“In the continuing quest for improved performance, which may be specified by various criteria including less weight, more strength and lower cost, currently-used materials frequently reach the limit of their usefulness. Thus materials scientists, engineers and scientists are always striving to produce either improved traditional materials or completely new materials. Composites are an example of the latter category.” [1]

This is how of one of the most famous books explaining the underlying science and enginnering performance of composite materials starts. In these few lines I found the reasons why composites have been very fascinating to me, since the very beginning. On the other hand, who would not wish to discover a new material or at least a new its application? Moreover, who would not want it lighter, stronger and cheaper, potentially at the same time, with respect to a typical solution? Who would not enjoy to collaborate with a broad range of engineers and scientists, as the authors of the book point out? If you are one of them, probably it is worth keeping on reading. There is more to come, even in the bio-medical sector being the context that we will be exploring here.

However, let me start with the basics, it will help in the future discussions. A composite is a material having two o more distinct constituents. Thus, we can classify even the very familiar concrete as composite. In fact, it is a mixture of stones held together by the cement. But, there are naturally occurring composites too, of which the best known example may be bones. For example, the femur in Fig.1 has an outer shell of high density and low fat content bone, the so called cortical bone. It has good mechanical properties with flexural strength in the range of 46 and156 MPa and Young’s modulus of the order of 20 GPa, where Young’s modulus is a stiffness measure. Contained within the shell of cortical bone is a softer and spongy bone which consists of a three-dimensional network of beams and sheets, known as trabaculae. Its compressive strength and Young’s modulus are tipically in the range 5 to 20 Mpa and 0.02 to 1.7 GPa respectively. These few numbers will be very useful to illustrate the high standards that synthetic composites have to achieve to be of comparable quality.

Continuing with the fundamentals, we have already stated that composites are mixtures of two or more distinct constituents or phases. However, there are three more criteria to be satified before a material can be said to be a composite. First, bot phases have to be present in reasonable proportions, say greater than 5%. Secondly, the composite properties have to be markedly different from the properties of its constituents. Lastly, a man-made composite is usually produced by intimately mixing and combining the constituents by various methods and processes.

Let us move to some classification by considering the most common composite configuration, i.e. with 2 constituents. The one being continuous is termed the matrix. The normal view is that is present in the greater quantity in the composite and that its properties are improved by incorporating another constituents and, not less important, by controlling the interface being created between them. A composite may have a ceramic, metallic or polymeric matrix then. Broadly speaking, ceramics are the strongest and  stiffest but are brittle. On the contrary, polymers have low strength and Young’s modulus. Metals have intermadiate strengths and moduli and are not brittle.

The second constituent is referred to as the reinforcing phase or reinforcement, as it should enhance the mechanical properties of the matrix, with the result of having a new material. In most cases, the reinforcement is harder, stronger and stiffer than the matrix. The main feature of the reinforcement is that at least one of the dimensions is small, say up to 500µm, i.e. 0.5mm. We usually describe it as being either fibrous or particulate. The mechanical properties of the composite are a function of the shape and dimensions of the reinforcement. The fibrous reinforcement may be discontinous (short fibres) or continuous (long fibres). The short fibres may have random or preferred orientation. The most frequent orientation for the long fibre is termed unidirectional, but bidirectional woven long fibre reinforcement also exists.

To summarize, we have seen so far that composites are completely new materials generally made by two different constituents: the matrix and the reinforcement. Their properties are markedly different from those of the costituents. The matrix can be ceramic, metal or polymer and the reinforcements may be particulates or short or long fibres.  How much the matrix properties are increased depends almost entirely by the reinforcement and the interface between them.

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Fig. 1 The composite structure of the femur [2

References

[1]. Matthews, FL & Rawlings, RD 2004, Composite Materials: Engineering and Science, CRC Press, London, UK.
[2]. Image from WWW, viewed 1 September, 2006,
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Bone/Bone.htm.