Download Composite Material and its uses and more Lecture notes Kinematics in PDF only on Docsity!
COMPOSITE MATERIALS
• Technology and Classification of Composite Materials
• Metal Matrix Composites
• Ceramic Matrix Composites
• Polymer Matrix Composites
• Guide to Processing Composite Materials
Composite Material Defined
A materials system composed of two or more physically distinct phases whose combination produces aggregate properties that are different from those of its constituents
• Examples:
• Cemented carbides (WC with Co binder)
• Plastic molding compounds containing fillers
• Rubber mixed with carbon black
• Wood (a natural composite as distinguished from a synthesized
composite)
Why Composites are Important?
- Composites can be very strong and stiff, yet very light in weight, so ratios of strength-to-weight and stiffness-to-weight are several times greater than steel or aluminum
- Fatigue properties are generally better than for common engineering metals
- Toughness is often greater too
- Composites can be designed that do not corrode like steel
- Possible to achieve combinations of properties not attainable with metals, ceramics, or polymers alone.
Disadvantages and Limitations of Composite
Materials
• Properties of many important composites are anisotropic - the
properties differ depending on the direction in which they are measured – this may be an advantage or a disadvantage
• Many of the polymer-based composites are subject to attack by
chemicals or solvents, just as the polymers themselves are susceptible to attack
• Composite materials are generally expensive
• Manufacturing methods for shaping composite materials are
often slow and costly.
One Possible Classification of Composite
Materials
1. Traditional composites – composite materials that occur in
nature or have been produced by civilizations for many years
- Examples: wood, concrete, asphalt
2. Synthetic composites - modern material systems normally
associated with the manufacturing industries, in which the
components are first produced separately and then
• When a load is applied, the matrix shares the load with the
secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent The Reinforcing Phase (Secondary Phase)
• Function is to reinforce the primary phase
• Imbedded phase is most commonly one of the following
shapes:
Fibers
Particles
Flakes
In addition, the secondary phase can take the form of an
infiltrated phase in a skeletal or porous matrix Example: a powder metallurgy part infiltrated with polymer
Secondary Phase..
Fibers
Filaments of reinforcing material, usually circular in cross-section
• Diameters range from less than 0.0025 mm to about 0.
mm, depending on material
• Filaments provide greatest opportunity for strength
enhancement of composites
• The filament form of most materials is significantly
stronger than the bulk form
• As diameter is reduced, the material becomes oriented in
the fiber axis direction and probability of defects in the structure decreases significantly
Continuous vs. Discontinuous Fibers
• Continuous fibers - very long; in theory, they offer a continuous
path by which a load can be carried by the composite part
• Discontinuous fibers (chopped sections of continuous fibers) -
short lengths (L/D = roughly 100)
• Important type of discontinuous fiber are whiskers - hair-like
single crystals with diameters down to about 0.001 mm (0. in.) with very high strength.
Fiber Orientation – Three Cases
• One-dimensional reinforcement, in which maximum strength
and stiffness are obtained in the direction of the fiber
• Planar reinforcement, in some cases in the form of a two-
dimensional woven fabric
• Random or three-dimensional in which the composite material
tends to possess isotropic properties
Fiber orientation in composite materials
(a) one-dimensional, continuous fibers; (b) planar, continuous fibers in the form of a woven fabric; and (c) random, discontinuous fibers
Materials for Fibers
- Fiber materials in fiber-reinforced composites:
- Glass – most widely used filament
- Carbon – high elastic modulus
- Boron – very high elastic modulus
- Polymers - Kevlar
Fibers Illustrate Importance of Geometric
Shape
Most materials have tensile strengths several
times greater as fibers than in bulk.
By imbedding the fibers in a polymer matrix, a
composite material is obtained that avoids the
problems of fibers but utilizes their strengths.
The matrix provides the bulk shape to protect the
fiber surfaces and resist buckling.
When a load is applied, the low-strength matrix
deforms and distributes the stress to the high-
strength fibers.
Other Composite Structures
• Laminar composite structure= conventional
• Sandwich structure
• Honeycomb sandwich structure
Laminar Composite Structure
Two or more layers bonded together in an integral piece
- Example: plywood in which layers are the same wood, but grains are oriented differently to increase overall strength of the laminated piece.
Sandwich Structure – Foam Core
Consists of a relatively thick core of low density foam bonded on both faces to thin sheet of a different material
Sandwich Structure – Honeycomb Core
• An alternative to foam core
• Either foam or oneycomb achieves high strength-to-weight and
stiffness-to-weight ratios.
