Question

Explain why the stiffness of a discontinuous fiber composite is less than that of a continuous fiber composite with the same constituents and same proportion of reinforcement.

Answer 1

A fiber has a length that is much greater than its diameter. The length-to-diameter (l/d) ratio is known as the aspect ratio and can vary greatly. Continuous fibers have long aspect ratios, while discontinuous fibers have short aspect ratios. Continuous-fiber composites normally have a preferred orientation, while discontinuous fibers generally have a random orientation. Examples of continuous reinforcements include unidirectional, woven cloth, and helical winding while examples of discontinuous reinforcements are chopped fibers and random mat . Continuous-fiber composites are
often made into laminates by stacking single sheets of continuous fibers in different orientations to obtain the desired strength and stiffness properties with fiber volumes as high as 60 to 70 percent. Fibers produce high-strength composites because of their small diameter; they contain far fewer defects. (normally surface defects) compared to the material produced in bulk. As a general rule, the smaller the diameter of the fiber
the higher its strength, but often the cost increases as the diameter becomes smaller. In addition, smaller-diameter high-strength. fibers have greater flexibility and are more amenable to fabrication processes such as weaving or forming over radii. Typical fibers include glass, aramid, and carbon,
which may be continuous or discontinuous. The type and quantity of the reinforcement determine the final properties. highest strength and modulus are obtained with. continuous-fiber composites. There is
a practical limit of about 70 volume percent reinforcement that can be added to form a composite. At higher percentages, there is too little matrix to support the fibers effectivelyThetheoretical strength of discontinuous-fiber composites can approach that of continuous-fiber composites if their aspect ratios are great enough and they are aligned, but it is difficult in practice to maintain good alignment with discontinuous fibers.
Discontinuous-fiber composites are normally somewhat random in alignment, which dramatically reduces their strength and modulus. However, discontinuous-fiber composites are generally much less costly than continuous-fiber composites. Therefore, continuous-fiber composites are used where higher strength and stiffness are required (but at a higher cost), and discontinuous-fiber composites are used when
cost is the main driver and strength and stiffness are less important.