# Characterization and Finite Element Modeling of Microperforated Titanium Grade 2
## 4 Conclusions  
The present study has described microperforated titanium grade 2 experimental testing and FE modeling. The non-perforated titanium grade 2 testing campaign consisted of nine tensile tests, while the microperforated testing campaign consisted of another nine tensile and several Nakajima tests. All of them were conducted at room temperature. In both \begin{table}
\begin{tabular}{c c c c|c c c} \hline \hline \multirow{2}{*}{**Case**} & \multicolumn{2}{c|}{**Inputs**} & \multirow{2}{*}{**Case**} & \multicolumn{3}{c}{**Inputs**} \\ \cline{2-2} \cline{5-8}  & **R\({}_{22}\)** & **R\({}_{33}\)** & **R\({}_{12}\)** & & **R\({}_{22}\)** & **R\({}_{33}\)** & **R\({}_{12}\)** \\ \hline
1 & 0.65 & 0.65 & 0.65 & 15 & 1.00 & 1.00 & 1.35 \\
2 & 0.65 & 0.65 & 1.00 & 16 & 1.00 & 1.35 & 0.65 \\
3 & 0.65 & 0.65 & 1.35 & 17 & 1.00 & 1.35 & 1.00 \\
4 & 0.65 & 1.00 & 0.65 & 18 & 1.00 & 1.35 & 1.35 \\
5 & 0.65 & 1.00 & 1.00 & 19 & 1.35 & 0.65 & 0.65 \\
6 & 0.65 & 1.00 & 1.35 & 20 & 1.35 & 0.65 & 1.00 \\
7 & 0.65 & 1.35 & 0.65 & 21 & 1.35 & 0.65 & 1.35 \\
8 & 0.65 & 1.35 & 1.00 & 22 & 1.35 & 1.00 & 0.65 \\
9 & 0.65 & 1.35 & 1.35 & 23 & 1.35 & 1.00 & 1.00 \\
10 & 1.00 & 0.65 & 0.65 & 24 & 1.35 & 1.00 & 1.35 \\
11 & 1.00 & 0.65 & 1.00 & 25 & 1.35 & 1.35 & 0.65 \\
12 & 1.00 & 0.65 & 1.35 & 26 & 1.35 & 1.35 & 1.00 \\
13 & 1.00 & 1.00 & 0.65 & 27 & 1.35 & 1.35 & 1.35 \\
14 & 1.00 & 1.00 & 1.00 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Full three-level factor design matrix. Figure 13: Comparative samples. Geometry G4 in longitudinal orientation. cases, different orientations were tested: longitudinal (rolling direction), transversal, and diagonal. The main conclusions obtained from the testing campaign can be summarized as follows:  
* Microperforation significantly reduces the formability and elongation of titanium grade 2 samples, between 17 and 31% depending on direction. * Both yield stress and ultimate tensile strength increase slightly (less than 5%) with microperforations. It is not considered significant due to the small difference and the small number of repetitions. * The orientation of specimens does not play a significant role in formability. Only the formability of transversal specimens is slightly worse than that of other specimens. * Anisotropy has a relevant role in punch load-stroke curves. * A crack always moves on the shortest path between two microholes regardless of the material's anisotropy. A dynamic explicit FE model has been built to characterize mechanical properties and anisotropy. The material characterization was carried out considering the results from the testing campaign. The most relevant conclusions obtained are listed in the following lines:  
* Anisotropic ratios play a significant role in load-stroke curves. * Hill anisotropic is not able to reproduce crossing plastic curves. However, the combination of Hill78 anisotropic and damage initiation criteria models accurately the behavior of the microperforated titanium grade 2. Conceptualization, D.M.-M. and R.E.-G.; methodology, D.M.-M., R.E.-G. and O.Z. ; validation, D.M.-M. and O.Z. ; investigation, D.M.-M. and R.E.-G.; writing--original draft preparation, R.E.-G.; writing--review and editing, D.M.-M., R.E.-G. and O.Z. ; project administration, R.E.-G. All authors have read and agreed to the published version of the manuscript. This work was carried out within the framework of an EU Project and was funded by the European Union (Clean Sky 2 EU JTI Platform) under the thematic call JTI-CS2-2019-CfP10-LPA-01-83 \"Forming of microperforated outer skin of HLFC wings assisted by FEM simulation/MICROFORM\" (grant agreement no: 886409). Not applicable. The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author. The authors acknowledge the significant contributions of the Department of Laser Processing and Structural Assessment at the Helmholtz-Zentrum Hereon to the Microform project and this paper; especially the work of [PERSON] and [PERSON].