes * Manufacturability: N/C Machining; Moulding
·    Multi-role or functionality	All Aerospace programmes	§ Efficient design § Use: composites with functional properties
·    Fly-by-wire	Aircrafts, mostly for fighters but also some in passenger a/c	§ Structure-control interactions * Aero-servo-elasticity § Extensive use of computers and electronics * EMI shielding
·    Stealth	Specific military aerospace applications	§ Specific surface and shape of aircraft * Stealth coatings
·    All-Weather operation	Aircraft	§ Lightning protection, erosion resistance

Further, the structure has to meet the requirements of fuel sealing and provide access for easy maintenance of equipments. Passenger carriage requires safety standards to be followed and these put special demands of fire-retardance and crash-worthiness on the materials and design used. For spacecraft the space environment–vacuum, radiation and thermal cycling-has to be considered and specially developed materials are required for durability.

Two key developments in scientific-technological world have had a tremendous influence on the generation and satisfaction of the demands raised by the aerospace community: one, the advances in the computational power and the other, composites technology using fibre reinforced polymeric materials.

3.  Use of composites in aerospace structure

It is to be realized that in order to meet the demands in table 1, it is necessary to have materials with a peculiar property-set. The use of composites has been motivated largely by such considerations.

The composites offer several of these features as given below:

§  Light-weight due to high specific strength and stiffness

§  Fatigue-resistance and corrosion resistance

§  Capability of high degree of optimization: tailoring the directional strength and stiffness

§  Capability to mould large complex shapes in small cycle time reducing part count and assembly times: Good for thin-walled or generously curved construction

§  Capability to maintain dimensional and alignment stability in space environment

§  Possibility of low dielectric loss in radar transparency

§  Possibility of achieving low radar cross-section

These composites also have some inherent weaknesses:

§  Laminated structure with weak interfaces: poor resistance to out-of-plane tensile loads

§  Susceptibility to impact-damage and strong possibility of internal damage going unnoticed

§  Moisture absorption and consequent degradation of high temperature performance

§  Multiplicity of possible manufacturing defects and variability in material properties

Even after accepting these weaknesses, the projected benefits are significant and almost all aerospace programmes use significant amount of composites as highlighted in the figure below.

All this is, of course, not without its share of hassles. Challenges of using composites on such a large scale are many. The composites are not only new but also non-conventional: they are anisotropic, inhomogeneous, have different fabrication and working methods and also different controls for quality assurance. They have a complex material behaviour under load requiring new and complicated analysis tools. Moreover, the behaviour is not always predictable by analysis and this makes reliance on several expensive and time consuming tests unavoidable.

The routes to meet these challenges have evolved around use of the advances in computer technology and analysis methods to implement schemes based on computer aided design, computer aided engineering, finite element methods of analysis and building computer interfaces amongst all aspects of development, namely, design, analysis and manufacturing.  These should provide fast transfer of information including graphics and accurate analysis methods for a reasonable prediction of complex behavioural patterns of composites.  It is only by harnessing the vast computational power for various purposes that the aircraft structural design of today can meet the challenges posed by the required performance.

4.  Materials for aerospace composites:

The materials systems which have been considered useful in aerospace sector are based on reinforcing fibres and matrix resins given in table 2 and 3, respectively.  Most aerospace composites use prepregs as raw materials with autoclave moulding as a popular fabrication process.  Filament winding is popular with shell like components such as rocket motor casings for launch vehicles and missiles.  Oven curing or room temperature curing is used mostly with glass fibre composites used in low speed small aircraft.  It is common to use composite tooling where production rates are small or moderate; however, where large number of components are required, metallic conventional tooling is preferred.  Resin injection moulding also finds use in special components such as radomes.  Some of the popular systems are given in table 4 along with the types of components where they are used in a typical high-performance aircraft.

Table 2.  Reinforcing fibres commonly use in aerospace applications.

Fibre	Density (g/cc)	Modulus (GPa)	Strength (GPa)	Application areas
Glass     E-glass     S-glass	2.55 2.47	65-75 85-95	2.2-2.6 4.4-4.8	Small passenger a/c parts, air-craft interiors, secondary parts; Radomes; rocket motor casings Highly loaded parts in small passenger a/c
Aramid     Low modulus     Intermediate     modulus     High modulus	1.44 1.44 1.48	80-85 120-128 160-170	2.7-2.8 2.7-2.8 2.3-2.4	Fairings; non-load bearing parts Radomes, some structural parts; rocket motor casings Highly loaded parts
Carbon     Standard modulus     (high strength)     Intermediate     modulus     High modulus     Ultra-high strength	1.77-1.80 1.77-1.81 1.77-1.80 1.80-1.82	220-240 270-300 390-450 290-310	3.0-3.5 5.4-5.7 2.8-3.0 4.0-4.5 7.0-7.5	Widely used for almost all types of parts in a/c, satellites, antenna dishes, missiles, etc. Primary structural parts in high performance fighters Space structures, control surfaces in a/c Primary structural parts in high performance fighters, spacecraft

Table 3.  Polymeric matrices commonly used in aerospace sector.

Thermosets				Thermoplastics
Forms cross-linked networks in polymerization curing by heating				No chemical change
Epoxies	Phenolics	Polyester	Polyimides	PPS, PEEK
§ Most popular § 80% of total composite usage § Moderately high temp. § Comparatively expensive	§ Cheaper § Lower viscosity § Easy to use § High temp usage § Difficult to get good quality composites	§ Cheap § Easy to use § Popular for general applications at room temp	§ High temp application 3000C § Difficult to process § Brittle	§ Good damage tolerance § Difficult to process as high temp 300-4000 C is required
§ Low shrinkage (2-3%) § No release of volatile during curing	§ More shrinkage § Release of volatile during curing	§ High shrinkage (7-8%)
§ Can be polymerized in several ways giving varieties of structures, morphology and wide range of properties	§ Inherent stability for thermal oxidation § Good fire and flame retardance § Brittle than epoxies	§ Good chemical resistance § Wide range of properties but lower than epoxies § Brittle § Low Tg
§ Good storage stability to make prepregs	§ Less storage stability-difficult to prepreg	§ Difficult to prepreg		§ Infinite storage life.  But difficult to prepreg
§ Absolute moisture (5-6%) causing swelling and degradation of high temp properties § Also ultra violet degradation in long term	§ Absorbs moisture but no significant effect of moisture in working service range	§ Less sensitive to moisture than epoxies		§ No moisture absorption
§ Density (g/cm3) 1.1-1.4	§ Density (g/cm3) 1.22-1.4	§ Density (g/cm3) 1.1-1.4		§ Density (g/cm3) 1.3-1.4
§ Tensile modulus 2.7-5.5 GPa	§ Tensile modulus 2.7-4.1 GPa	§ Tensile modulus 1.3-4.1 GPa		§ Tensile modulus 3.5-4.4 GPa
§ Tensile strength 40-85 MPa	§ Tensile strength 35-60 MPa	§ Tensile strength 40-85 MPa		§ Tensile strength 100 MPa