Application ofWeibull Probability to Predict the Size of Inclusion in Metallic Material

Large inclusions can be the initiation site for fatigue failure in metal components. As
melting processes are becoming more refined, the size of the inclusions falls below the
level of detectability of the non-destructive testing methods. This final year project is
divided into three parts. In the first part of the project, Weibull probability was applied to
predict largest oxide inclusion size and compare to the actual observation under scanning
electron microscope. The results showed that Weibull probability prediction is accurate
with margin of ± 3 microns. In the second part of project, the Weibull probability was
tested using nodular cast iron. The nodules were measured for their true and apparent
sizes, respectively. Based on the data, the effect on Weibull probability was found to be
negligible. In the third part of the project, rotating fatigue test was performed under
cantilevered loading by using two sets of medium carbon steel specimens. The specimens
were annealed at 840 °C, held for one hour and furnace cooled before being polished and
tested. Step-size method was selected where each specimen was subjected to 2.52 x 10
cycles at initial load of 5 N. The load was increased progressively until the specimen
eventually fails. Only those specimens failed due too oxide inclusion at fatigue initiation
site were regarded. Based on observational results, the two sets had different probability
of survivalwhich corresponded to their respectively largestoxide inclusion size.