Composites Material- Classification Based on Matrix and Topology

 

Classification of Composite Materials | Fibers

Composite material is a material system composed of two or more dissimilar constituents, differing in forms, insoluble in each other, physically distinct and chemically inhomogeneous. The resulting product possesses properties much different from the properties of constituent materials.

Classification of Composite Materials

Composite materials, also referred as composites, are broadly classified as

1. Agglomerated composite materials,
2. Laminated composite materials, and
3. Reinforced composite materials.

Laminated composite

Lamina and Laminate: Laminated materials also referred as laminates are layered composites made-up of many laminae. A lamina also known as a ply or a layer is very thin, about 0.1 mm to 1 mm thick. A single lamina is unsuitable for any purposeful application. They are, therefore, joined or glued together to form a laminate of desired thickness. Thus a laminate is made up of an arbitrary number of lamina.

Number of lamina, in a laminate, can be few to many tens. A few examples of common laminates are the following.

1. Plywood
2. Metal to metal laminate viz. cladded metals
3. Sheet moulding compounds (SMC)
4. Bulk moulding compounds (BMC)
5. Linoleum etc..
6. Tufnol

Now we shall discuss main among them, one by one.

Laminate and delamination: A laminate is a stack of lamina with various orientations of the directions of the principal materials in the lamina. Laminates can be built up with plates or plies of different materials or of the same material, such as glass fibers.

Shear stresses are always present between the layers of a laminate because each layer tends to deform independently of its neighboring layers due to each layer having different properties. These shear stresses, including the transverse normal stresses, are a cause of delamination.

Bulk molding compounds (BMCs) are a premixed material of short fibers (chopped-glass strands) pre-impregnated with resin and various additives. A dough molding compound (DMC) is an alternative term for a BMC. Some thermoset resins are quite thick and are called moulding doughs. Parts made by BMCs are limited to about 400 mm in their longest dimension due to problems with separation of the components of moulding compound during molding.

Sheet moulding compounds (SMC) are nonmetallic plastic-rein-forced-composite laminates made-up by pressing together many unidirectional (U/D) laminae one over the other. Laminate of desired properties may be prepared by placing U/D laminae in different orientations. Generally a prepreg is used for that.

Prepreg SMC is an intermediate compound between raw material and the final product. Its pot life is 3 to 4 days, and cannot be used after that. SMC is a blend of resin, hardener, fibers, catalyst and accelerator. Its curing time can be further decreased by adding more than specified quantity of hardener.

Reinforced Composite Materials

Reinforced composites are made-up of two basic constituents viz.

1. Matrix or body constituent, and
2. Reinforcing constituent.

The matrix constituent comprises of the following materials.

• Metals,
• Ceramics,
• Polymers,
• Concrete,
• Elastomers, and
• Cements.

Commonly used reinforcing agents are the following.

• Metals such as steel in cement concrete.
• Organic fibers such as carbon, graphite, kevlar etc.
• Inorganic fibers such as glass, ceramic etc.

Classification of Reinforced Composite Materials

Based on various considerations, the reinforced composites are classified as follows.

Based on the nature:

• Organic Composites e.g. polymeric based
• Inorganic Composites e.g. reinforced cement concrete (RCC)

Based on the matrix material used:

• Polymer Matrix Composites (PMC)
• Metal Matrix Composites (MMC)
• Ceramic Matrix Composites (CMC)
• Cement Matrix Composites.

Based on the type of reinforcing material used:

• Particulate Composites
• Fiber Reinforced Composites (FRC)
• Flake Reinforced Composites or Flake Composites
• Whisker Reinforced Composites
• Hybrid Composites
• Sandwich Composites.

Based on the existence of fiber:

• Natural Fibers Composites
• Synthetic Fibers Composites  

Based on the aspect ratio of fiber:

• Continuous Fiber Composites
• Short Fiber Composites

Based on specialized material system:

• Fiber-Fiber Composites
• Toughened Composites, such as rubber-toughened-plastics
• Fiber Reinforced Glass (FRG)
• Glass Ceramic Matrix (GCM) Composites
• Polymer Concretes (PCs)
• Fiber Reinforced Concrete (FRC)
• Polymer Cement Concrete (PCC)
• Asbestos Reinforced Plastics (ARP)
• Nylon Reinforced Elastomers (NRE)
• Fiber Reinforced Metals (FRM)
• Wood-Plastics Composite

Based on the arrangement of fiber lay:

• Unidirectional (U/D) Composites
• Bidirectional or Cross-plied Composites
• Angle-plied Composites
• Off-axis Composites
• Randomly Oriented Composites

We shall now discuss some main composites out of the above classification.

Particulate Composites

Particulate composites have one or two dimensional macroscopic additive constituents randomly embedded in the matrix. The size, nature and function of these particles vary widely. The particles are 1µm or more in size and having volume concentrations of 20 to 40 percent.

 Small particles of uniform size with proper orientation exhibit a more strengthening effect. Elastic modulus of a particulate composite may be obtained by simple rule of mixture.

 Applications: Particulate composites are made by sintering which is a powder metallurgy technique. The W-Ni-Fe and W-Ni-Cu systems are the examples of particulate composites. Cermets described below are similar to particulate composites.

