THE MICROMECHANICS OF FIBER PULL-OUT

Results of experimental and modeling studies on the micromechanics of fiber pull-out are reported. For the experiment an optical glass fiber coated with a layer of acrylate or gold-palladium alloy is embedded i n an epoxy matrix and then pulled out at various speeds. The fiber has a diameter of 125 or 200 mu m. While the fiber is pulled out, the coa ting is left embedded in the epoxy matrix, producing frictional slidin g between the contact surfaces of the glass fiber and the coating. As the thin and long fiber is pulled out, the trace of pull-force versus displacement shows several distinct stages corresponding to different pull-out processes. In the debonding process of the glass-acrylate int erface, stable crack growth was observed prior to unstable sliding. Th e stable crack growth behavior is believed mainly to be caused by the fact that the interface fracture toughness is strongly mode dependent and the mode mixity of the debonding crack varies towards tougher mode as the crack advances. After the interface is completely debonded, th e trace of pull-force versus displacement shows stick-slip oscillation s about a constant mean full force. Through the use of photoelasticity it is found that the unstable stick-slip sliding of the glass-acrylat e interface is caused by the propagation of a highly concentrated acti ve sliding zone, a dislocation, along the interface. When a thin gold- palladium coating is introduced at the interface to produce debonding and sliding along the glass-gold-palladium interface, the initial stab le crack growth is not observed and the interface dislocation emission is suppressed. The interface fracture toughness and the frictional sl iding resistance are found to depend on the thickness of the coating; the interface fracture toughness is higher for thicker gold-palladium coatings, while the frictional sliding resistance is higher for thinne r gold-palladium coatings. The sliding at the glass-gold-palladium int erface also shows a stick-slip behavior under certain conditions. Howe ver, unlike the stick-slip process accompanying dislocation emission o bserved in the sliding process of glass-acrylate interface, the stick- slip is generated, while the entire contact interface slides simultane ously, by rate-dependent softening and hardening of the frictional int erface. It is demonstrated that significant features of this type of s tick-slip process can be predicted using a phenomenological friction l aw with an internal state variable. Copyright (C) 1996 Elsevier Scienc e Ltd