COMPOSITES

AIRCRAFT MATERIALS

INTRODUCTIONINTRODUCTION

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What is Composite:What is Composite:

The term composite is used to describe two or more materials that are combined to form a much stronger structure than either material by itself.

The most simple composite is composed of the elements: a matrix (fabric) which serves as a bonding substance (adhesives or resins), and a reinforcing material. Prior to combination, the matrix is generally in liquid form and the reinforcing material is a solid. When the substances are combined and cured, the part is stronger than fabric is by itself, and stronger than the resin is by itself.

Modern composites are advanced to the point that they are strong enough to be used in primary airframe components like rudders and floor beams. In some cases the whole airframe is designed of advanced composite materials. Composites are used because overall properties of the composites are superior to those of the individual components. While the structural value of a bundle of fibres is low, the strength of individual fibres can be harnessed if they are embedded in a matrix that acts as an adhesive, binding the fibres together and lending solidity to the material.

The rigid fibres impart structural strength to the composite, while the matrix protects the fibres from environmental stress and physical damage and imparts thermal stability to them. The fibre-matrix combination also reduces the potential for a complete fracture; if one fibre fails the crack may not extend to other fibres, whereas a crack that starts in a monolithic (or single) material generally continues to propagate until that material fails.

Most conventional composites resemble plywood in that they are built in thin layers, each of which is reinforced by long fibres laid down in a single direction. Such materials exhibit enhanced strength only along the direction of the fibres. To produce composites that are strong in all directions, the fibres are woven into a three-dimensional structure in which they lie along three mutually perpendicular axes.

. Composites are of greatest use in the aerospace industry, however, where their stiffness, lightness, and heat resistance make them the materials of choice in reinforcing the engine cowls, wings, doors, and flaps of aircraft. Composite materials are also used in rackets and other sports equipment, in cutting tools, and in certain parts of automotive engines.

TYPES OF COMPOSITESTYPES OF COMPOSITES

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Polymer-matrix composites Polymer-matrix composites

(PMCs) are the lightest of the three types of composite materials. Recent applications of PMCs in aircraft propulsion systems, such as General Electric`s F-404 engine, have resulted in substantial reductions in both engine weight and manufacturing costs. Unfortunately, the low thermal-oxidation stability of PMCs severely limits the extent of their application. Commercially available state-of-the-art high-temperature PMCs, such as graphite fiber/PMR-15 and graphite fiber/PMR-11-55, are capable of withstanding thousands of hours of use at temperatures between 290 and 345°C).

 

Metallic-matrix compositesMetallic-matrix compositesSeveral major problems limit the

development of inter-metallic-matrix composites (IMCs), including chemical incompatibility and CTE mismatch between potential reinforcing fibers and matrix materials, poor low-temperature ductility, and marginal high-temperature oxidation resistance of intermetallic materials. Composite fabrication and joining processes that do not result in excessive fiber/matrix reaction or matrix contamination is an additional need.

Ceramic-matrix composites. Ceramic-matrix composites.

Ceramic matrix composites (CMCs) combine reinforcing ceramic phases with a ceramic matrix to create materials with new and superior properties. In ceramic matrix composites, the primary goal of the ceramic reinforcement is to provide toughness to an otherwise brittle ceramic matrix. Fillers can also be added to the ceramic matrix during processing to enhance characteristics such as electrical conductivity, thermal conductivity, thermal expansion, and hardness.

The desirable characteristics of CMCs include high-temperature stability, high thermal shock resistance, high hardness, high corrosion resistance, light weight, nonmagnetic and nonconductive properties, and versatility in providing unique engineering solutions. The combination of these characteristics makes ceramic matrix composites attractive alternatives to traditional processing industrial materials such as high alloy steels and refractory metals.

PROPERTIESPROPERTIES(Advantages & Disadvantages)(Advantages & Disadvantages)

  

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AdvantagesAdvantages

High strength to weight ratio (low density high tensile strength)

High tensile strength at elevated temperatures

High toughnessLight Weight Chemical Resistance/ Corrosion resistanceColour Translucency Design flexibility Manufacturing economy

ExpenseApplicationProcessabilityReduction of parts and fastenersFire resistant

DisadvantagesDisadvantages

General expensiveNot easy to repair

MATERIALSMATERIALS

REINFORCING MATERIALSREINFORCING MATERIALSGeneral:When combined with a matrix, the reinforcing

material (fibres) are what give the major strength to the composite components. There are several types of reinforcing fibres; the most commonly used are outlined as follows:

Fibreglass – Fibreglass is made from small strands of molten silica glass and then spun together and woven into cloth. There are many different weaves of fiberglass available, depending on the application. The wide range of application of the material and its low cost make it one of the most popular used. Fibreglass weighs more and has strength than most other fibre material.

