MIXED-MODE FIBER PULL-OUT EQUIPMENT - APPLICATIONS AND RESULTS

Crack propagation in fiber reinforced composites is governed by bridgi ng stresses provided by fibers along the crack. To predict the behavio r of fiber reinforced structural components under general loading cond itions (where mixed mode crack propagation is possible), the normal an d shear stresses acting on a crack need to be obtained as a function o f crack opening and shearing displacements. In this investigation, a m ixed mode fiber pull-out equipment is developed to measure both the no rmal and shearing bridging forces as a fiber is being pulled and shear ed. With slight modifications, the setup can also be employed to study the effect of lateral stresses on fiber pull-out. To demonstrate the applicability of the novel equipment, pull-out tests were carried out with steel fiber reinforced mortar specimens. Experimental results ind icate: 1) high sensitivity of fiber pull-out behavior to lateral stres ses; 2) dependence of both normal and shear bridging forces on crack o pening as well as shearing displacements; and 3) dependence of crack b ridging farces on crack opening/shearing displacement history. These e xperimental observations provide new insights and challenges to the mi cromechanical modeling of fiber composites.