Investigations on fatigue cracks in an aircraft engine crankshaft

Abstract

Internal combustion engines are widely used in small and light vehicles due to their efficient fuel consumption and compact design. However, their dynamic components are susceptible to various mechanical failures. The crankshaft is particularly prone to fatigue due to its intricate structure and the significant loads it must endure. Despite design efforts to minimize this issue, such failures still occur in modern engines. This paper presents a failure analysis of a trainer aircraft crankshaft following in-flight vibrations and flame-out that led to a crash. Macroscopic examination and fractographic evaluations identified the initial fracture at one of the crankwebs, characterized by well-delineated, single-origin beach marks. A secondary fracture was observed at a second crankweb, exhibiting features consistent with rapid, multi-origin fatigue crack growth under elevated stress levels. Additionally, the forward displacement at one of the journal bearings was identified as a critical contributing factor. This displacement caused significant damage to the journal bearing in question, including forward tip wear, which led to rubbing against the fillet radius of the crankweb where the initial fracture was discovered. This localized contact initiated cyclic thermal cracks on the fillet radius and they progressively evolved into surface wear, spalling, and pitting. These surface discontinuities acted as stress raisers and ultimately nucleated the fatigue crack at the aforementioned crankweb.