Damage Due to Anchorage Failure in Deformed Fibres
Nieuwoudt and Boshoff  recently showed that fracture may occur in the matrix surrounding the fibre as it is pulled-out, especially in the case of deformed steel
Fig. 4 CT scan images of a single fibre at different stages of pull-out; the lower images are magnifications of the zones indicated in the upper images (with permission of Dr. Boshoff from )
fibres (Fig. 4), both hooked-ended, as shown here, or undulated. When such damage progresses with time and load level, due to microcracking and local crushing produced by sustained loading, the pull-out is further facilitated. In the case of straight fibres and lower modulus fibres, like polymeric fibres, such damage is not expected to be significant.
Filament or Individual Fibre Creep or Relaxation
Compared to steel fibres that hardly experience any creep at ambient temperature, filaments of polymeric fibres could experience creep, especially at higher temperatures, due to their viscoelastic nature. Nevertheless, cross-linked and more crystalline polymers are less susceptible to creep. The general trends in creep and relaxation response of polymers are shown in Fig. 5a, b, respectively . When the fibre undergoes creep strain, the crack width will tend to increase. Moreover, in PFRC, the relaxation of the stress in the fibre will decrease the bridging effect, leading to crack widening and propagation. Obviously, such effects will be more prominent as the temperature increases, especially above the glass transition temperature of the polymer.
At this point, it must be emphasized that there are significant differences among polymeric fibres, as a function of the polymer family, shape, surface texture and the production process, and, therefore, generalizations should be made with caution.
Fig. 5 a Creep of a typical polymer under constant stresses; b Stress relaxation of a typical polymer under different strain (adapted from )