Issue 21

A. De Iorio et alii, Frattura ed Integrità Strutturale, 21 (2012) 21-29; DOI: 10.3221/IGF-ESIS.21.03 26 Figure 12 : Crack length vs. Number of Cycles (left) and corresponding residuals (right). In any case, the best-fitting with the proposed method appears visually acceptable and the coefficient of determination R 2 is very close to one. It follows that if the normality test of the residuals distribution does not verify the optimum conditions for the approximation of the experimental data with the proposed model, it is due to the defectiveness of some experimental points or of short parts of some experimental curves. This result, apparently negative, can even be profitably used to filter the experimental data, that is eliminate all data that move away in an excessive or anomalous manner from the unique natural trend of all the other data points, as unique must be the physical law that characterizes the crack propagation phenomenon in each material that is free of geometrical singularities and structural inhomogeneities. This way to see the experimental data allows minimizing and improving the information derivable from testing and, in this particular case, to improve the evaluation of the model parameters α, β, γ using all available data. I DENTIFICATION AND VARIABILITY OF THE PARAMETERS α, β, γ he interpolation of the experimental data of each test, through the proposed model and best-fitting technique, allows identifying the parameters α, β, γ of the model. The numerical values of these parameters are represented in three distinct groups of three diagrams, each group being relative to a data set obtained under the same loading condition (Fig. 13-15). The parameters values assigned to each data set are the statistical averages of the values obtained for each experimental curve of the set. They are given in Tab. 2, where the standard deviations of the three samples of parameter estimates are also reported. β α γ Set _ I µ 1.37 0.47 -0.062 σ 0.35 0.078 0.073 Set _ II µ 1.55 0.43 -0.105 σ 0.42 0.069 0.15 Set _ III µ 1.62 0.43 -0.15 σ 0.53 0.01 0.176 Table 2 : Model parameters values. T