Variance of adhesive strength and fracture mechanism of fibre/matrix joints

A study was made of the fracture mechanism in fibre/polymer joints tested by the pull-out technique. The shear adhesive strength τ of joints between epoxy matrix EDT-10 (based on DGEBA) and high-strength steel wire of diameter 150 μm was determined at room temperature and the variance D of the τ values was analyzed. The number of specimens tested reached 1269. The areas of joints S were varied from 0.04 to 1.2 mm2. The large number of specimens and the wide range of areas S gave us the possibility to construct not only the τ–S curve but also the D–S one. Whereas the τ–S function decreases non-linearly and significantly as the joint area S increases, the values of variance D do not markedly depend on the joint sizes when those sizes are not so small (S>0.2 mm2). This independence and other obtained data support the conjecture that the fracture of fibre/polymer joints starts, near that end of joint where external force is applied.