PSI - Issue 2_B

Andreas J. Brunner et al. / Procedia Structural Integrity 2 (2016) 088–095 Author name / Structural Integrity Procedia 00 (2016) 000–000

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The quantities of equation (1) are defined in the nomenclature and A,  , D and G thr are the four fit parameters to describe the dependence of da/dN on the square-root of G. The analysis by Murri 2013 compared 2-point and 7 point polynomial fits (ASTM E647) and this contribution explores a modified Hartman-Schijve fitting for selected data from the same set. A consistent procedure is proposed that treats G thr that is of interest in composite design as parameter depending on explicitly chosen values of the other fit parameters in order to estimate variation of G thr and allows determining a design limit from G thr based on scatter in experimental data. 3. Results and Discussion Fig. 1 compares the same data (Murri 2013) in Paris-type and modified Hartman-Schijve graphs, the latter with one set of fit parameters (A = 240 J/m 2 ,  = 6.9617, D = 1.3628x10 -12 (mm/cycle) (J/m²) (-n/2) , and G thr = 77 J/m 2 ). Data were evaluated with the MBT method. Both methods for the presentation of the data yield curves that show little scatter, but some curvature, even though both axes have logarithmic scales. If the Paris-type data are extrapolated to a da/dN-value of 10 -10 mm/cycle, a rough estimate for G thr is obtained. This amounts to about 80-90 J/m 2 , if the lower part of the curve (i.e., steeper slope), and to about 60-70 J/m 2 , if the upper part (lower slope), respectively, is extrapolated. If the Hartman-Schijve data are extrapolated to the same value of da/dN, the x-axis value amounts to about 1.5 in units of square-root (J/m 2 ). Backcalculating G thr will depend on the values of the other fitting parameters, but the fitting value of G thr = 77 J/m 2 used in Fig. 1(b) is roughly consistent with the extrapolated values from the Paris-type graph. While an estimate of the scatter in G thr for the Paris-type curve can be obtained by 3.1. Comparison between Paris-type and Hartman-Schijve type data presentation

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(b)

Fig. 1. (a) Fatigue fracture data in Paris-type plot; (b) the same data in a modified Hartman-Schijve plot.