A composite is a material which consists of two or different phases. We have a reinforcement phase, the part which provides strength to overall material and a matrix phase which completely surrounds/binds the reinforcement phase. These two phases have to be dissimilar, meaning they have to be different materials. This dissimilarity is usually very easily identifiable. To understand this let’s take the example of fiberglass. The reinforcement here is glass fibers which are thin long thread-like glass components. These glass fibers are embedded in a thermosetting polymer such as epoxy. When load is applied on to this composite, the matrix (epoxy) transfers the load to the glass fibers which are able to bear the force. Fundamentally, that is how a composite works. The matrix holds the fibers and transfer load on to them. A small fraction of the load is borne by the matrix. But that’s not it. The matrix also protects these fibers from environment. Chemical reactions or mechanical damage such as scratching can create flaws in the reinforcement which compromises their load bearing capabilities. The matrix material needs to be ductile, more so than the reinforcement. This behavior allows the fibers to be isolated once they form a composite. As a result if one fiber cracks, it will not be able to spread to the adjacent fibers and the composite remains intact. Failure will only occur once sufficient adjacent fibers fracture.
How do composites handle load?
The stress in a fiber depends on its length, the interface it forms with the matrix and the elastic characteristics of fiber and matrix. The mechanical characteristics such as strength, stiffness/modulus depends on both the reinforcement and the matrix, mainly rests on the reinforcement. The fibers can be either long or short, but for fiber to effectively translate the aforementioned characteristics there is a minimum fiber length or critical fiber length required. Fibers longer than the critical length are defined as continuous while those smaller are termed as discontinuous. These discontinuous fiber composites work as particulate composite. Practically, we want the fiber to bear most of the stress and that will depend on its length, the interface it forms with the matrix and the elastic characteristics of fiber and matrix. The composites with continuous fibers will therefore be able to bear more weight compared to those with discontinuous fibers.
The fiber-matrix interface
Simply, the interface is the interfacial region between the fibers and the matrix. Chemical bonding between fiber and matrix, mechanical bonding, electrostatic forces, anion-cation interaction and various other physical interactions can contribute to formation of interface. A good interface ensures most of the load is transferred to the fiber. Naturally, greater exposed surface area of the fiber to the matrix will lead to a good interface because the previously mentioned interactions will occur at on a larger scale, hence fibers which are long and thin will contribute to a better interface. This is one of the primary reasons that the critical fiber lengths increase as the diameters of the fibers increase. With a larger diameter, the surface area to volume ratio decreases and a good interface cannot form.
Reinforcement geometries and orientation
Reinforcement can come in a variety of geometries. We have fibers, as mentioned previous, which can be continuous and discontinuous. If all these fibers are aligned in longitudinal direction then they will be more effective in dealing with tensile loads. Randomly aligned fibers will also provide strength but under directional loading only the fibers in the direction of applied stress will provide effective strength. However, the randomly aligned fiber comparatively will fare much better under transverse loading than the aligned fibers which will virtually offer no amount of strength in the transverse direction. Particulate reinforcement works in a differently than fibers. Here the volume fraction of the particulates mainly governs the strength bearing ability of the composite. For best results, the particulates should be approximately of the same size and geometry as well as evenly distributed in the composite. Laminates or mats are another largely used form of reinforcements. These structures are formed of fibers which are held together by a binder material. These mats are composed of fibers bound by a binder such as chopped strand glass fiber mats. They prove advantages by providing strength in 2-D direction (in a plan). In addition, the mats come handy in composite fabrication techniques such as the simple hand lay-up.
Composite Matrix
In the same way the reinforcement can be of different materials, composite matrix can be a metallic (metal matrix composite), ceramic (ceramic matrix composite) or a plastic/polymer (polymer matrix composite), though matrix will be weaker than the reinforcement. Matrix materials are chosen depending on the conditions where the composite will be employed. In case of refractory conditions, a ceramic matrix would usually be employed as ceramics can withstand high temperatures. It should be kept in mind that a good interface is vital to a composite and keeping that in view, some reinforcements might not interact all too well with a composite such as pairing steel with polyester (sounds almost ridiculous). This is one of the primary reasons that we do not see natural fiber composites. With a poor interface, the load transfer from matrix to reinforcement does not take place. Some times to improve adhesion between these phases the fibers are surface treated to make them more compatible with matrix in question. There is a lot more to composites and how they work. If you readers have any queries please leave a comment or contact us we will get back to you as soon as possible.
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