**4.1 Visual inspection**

The failed "as-received" brake rotor was carefully examined before cutting samples out of it to observe any signs of aggressive use or inappropriate installation of the rotor with other brake system components, such as pads and calipers. As shown in **Figure 4**, a circumferential crack was noted at the swan neck region of the rotor, indicated by the arrows. The damage covered over 75% of the disc neck perimeter. It is also observed that the in-board area and the rotor's vents have been rusted, while the out-board region is still coated with graphite black paint that protects it against corrosion. A thorough examination of the brake surfaces (in-board and out-board) does not suggest any symptoms of abusive use of the rotor (**Figure 2a** and **b**). The friction planes do not show a blue discolouration marking. Blue discolouration of the friction surface emanates from excessive heat generated from braking due to the misalignment of brake calipers or inadequate heat dissipation by typical brake components [27]. This discolouration could occur due to extreme pressure mounted on the brake pads even when the brake system is not used. However, the brake surface blue marks

**Figure 4.** *A circumferential crack around the swan neck of the rotor.*

can suggest faulty calipers and brake pads, disc cracks, and non-uniform brake pad wear. This analysis does not indicate any of the abovementioned issues as the cause of the failure, implying that the rotor was installed correctly and operated normally.
