Weibull Analysis of Mechanical Data for Castings: A Guide to the Interpretation of Probability Plots

The Weibull statistics used in the analysis of mechanical data from castings are reviewed. The insight that can be gained from the three-parameter analysis is introduced, and in particular, the meaning of the threshold value in the three-parameter Weibull distribution from a casting process standpoint is discussed. Weibull mixtures, which are necessary when different defect populations are present in castings, are discussed in detail. Data sets from the literature are reinterpreted for demonstration of the guidelines. Step-by-step procedures are outlined for analysis of Weibull mixtures and for fatigue life data sets with run-outs. The results indicate that knowledge of the casting process is necessary for the accurate interpretation of Weibull probability plots. Because the filling systems of most castings currently are designed poorly, a mixture of Weibull distributions can be expected to be the norm rather than the exception. A single Weibull distribution can be expected only in castings whose filling systems were designed so that no new damage to liquid metal takes place during filling.