# Modelling of Failure Behaviour of 3D-Printed Composite Parts
## 3 Results and Discussion
### Composite Material (ABS + sCF)  
In this section, we considered 3D-printed parts made of composite material for characterizing the nonlinear behaviour using computational models. The computational models are applied for four different test coupons as described in the previous section. Experimental work on 3D-printed test coupons revealed that the nonlinearity during the deformation of the printed composites is attributed to only the ABS polymer. Therefore, the nonlinear behaviour of ABS materials alone is accounted for in the modelling of the 3D-printed composites. In contrast, the sCF reinforcements material's behaviour remains \begin{table}
\begin{tabular}{l l l l l} \hline \hline  & **Experimental [6]** & **CLT and Tsai–Hill [6]** & \multicolumn{3}{c}{**Computational Modelling**} \\ \cline{3-5}  & & & **CM-1** & **CM-2** \\ \hline
**Cross-ply** & & & & \\ \(E_{x}\), in MPa & 1783.9 \(\pm\) 2.7 & 1673.0 & 1865.7 & 1751.1 \\ \(U_{t}\), in MPa & 29.7 \(\pm\) 0.7 & 25.2 & 34.5 & 33.3 \\ \(\epsilon_{t}\) & 0.0367 \(\pm\) 0.0135 & 0.0135 & 0.09 & 0.07 \\
**Angle-ply** & & & & \\ \(E_{x}\), in MPa & 1728.7 \(\pm\) 16.4 & 1645.6 & 1810.2 & 1720.2 \\ \(U_{t}\), in MPa & 28.0 \(\pm\) 1.3 & 25.5 & 35.4 & 33.8 \\ \(\epsilon_{t}\) & 0.0435 \(\pm\) 0.0049 & 0.0143 & 0.09 & 0.069 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of computational results with the results of Tsai–Hill failure criterion and experimental work for 3D-printed parts made of ABS material and subjected to uniaxial tensile loading. Figure 6: Stress–strain relation for 3D-printed parts made of ABS material for: (**a**) cross-ply and (**b**) angle-ply printing strategies. linear while the printed composites are subjected to deformation. Finite element model preparation of RVE for 3D-printed composites was followed as explained in this work [37]. The results of the present computational models of CM-1 and CM-2 are compared with the stress-strain curves of experimental work [16]. The comparisons of results are presented in Figure 7a for cross-ply 3D-printed parts and Figure 7b for angle-ply 3D-printed parts. Comparing the results of computational models with the results of experimental work reveals that the computational model based on CM-1 predicted higher values. However, the computational model based on CM-2 predicted accurate damage behaviour, and this is because this model employed actual mechanical testing data as described earlier. The difference between experimental and computational models of CM-1 is significant, mainly due to the process of inherited defects such as voids and misalignment of reinforcements' orientation in the printed parts is not accounted for in the FE modelling of RVEs. It is important to note from the stress-strain curves of 3D-printed3D-printed composites that during deformation, the printed parts initially follow linear behaviour and then exhibited hardening behaviour, and then the parts fail suddenly. On the contrary, 3D-printed ABS material has seen softening behaviour before the fracture. The hardening material behaviour in 3D-printed composites is mainly because of the presence of sCF reinforcements. Further, nonlinearity behaviour starts when the strength of the bonding between the ABS matrix and the sCF starts degrading during the deformation. After the hardening behaviour, the parts undergo sudden failure. This is due to the formation of larger discontinuity in the material and is caused by the coalescence of enclosed voids in the printed composites. This failure behaviour in the printed composites can be replicated in the computational models by considering mechanical testing data of the materials. The computational model, CM-2, that considered the mechanical testing results of the unidirectionally 3D-printed parts predicted the actual nonlinear behaviour of the parts. Further, the present computational result is also compared with Tsai-Hill py-based failure theory and existing experimental work.