Aramid – Aramid fibres are general characterized by its yellow colour, light weight and its excellent tensile strength. Aramid is a registered tradename of the Du Pont Company and is an ideal material for aircraft parts that are subject to high stress and vibration (e.g. rotor blades). It is also used in bullet-proof-vests. Damage to Aramid Structural components will, in general, be repaired with fiberglass.

Graphite – Black graphite/carbon fibre is very strong and stiff and is used for its rigid, strong properties. This material is used to manufacture primary structural components like ribs and floor beams. Graphite is stronger in compressive strength than Kevlar. however it is more brittle than Kevlar. It has the problem of being corrosive when bonded to aluminium.

MATRIX MATERIALSMATRIX MATERIALS

General:The matrix is the bonding material the

completely surrounds the fibre to give strength and transfer the stress to the fibre. The newer matrix materials have good stress-distribution, heat resistant, chemical resistant and durability properties. Most of these newer matrix materials are epoxy resins.

Resin matrix are two-part systems consisting of a resin and a hardener or catalyst, which acts as curing agent.

Resins are a type of plastic and are broken down into two categories:ThermoplasticThermoset

Thermoplastic – Thermoplastic resins use heat to form the part into a specified shape, and this shape in not permanent. That means, if we add heat again it will flow again to another shape. So Thermoplastics can only be used in areas where the temperature do not exceed 750°F.

Thermosets – Thermoset use heat to form and set the shape permanently. The plastic, once formed, cannot be reformed even if it is heated. Most composite structural components are made from thermoset resins.

Epoxy Resins – Epoxy resins are one type of thermosetting plastic resin. They have good adhesion, strength and resistance to moisture and chemical properties. They are used to bond non-porous and dissimilar materials, like metal to composite components.

Prepeg – Prepeg is the abbreviation of pre-impregnated fabrics, and they are fabrics that have the resin already impregnated into them.

USESUSES

CHOOSING MATERIALS CHOOSING MATERIALS

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For Matrix:For Matrix:For the matrix, many modern composites

use thermosetting or thermosoftening plastics (also called resins). (The use of plastics in the matrix explains the name 'reinforced plastics' commonly given to composites). The plastics are polymers that hold the reinforcement together and help to determine the physical properties of the end product.

Thermosetting plastics are liquid when prepared but harden and become rigid (ie, they cure) when they are heated. The setting process is irreversible, so that these materials do not become soft under high temperatures. These plastics also resist wear and attack by chemicals making them very durable, even when exposed to extreme environments.

Thermosoftening plastics, as the name implies, are hard at low temperatures but soften when they are heated. Although they are less commonly used than thermosetting plastics they do have some advantages, such as greater fracture toughness, long shelf life of the raw material, capacity for recycling and a cleaner, safer workplace because organic solvents are not needed for the hardening process.

Ceramics, carbon and metals are used as the matrix for some highly specialised purposes. For example, ceramics are used when the material is going to be exposed to high temperatures (eg, heat exchangers) and carbon is used for products that are exposed to friction and wear (eg, bearings and gears).

For Reinforcement:For Reinforcement:

Although glass fibres are by far the most common reinforcement, many advanced composites now use fine fibres of pure carbon. Carbon fibres are much stronger than glass fibres, but are also more expensive to produce. Carbon fibre composites are light as well as strong. They are used in aircraft structures and in sporting goods (such as golf clubs), and increasingly are used instead of metals to repair or replace damaged bones. Even stronger (and more costly) than carbon fibres are threads of boron.

Polymers are not only used for the matrix, they also make a good reinforcement material in composites. For example, Kevlar is a polymer fibre that is immensely strong and adds toughness to a composite. It is used as the reinforcement in composite products that require lightweight and reliable construction (eg, structural body parts of an aircraft). Composite materials were not the original use for Kevlar – it was developed to replace steel in radial tyres and is now used in bulletproof vests and helmets.

WORKING WITH COMPOSITE WORKING WITH COMPOSITE MATERIALSMATERIALS

SafetySafety

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General:

When working with composite materials, correct safety precautions must be observed. Pay attention to the material safety data sheets (MSDS). On the MSDS you will find the following information:◦Health precautions◦Flammability of the material◦Ventilation requirements◦Information for health professionals in case of an

accident.

Safety PrecautionsSafety PrecautionsSome of the materials are very dangerous

and can cause allergic reactions, so take care if you are working with these materials and observe the safety precautions: Do not let any of the materials come into

contact with your skin or with your clothesDo not inhale vapors Do not wash your skin with powerful solventsDo not eat, drink or smoke in work areasDo not machine materials without wearing

protective clothing and a dust mask.

DAMAGE TO COMPOSITE DAMAGE TO COMPOSITE STRUCTURESTRUCTURE

General:

When damage is discovered on a composite structural part, and before any further repair work is performed, a complete investigation of the affected area is to be carried out.

The investigation of damage is done using the related chapters and pages of the SRM (Structural Repair Manual) in order to determine whether the damage is repairable or not, and if so, the applicable type of repair.

