# Recent Advances in Additive Manufacturing of Fibre-Reinforced Materials: A Comprehensive Review
## 3 Evaluation
### Quasi-Static Mechanical Testing
#### 3.1.1 Tensile Testing  
Tensile testing involves clamping a standardized specimen on a universal testing machine and applying uniaxial forces gradually until the specimen fractures Figure 3[45; 46; 47; 48; 49]. During the testing, tensile strength, tensile modulus, and elongation are the most commonly measured values. In Figure 3a,b, pictures of tensile specimens and the universal testing machine apparatus are provided. Tensile mechanical properties depend on the layer thickness, printing speed, temperature, building direction/orientation, and the raster angle, among others [50; 51; 52; 53]. In some of the reviewed literature, sensitivity analysis for different printing factors occurs, but these will not be presented in this review. The reviewed literature mainly used ASTM D-638 [54] and ISO 527 [55] for polymers and reinforced polymers, ASTM D3039 [56] for composite polymers, and EN 2561 [57] for carbon fibre-reinforced plastics. All the reviewed experiments were conducted at a low strain rate of 1-2 mm/min. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Techniques** & **Matrix Material** & **Reinforcement Material** \\ \hline Prepreg Extrusion [1; 6; 30; 40] & Thermoplastics: ABS, PA, PC, PETG, PLA, ONYX & Glass, Carbon, Aramid \\ \hline Prepreg Co-Extrusion [1; 6; 33; 34; 40] & Thermoplastics: ABS, PA, PC, PETG, PLA, TPU & Glass, Carbon, Basalt \\ \hline In situ Impregnation [1; 6; 35; 36; 37; 41] & Thermoplastics: ABS, PA, PC, PETG, PLA, ONYX & Glass, Carbon, Aramid \\  & Thermosetting: Epoxy resins & Natural Fibres \\ \hline In-Line Impregnation [1; 6; 11] & Thermoplastics: ABS, PA, PC, PETG, PLA, ONYX & Glass, Carbon, Basalt, Aramid \\  & Thermosetting: Epoxy resins & Natural Fibres \\  & Vinyl esters & Natural Fibres \\ \hline In situ Consolidation [1; 6; 38; 39] & Thermoplastics: ABS, PA, PC, PETG, PLA, PAEK & Glass, Carbon \\  & family (PEK, PEEK, PEKK, etc.) & Glass, Carbon \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Techniques and used materials. Polylactic acid (PLA) is the most commonly used material for 3D-printed parts, with a tensile strength of 45 MPa, according to [PERSON] et al. [58], and a 2.69 GPa tensile modulus. With the addition of carbon fibre (CF) and the use of prepreg extrusion, there is an increase in the ultimate strength to 61.4 MPa and 8.28 GPa for the tensile modulus. [PERSON] et al. [59] conducted experiments with PLA-CF specimens but used AM technology in situ impregnation and achieved an increase to the ultimate strength by 400% and a 150% increase in modulus, while [PERSON] et al. [60], with the same technology, achieved a 10-times increase in ultimate strength and seven-times increase in tensile modulus. The findings from [PERSON] et al. [61] should also be mentioned, where [PERSON]'s modulus improved to almost 850% for a PLA/CF composite compared to pure PLA. The same increase in strength was observed in specimens that used acrylonitrile butadiene styrene (ABS) as the matrix and CF for reinforcement, according to the review conducted by [PERSON] et al. [62], from a value of 24 MPa of ultimate tensile strength to a 70 MPa increase, respectively. The printing method used was prepreg extrusion, while [PERSON] et al. [60] combined ABS/basalt and prepreg co-extrusion, achieving a 170% increase in tensile strength. It is unclear if this happens due to the materials or method used. To make a better comparison among the same methods but using different material combinations, [PERSON] et al. [60] will be used, where prepreg co-extrusion was used as the method, polyamide (PA) and basalt were used for the materials, and the observed strength was 28.48 MPa for plain PA, 144 MPa for PA/basalt, and in addition the tensile modulus was 1.07 GPa and 5.34 GPa, respectively. [PERSON] et al. [63], with the same method but for the combined materials PA/CF, showed an increase in both tensile strength and modulus of 18% and 145%. It is plausible that the difference between ABS/CF and ABS/basalt, which gave the basalt core the edge, occurred due to the printing method.