Microstructure evaluation of the damage and wear characteristics of a failed disc brake of a provincial bus

Abstract

Abstract One of the most important part of a vehicle is the brake system. This is responsible for halting a vehicle into a stop. This is done by converting the moving object’s kinetic energy into another form such as heat. The brakes convert this energy mainly by friction and absorbs all this heat before finally dissipating to the surrounding air. For this reason, the parts of a braking system are most prone to mechanical wear. Disc brake assembly is one type of braking system. It consists of a master cylinder, brake pads, brake discs and rotor as its main componentsThe study examined a disc brake which experienced premature cracking. By observing the microstructure of the failed disc brake, this paper aims to determine stress points, early signs of failure, crack propagation, and most importantly, the possible cause of cracking. It is of utmost importance that provincial bus companies determine the cause of cracking to prevent premature failure of other disc brake assembly and ultimately, to avoid road accidents. The disc brake’s key features were visually inspected and measured. Samples were taken and prepared for metallographic study. Small hairline cracks observed on the disc surface indicates thermal cracking. The disc’s microstructure reveals graphite flakes in a pearlite matrix. This type of microstructure is observed in grey cast iron, a type of steel commonly used for brake assemblies. The flakes were suspected to be sites for hairline crack formation, which then propagated and caused premature cracking. The observed microstructure of the disc surface showed that carbon flakes were significantly larger at the mid-diameter of the disc and could have been sites for crack initiation. The hairline cracks did not penetrate through the thickness of the disc which indicates short, sharp braking.