Composite Structures (continued)
Classification of Composite Materials
Since the reinforcement material is of primary importance in the strengthening mechanism of a composite, it is convenient to classify composites according to the characteristics of the reinforcement. The following three categories are commonly used.
- Fiber Reinforced – In this group of composites, the fiber is the primary load-bearing component.
- Dispersion Strengthened – In this group, the matrix is the major load-bearing component.
- Particle Reinforced – In this group, the load is shared by the matrix and the particles.
Fiber Reinforced Composites
Fiberglass is likely the best know fiber reinforced composite but carbon-epoxy and other advanced composites all fall into this category. The fibers can be in the form of long continuous fibers, or they can be discontinuous fibers, particles, whiskers and even weaved sheets. Fibers are usually combined with ductile matrix materials, such as metals and polymers, to make them stiffer, while fibers are added to brittle matrix materials like ceramics to increase toughness. The length-to diameter ratio of the fiber, the strength of the bond between the fiber and the matrix, and the amount of fiber are variables that affect the mechanical properties. It is important to have a high length-to-diameter aspect ratio so that the applied load is effectively transferred form the matrix to the fiber.
Fiber materials include:
Glass – glass is the most common and inexpensive fiber and is usually use for the reinforcement of polymer matrices. Glass has a high tensile strength and fairly low density (2.5 g/cc).
Carbon-graphite - in advance composites, carbon fibers are the material of choice. Carbon is a very light element, with a density of about 2.3 g/cc and its stiffness is considerable higher than glass. Carbon fibers can have up to 3 times the stiffness of steel and up to 15 times the strength of construction steel. The graphitic structure is preferred over the diamond-like crystalline forms for making carbon fiber because the graphitic structure is made of densely packed hexagonal layers, stacked in a lamellar style. This structure results in mechanical and thermal properties are highly anisotropic and this gives component designers the ability to control the strength and stiffness of components by varying the orientation of the fiber.
Polymer – the strong covalent bonds of polymers can lead to impressive properties when aligned along the fiber axis of high molecular weight chains. Kevlar is an aramid (aromatic polyamide) composed of oriented aromatic chains, which makes them rigid rod-like polymers. Its stiffness can be as high as 125 GPa and although very strong in tension, it has very poor compression properties. Kevlar fibers are mostly used to increase toughness in otherwise brittle matrices.
Ceramic – fibers made from materials such as Alumina and SiC (Silicon carbide) are advantageous in very high temperature applications, and also where environmental attack is an issue. Ceramics have poor properties in tension and shear, so most applications as reinforcement are in the particulate form.
Metallic - some metallic fibers have high strengths but since there density is very high they are of little use in weight critical applications. Drawing very thin metallic fibers (less than 100 micron) is also very expensive.
Dispersion Strengthen Composites
In dispersion strengthened composites, small particles on the order of 10-5 mm to 2.5 x 10-4 mm in diameter are added to the matrix material. These particles act to help the matrix resist deformation. This makes the material harder and stronger. Consider a metal matrix composite with a fine distribution of very hard and small secondary particles. The matrix material is carrying most of the load and deformation is accomplished by slip and dislocation movement. The secondary particles impede slip and dislocation and, thereby, strengthen the material. The mechanism is that same as precipitation hardening but effect is not quite as strong. However, particles like oxides do not react with the matrix or go into solution at high temperatures so the strengthening action is retained at elevated temperatures.
Particle Reinforced Composites
The particles in these composite are larger than in dispersion strengthened composites. The particle diameter is typically on the order of a few microns. In this case, the particles carry a major portion of the load. The particles are used to increase the modulus and decrease the ductility of the matrix. An example of particle reinforced composites is an automobile tire which has carbon black particles in a matrix of polyisobutylene elastomeric polymer. Particle reinforced composites are much easier and less costly than making fiber reinforced composites. With polymeric matrices, the particles are simply added to the polymer melt in an extruder or injection molder during polymer processing. Similarly, reinforcing particles are added to a molten metal before it is cast.
- The interface is a bounding surface or zone where a discontinuity occurs, whether physical, mechanical, chemical etc.
- The matrix material must "wet" the fiber. Coupling agents are frequently used to improve wettability. Well "wetted" fibers increase the interface surface area.
- To obtain desirable properties in a composite, the applied load should be effectively transferred from the matrix to the fibers via the interface. This means that the interface must be large and exhibit strong adhesion between fibers and matrix. Failure at the interface (called debonding) may or may not be desirable. This will be explained later in fracture propagation modes.
- Bonding with the matrix can be either weak van der Walls forces or strong covalent bonds.
- The internal surface area of the interface can go as high as 3000 cm2/cm3.
- Interfacial strength is measured by simple tests that induce adhesive failure between the fibers and the matrix. The most common is the Three-point bend test or ILSS (interlaminar shear stress test)
We will consider the results of incorporating fibers in a matrix. The matrix, besides holding the fibers together, has the important function of transferring the applied load to the fibers. It is of great importance to be able to predict the properties of a composite, given the component properties and their geometric arrangement.
Isotropy and Anisotropy in Composites
- Fiber reinforced composite materials typically exhibit anisotropy. That is, some properties vary depending upon which geometric axis or plane they are measured along.
- For a composite to be isotropic in a specific property, such as CTE or Young’s modulus, all reinforcing elements, whether fibers or particles, have to be randomly oriented. This is not easily achieved for discontinuous fibers, since most processing methods tend to impart a certain orientation to the fibers.
- Continuous fibers in the form of sheets are usually used to deliberately make the composite anisotropic in a particular direction that is known to be the principally loaded axis or plane.