THE ENERGY-RELEASE RATE OF THE FIBER POLYMER MATRIX INTERFACE - MEASUREMENT AND THEORETICAL-ANALYSIS/

A new method for the experimental determination of the fracture toughn ess in the fiber/matrix interface by means of the single fiber pull ou t test is presented. To achieve this aim, two problems had to be overc ome: the measurement of the compliance of a partial debonded fiber, wh ich necessitates a stable crack propagation, and the determination of the corresponding crack length. Stable crack propagation along the int erface of a single fiber is achieved using an advanced test equipment exhibiting an extremely high stiffness. This is obtained by using a pi ezo translator and a piezo force cell in combination with a very short free fiber length. The experimental data and a theoretical analysis o f 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 b rittle interfacial fracture the maximal force cannot be used for the d etermination of an interfacial strength as it is common practise till now. The crack length is measured with the aid of a polarisation micro scope. A combination of the advanced pull out experiment with a simult aneous monitoring of the photoelastic patterns of the embedded fiber e nables the determination of G(c)-values as a function of the crack len gth. For the calculation of G(c) the compliance data are obtained by t he force displacement trace and the crack length data by the correlate d 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 pho toelastic patterns give evidence, that in the case of a glass fiber th e crack starts at the matrix surface and in the case of a carbon fiber with an embedded length < 150 mu m the crack starts at the fiber tip.