## 4 Discussion  
The experimental evaluation of fracture mechanics parameters for Ti-6 Al-7 Nb alloys produced by drawing and selective laser melting confirmed that there are critical differences in crack initiation and propagation behavior. The test specimens were produced by both production processes according to ISO 148-1:2009 [17] and ASTM E466-96 [18] standards, with rectangular specimens featuring V-notches and cylindrical specimens with ring-shaped notches. Threshold stress intensity factor measurements showed that SLM-produced specimens exhibited higher initial resistance to crack initiation than the drawn alloy. However, Paris' law analysis revealed fundamental differences, as the SLM material demonstrated a \(C\) factor that is two orders of magnitude greater as well as a lower \(m\) exponent, indicating accelerated low-\(\Delta K\) crack growth rates despite reduced sensitivity to stress intensity increases at higher loads. This can be attributed to the well-known porosity issue for SLM-produced parts and a large grain structure, which create preferential crack paths, while the drawn material's homogeneous \(\alpha\)+\(\beta\) microstructure ensures more uniform deformation [28]. Recent studies confirm that the application of functionally graded coatings on titanium alloys significantly alters the tangential stress distribution and affects the paths of fatigue crack propagation, providing a potential way to improve fatigue resistance [29]. The fracture toughness values obtained by the circular test specimens [25] differ from the values obtained by the ASTM E1820 [23] standard but present the same difference between production processes, in favor of the drawn alloy. The bending moment range and \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Production Process** & \(\mathbf{F_{r}}\)**N** & \(\mathbf{D_{r}}\)**mm** & \(\mathbf{D_{r}}\)**mm** & \(\mathbf{K_{c}}\)**, MPam\({}^{0.5}\)** \\ \hline Drawing & 23,114 & 5.95 & 3.91 & 61.34 \\ Selective laser melting & 9468 & 5.78 & 3.92 & 24.64 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Fracture toughness values determined according to [25]. medium bending moment values used for threshold stress intensity factor determination indicate a higher fracture toughness of the drawn Ti-6 Al-7 Nb alloy. This can be concluded since the higher value of the bending moment had to be applied to the drawn specimen in order to initiate the crack. The drawn alloy's favorable performance was further confirmed by stable J-R and CTOD-R resistance curves, whereas the SLM specimens exhibited highly brittle behavior, leading to crack instability. This resulted in significant data scattering and prevented the collection of reliable crack extension measurements necessary for constructing a valid J-R curve. Crack growth dynamics showed process-dependent characteristics, since the SLM-produced alloy exhibits propagation rates that are two orders of magnitude faster at \(\Delta K\) = 0.1 MPam\({}^{0.5}\), decreasing to an order of magnitude at \(\Delta K\) = 30 MPam\({}^{0.5}\), while the drawn alloy demonstrated greater high-\(\Delta\)K sensitivity. The results show that conventionally drawn Ti-6 Al-7 Nb offers higher fracture toughness and more predictable crack growth than SLM-produced specimens. This indicates that the implants produced via SLM may need higher safety margins due to lower fracture resistance, while the drawn material allows less conservative margins. Therefore, production method selection is critical for ensuring implant safety. Although a comprehensive microstructural analysis was not conducted in this study, preliminary inspection of fracture surfaces, even at a low magnification, revealed clear differences between the drawn and SLM-produced Ti-6 Al-7 Nb specimens. The drawn specimens exhibited uniform, ductile fracture surfaces, consistent with their higher measured fracture toughness and stable crack propagation. In contrast, the SLM specimens displayed visible irregular porosity on the fracture surfaces.