Thermal fatigue cracks of fire fighting vehicles gray iron brake discs

A considerable number of brake discs of fire fighting vehicles used in supporting to the aeronautical activities were found cracked during NDT. Macrofractography of the discs, manufactured in gray iron, showed several cracks running along the radius of the friction surfaces having length approximately in the range 1–7.5 cm. Further cracks examination by using optical microscope and field emission scanning electron microscope (FESEM) revealed the cracks were propagated in semi elliptical shape through the thickness of the friction surfaces in accordance with a thermal fatigue mechanism. Two zones that exhibited defined signs of heating were also observed. In addition, no sign of defects that could have originated the cracks were found. The discs were found to be confirmed to the chemical, microstructural and hardness measurements requirements for the selected material. The finite element analysis (FEA) performed to determine the temperatures profile in the disc as well as to estimate the stresses distribution during the braking also confirmed the relatively high temperatures reached during the braking actions and attributed to the residual tensile stress due to the repetition of the thermal stress cycles the cause of the cracks, respectively. It was recommended all fire fighting vehicles used be examined, the brake discs replaced and an improvement in terms of material choice eventually be developed.