Failure phenomena in fibre-reinforced composites. Part 6: a finite element study of stress concentrations in unidirectional carbon fibre-reinforced epoxy composites

A three-dimensional (3-D) finite element (FE) analysis of the stress situation around a fibre break in a unidirectional carbon fibre-reinforced epoxy composite has been performed. Two cases were considered: (i) good fibre/matrix adhesion and (ii) fibre/matrix debonding. In the case of good adhesion, three matrix systems differing in yield stress were investigated. For all three matrix systems stress concentration factors (SCFs) of approximately 1.07 are found for a fibre volume fraction ranging from 25 to 70%. According to a fibre interaction criterion reported earlier (Composites Science and Technology 1997;57:899), this is large enough to cause coordinated fibre failure. In the case of poor fibre/matrix adhesion, it was found that in addition to the stress concentration at the plane of fracture, a second stress concentration is located at the end of the debonded zone. For a 60 fibre vol.% composite with a debond length of ten fibre diameters, these SCFs are approximately 1.03 and 1.05, respectively. Furthermore, in order to experimentally obtain an indication of the SCFs in a full composite, Raman experiments were performed on 2.5-D carbon/aramid/epoxy hybrid model composites in which a single carbon fibre was placed on top of a bundle of ten aramid fibres. The results indicated that in such a hybrid model composite the SCF is slightly larger than in a full (3-D) unidirectional composite. Furthermore, these experiments confirmed the existence of the second SCF in the case of debonding.