# Recent Advances in Additive Manufacturing of Fibre-Reinforced Materials: A Comprehensive Review
## 3 Evaluation
### Dynamic Testing  
The implementation of reinforcing fibres in AM can give a significant advantage in dynamic viscoelastic behaviour. Viscoelastic creep analysis tests were conducted in several studies [92; 93; 94; 95; 96], but mostly for the technology of prepreg extrusion. According to these studies, the addition of the fibres increases the creep resistance significantly from the specimens without reinforcement. In more depth, as the percentage of fibre infill increases, the specimens display more linear creep behaviour, and the creep strain is lower. This linear  
Figure 7: Building orientation and ONYX compression diagrams for the respective orientation [79]: (**a**) Building orientation U is along the z-axis, and F is along the x-axis; (**b**) stress–strain diagrams for different building orientations and speeds of ONYX specimens; (**c**) stress–strain diagrams for different building orientations and speeds of pure nylon specimens. creep behaviour is further illustrated in Figure 8, where in Figure 8a the strain increases for rising temperatures, while in Figure 8b the strain decreases as the infill percentage is reduced. The different raster angles also implement different creep resistance. By way of illustration, if the raster angle is at \(0^{\circ}\), then the creep exhibits a more linear behaviour rather than if the raster angle is at \(45^{\circ}\). According to [PERSON] et al. [97], temperature is another variable that affects strain, where the results exhibit an increase of strain with temperature increase. Except for creep analysis, dynamic mechanical analysis (DMA) is also a technique used to measure the mechanical properties of polymers, and other materials, as a function of time, temperature, and frequency. This method provides information about the materials' viscoelastic behaviour, which includes elastic (storage) and viscous (loss) characteristics. The main focus of DMA is on the viscoelastic properties, where polymers exhibit elastic and viscous behaviour; on the storage modulus (E\({}^{\prime}\)), which represents the stored energy in the material, reflecting its elastic response; on the loss modulus (E\({}^{\prime\prime}\)), which represents the energy dissipated as heat, reflecting the viscous response of the material; and finally on the tan delta (tan \(\delta\)), which is the ratio of the loss modulus to the storage modulus and provides an insight in the damping properties of a material. According to the given DMA definition, the results from studies are presented. [PERSON] et al. [98] evaluated the storage modulus and loss factor (tan \(\delta\)) for different materials and for temperatures from \(-100\)\({}^{\circ}\)C to \(150\)\({}^{\circ}\)C. Nylon and ONYX face three stages of different peaks at \(25\)\({}^{\circ}\)C, with a significant decrease in modulus between \(-60\)\({}^{\circ}\)C and \(-70\)\({}^{\circ}\)C and finally at \(75\)\({}^{\circ}\)C. ONYX exhibits a slightly higher storage modulus than Nylon, probably due to the presence of short carbon fibres, while the specimens with continuous fibres exhibit a higher storage modulus due to the long fibres, which translates as a stiffer material. In Alarifi [99], nylon embedded with glass and carbon fibres was evaluated for different temperatures. For the storage modulus, the nylon/glass fibre (GF) composite showed higher modulus values at the high temperatures than nylon/CF, which indicates better mechanical properties and better interface bonding with the nylon matrix. The glass transition temperature, derived from the storage modulus curves, is the limit temperature above which the mechanical properties, like stiffness, start deteriorating and behaving like rubber, confirming the stiffness differences, with the nylon/GF composite showing greater restrictions on polymer chain motions, resulting in enhanced stiffness. Both composites display a decrease in storage modulus at higher temperatures, illustrating the effect of fibre inclusion on the phase transition of nylon.