## 3 Materials
### Polymer Materials  
Leading edge repair technology involves the local heating of the structure, both for the complete detachment of the shield from the rubber, which is necessary to prepare the bonding surface, and for the subsequent post-curing of the new epoxy resin-based adhesive. Both processes take place at temperatures as high as 110 \({}^{\circ}\)C. Consequently, it is essential for the rubber and adhesive that the polymer components of the blade remain thermally stable after the annealing operations performed during the repair process. Thermal stability is a general term used to describe the changes (or lack of changes) in material properties as a function of temperature [16]. A differential scanning calorimetry (DSC) study was conducted to verify the behavior of the leading edge rubber and the bonding ply that connects the rubber to the shield. The study of the thermo-physical properties was carried out on one sample of each material. According to [20], one sample of a given material is sufficient to estimate the properties associated with thermo-physical transformations. The bonding ply sample was taken from the rubber surface after the shield has detached from the rubber, as per Figure 5a. The rubber sample was cut out from the top of the leading edge, as shown in Figure 5b. The samples were cut to fit a standard 146.33 mg aluminum DSC crucible. The bonding ply sample weight was 75.82 mg and the rubber sample weight was 92.45 mg. \begin{table}
\begin{tabular}{c c c c c} \hline
**Test 1** & **Test 2** & **Test 3** & **Average** & **Standard Deviation** \\ \hline Ti & Ti & Ti & Ti & Ti \\
96.60\% & 97.74\% & 97.55\% & 97.96\% & 0.60\% \\ Mn & Mn & Mn & Mn & Mn \\
1.29\% & 1.06\% & 1.15\% & 1.16\% & 0.11\% \\ Al & Al & Al & Al & Al \\ - & 1.12\% & 1.18\% & 0.76\% & 0.04\% \\ Fe & Fe & Fe & Fe & Fe \\ - & 0.07\% & 0.07\% & 0.05\% & 0\% \\ Si & Si & Si & Si & Si \\
0.11\% & - & 0.05\% & 0.13\% & 0.05\% \\ \hline \end{tabular}
\end{table} 
Table 1: XRF-identified composition of the sheath material. Figure 4: Sample of a shield for the XRF identification tests. DSC was performed using a TA Instrument DSC Q1000. The samples were tested in the temperature range of \(-90\div 110\)\({}^{\circ}\)C.  
The lower limit of \(-90\)\({}^{\circ}\)C was adopted for DSC testing to check whether the materials crystallize--enter a brittle state--which is unacceptable for materials used in aviation. This study was based on the conclusion of [22; 23], where the author of the work showed that transformations at temperatures below the aircraft operating limit can impinge on the material properties in the operating range of the tested material, despite the presence of a reversible phase transition. The DSC test included four thermal cycles carried out consecutively on one specimen:  
1. Cycle 20 \({}^{\circ}\)C \(\rightarrow\)\(-90\)\({}^{\circ}\)C \(\rightarrow\) 50 \({}^{\circ}\)C \(\rightarrow\) 20 \({}^{\circ}\)C--fly simulation I;
2. Cycle 20 \({}^{\circ}\)C \(\rightarrow\)\(-90\)\({}^{\circ}\)C \(\rightarrow\) 50 \({}^{\circ}\)C \(\rightarrow\) 20 \({}^{\circ}\)C--fly simulation II;
3. Cycle 20 \({}^{\circ}\)C \(\rightarrow\)\(110\)\({}^{\circ}\)C \(\rightarrow\) 20 \({}^{\circ}\)C--process repair simulation;
4. Cycle 20 \({}^{\circ}\)C \(\rightarrow\)\(-90\)\({}^{\circ}\)C \(\rightarrow\) 50 \({}^{\circ}\)C \(\rightarrow\) 20 \({}^{\circ}\)C cycle--fly simulation III. The rate of temperature change for each cycle was 5 \({}^{\circ}\)C/min. The sample was in a helium atmosphere during the test. The DSC results are shown in Figure 6.  
The glass transition temperatures were determined for both the rubber and bonding ply. They were \(-26\)\({}^{\circ}\)C for rubber and \(-11\)\({}^{\circ}\)C for bonding ply. Moreover, the DSC curves for the rubber showed an endothermic peak at 65 \({}^{\circ}\)C with low enthalpy. It can be supposed that this peak is related to the moisture content of the rubber.