PSI - Issue 18

Matus Margetin et al. / Procedia Structural Integrity 18 (2019) 663–670 Matus Margetin at al / Structural Integrity Procedia 00 (2019) 000–000

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Findley material parameter which is determined experimentally by performing fatigue testing procedure under two different stress states. In case of fully reversed normal and torsion tests it can be calculated by solving following equation: 2 ���� � ������ � � � ��� � ��� (2) One of the problems with Findley criterion (as with many criterions defining the critical plane as plane with maximal damage parameter) is that its predicted failure plane orientation is inconsistent with observation. For example, there are observation (Polak at al. (2009)) that under uniaxial tension/compresion cyclic loading the fatigue crack initiate in plane with maximal shear strain (stress) amplitude (under angle of 45° to free surface). As can be seen from fig. 1 the Findley prediction (based on material parameters given in table 1) are different.

Fig. 1. Damage distribution corresponding to Findley criterion as a function of plane orientation (a) S355; (b) C55.

2.2. Plane orientation defined by maximal range of shear stress (strain) Unlike the previous mentioned the criteria in this section define critical plane orientation based on experimental observation and definition of fatigue as a process caused by dislocation movement. The criteria defined on these principles define critical plane as a plane exposed to maximal shear stress (or strain) amplitude. However, this definition leads to some inconsistency when used for fatigue lifetime estimation under nonproportional variable amplitude loading. As a matter of fact, the definition of critical plane in the case of such a signal is based again on