Abstract The failure of polymer composites is a very complex process, which mainly occurs as a result of interactions between different types of damage in the material, often at random locations and without any particular signs. To increase the reliability of composites and enable their wider use in safety-critical components, it is crucial to make their failure processes more controllable and predictable, which could be achieved by the local modification of the interfacial adhesion. The aim of our research is to develop and investigate a method for designing the failure process of polymer composites in terms of the location and the mode of failure. We produced interfacially engineered composites containing a weakened adhesion zone by adding polycaprolactone (PCL) thermoplastic interlayer material, which was applied directly on the surface of the reinforcing material by fused filament fabrication (FFF). Then, material tests were carried out, and the failure mode and position were evaluated as a function of the geometry of the interlayer material.