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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modifies the original material S-N curve at the notch root to the local component curve (see also Fig. 1). The stress distribution at the notch is taken into account by the fatigue factor n , which describes the multiple by which the fatigue limit of the material S-N curve should be increased to derive the local component curve: = � � � � (7) Because generally K t > K f , the fatigue factor is higher than 1.0, and the fatigue limit of the newly created local component curve lies above the material S-N curve. There are multiple theories, how to define the fatigue factor based on the relative stress gradient. Some of them modify not only the fatigue limit position, but also the other parameters of the component S-N curve (its exponent w ~ slope, and the number of cycles at the fatigue limit N FL ). Due to the limited space in this paper the variants of the method proposed by Femfat v. 4.4 (2006) and FKM-Guideline, Rennert et al. (2012) are provided in Tab. 1 to refer to various formulas defining component S-N curves. Table 1. Variants of RSG methods evaluated here. The formulas and coefficients should relate to the aluminum alloy covered in this paper. The only exception is the definition of the n parameter by the IABG method. The parameters relevant to steel had to be used in it, while no information for aluminum alloys was provided. 2.3. Critical distance approach (TCD) The core decision of the critical distance approach is the distance from the notch root, at which or along which the effective stress is to be read. The variants of the critical distance approach do not limit themselves only to reading the stress state at a given distance (the point method), but also variants computing the integral mean along the distance or within a given area or area can be found - see Susmel and Taylor (2007). The study here is limited to the point method providing the highest flexibility and quick applicability to practical industrial problems. The point method suggests that the effective stress can be found at the critical distance from the apex of the notch. The concept assumes that failure in the assessed component occurs when the effective stress, σ eff (see Fig. 1) exceeds the reference fatigue strength of material (of the unnotched component). The critical distance L CD can be derived according to Susmel and Taylor (2007) for the point method from this formula: ��,�� = � � � � �� �� �� �� � � (8) The above definition shows that L CD depends on two material properties, the threshold of the stress intensity factor Δ K th and the material fatigue limit  σ FL . Both should be determined at the same load ratio R as the evaluated load case. In this report, only experiments without any mean stress are evaluated, i.e. R =-1. Retrieval of the Δ K th parameter gets crucial, because it is often not determined experimentally even in the crack growth experiments. L CD,FL is relevant only for the fatigue limit evaluation, so Susmel and Taylor (2007) propose to define its complete dependency on the number of cycles by a power law similar to the S-N curve: �� = � (9) Method Version Siebel-Stiller 1+ �γ'∙10 -�.��∙� � ⁄��� IABG 1+0.45 ·  ‘ 0.3 Eichlseder 1 + � ��,���� ��,���� − 1� � ∙ � 2 � �.� 1 + � � 10 ��� � � � � � � � n w notched − 3 �1 + 1.8 ′ �.� � �.� + 3 N FL,notched Femfat v.4.4 Femfat v.4.4 �� ∙ 10 � � .� � � �.� ������� Femfat v.4.4 FKM v. 6 (2012) 5 N FL