2. Fiber Volume Fraction

The fibre volume fraction is an important factor in composites as it governs the properties of a composite. The usual fibre volume fraction ranges from 30% to 65%. As we know, the lower end depends upon the significance of property contribution of the fibres where as the upper depends upon the effective, defect-free packing.

In the following we will see some methods to determine the fibre volume fractions.

1st Method:  In this method the number of fibers is counted in several measured representative areas of a polished surface of the composite under magnification. Then measure the diameter or the cross sectional area of one or more fibers. Then calculate the average fiber volume fraction as the percentage of area that is fiber. The advantage of this method is that it is simple and provides information about type and uniformity of fiber spacing as well as indication of the void content. However, it should be noted that it is a crude method.

2nd Method: In this method the matrix material is digested or dissolved by putting a measured volume of composite in an acid bath. Then weigh the (dry) fibers remaining after digestion. Thus, knowing the density of the fibers, the volume of fibers and the fiber volume fraction can be determined. One should be careful to choose the liquid for digestion such that the fibers are not digested. Generally, hot nitric acid is used for carbon/epoxy. The ASTM standards used for digestion method are D3171-76 (1990) for polymeric composites and D3553-76 (1989) for metal matrix composites.

3rd Method: In this method one determines the density of the composites and then calculates the fiber volume fraction knowing the density of the fiber and the matrix. This method makes an assumption that the void content is negligible. However, which is not true for any composite. Hence, the results of this method may vary with the results of earlier two methods.