# Regolith-Rich PEEK Composite Bricks: Steps Towards Space-Ready Lunar Construction Materials
## 3 Mechanical Characterization Campaign
### Bending and Compression Tests  
For both bending and compression tests, standardized procedures referenced from ASTM and ISO are followed, specifically targeting reinforced rigid plastics and concrete. To keep consistency with [5], the same prism-shaped and cylindrical molds are used. The internal surfaces of these molds are coated with a high-temperature release agent (Cirex Si 041 WB) to ensure easy demolding. After manufacturing, all specimens are measured for weight and dimensions before testing. Data on load and displacement are recorded at a frequency of 10 Hz. Prism specimens with rectangular cross sections (see Figure 2(a)) are prepared in molds with internal dimensions of 150 mm in length, 25 mm in width, and 25 mm in depth. The mold is filled with powder to a depth of 10 mm; powders are leveled with a spatula before compaction takes place. Each specimen undergoes a three-point bending test at a constant speed of 2 mm/min until failure, using an MTS QTest10 Universal Testing Machine equipped with a 1 kN load cell. Bending stress and strain are calculated according to [61], and bending strength is determined at the maximum load each sample can bear. Cylindrical specimens (see Figure 2(b)) are produced using molds with nominal dimensions of 14 mm in diameter and 30 mm in height. The mold is filled completely with powder, leveled before compaction takes place. Each specimen undergoes a compression test at a constant speed of 1.3 mm/min until failure, using an Instron 8801 Universal Testing Machine equipped with a 10 kN load cell. Compression stress and strain are calculated following [62], and compression strength is determined at the maximum load each sample can withstand.  
The elastic moduli of the samples from both compression and bending tests are also evaluated. An adaptive and robust data analysis method is implemented to compensate for the variable behavior exhibited by specimens, due to differences in the manufacturing parameters. The heterogeneity of the samples results in stress-strain curves with varying trends and characteristics, particularly in the initial toe region, which often displays non-linear behavior due to the initial slack or settling of the material under stress. The algorithm  
Figure 3: Example of reference bending and compression specimens used throughout the experimental campaign. The geometries shown are representative of all specimens, which were ideally identical and used for both air-cured and vacuum-cured samples.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \multicolumn{2}{c}{**DoE Run No.**} & **01** & **02** & **03** & **04** & **05** & **06** & **07** & **08** & **09** & **10** & **11** & **12** \\ \hline Organic phase & wt\% & 5\% & 5\% & 5\% & 5\% & 10\% & 10\% & 10\% & 10\% & 15\% & 15\% & 15\% & 15\% \\ Comp. press. & MPa & 0.50 & 2.50 & 5.00 & 7.50 & 0.50 & 2.50 & 5.00 & 7.50 & 0.50 & 2.50 & 5.00 & 7.50 \\ \hline \end{tabular}
\end{table}
Table 3: Processing parameters for the L12 DoE characterization campaign. The table lists the levels of each factor varied during the experimental design. The combinations follow the orthogonal array structure of an L12 design, ensuring a systematic exploration of the parameter space.  
dynamically identifies the linear region within each stress-strain curve by performing a rolling linear regression. A windowed approach is used, wherein the slope of the curve is calculated over multiple and overlapping windows. The consistency of these slopes is then assessed, identifying the most stable linear portion of the curve. This stability is quantified by requiring that the variation among the slopes does not exceed 10%. By progressing through the curve and recalculating the regression for each window, the algorithm can effectively adjust to variations in curve trends, allowing for the identification of the true linear region regardless of the initial non-linear characteristics presented by different specimens.