Failure Analysis of the Minibus Crankshaft Using Experimental Methods

This study aims to analyze the causes of crankshaft failure. The methods implemented to examine the value of stress acting on the crankshaft include visual observation, SEM observation, chemical composition testing, and hardness testing. Furthermore, the visual observation of the fourth crankpin fracture surface showed the presence of beachmarks and rivermarks that become characterize fatigue fractures. The SEM observations showed that the crack originated around the fillet on the fourth crankpin. Furthermore, the chemical composition test showed that the type of material used on the shaft was Steel Alloy ASTM A29 standard Grade AISI 4140. The hardness test showed that its value is not homogeneous at each test point with an average value of 99.67 HRB. This exceeds the standard hardness of the AISI 4140 material, which is 92 HRB, leading to the decrease in the toughness of the crankshaft material. The lower surface hardening in the crack initiation region triggers initial defects in the material, while there was friction with the bearing. Because the crankshaft experiences dynamic loads, over time crack propagation occurs from the initial crack to the final fracture.