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1. Introduction
Additive manufacturing is the general term for the wide range of manufacturing techniques where successive addition of layers forms the part. As a result of advances in enabling technologies and with the expiration of some essential patents on the subject, additive manufacturing has started to generate a paradigm shift in a wide range of manufacturing industries in the past two decades. In the meantime, three-dimensional (3D) printing has started to become a popular umbrella term for describing additive manufacturing techniques. Currently, there exist more than a dozen different technologies that allow 3D printing of almost any material including metals, polymers, ceramics and their composites (Ngo et al., 2018). Among these, one of the most influential and popular techniques is fused deposition modeling (FDM), especially for printing polymers. Figure 1 shows a schematic description of the method. FDM involves the extrusion of a polymer filament through a heated nozzle. The printer head that contains the nozzle moves in the XY-plane to deposit the current layer of the printed part, and the sample stage moves down upon the completion of each layer.
Earlier industrial use of FDM had been mostly limited to design and prototyping activities. Recent advances in instrumentation and filament materials improved reliability, simplified the use and reduced the cost of printing. As a result, FDM-produced parts are now also used as structural components and load-bearing elements. While the mechanical behavior of printed parts is not the most critical aspect of design and prototyping applications, it is one of the major concerns for structural use. The rapid expansion of the use of printed parts in structural applications has triggered a vast amount of research efforts in the past decade with two objectives in mind: understanding the fundamentals of the mechanical behavior of FDM-produced parts. Optimization of the process parameters and the material properties for the best mechanical performance.
Figure 2 shows the rapid expansion of the field through the number of relevant publications since 2000. In the period between 2000 and 2013, there were about 10 publications each year and in 2018, this number exceeded 200. The data implies that many new researchers and research groups are entering the field every year. Review articles are especially useful during these times, to give...