Other Laminar Composite Structures
• Automotive tires - consists of multiple layers bonded
together
• FRPs - multi-layered fiber-reinforced plastic panels for
aircraft, automobile body panels, boat hulls
• Printed circuit boards - layers of reinforced plastic and
copper for electrical conductivity and insulation
• Snow skis - composite structures consisting of layers of
metals, particle board, and phenolic plastic
• Windshield glass - two layers of glass on either side of a
sheet of tough plastic
Metal Matrix Composites (MMCs)
A metal matrix reinforced by a second phase
mining tools, dies for powder metallurgy, indenters for hardness testers
• Titanium carbide cermets (Ni binder) - high temperature
applications such as gas-turbine nozzle vanes, valve seats, thermocouple protection tubes, torch tips, cutting tools for steels
• Chromium carbides cermets (Ni binder) - gage blocks, valve
liners, spray nozzles, bearing seal rings
Ceramic Matrix Composites (CMCs)
A ceramic primary phase imbedded with a secondary phase, which usually consists of fibers
• Attractive properties of ceramics: high stiffness, hardness, hot
hardness, and compressive strength; and relatively low density
• Weaknesses of ceramics: low toughness and bulk tensile
strength, susceptibility to thermal cracking
• CMCs represent an attempt to retain the desirable properties
of ceramics while compensating for their weaknesses
Polymer Matrix Composites (PMCs)
A polymer primary phase in which a secondary phase is imbedded as fibers, particles, or flakes
• Commercially, PMCs are more important than MMCs or CMCs
• Examples: most plastic molding compounds, rubber reinforced
with carbon black, and fiber-reinforced polymers (FRPs)
• FRPs are most closely identified with the term composite
Fiber-Reinforced Polymers (FRPs)
A PMC consisting of a polymer matrix imbedded with high-strength fibers
- Polymer matrix materials:
- Usually a thermosetting (TS) plastic such as unsaturated polyester or epoxy
- Can also be thermoplastic (TP), such as nylons (polyamides), polycarbonate, polystyrene, and polyvinylchloride
- Fiber reinforcement is widely used in rubber products such as tires and conveyor belts Fibers in PMCs
- Various forms: discontinuous (chopped), continuous, or woven as a fabric
- Principal fiber materials in FRPs are glass, carbon, and Kevlar 49
- Less common fibers include boron, SiC, and Al 2 O 3 , and steel
- Glass (in particular E-glass) is the most common fiber material in today's FRPs; its use to reinforce plastics dates from around 1920 Common FRP Structure
- Most widely used form of FRP is a laminar structure , made by stacking and bonding thin layers of fiber and polymer until desired thickness is obtained
- By varying fiber orientation among layers, a specified level of anisotropy in properties can be achieved in the laminate
- Applications: parts of thin cross-section, such as aircraft wing and fuselage sections, automobile and truck body panels, and boat hulls FRP Properties
- Examples: wood flour in phenolic and amino resins; and carbon black in rubber
- Extenders – used to increase bulk and reduce cost per unit weight, but little or no effect on mechanical properties Guide to Processing Composite Materials
- The two phases are typically produced separately before
being combined into the composite part
- Processing techniques to fabricate MMC and CMC
components are similar to those used for powdered metals
and ceramics
- Molding processes are commonly used for PMCs with
particles and chopped fibers
- Specialized processes have been developed for FRPs coupling agent A coupling agent is a chemical which improves the adhesion between two phases in a composite material. The term 'composite' is used here to denote a material which has two or more distinct constituents, not chemically bound to each other. The two examples of composite materials that are especially relevant here are: A. resins containing glass fibre reinforcement; B. thermosetting resins and thermoplastics containing particulate fillers. The most important coupling agents to-day are the organosilanes, applied to the surface of glass filaments in the form of an aqueous or nonaqueous solution, to promote adhesion to resins. The organosilanes completely dominate this market, although there have been earlier coupling agent technologies based on the chemistry of chromium complexes. Zirconates and titanates have much more varied applications, mostly related to fillers.
Fillers Fillers not only reduce the cost of composites, but also frequently impart performance improvements that might not otherwise be achieved by the reinforcement and resin ingredients alone. Fillers are often referred to as extenders. In comparison to resins and reinforcements, fillers are the least expensive of the major ingredients. Fillers can improve mechanical properties including fire and smoke performance by reducing organic content in composite laminates. Also, filled resins shrink less than unfilled resins, thereby improving the dimensional control of molded parts. Important properties, including water resistance, weathering, surface smoothness, stiffness, dimensional stability and temperature resistance, can all be improved through the proper use of fillers. Use of inorganic fillers in composites is increasing. When used in composite laminates, inorganic fillers can account for 40 to 65% by weight. There are a number of inorganic filler materials that can be used with composites, including:
- Calcium carbonate is the most widely used inorganic filler. It is available at low cost in a variety of particle sizes and treatments from well-established regional suppliers, especially for composite applications. Most common grades of calcium carbonate filler are derived from limestone or marble and very common in automobile parts.
- Kaolin (hydrous aluminum silicate) is the second most commonly used filler. It is known throughout the industry by its more common material name, clay. Mined clays are processed either by air flotation or by water washing to remove impurities and to classify the product for use in composites. A wide range of particle sizes is available.
- Alumina trihydrate is frequently used when improved fire/smoke performance is required. When exposed to high temperature, this filler gives off water (hydration), thereby reducing the flame spread and development of smoke.
fillers or flame retardant additives. Included in this category are materials containing ATH (alumina trihydrate), bromine, chlorine, borate and phosphorus.
- Suppressants: In open mold applications, styrene emission suppressants are used to block evaporation for air quality compliance. These wax-based materials form a film on the resin surface and reduce styrene emissions during curing.
