Influence of fiber orientation on the behavior of macro synthetic fiber in short- and long-term pullout tests

Synthetic fibers deforming over time can be a concern in structural design, particularly in serviceability limit states. Short-term pullout tests are commonly used to predict fiber–matrix interactions, but even in this case, an individualized evaluation of the pullout behavior of single fibers oriented parallel to the load direction may not be sufficient to predict the efficiency of the composite. In the present work, short- and long-term pullout tests were performed with fibers oriented at angles of 15°, 30°, and 45° to the direction of the load to investigate the influence of macro synthetic fibers orientation on fiber–matrix interactions. In short-term tests, optical microscopy images were obtained on the pulled-out fibers to correlate the surface degradation of the fibers with the stress versus strain curves. In quasi-static pullout (short-term), small reductions in pullout strength were observed for all fibers and angles, in addition to an intensive degradation of their surfaces owing to the significant snubbing effect of this type of fiber. In contrast, for the long-term tests, a creep reduction was observed with increasing fiber inclination angle caused by the creep reduction of the fiber due to non-axial loading and additional force components produced by the deviation of the axial force. The parameters of Burgers rheological model were written as a function of the fiber orientation angle, with excellent adjustment to the experimental data.