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A. De Iorio et alii, Frattura ed Integrità Strutturale, 26 (2013) 57-68; DOI: 10.3221/IGF-ESIS.26.07 67 ΔK (MPa  m) da/dN (m/GC) Paris 10 13.48 13.5 34.70 Table 9 : Crack growth rates computed using the procedure V. All the crack growth rates computed using the discussed procedures are shown in Fig. 14 together with the fatigue crack propagation curves obtained analysing the experimental data by the procedure suggested by the ASTM [10]. Figure 14 : Comparison among crack growth rates corresponding to the different  K. ΔK (MPa  m) μ (MPa  m) Σ (MPa  m) 10 13.92 0.37 13.5 32.26 1.05 Table 10 : Estimated crack growth rates: means and standard deviations. Means and standard deviations of the crack growth rate estimates computed with the different procedures are given in Tab. 10 for several values of  K. The greatest deviation from the mean value occurs at the maximum ΔK (13.5 MPa  m). However, this result is essentially due to the excessive deviation of the value computed using the Paris model (see Fig. 14 right) from the crack propagation curve. For ΔK = 10 MPa  m the maximum deviations occur for the procedure identified as I and II, probably due to the adoption of the three-parameters model to fit the experimental data points. C ONCLUSIONS uring the experimental activities carried out for the certification of rail steel, significant lacks in the current standard that regulates the principal tests arose. In the present paper the main results of the testing activities and the adopted procedures are reported. Also, the adopted choices in order to either implement the guidelines given by the reference Standards or to fill the aforementioned gaps left by these latter are described. In particular, due to the lacks in all current standards, some alternative procedures to compute the crack growth rates to be compared with the reference values prescribed by the standard have been proposed and verified. The final considerations about the obtained results represent a contribution to provide a mean for better understand and apply the current Standards to produce reliable results in the complex task of certifying a rail steel. D