EFFECT OF LATERAL STRESSES ON FIBER DEBONDING/PULL-OUT

The performance of fiber reinforced composites is strongly dependent on the behavior of the fiber/matrix interface. The fiber pull-out test has been widely used to determine interfacial properties, from which fiber debonding/pull-out behavior in the composite can be deduced. Pull-out test results reported in the literature are almost always obtained under the condition of zero far-field lateral stresses. However, in many practical applications, crack bridging fibers can be under significant lateral compression (e.g. splitting cracks, shear cracks) or tension (cracks at the bottom of plates under biaxial bending). In this investigation, a novel experimental set-up is developed to study the effect of lateral stresses on fiber debonding and pull-out. Steel fiber reinforced mortar specimens were tested to provide an example. With lateral compression, both the initial interfacial friction and the effective interfacial shear strength are found to increase. A higher lateral compression, however, also results in a more rapid decrease in the interfacial friction during fiber pull-out. Therefore, while lateral compression can significantly increase the peak pull-out load, the energy absorption capacity (denoted by the area under the pull-out curve) does not increase to the same degree. Qualitatively, lateral tension imposes opposite effects to lateral compression. Quantitatively, a small lateral tension can result in changes in interfacial properties comparable in magnitude to those caused by a much higher lateral compression. Therefore, although the lateral tension that can act on a fiber is limited by the low tensile strength of the matrix, it may still impose a noticeable effect on the fiber debonding/pull-out behavior.