Tests carried out recently by TWI for a large oil company confirmed the satisfactory design of a major diesel generator component.
During commissioning of a 5MW V16 generator at one of their refineries, some large machined notches in the crankshaft main bearing housing were noticed. The company concerned were worried that these may provide initiation sites for cracking, so they called in TWI's on-site stress analysis team.
In order to quantify the risks of fatigue cracking the team needed first to establish the stress distribution around the notches during generator operation. Measurements were taken using strain gauges. Attaching them proved to be difficult, access was restrictive and the gauge installation needed to be robust in order to withstand a hot oil environment.
Data were collected from several hours of generator operation, under full and part-load running conditions. From these figures, and from known materials properties data and published stress concentration factors, a cyclic stress history at the notch was calculated. The maximum stress range was little more than half that required to initiate a fatigue crack.
To confirm this, TWI also ran a supporting fracture mechanics analysis. This showed that the flaw size necessary to propagate a crack by fatigue was much greater than any that could have escaped detection. Risk of fatigue failure from the detail in question was insignificant, therefore confidence was restored. Life expectancy of this generator had not been foreshortened.