# Fracture Toughness of Hybrid Components with Selective Laser Melting 18 Ni300 Steel Parts
## Material and Methods
### Fracture Toughness Tests  
The fracture toughness tests were performed according to the BS 7448-3 standard [20] using CT specimens in loading Mode I. Before the fracture toughness tests a fatigue pre-crack was created at room temperature in an Instron Electroplus machine under constant amplitude tensile loading with sinusoidal load wave and stress ratio R = 0.05. In accordance with this standard, a pre-crack of 4 mm length was created from the notch, in order to comply with the two standard requirements: the pre-crack has to be higher than 2.5% of the W parameter and greater than 1.3 mm from the notch. The crack length was measured using a travelling microscope (45\(\times\) of magnification) with an accuracy of 10 \(\upmu\)m, as shown in Figure 2.  
The fracture toughness tests were carried out in an Instron machine, model 4206, using a displacement rate of 2 mm/min. The crack opening displacement was measured using an MTS clip gauge model 632.02C-221 (MTS Systems, MN, USA) (range: \(-2.5\) mm to 2.5 mm) fixed with an apparatus especially created for this purpose. Figure 3 shows the CT sample and \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Steel** & **C** & **Ni** & **Co** & **V** & **Mo** & **Ti** & **Al** & **Cr** & **P** & **Si** & **Mn** & **Fe** \\ \hline
18 Ni300 (powder) & 0.01 & 18.2 & 9.0 & - & 5.0 & 0.6 & 0.05 & 0.3 & 0.01 & 0.1 & 0.04 & Balance \\ AISI H13 (CM) & 0.4 & - & - & 0.94 & 1.30 & - & - & 5.29 & 0.017 & 1.05 & 0.036 & Balance \\ AISI 420 (CM) & 0.37 & - & - & 0.17 & - & - & - & 14.22 & 0.021 & 0.64 & 0.037 & Balance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of steels (CM-Conventional Manufacturing). Reproduced with the permission from [17], copyright Elsevier, 2016. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimen** & \multicolumn{3}{c}{**SM**} & \multicolumn{3}{c}{**SS/SM**} & \multicolumn{3}{c}{**HS/SM**} \\ \hline
**Scan rate (mm/s)** & 200 & 400 & 200 & 400 & 200 & 400 \\
**Ultimate Tensile Stress [MPa]** & 1147 & 1032 & 1135 & 1000 & 1146 & 1000 \\
**Yield stress [MPa]** & 1000 & 910 & 1115 & 900 & 1125 & 900 \\
**Extension at break [\%]** & 6 & 2.2 & 3.6 & 1.5 & 4.2 & 1.7 \\
**Young’s modulus [GP]** & 180 & 180 & 190 & 180 & 190 & 180 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Tensile properties for SS/SM, HS/SM and SM specimens. Reproduced with the permission from [17], copyright Elsevier, 2016. Figure 2: Pre-crack length measurement apparatus. The fracture toughness \(\mathrm{K_{L}}\), and the critical integral, \(\mathrm{J_{c}}\), were calculated according the BS 7448-3 [20] standard through the Equations (1)-(4), respectively. Equation (5) was used for the validation of the critical stress intensity factor values in Mode I loading (\(\mathrm{K_{L}}\)) by the criteria of excessive plastic deformation at the crack tip. \[\mathrm{K_{Q}}=\frac{F_{\mathrm{Q}}}{BW^{0.5}}\times f^{\prime}\left(\frac{a_{ 0}}{W}\right) \tag{1}\]  
\[f^{\prime}(\frac{a_{0}}{W})=\frac{\left(2+\frac{a_{0}}{W}\right)\left[0.886+4.6 4\frac{a_{0}}{W}-13.32\frac{a_{0}^{2}}{W^{2}}+14.72\frac{a_{0}^{3}}{W^{3}}-5.6 \frac{a_{0}^{4}}{W^{4}}\right]}{\left(1-\frac{a_{0}}{W}\right)^{1.5}} \tag{2}\]  
\[J=\left[\frac{F}{BW^{0.5}}\times f^{\prime}\left(\frac{a_{0}}{W}\right)\right] ^{2}\frac{\left(1-\delta^{2}\right)}{E}+\frac{\eta_{p}L_{\mathrm{P}}}{B(W-a_ {0})} \tag{3}\]  
\[\eta_{p}=2+0.522\left(1-\frac{a_{0}}{W}\right) \tag{4}\]  
\[2.5\left(\frac{\mathrm{K_{Q}}}{\sigma_{\mathrm{YS}}}\right)^{2}\geq(W-a_{0}) \tag{5}\]  
where \(a_{0}=14\) mm is the crack length, \(W\) the distance indicated in Figure 1, \(B\) is the sample thick, \(F_{\mathrm{Q}}\) is a particular load value determined from the fracture test chart, \(U_{\mathrm{P}}\) correspond to the plastic component of the area under load vs. elongation curve, \(E\) is the Young modulus, \(\
u\) the Poisson coefficient and \(F\) is the maximum load. Complementary analyses were performed to characterize the microstructure and the fracture mechanisms.