MECHANICS OF THE FIBER PUSHOUT TEST

Model equations governing the debonding and the pushout phases of the fiber pushout test are presented and are then evaluated for accuracy by comparing them with detailed numerical analyses of some specific examples. It is assumed that a residual compressive stress acts across the fiber/matrix interface and residual axial stress in the fiber is taken into account. The interface is characterized by a mode 2 debond toughness GAMMA and is assumed to develop a frictional stress upon sliding according to tau = tau0 - musigma(r), corresponding to a constant stress contribution and a Coulomb term. The model applies either to pushout of a single fiber embedded in a homogeneous matrix or to a fiber selected for pushout from a specimen sliced from a fiber reinforced composite. The effect of redistribution of residual stress due to slicing the composite in preparation of the specimen is addressed in numerical examples. The detailed numerical work establishes that the debond crack advancing down the fiber becomes unstable and breaks through to the bottom of the specimen when the debond tip reaches a distance about one and one half fiber radii from the bottom. The model equations provide a reasonably accurate description of the dependence of the pushout test on its many parameters.