AbstractThis paper is concerned with a numerical investigation of different size effects and their interactions in fibre reinforced PMMA. Focus is on the mechanical response – particularly on the damage and the fracture behaviour. The performed numerical studies are based on finite element simulations in which representative volume elements with different microstructures have been virtually mechanically tested and compared to each other. The underlying numerical model captures the most relevant mechanical mechanisms such as damage evolution, crack propagation and failure by a cohesive zone model. Previous studies have shown that the effective macroscopic fracture properties can be changed by varying the thickness of the fibres. In this paper, an additional size effect resulting from a variation of the fibres’ lengths and the interaction between both size effects is carefully analysed. By understanding such size effects, the observed failure mechanisms can be changed effectively and the properties of the considered composite can be improved significantly. For instance, it will be shown that a composite can be designed which shows a high strength as well as a high fracture energy.