*2.1. Experiment Materials*

Material was taken from a train brake disc cracked due to thermal fatigue, which was made of nodular iron. Additionally, the chemical composition is shown in Table 1, and the microstructure is shown in Figure 2. The microstructure was composed of spheroidal graphite (G), pearlite (P), and ferrite (F). Figure 3 shows that the surface crack of the damaged brake disc was about 0.33 mm wide and about 0.54 mm deep.

**Table 1.** Chemical compositions of nodular cast iron.


**Figure 2.** Microstructure of ductile iron.

**Figure 3.** Crack of unrepaired thermal fatigue sample.