Damage DetectionDamage may be discovered during scheduled

inspections required by the maintenance program, or in unscheduled inspections when the part has been subjected to accidental damage.

Damage EvaluationA complete inspection of the damaged area or

component will give the required information concerning the extent and type of damage. Depending on the type, extent and importance of the affected zone, the damage acceptance level may be determined.  

Acceptance Level

In order to define the applicable repair type and its associated limits (time and size), it is necessary to initially determine whether the damage is:AllowableRepairable orNot – repairable

General: For composites surfaces (as per the SRM),

damage is divided into two main categories:Skin not-perforated damageSkin perforated damage.

DAMAGE CLASSIFICATIONDAMAGE CLASSIFICATION  

Skin not-perforated damage includes:

AbrasionScratchesGougesNicksDebondingDelaminationDents

Skin perforated damage includes:

◦Lightning strikes◦Holes ◦Impact by foreign object, requiring investigation

for delamination moisture contamination.

Abrasion

Abrasion is damage to a surface caused by scuffing, rubbing or scrapping of the component. Fibres are not damaged and mechanical performance is not affected. Abrasion damage is repaired by restoration of the surface protection, in order to avoid any fluid ingress.

Corrosion

Galvanic corrosion may occur when an aluminium alloy part is in direct contact with a carbon fibre surface in the presence of a corrosive environment. In this case it is the aluminium ally part (e.g. fitting, lightning strike protection straps) which corrodes and which needs replacing or repaired if possible.

Erosion

Erosion could affect all the leading edge surfaces, especially when the initial surface protection system has been damaged. Erosion, when undetected or unrepaired, may generate composite deterioration. The component may be completely perforated and fluid ingress likely to occur. Restore the protection of the area and install additional protection if necessary.

Scratches / Gouges

A scratch is the result of contact with a sharp object and only surface fibres are affected. While a gouge is wider and deeper than a scratch, several plies are affected, but the edges of a gouge are generally smooth. For scratches in general, only surface restoration is necessary to prevent any fluid ingress. Gouges affect structural strength and have to repaired by removing the damaged plies and performing a hand lay-up.

Water Absorption

Any detected moisture has to be removed to avoid further damage.

During any repair procedure, ensure that repair parts are completely dry, in order to avoid any material delamination during heat application. Water ingress in sandwich structures is due to porosity of the skin. It reduces performance and increases the weight of the affected structure. Water absorption is a phenomenon of resin properties. The absorption stops once the resin is saturated.

Chemical Degradation

Chemical degradation principally affects the resin and is generally due to accidental contact with aggressive chemical liquids or products. In case of chemical degradation detection, the whole contaminated area must be repaired.

Dent/ Depression

A depression or a dent is a deformity in the thickness of an area. It may be caused by impact. This type of defect requires further NDT investigation to detect delamination or debonding. On sandwich structure, the honeycomb is generally damaged and requires a repair.

Lightning Strike Damage

Carbon fibre is a conductive material while glass or Aramid fibres are non-conductive materials. The effect of a lightning strike will not be the same for non-conductive materials. (glass, Aramid) a large part of the component, if not completely protected, may be blown out because both skins are affected and the core generally vaporized due to the extreme heat.

Damage on carbon fibre structures will be less significant (sports, small holes, or charring).

Allowable damage

For each of the defined zones, a graphic is to be used to determine allowable damage limits, recommended repair types and repair associated limitations. Damage type and dimensions, as well as initial thickness, have to be know in order to select and work with these graphics.

Visual inspection is the principal method for damage detection. Delamination or debonding can be used by impact, abnormal loading or an undetected manufacturing defect.

NOTE: SUCH DAMAGE IS NOT ALWAYS VISIBLE ON THE SURFACE. THE COMPRESSION STRENGTH OF THE COMPONENT IS AFFECTED AND WATER OR FLUID INGRESS IS VERY LIKELY TO OCCUR.

Extent of Damage

Close visual and non-destructive testing methods such as tap-testing, ultrasonic and X-Rays are used to determine the amount of damage.

For delaminated/debonded area of determination, a minimum inspection area is defined. In case of indication, the inspection area must be extended until the limits of the affected zone are located.

Surfaces Zones

As damage is not of the same significance in each area of the component, each composite surface of the aircraft is divided into zones of different structural importance.

REPAIR TYPES

General:Recommended repairs can be of three

types:

◦Temporary repairsPermanent cosmetic repairsPermanent structural repairs

PREPARATION BEFORE REPAIR

Before any repair action can be performed, it is necessary to ensure that the surface of the repair area has been correctly prepared. This will ensure the maximum bonding strength and durability.

 

INSPECTION OF COMPOSITE MATERIALS

Tap TestingVisual or Optical InspectionHolographyAcoustic EmissionUltrasonicRadiography