BDCS Notes - Microscopic Composites

Microscopic Composites

Two constituent phases of microscopic composites:

1. A Continuous Phase (Matrix)

2. Dispersed Phase (Reinforcing Phase) (Surrounded by Matrix)

The dispersed phase is harder and stiffer than the matrix. The properties of the composite depend on the properties of both component phases, their relative properties and the geometry of the dispersed phase (such as particle shape, size, distribution and orientation).

The Dispersed Phase

Two Basic Classes of Microscopic Composites:

1. Fiber-Reinforced

2. Particle-Reinforced

The classification above is based on the shape of the dispersed phase. The microscopic composites have three different schematics:

1. Aligned Fibers

2. Random Fibers

3. Random Particles

Fiber-Reinforced Composites – Include fibers dispersed in a matrix such as metal or polymer. Fibers have a high strength-to-diameter ratio, with near crystal-sized diameters. That is to say, because of the small diameter of the fibers, they are much stronger than the bulk material. Fibers are much stronger than the bulk form, because they have fewer internal defects.

Classification of Fibers – On the basis of their diameter and character as whiskers, fibers and wires.

Whiskers – Very thin single crystals, have extremely large length-to-diameter ratios. Very strong. High degree of crystalline perfection. Not commonly used in reinforcement, because of high costs and poor bonding, not to mention the difficulty of incorporating them into the matrix.

Fibers – Manufactured from many materials, such as glass, carbon and graphite. Larger diameters than whiskers. Because of low cost and high strength, glass fibers are most common of all reinforcing fibers. Glass fibers are available in several forms suitable for different applications. Common glass fibers include veils, rovings (continuous fibers) and mats. A common fiber-reinforced composite is fiberglass (glass fibers in a plastic matrix).

Wires – Have a larger diameter than fibers. Least used of all three.

Particle-Reinforced Composites – Particles dispersed in a matrix phase. The strengthening mechanism of particle-reinforced composites varies with the size of the reinforcing particles. When the particles are small (in microns) the matrix bears most of the applied load. When the particles are larger than 1 micron, particles act as fillers to improve the properties of the matrix phase and/or to replace some of its volume. Applied load is then shared by matrix and dispersed phases. The stronger the bond between dispersed particles, the larger the reinforcing effect.

The Matrix Phase

Matrix – Polymers (plastics) or metals are used in most microscopic composites. The matrix binds the dispersed materials (particles and fibers) together. It protects the materials from environment and damage.

Benefits of Polymers:

1. Low Cost

2. Easy Possibility

3. Good Chemical Resistance

4. Low Specific Gravity

Shortcomings of Polymers:

1. Low Strength

2. Low Modulus

3. Low Operating Temperatures

4. Low Resistance to Prolonged Exposure (UV Rays)

Metals tend to excel in the areas where polymers are lacking. The metals most commonly used as a matrix phase in composites are aluminum and titanium alloys.

Fabrication –Composites are formed by combining matrix and dispersed material. Fabrication methods are based on the chemical nature of the matrix, dispersed phases and temperature to form, melt and cure the matrix.

Pultrusion – An automated process for manufacturing fiber-reinforced composite materials into continuous, constant-cross-section profiles.

Pultrusion Process:

1. Reinforcement Material

2. Resin Bath

3. Heated Die

4. Puller

5. Saw

6. Finished Product

Civil Engineering Applications:

1. Structural Shapes

2. Concrete Reinforcement (Instead of Rebar)

3. Tanks

4. Industrial Flooring

5. Trusses and Joints

6. Walkways and Platforms

7. Waste Treatment Plants

8. Handrailings

9. Plastic Pipes

10. Light Poles

11. Door and Window Panels and Frames

12. Electrical Enclosures

13. Strengthen and Wrap Columns and Bridge Supports

Fiber-Reinforced Concrete – Fiber can impede the progression of cracks in concrete. They also increase the tensile and flexure strength of concrete so that a more efficient structural member can be designed (up to two to three times the flexural strength of unreinforced concrete). The material under increased load does not fail abruptly, but yields gradually because of the fibers.

Entrained Air – A component in a microscopic composite material. Entrained air increases the durability of concrete since it releases internal stresses due to freezing of water within the concrete. For a same water-to-cement ratio, however, air bubbles reduce the concrete strength by about 20%. Entrained air helps workability in concrete, the water-to-cement ratio can be reduced to compensate for some of the strength reduction.