# Photoelasticity as a Tool for Stress Analysis of Re-Entrant Auxetic Structures
## 5 Discussion and Results
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Point**} & **FEM** & **EXP** & **Diff.** & **FEM** & **EXP** & **Diff.** & **Point** & **FEM** & **EXP** & **Diff.** \\  & **(MPa)** & **(MPa)** & **(\%)** & **Point** & **(MPa)** & **(MPa)** & **(\%)** & **Point** & **(MPa)** & **(MPa)** & **(\%)** \\ \hline
1 & 4.49 & 4.59 & 2.2 & 11 & 38.47 & - & - & 21 & \(-\)24.29 & - & - \\
2 & 5.63 & 5.72 & 1.6 & 12 & 39.26 & - & - & 22 & \(-\)19.51 & - & - \\
3 & 7.36 & 7.46 & 1.4 & 13 & 29.38 & - & - & 23 & \(-\)13.98 & - & - \\
4 & 1.90 & 2.01 & 5.4 & 14 & 27.11 & - & - & 24 & \(-\)15.40 & - & - \\
5 & 4.09 & 4.22 & 3.0 & 15 & 13.72 & - & - & 25 & \(-\)5.11 & 5.29 & 3.4 \\
6 & 0.34 & 0.37 & 9.1 & 16 & 9.92 & - & - & 26 & \(-\)6.05 & 6.15 & 1.6 \\
7 & 1.45 & 1.50 & 3.1 & 17 & \(-\)2.22 & 2.26 & 1.9 & 27 & 1.87 & 1.94 & 3.8 \\
8 & \(-\)2.47 & 2.56 & 3.3 & 18 & \(-\)6.10 & 6.21 & 1.8 & 28 & 1.59 & 1.65 & 3.7 \\
9 & \(-\)1.04 & 1.08 & 3.4 & 19 & \(-\)12.04 & - & - & 29 & 6.59 & 6.72 & 1.9 \\
10 & \(-\)2.30 & 2.43 & 5.2 & 20 & \(-\)15.42 & - & - & 30 & 8.88 & 8.94 & 0.7 \\ \hline \hline \end{tabular} Color scheme corresponds to Figure 6.  
\end{table} 
Table 6: A comparison of the principal stresses from FEM and the experiment for Variant 1—asym metrical loading. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Point**} & **FEM** & **EXP** & **Diff. ** & **FEM** & **EXP** & **Diff. ** & **Point** & **FEM** & **EXP** & **Diff. ** \\  & **(MPa)** & **(MPa)** & **(\%)** & **Point** & **(MPa)** & **(MPa)** & **(\%)** & **Point** & **(MPa)** & **(MPa)** & **(MPa)** & **(\%)** \\ \hline
1 & 11.05 & - & - & 11 &  
isidered a valuable tool for designers and analysts in validating the results obtained by numerical modeling. It is important to note that every experimental measurement requires thorough preparation of the measurement methodology, which in this case also considered the selection of a suitable optically sensitive material for the maximum load chosen by the authors, F = 150 N. In practice, there may be cases where the measurement setup is correctly designed but the sensitivity of the chosen optically sensitive material is outside the measured range. The presented article documents that the experimental results obtained by photoelasticity can be used even without the use of state-of-the-art hardware and software. On the other hand, their use would allow a more detailed strain-stress analysis not only at the edge of the model but also at any point of the investigated object. Based on the findings, future research will focus on exploring the applicability of photoelasticity to a broader range of auxetic and geometrically complex materials, considering varying load conditions and material sensitivities. At the same time, there are possibilities for validation of the results on the same samples using modern optical methods (digital image correlation). Conceptualization, B.S., M.P. and P.L. ; methodology, M.P. and P.F. ; software, B.S. and P.L. ; validation, B.S., M.P. and P.L. ; formal analysis, B.S. and P.F. ; investigation, B.S. ; resources, P.F. ; data curation, M.P. ; writing--original draft preparation, B.S., M.P. and P.L. ; writing--review and editing, P.F. ; visualization, P.F. and P.L. ; supervision, M.P. ; project administration, M.P. and P.F. ; funding acquisition, P.F. All authors have read and agreed to the published version of the manuscript. This work was supported by the following projects: APVV-23-0461, VEGA 1/0516/22, VEGA 1/0152/24. Not applicable. The data that support the findings of this study are available from the corresponding author upon request. The authors declare no conflicts of interest.