- UV Inhibitors & Stabilizers: Both thermoset and thermoplastic composites may use special materials which are added to prevent loss of gloss, crazing, chalking, discoloration, changes in electrical characteristics, embrittlement and disintegration due to ultraviolet (UV) radiation. Additives, which protect composites by absorbing the UV, are called ultraviolet absorbers. Materials, which protect the polymer in some other manner, are known as ultraviolet stabilizers. In the event that a non-gel coated resin will be exposed to sunlight, the addition of a UV stabilizer will slow the surface degradation.
- Conductive Additives: Most composites do not conduct electricity. It is possible to obtain a degree of electrical conductivity by the addition of metal, carbon particles or conductive fibers. Electromagnetic interference shielding can be achieved by incorporating conductive materials.
- Release Agents: Release agents facilitate removal of parts from molds. These products can be added to the resin, applied to molds, or both. Zinc stearate is a popular mold release agent that is mixed into resin for compression molding. Waxes, silicones and other release agents may be applied directly to the surface of molds. Corrosion Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment. In the most common use of
the word, this means electro-chemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides is a well-known example of electrochemical corrosion. This type of damage typically produces oxides or salts of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and ability to contain a vessel's contents. Many structural alloys corrode merely from exposure to moisture in the air , but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromateconversion , can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable. GALVANIC CORROSION Galvanic corrosion occurs when two different metals have physical or electrical contact with each other and are immersed in a common electrolyte , or when the same metal is exposed to electrolyte with different concentrations. In a galvanic couple , the more active metal (the anode) corrodes at an accelerated rate and the more noble metal (the cathode) corrodes at a retarded rate. When immersed separately, each metal corrodes at its own rate. What type of metal(s) to use is readily determined by the galvanic series. For example, zinc is often used as a sacrificial anode for steel structures. Galvanic corrosion is of major interest to the marine industry and also anywhere water (via impurities such as salt ) contacts pipes or metal structures. Factors such as relative size of anode , types of metal, and operating conditions ( temperature , humidity , salinity , etc.) affect galvanic
Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. Rust is a general term for a series of iron oxides, usually, red oxides, formed by the reaction of iron with oxygen in the presence of water or air moisture. Rusting is the common term for corrosion of iron and its alloys, such as steel. Many other metals undergo similar corrosion, but the resulting oxides are not commonly called rust. Rusting is a special term for corrosion of the metal Iron. Rusting happens on the surface of iron objects making it coarse and flaky. It also makes the iron objects fragile. Rusting happens quicker in a humid environment. As a result, it is easier for water tanks and pipes to get rusted. Therefore, various methods are used to reduce the effect of rusting. Getting wounded from a corroded metal object especially rusted iron objects can prove to be dangerous. Prevention from Rusting Methods used to prevent Rusting of Iron are as follows: Alloying – Since Rusting of Iron is a chemical process that happens because the metal is attaining more stable chemical state, alloying (mixing) the iron with other stable metals or alloys can slow down the process of rusting. Galvanizing – Galvanizing a metal object means to coat the surface of that object with a layer of metallic zinc. Also, it is an inexpensive procedure. In conclusion, it will provide it with protection against rusting. Coating and Painting – Coating the surface of a metal object with a layer of either Paint or Varnish will break the contact between the surface and atmospheric oxygen making it consequently immune to rusting. Humidity Control – Controlling the humidity of the environment is also a solution. Therefore, the chances of the metal object rusting will reduce. Corrosion in passivated materials
Passivation (use of a light coat of a protective material, such as metal oxide, to create a shell against corrosion) is extremely useful in mitigating corrosion damage, however even a high-quality alloy will corrode if its ability to form a passivating film is hindered. Proper selection of the right grade of material for the specific environment is important for the long-lasting performance of this group of materials. If breakdown occurs in the massive film due to chemical or mechanical factors, the resulting major modes of corrosion may include pitting corrosion , crevice corrosion and stress corrosion cracking. Pitting corrosion: Certain conditions, such as low concentrations of oxygen or high concentrations of species such as chloride which complete as anions , can interfere with a given alloy's ability to re- form a passivating film. In the worst case, almost all of the surface will remain protected, but tiny local fluctuations will degrade the oxide film in a few critical points. Corrosion at these points will be greatly amplified, and can cause corrosion pits of several types, depending upon conditions. In extreme cases, the sharp tips of extremely long and narrow corrosion pits can cause stress concentration to the point that otherwise tough alloys can shatter; a thin film pierced by an invisibly small hole can hide a thumb sized pit from view. These problems are especially dangerous because they are difficult to detect before a part or structure fails. Pitting remains among the most common and damaging forms of corrosion in passivated alloys, but it can be prevented by control of the alloy's environment.
Crevice corrosion
Crevice corrosion is a localized form of corrosion occurring in confined spaces (crevices) to which the access of the working fluid from the environment is limited and a differential aeration cell is set up, leading to the active corrosion inside the crevices. Examples of crevices are gaps and contact areas between parts, under gaskets or seals, inside cracks and seams, spaces filled with deposits and under sludge piles. Example: corrosion occurred in the crevice between the
