Pullout of inclined flexible fiber in cementitious composite

The performance of fiber-reinforced composites is strongly dependent on the debonding/pullout behavior of the fibers. In cementitious composites, fibers are usually distributed in a quasi-random manner. It is therefore important to study the pullout of fibers that are inclined to a crack. For flexible fibers, a simple model of a String passing over a frictional pulley has been developed to describe inclined fiber pullout. The existing model is based on experimental results obtained from specimens with initially bent fibers. To evaluate the validity of the experimental results and the model, pullout tests are carried out with polypropylene fibers using (1) specimens with initially straight fibers; and (2) specimens with initially bent fibers. Significant differences are found between the two sets of results. Since the pullout behavior of initially straight fibers is more representative of that in real composites, a new model for flexible fiber pullout is developed. By considering (1) the increase of both interfacial friction and snubbing factor with sliding; and (2) spalling of matrix at fiber exit point in the new model, excellent fitting of the experimental results can be obtained.