PSI - Issue 18

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

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Matus Margetin at al./ Structural Integrity Procedia 00 (2019) 000–000

accumulated damage in each particular plan. The critical plane is than again the plane with maximal damage accumulated during the whole loading segment. The inconsistency isn’t only problem, the bigger problem is that the criteria often leads to overestimate damage in noncritical plane as the formulation of damage parameter is derived for specific plane (figures 2-3.). This leads to over conservative results. Two criteria (namely McDiarmid and Matake) from this category have been chosen in this study. McDiarmid criterion McDiarmid (1991) as well as Findley proposed that his criterion will be based on linear combination of shear stress amplitude and maximal normal stress acting on the plane during one load cycle. He defined that critical plane determining the fatigue lifetime will be the plane ongoing maximal shear stress amplitude. His formulation for final fatigue lifetime estimation can be described as: ���� � � ��� �� � ����� � � ′ ��� � � � � (3) Where t mcd is the material parameter which need to be experimentally obtained from fatigue tests. Unlike for Fyndley only one set of fatigue test is needed with combination with one static material parameter. Figures 2 show relative damage (ratio between damage in particular plane and damage in critical plane) as a function of plane orientation for both material parameters from table 1 in case of pure axial and pure torsion loading.

Fig. 2. Damage distribution corresponding to McDiarmid criterion as a function of plane orientation (a) S355; (b) C55.