Among the several test methods to characterize the fibre-matrix interface in fibrous composite materials, the single fibre fragmentation test is one of the most simple in terms of experimental setup, and at the same time the amount of data that can be extracted from one single test is significant. In this work, the single fibre fragmentation test method was implemented to assess the interface shear strength obtained for a single steel filament embedded in an unsaturated polyester resin. Furthermore the single fibre fragmentation test result was modelled and the results substantiated using finite element analysis (FEA) considering the material behaviour as being linear elastic. The stress distributions obtained from the FEA and the one-dimensional Cox model were analyzed and compared. A comparison of obtained average interface shear stresses at filament failure shows, that the different models have certain and different ranges of applicability, which in some cases can lead to misinterpretations and erroneous conclusions regarding the filament/resin interface properties.