Critical discussion of the single-fibre pull-out test: does it measure adhesion?

With a comprehensive finite-element model the interface failure process of the single-fibre pull-out test, for the measurement of fibre/matrix adhesion, is investigated on the basis of a fracture-mechanics debonding criterion. Special emphasis is placed on the interface local mixed-mode load, which is shown to have an important influence on the debonding process and is taken into account by a fracture ellipsoid criterion. Additional features investigated are residual thermal stresses, specimen geometrical details (wetting meniscus, drop shape) and a simplistic model of fibre/matrix interfacial friction. For medium debonding lengths the energy release rate runs through a plateau range that can be approximated by a simple analytical approach and can be observed experimentally with a very stiff loading configuration. The mixed-mode state in the plateau range is uniform and dominated by mode 2, but its actual value is quite uncertain. From experimental experience the actual adhesion failure is closely connected with the interface local normal load, while local shear load induces submicroscopic friction and matrix inelasticity which strongly reduce the interface sensitivity, resulting in G_1c<G_2c. G_1c seems to be more significant for adhesion. The interpretation of the plateau range may provide the total critical energy release rate, G_c, for the debonding process, but from a region where mode II prevails. G_c will therefore be far from G_1c, reducing the significance of the tests results for characterization of adhesion.