Composite Materials: Types and Classifications

• Composite materials consist of two or more physically distinct and mechanically separable materials.

Composites

• Composites have a continuous phase (matrix) and a reinforcing phase (fiber/particles).
• The reinforcing phase carries 90-95% of the load.

Continuous Phase (Matrix)

• The matrix can be a polymer, metal, or ceramic.
• Examples of polymers include plastic and epoxy.
• Thermoset polymers burn and are not affected by temperature.
• Thermoplastic polymers melt and can be reshaped.

Reinforcing Phase (Fiber/Particles)

• Reinforcing architectures are fibers or particles, arranged discontinuously or continuously.
• Reinforcing materials include glass, carbon, and aramid.
• Discontinuous architectures include particles and short fibers.
• Continuous architectures include unidirectional and textile.

Broad Classification of Composite Materials

• Natural composite materials include wood, bones, bamboo, and muscles.
• Microscopic composites include metallic alloys (steel, bronze) and toughened thermoplastics (impact polystyrene).
• Macroscopic composites (engineering products) include galvanized steel, reinforced concrete beams, and helicopter blades.

Alloys vs. Composites

• Alloys are not considered composites because their manufacture requires heating, whereas composites are formed without heating.

Factors Governing Fiber's Contribution

• The main factors governing the fiber's contribution are the basic mechanical properties of the fiber, the surface interaction of fiber and resin (the interface), the amount of fiber (fiber volume fraction), and the orientation of the fiber in the composite.
• Fiber volume fraction (Vf) is expressed as Vf = Vf / (Vf + Vr), typically in the range of 50-70%.

Fiber Types

• Glass fiber is white and has high strength.
• Carbon fiber is used in military applications such as missiles and is often paired with an epoxy matrix; it is not abundantly available.

Matrix Properties

• The matrix must have good mechanical and adhesive properties, good toughness, and good resistance to environmental degradation.

Matrix Functions

• The matrix binds fibers together, transmits and distributes applied stress to the fibers, and protects fibers from damage.
• Ductility is essential in the matrix phase because the fiber is brittle.
• The matrix carries a small portion of the load (10-5%) and usually determines the temperature limit.
• If Tfiber is less than 1000°C and Tmatrix is less than Tfiber, then the matrix limits the temperature.

Matrix Materials

• Thermoplastics are not commonly used; they are affected by temperature and become flexible, such as teflon/poly-ethylene.
• Thermosets are most common and are not affected by burning, such as epoxy.

Mechanical Behavior

• Composites are anisotropic materials.
• Isotropic materials are homogeneous, with the same properties in all directions (e.g., steel, metals).
• Anisotropic materials are non-homogeneous (e.g., composite materials).

Manufacturing Processes: Spray Lay-up

• Spray lay-up involves pressurized resin, a chopper gun for fibers, a resin catalyst pot, and an optional gel coat, all applied to a mold shape.
• Advantages of spray lay-up include its widespread use, low cost, and quick desposing of fiber and resin.
• Disadvantages include laminates tending to be very resin-rich, therefore very heavy.
• Applications include bathtubs and shower trays.

Wet Lay-up/Hand Lay-up

• Wet lay-up involves dry reinforcement fiber, consolidation roller, resin, and an optional gel coat applied to a mold tool.
• Advantages include widespread use and simplicity to teach.
• Low-cost tooling and higher fiber content than spray lay-up are also advantages.
• A disadvantage is resins need to be low in viscosity to be workable by hand, compromising their mechanical and thermal properties.
• Applications include wind-turbine blades and production boats.

Resin Transfer Moulding

• Resin transfer molding uses pressure or a clamp to hold the halves together.
• It includes a mold tool, resin injected under pressure, and optional vacuum assistance, with dry reinforcement fibers.
• Advantages include good health, safety, and environmental control because of the enclosure of resin, possible labor reductions, and both sides having a molded surface.
• Disadvantages include matched tooling being expensive and heavy to withstand pressures and generally being limited to smaller components.
• Applications include small complex air craft and automotive components.

Advantages & Disadvantages of Composite Materials

• Advantages include high strength or stiffness to weight ratio, tailor-able properties, long life, no corrosion, and good damping resistance.
• Disadvantages include high cost of raw materials, difficulty to attach, difficulty to repair, difficult stress analysis, and weak transverse properties.

Fatigue of Composites

• Fatigue in composites involves matrix cracking, delamination, fiber breakage, and fracture.

Failure Mechanisms

• Failure mechanisms include matrix cracking, fiber breakage, delamination, fiber bridging, interfacial shear, and debonding.

Fatigue Failure Definition

• Fatigue failure results from variable fatigue load, forming micro cracks, growth delamination, and subsequent mixed-mode crack propagation until complete fracture occurs.

Steps for Complete Fracture

• The steps for complete fracture are micro crack formation (scratches), crack propagation in the matrix, and rupture (fracture of fiber).

Factors Affecting Fatigue Behavior in Composites

• Type of loading, loading frequency, volume fraction, fiber orientation, mean stress and stress ratio, size and stress gradient are factors.

Effect of Type of Loading

• The best load for composites (fiber) is tension-tension.
• The worst load is shear, which requires methods to fix it.
• Shear can be converted to tension or compression by changing the angle of the fibers.

Loading Frequency

• As the frequency increases, the life period decreases.
• This is because as frequency increases, heat generation occurs (60-70°C).
• Heat will make the strength decrease, thus life will decrease.

Volume Fiber Fraction

• Since Vf = Wf / (Wf + Wr), where Wf is the weight of the fiber and Wr is the weight of the resin.
• Vf < 1, optimum value ranges from 50-70%.
• Decreasing strength decreases if Vf decreases from the optimum value.
• If Vf < optimum, there will be not enough fiber to carry the load.
• If Vf > optimum, the matrix will be small, so it won't be enough to hold the fiber (delamination).
• Relationship between load direction and Vf include transverse and longitudinal directions.

Fiber Orientation

• Includes (MPa), with sigma rupture.
• Relationships between orientation, stress, and Modulus of Elasticity.
• Common Orientations at 90, 45, and 0 direction.

For Orientation 0°

• Longitudinal tension is the best load, good.

Transverse Loads

• Transverse Loads are bad.

Shear

• Shear is very bad

Shear at 45°

• Shear at 45° is medium.
• The best Orientation for shear is [±45°].

Mean Stress & Stress Ratio

• Since Omax = Omax + Omin
• Effect in om the greater the min the shorter the life?
• Better tensile strength than compressive.
• Compression is better than tension.
• Decreases cracks..

Surface Finish

• Surface finish effects.
• If size, the diameter increases, life decreases
• Surface finish effects like soft surface is better than rough surface.
• No Stress concentration on soft and smoother surface.

Environmental Factors

• Higher Humidity reduces the strength and lowers Life.
• Temperature, as temperature increases strength decreases and life decreases as well.

Stress Concentration

• Hole (stress riser) makes concentration higher.
• Rough surface finish also raises stress concentration.