can be found in the aramid family, where production technology has been developed to alter the balance between the elongation and modulus of fibers to meet the demands of applications that are driven by fatigue (e.g., reinforcement of rubber) as well as polymer matrix composite applications, where stiffness is a controlling factor. These two extremes cover the range of elongation and modulus described in Table 1.2.
Current estimates of theoretical strength and stiffness for p-aramids and polyethylene are shown in Figure 1.4. The elastic modulus realized in commercially available fibers approaches 80 percent of the theoretical values for these fibers. Thus, a dramatic increase in the stiffness of these organic fibers is not expected. For higher stiffness, one would have to look at the fibers described in Table 1.3. Estimating the theoretical strength of a material is more difficult and less reliable than estimating the theoretical modulus. However, there is general agreement that the strength values attained for organic fibers are further from theoretical values than are the modulus values. For example, demonstrated values of strength are estimated at 30 to 50 percent of theoretical expectations. Thus, it is reasonable to expect incremental improvements in strengths of existing commercial fibers over the next several years.