Determination of the strength of dental phosphate-bonded investments by 2 and 3 parameter Weibull analysis

Objective: Phosphate-bonded investments are being investigated for use as d ie materials for dental superplastic forming. The effects of handling techn ique on the strengths of these investments needs to be determined. The purp ose was to use methods of Weibull analysis to fit measured 4 point bend str ength data and determine strength and Weibull modulus, where modulus repres ents the scatter of the data. Materials and methods: Weibull parameters wer e determined using methods of regression and maximum likelihood using 2 and 3 parameters. The parameters were x(0) the lower bound of strength, m the Weibull Modulus, and theta the characteristic strength. x(0) is zero for th e 2 parameter models. In addition to plots of the measured data and curves generated using the Weibull parameters, the fit of the models to the measur ed data was determined using correlation coefficient and the chi (2) statis tic. Results: The results showed that characteristic strength was similar w hether determined using regression or maximum likelihood, 2 parameter or 3 parameter models. However, Weibull Modulus was significantly lower for 3 pa rameter models than for 2 parameter models. Visually the 3 parameter models give the best fit for the majority the data sets. Correlation coefficient, r, showed whether the method of maximum likelihood or the method of regres sion was more appropriate. chi (2) and correlation coefficient did not nece ssarily give the same indication of goodness-of-fit of these data sets. Sig nificance: Strengths of dental phosphate-bonded investment materials can be described well using weibull analysis which gives characteristic strengths and quantitative measures of scatter of the data in the form of Weibull Mo dulus. These may then be used to select a handling technique to produce die s for dental superplastic forming. (C) 2001 Kluwer Academic Publishers.