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A new method for the experimental determination of the fracture toughness in the fiber/matrix interface by means of the single fiber pull out test is presented. To achieve this aim, two problems had to be overcome: the measurement of the compliance of a partial debonded fiber, which necessitates a stable crack propagation, and the determination of the corresponding crack length. Stable crack propagation along the interface of a single fiber is achieved using an advanced test equipment exhibiting an extremely high stiffness. This is obtained by using a piezo translator and a piezo force cell in combination with a very short free fiber length. The experimental data and a theoretical analysis of the pull out process under these conditions reveal that the commonly used compliant pull out equipment is hiding important details of the force displacement trace and, thus, leading to a wrong interpretation and wrong results. The most important result is that in the case of a brittle interfacial fracture the maximal force cannot be used for the determination of an interfacial strength as it is common practise till now. The crack length is measured with the aid of a polarisation microscope. A combination of the advanced pull out experiment with a simultaneous monitoring of the photoelastic patterns of the embedded fiber enables the determination of G -values as a function of the crack length. For the calculation of G, the compliance data are obtained by the force displacement trace and the crack length data by the correlated photo-elastic patterns. The discrimination between the failure modes is obtained by finite element analysis. The energy release rates for different fiber/matrix combinations are presented. In addition the photoelastic patterns give evidence, that in the case of a glass fiber the crack starts at the matrix surface and in the case of a carbon fiber with an embedded length < 150 jAm the crack starts at the fiber tip.

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