Abstract
Failure analysis on a diesel engine crankshaft that failed due to fatigue was carried out by means of fractography. The value of the applied stress was estimated by the following procedure. First, the crack growth rate was estimated from the percentage of each fracture mode using the reported relationship between them. Then, the stress intensity factor range was estimated from the crack growth rate. Finally, the applied stress was estimated from the stress intensity factor range in a semi-elliptical surface crack.
The engine analyzed had been used to drive an emergency electric power supply. Therefore, the total running time was only about 300 hours. The material of the crankshaft was JIS SF55A (normalized carbon steel forged for general use, tensile strength 55kgf/mm2). A fatigue crack formed at the web and grew until the pin finally fell off. A beach-mark was observed on the fractured surface of the web initially damaged. Microfractographs of this surface showed facet-like fracture mixed with intergranular fracture in sequence with fatigue striations. From these observations, the applied stress on the web was estimated to be 8 to 11kgf/mm2. Microfractographs of the second web showed fatigue striations followed by a river pattern. The striation spacing was about 1μm. The stress value was estimated to be 38 to 54kgf/mm2. In the first web, the stress value was more than the web's fatigue limit, about 9kgf/mm2. The failure of the crankshaft was caused by overload during operation.
