PSI - Issue 19

Jan Papuga et al. / Procedia Structural Integrity 19 (2019) 405–414 Author name / Structural Integrity Procedia 00 (2019) 000–000

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based on the material S-N curve – tends to be too conservative (see Fig. 1). The trend today shifts in the direction of light-weight structures saving not only the weight but also production and in-service costs. The two more modern approaches differ in the way they operate with local stresses and with the material S-N curve derived from experiments on unnotched specimens. The RSG approach uses the value of the relative stress gradient at the notch root to define the local component S-N curve (see Fig. 1). This curve lies above the material S-N curve, so the maximum stress at the notch root leads to higher number of cycles then with the material S-N curve. The TCD approach uses the material S-N curve, but the acting effective stress is obtained in a given (critical) distance from the notch root instead of the maximum stress at the notch root. Thanks to that, the effective stress is smaller, and thus it leads to longer lifetime than the maximum stress (see Fig. 1).  FI relative stress error (positive for safe prediction results, negative for unsafe prediction results)  ’ relative stress gradient K f notch factor (ratio of the fatigue limit of an unnotched specimen to the fatigue limit of a notched specimen K Ic stress intensity factor K t stress concentration factor (ratio of the maximum stress at the notch root to the nominal stress there) K th threshold of the stress intensity factor (at given R ) L CD critical distance n fatigue factor N number of cycles till break NOM nominal; the nominal stress approach in general PS1 first principal stress R stress ratio  min /  max  notch root radius RSG relative stress gradient; the stress gradient approach in general  stress s logN standard deviation of logarithms of cycles to failure related to the S-N curve regression from experiments S u tensile strength S y yield stress TCD theory of critical distance; the critical distance approach in general w S-N curve exponent Nomenclature C S-N curve constant

Fig. 1. The scheme of three evaluated approaches: nominal (NOM, brown), critical distance (TCD, green), relative stress gradient (RSG, violet).