Application of debond length measurements to examine the mechanics of fiber pushout

The interface failure sequence was observed during fiber pushout tests on several model composites. Composites with varying fiber-to-matrix moduli ratio (E_f/E_m), sample lengths, interface bond strength, and processing residual stresses were tested to determine which composites would debond from the top and which from the bottom. The present pushout experiments combined with previous work in the literature indicate that only composites with an E_f/E_m ratio greater than four tend to debond from the bottom during pushout testing, and debonding tends to initiate at the top of the interface when the E_f/E_m ratio is less than three. The debond length as a function of force and displacement was also measured in a polariscope for two of the model composites - steel/epoxy and polyester/epoxy. The pushout data from a polyester/epoxy system that debonded from the top was fit to a shear lag solution of the fiber pushout problem to obtain the mode II toughness (G_IIc) of the fiber-matrix interface. The resulting interface toughness was then used to check the predicted debond length as a function of pushout force. The debond length calculated from the shear lag model was less than the measured debond length by a nearly constant 1.5 fiber radii.