In the course of evolution, nature has developed high-performance materials that now serve as inspiration and role models for future areas of application. These usually have a combination of very high stiffness and high toughness values. The special properties of biological materials are due to their microstructure and a combination of materials. The big difference to technical materials is their inner structure. While technical materials are mostly homogeneous, biological materials have a hierarchical structure, with a material inhomogeneity. This complexity causes different mechanisms that lead to a very high toughness with consistently high stiffness values. In order to mimic such properties, on the one hand, methods have to be found which make it possible to produce materials with such special structures and, on the other hand, to develop test methods to describe and quantify these properties. In the present work, polymer multi-layer composites were produced by additive manufacturing (“3d printing”) using the material extrusion, according to the FFF-method (fused filament fabrication) and tested using fracture mechanical methods. For this purpose, the influence of a thin inlayer on the material behaviour of an otherwise brittle base material was investigated to determine the effects of the material inhomogeneity. In order to determine an improvement in properties, the behaviour of the crack propagation and the resulting crack toughness of the compound was determined. Two different concepts of fracture mechanics were applied, first the linear elastic fracture mechanics, with the requirement of a brittle material behaviour, whereby due to the test arrangement and the fracture behaviour of the test specimens, there was no meaningful improvement in properties. However, it was found that the test direction had a major influence on the material properties with regard to the arrangement of the individual layers of the 3D-printed test specimens, with better mechanical properties of the transverse arrangement of the layers. In addition, tests based on elastic plastic fracture mechanics were carried out and the so-called J-Integral J was determined. The combination of materials with very similar properties did not result in an improvement in the mechanical properties of the overall composite. If a thin and very soft intermediate layer (thermoplastic copolymer) is combined with a very brittle material (glycolyzed PET), the overall material properties resulted in great improvements, with an approximately 3 times higher toughness of the composite compared to pure PET-G.