## 3 Results and Discussion  
### Compression and Fracture Tests  
Table 1 shows the main compressive properties for both polymeric materials, whereas Figure 6 shows examples of the obtained compressive engineering stress-strain curves. The results show a clear non-linear behavior in the compression curves of the two materials, although PMMA is significantly more non-linear (ductile) than GPPS.  
Table 2 shows the LCCs of the different tested fracture specimens, which can be interpreted as the peak points of the corresponding load-displacement curves. In the table, the notch tip radius, the three repetitions, and the average critical load are indicated by \(\rho\), P\({}_{1}\) (i = 1, 2, 3), and P\({}_{\text{avg}}\), respectively. Figure 7 shows two of the keyhole notched specimens after testing. It can be observed that at the neighborhood of the blunt tip of the keyhole notch under compressive loading an evident damage zone is created, from which a crack initiates and propagates abruptly. This phenomenon is in accordance with the findings reported in [32]. Herein, it is worth mentioning that such an event would not happen in the tensile mode I loading (also known as the notch opening mode), under which only a single crack initiates from the notch tip and grows promptly.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Material** & **E (GPa)** & \
\begin{tabular}{c} **Standard Deviation** \\ **(GPa)** \\ \end{tabular} & \(\sigma_{\pi}\) **(MPa)** &
\begin{tabular}{c} **Standard Deviation** \\ **(MPa)** \\ \end{tabular} \\ \hline PMMA & 1.70 & 0.09 & 86 & 2.94 \\ GPPS & 2.14 & 0.12 & 68 & 2.30 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main mechanical properties of the two polymers tested under compression.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **\(\rho\) (mm)** & **P\({}_{1}\) (N)** & **P\({}_{2}\) (N)** & **P\({}_{3}\) (N)** & **P\({}_{\text{avg}}\) (N)** \\ \hline \multirow{4}{*}{PMMA} & 0.5 & 16,636 & 16,553 & 17,070 & 16,753 \\  & 1 & 17,093 & 17,288 & 17,537 & 17,306 \\  & 2 & 19,032 & 19,203 & 19,329 & 19,188 \\  & 4 & 20,287 & 20,328 & 19,973 & 20,196 \\ \hline \multirow{4}{*}{GPPS} & 0.5 & 7360 & 6933 & 7244 & 7179 \\  & 1 & 7638 & 7668 & 7272 & 7526 \\ \cline{1-1}  & 2 & 8941 & 8646 & 8448 & 8678 \\ \cline{1-1}  & 4 & 9430 & 9186 & 9657 & 9424 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Experimental LCCs for the keyhole notched specimens.  
Figure 5: Detail of the mesh pattern around the keyhole notch.  
The results show a significantly larger LCC in the case of PMMA, which is more than double the LCC of GPPS for all the different notch radii considered in this work. Moreover, the two polymers show a clear notch effect, developing progressively larger LCCs when the notch radius increases. The notch effect is more significant in GPPS, for which the LCC increases by 31.2% when comparing the results for 0.5 mm and 4 mm radii, this increase being 20.5% in the case of PMMA.  
Figure 8 gathers some of the load-displacement curves obtained in tests of the key-hole notched specimens under compression. It is interesting to note that the shape of the curves  
Figure 6: Compressive stress–strain curves for **(a)** PMMA and **(b)** GPPS.  
Figure 7: Two of the keyhole notched polymeric specimens after the compressive fracture test: **(a)** PMMA, \(\rho\) = 4 mm; **(b)** GPPS, \(\rho\) = 4 mm.  
is different from that observed in fracture tests of notched components either under tension or compression. Most probably, this is because of the contact between the key-hole notch parallel faces during loading.