CRITICAL DISCUSSION OF THE SINGLE-FIBER PULL-OUT TEST - DOES IT MEASURE ADHESION

With a comprehensive finite-element model the interface failure proces s of the single-fibre pull-out test, for the measurement of fibre/matr ix adhesion, is investigated on the basis of a fracture-mechanics debo nding criterion. Special emphasis is placed on the interface local mix ed-mode lend, which is shown to have an important influence On the deb onding process and is taken into account by a fracture ellipsoid crite rion. Additional features investigated are residual thermal stresses, specimen geometrical details (wetting meniscus, drop shape) and a simp listic model of fibre/matrix interfacial friction. For medium debondin g lengths the energy release rate runs through a plateau range that ca n be approximated by a simple analytical approach and can be observed experimentally with a very stiff loading configuration. The mixed-mode stare in the plateau range is uniform and dominated by mode 2, but it s actual value is quite uncertain. From experimental experience the ac tual adhesion failure is closely connected with the interface local no rmal load, while local shear load indices submicroscopic friction and matrix inelasticity which strongly reduce the interface sensitivity, r esulting in G(1c) < G(2c). G(1c) seems to be move significant for adhe sion. The interpretation of the plateau range may provide the total cr itical energy release I ate, 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. (C) 1998 Elsevier Science Ltd. All rights reserved.