Effect of Interfacial Roughness on the Frictional Stress Measured Using Pushout Tests

A fiber which is partially pushed out of a surrounding matrix and subsequently pushed in the opposite direction exhibits a substantial decrease in sliding friction as it passes through its original position (its “origin”). This is manifest by a decrease in the load required to push the fiber. It is suggested that interfacial roughness causes this phenomenon and that the decrease in load (friction) is associated with the fiber seating back into its original position. The period of the drop has been correlated with the spatial extent of the interfacial surface roughness, and the magnitude of the drop (referred to hereafter as the seating drop) has been correlated with the amplitude of the interfacial roughness. Observation of the seating drop allows separation of the friction associated with interfacial irregularities from that resulting primarily from residual stresses at the interface. Implications for composite design and use are discussed. The effect of abrasion at the sliding interface is also addressed.