PSI - Issue 46

Przemysław Strzelecki et al. / Procedia Structural Integrity 46 (2023) 99– 104 Przemys ł aw Strzelecki / Structural Integrity Procedia 00 (2021) 000–000

101

3

2. Proposed method To describe the S-N curve can be used Basquin equation, which is suggested by ISO-12107, (2012) and is expressed as follow: log� � � � � � ���� � ��� (1) The least square method is recommended to estimate these parameters, which is describe in ISO-12107, (2012). Such estimated curve has a normal distribution, which is written as: ��log� � ��� �� � �� �� ���� ������ �� ���������� � ����� � � �� � (2) Results of the yield strength can be also expressed by the normal distribution, which has form as follow: � �log� � ��� �� � �� �� ���� ������ �� ������� � ������ � � �� � (3) The proposed method assumes that the S-N curve for 50% probability of failure is estimated for the small number of specimens e.g. 8 (eq. (2)) and the standard deviation is taken from the distribution of the yield strength (eq. (3)). Such distribution was written below. ��log� � ��� �� � �� �� ���� ������ �� ���������� � ����� � � �� � (4) The idea of developing the distribution given by eq. (4) was presented in Fig. 1. 3. Experimental results and verification of proposed method The proposed method was verified for tests results of steel S355J2+C (Strzelecki & Tomaszewski, (2016)) and stainless steel 1.4301 (Strzelecki et al., (2019)). Fatigue tests were performed by rotating bending machine, which was presented and verified in paper Strzelecki & Sempruch, (2012). Loads were applied with 28.5 Hz and 50 Hz frequencies for S355J2+C and 1.4301, respectively. Test results were presented in Fig. 2 (a) – (b) and Fig 3 (a) – (b) for S355J2+C and 1.4301, respectively. The estimated parameters for eq. (1) and (2) are shown in Table 1. A standard method is named S-N curve obtained acc ISO-12107, (2012) with at least 30 specimens. The tensile tests were performed by an Instron 8874 testing machine, which was presented in Fig. 2 (c) and 3 (c) for S355J2+C and 1.4301, respectively.

Table 1. Value of parameters for the estimated S-N curves. Material

Parameter Standard method Proposed method

-0.084 3.084 0.015 -0.089 3.118 0.021

-0.089 3.093 0.030 -0.094 3.137 0.024

m

S355J2+C

b σ

m

1.4301

b σ