PSI - Issue 13

Stanislav Seitl et al. / Procedia Structural Integrity 13 (2018) 1494–1501 Author name / Structural Integrity Procedia 00 (2018) 000–000

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Table 1. Chemical composition in percentage by weight (wt. %) of the S355 J0 steel grade according to producer’s paper (corresponding to EN 10025-2:2004 standard). Steel grade C (max. %] Mn (max. %) Si (max.%) P (max. %) S (max. %) N (max. %) Cu (max. %) CEV (max. %) S 355 J0 0.2 1.6 0.55 0.035 0.035 0.012 0.55 0.47

Fig. 1. Rolling direction of (a) Compact tension specimens and (b) S-N curve specimens.

a) S355 J0 A b) S355 J0 B Fig. 2. Structure of the S S355 J0 steel grade photos taken by light optical microscope. The crack propagates in the horizontal direction. 3. Theoretical background 3.1. Traditionally used models for evaluation of fatigue properties Fatigue life evaluation of structural details may be performed based on the stress based approach, which is supported by experimental S-N field results relating directly a global definition of stress range, Δ σ , to the total number of cycles to failure, N f , e.g. Ellyin (1997). Alternatively, the fracture mechanics based approach, see e.g. Klesnil & Lukáš (1992) allows the crack propagation from an initial crack size, a i , to final crack size, a f to be predicted using fatigue crack growth curves. In the following, both traditionally models are introduced along with some possible enhancements. S-N curve Often, Basquin’s equation (Basquin 1910) is adopted for representing the Wöhler field as a straight line in a double logarithmic plot, according to the following equation: ߪ ൌ ܣ ܰ ஻ (1) where the parameters A and B denote the independent term and slope, respectively, of the resulting straight line in double logarithmic scale. The scatter of the test results at the same level of loading is a characteristic inherent to the fatigue phenomenon due to several factors, such as material defects during the process of extrusion and machining process of specimens etc. Crack propagation rate curve The fatigue crack growth rate for the applied loading is defined by the crack length increment for given number of loading cycles. Often the Paris-Erdogan law, (Paris & Erdogan 1963), representing straight lines in a double logarithmic scale as expressed by Eq. 2, is used for description of the crack growth: ௗ ௗ ே ௔ ൌ ܥ ሺο ܭ ሻ ௠ , (2) where C and m are material constants, d a /d N is the fatigue crack growth rate, referred to the number of cycles, and  K is the stress intensity factor range.