Pull-out of short rods and fibres

Abstract

An experimental study has been carried out of the mechanics of pull-out of short nylon rods embedded in a rubber block, considered as a model of fibres embedded in an elastic resin. Two stages of pull-out were observed. First, rubber became detached from the base of the rod creating a large internal cavity. This process was apparently initiated by internal rupture of rubber under the rod, and occurred when the local triaxial tension reached a value similar to the tensile (Young's) modulus of the rubber. After cavitation, a cylindrical debond propagated up the rod, starting at the embedded end and ending in complete pull-out. Pull-out forces were found to be consistent with a simple fracture mechanics treatment based on the elastic compliance of partially debonded specimens, both measured and calculated by finite element analysis. The fracture energy for debonding is deduced to be about 300 J m⁻². The effects of varying rod diameter and depth of embedment are shown to be in satisfactory agreement with theory.