Cermets: Cermet is made of cer and met. Here ‘cer’ stands for ceramic and `met’ for metal. Cermet is a combination of ceramic and metal. A cermet consists of ceramic matrix and the reinforcing metal particles. They are generally made by powder metallurgy technique.

We know from that the refractories (a type of ceramics) are high temperature resisting, brittle materials. Contrary to this, metals are ductile and have melting points varying from low to very high. They also have higher mechanical strengths.

A composition of ceramics and metals in different proportions gives desired thermal and mechanical properties in cermets. They are very good wear and abrasion resistant materials, and possess high hardness.

Applications: Some notable applications of cermets are the following.

• Rotary drills in mining industries.
• Cutting tools in metal cutting industries.
• Shaping tools for refractories.
• Cemented carbide in high speed cutting.
• Components in satellites and space-going vehicles.

Flake Composites

These are composites of two-dimensional nature, and are preferred when planer isotropy is also desired in components of structures and machines. It should be noted that the composites generally possess orthotropy; or anisotropy and not isotropy. Moreover, flakes of two-dimensional geometry can be more closely packed than the fibers.

These qualities make them suitable for various applications. Mica flakes-glass matrix composites are easily machinable, and are used in heat and electrical insulating applications. Silver flakes are used where good conductivity is desired.

Whisker Reinforced Composites

Whiskers are a form of materials possessing extremely high strength and moduli. Strength and moduli of whiskers are much superior than the bulk and fiber form of the same material.

Hence their reinforcement in composites will impart too higher strengths and moduli. Whisker reinforced composites are in the initial stage of development. It is a likely material to be used in the near future.

Hybrid Composites

Fiber composites are made up of only a single type of fiber such as glass, carbon or kevlar etc. Each type of fiber has its own limitations in terms of strength, cost and other material properties. These limitations can be eliminated by using a combination of two or more types of fibers in the same matrix.

Mixing of two or more different-types of continuous fibers in the same matrix is called hybridization and the resulting product is called hybrid composite. Improvement in properties of such composites is due to the hybrid effect.

Idealized stress-strain curve of a hybrid composite showing hybrid effect.

Characteristics: Above Figure shows an idealized stress-strain characteristic curve of two fiber system. The initial slope OA gives initial modulus and slope BC gives the final modulus. The fall AB in the curve is due to failure of first fiber system at strain LA.

The second fiber system continues to take the load until final fracture occurs at strain Ec. In a typical case, the first and second fiber systems may be those of glass and carbon.

Types of Hybrid Composites: Hybrid composites are subdivided into following four types:

• Interply hybrid composites
• Intraply hybrid composites
• Inter-Intraply composites
• Super hybrid.

The interply hybrid composite consists of alternate lamina (layer or ply) of the same matrix but different fibers. Intraply hybrid composite contains each lamina having two or more kinds of fibers system. The inter-intraply composite is a combination of the above two types.

Applications: Practical utilization of hybrid composites have been widely made. Out of these some important applications are given below.

• Antenna dishes of CFRP (carbon fiber reinforced plastics) and aluminum honeycomb.
• CFRP and CRP leaf springs and drive shafts for automobiles.
• Busbars of aluminum, reinforced by CFRP.
• Squash racquets and golf clubs with shafts of CFRP/GRP/wood hybrid.
• Helicopter rotors and thin-walled tubes of CFRP and GRP.
• Coil springs of glass, aramid and carbon fiber hybrid.
• Artificial limbs and external bracing systems having CFRP base.

Sandwich Composites

A sandwich composite is constructed by sandwiching foam core between two skins of FRP laminates as shown in Figure given below. The thickness of the skin is ts kept up to 3 mm, and the thickness of core tc, is kept deeper.

A sandwich composite construction showing sandwiched core between FRP skins.

The core is either foamed or made of honeycomb material so that its density is very less. As tc is deep (tc>> ts), the area moment of inertia of the cross-section is enhanced too much; due to which the flexural rigidity of sandwich beam becomes more. This higher flexural rigidity construction along with very lightweight makes the sandwich composite most suitable as a beam.

Honeycomb Materials: Sandwich constructions are widely employed as beam component of structures in airplanes, spacecrafts, satellites etc. A non-sandwiched, FRP composite beam will be much heavier than a sandwiched beam of same dimensions.

Advantages and of Composites

• They posses combination of excellent mechanical, chemical, structural, electrical, optical and other desired properties.
•  They are lightweight materials possessing higher specific strength and specific modulus than the conventional materials.
• Power by weight ratio in aeroplanes is approximately 5 with the conventional materials while it is about 16 with composites. This will require prime mover of reduced power resulting in fuel economy, or more pay-load carrying capacity. Alternately it helps in weight reduction.

• Composites can be moulded to any shape and size and according to any desired specification.
• They possess excellent anti-chemical and anti-corrosion properties.
• Making, repairing and fabricating of composites are easier than the metals and RCC.
• Assembling and de-assembling of components is easy and quick. Efficient utilization of material may be done. The fibers may be oriented in such a way so as to provide greatest strength and stiffness in the desired direction.
• Seepage and weathering problems are negligible.
• Composites may be designed to obtain aesthetic appearance.

Disadvantages of Composites

• They have low flash and fire points.
• They may develop undesired biological effects seen in polymers.
• Polymeric composites are not suitable for high temperature applications.
• Cost of composites is still higher than many conventional materials.
• On prolonged exposure to sunlight, the colors of composites generally fade-out.