# Recent Advances in Additive Manufacturing of Fibre-Reinforced Materials: A Comprehensive Review
## 3 Evaluation
### Quasi-Static Mechanical Testing
#### 3.1.3 Compression and Energy Absorption  
Most of the literature on composite AM investigates the tensile and flexural properties and overlooks the compression and energy absorption properties [78]. Knowledge of these properties can provide advanced solutions to industries like the automotive industry. Fibre composites manufactured by AM often exhibit poor compressive strength due to anisotropy in the structures. CF, for instance, is brittle and can fail before the buckling of the composite. Also, the bubbles of air that can be trapped between the layers during the printing and the poor interlayer adhesion may cause layer separation and make the part more prone to buckling. Thus, a presentation of some of the studies on compression and energy absorption must be conducted for a better understanding of the failure mechanism under compression and impact loads. Figure 4: (**a**) Three-point bending apparatus; (**b**) four-point bending apparatus [70]. Quasi-static compressive tests are carried out to study and analyse the response and crushing behaviour of the specimens. The specimens are subjected to uniaxial compressive loads using a universal testing machine, where a steady displacement rate of 1-5 mm/min per study is applied on the crosshead of the testing machine. During the test, the displacement of the specimen and the applied force are measured to establish the strength and modulus from the stress-strain curves. [PERSON] et al. [79] examined the impact that the addition of short carbon fibres to pure nylon filaments would have; for more context, the compression properties of ONYX compared to pure nylon specimens were evaluated and presented. In the specific study, an increase in compressive strength of 130% was observed when the printing path was parallel to the loading direction, as shown in Figure 7, where the coding of each curve is F/U for the printing method, number, eg 1.3 the compressive speed of the test and a-d the number of the printed specimen. The effect of carbon fibre reinforcement became more apparent when examining the compressive Poisson's ratio. The  
Figure 5: Flexural strength (MPa) for various material combinations [58; 60; 63; 64; 71; 72; 73]. For the representation of PLA dark grey was chosen, for PETG light blue, for PA light grey and for ABS dark blue. Figure 6: Flexural modulus (GPa) for various material combinations [58; 60; 63; 64; 71; 72; 73]. For the representation of PLA dark grey was chosen, for PETG light blue, for PA light grey and for ABS dark blue. nylon specimens displayed a relatively stable Poisson's ratio, while the ONYX specimens with fibres oriented laterally, showed a reduction in Poisson's ratio by up to 43% compared to the parallel-laid fibres that resisted the development of lateral strains. Additionally, the compressive modulus of ONYX demonstrated a significant positive correlation with strain rate, increasing by up to 50%. In the study by [PERSON] et al. [80], ABS specimens reinforced with CF were tested (ASTM D695) with different volumetric ratios of CF varying from 0-30%. It was observed that while the percentage of fibre in the specimen was increasing, a reduction of 30% in the compressive strength occurred. [PERSON] et al. [81] also observed that for PETG with CF and aamid fibre composites, there was a decrease of 9.9% and 68.7%, respectively, compared to pure PETG for the optimised printing parameters [49]. A decrease was also observed in the compressive modulus with the increase of \(V_{r}\), which is correlated to the plasticity of the material under compressive loads. Various studies reached the same outcome [82; 83; 84; 85; 86; 87], where different specimens reinforced with CF were tested in correspondence to the fibre-laying direction. The results showed better behaviour in compressive strength and modulus, with an increase in their respective values by increasing \(V_{r}\) due to better interlaminar adhesion, and better behaviour when the fibres were parallel to the load direction.