PSI - Issue 23

Krzysztof Kluger et al. / Procedia Structural Integrity 23 (2019) 89–94 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The specimens were manufactured from the rolled bar with a diameter of 22 mm of S355 steel at the delivered condition. In the final manufacturing process, the specimen surfaces were ground manually with abrasive paper with decreasing gradation and until the Ra surface roughness parameter within the range of 0.20 – 0.40 was obtained. The selected specimens were also polished using abrasive paste until the surface roughness parameter Ra = 0.12 was obtained. However, the fatigue lifetime of the specimens with a polished surface does not statistically differ from the results obtained for unpolished specimens. The total number of specimens is equal to 78. The results of experimental research were used for the determination of the fatigue characteristics for cyclic bending and torsion that are presented in Fig. 2 along with the values of the coefficient of the scatter band of lifetime at the confidence level equal to 0.95 ASTM E739-91 (1998). In this figure also the linear regression coefficients of the fatigue characteristics are shown along with the confidence band at the confidence level of 0.95. (a) (b)

Fig. 2. S355 steel fatigue characteristics for fully reversed cyclic: (a) bending (20 specimens), (b) torsion ( 26 specimens)

3. Multiaxial criteria For the verification of the concept of life dependent material parameters, the criteria fundamentally differing by the multiaxial stress reducing function and the definition of the critical plane were selected. Because the selected criteria are well documented in many scientific studies only a short description of them is presented below. Matake (1977) proposed a linear function reducing multiaxial stress state taking into consideration only the shear stress amplitude τ ns,a and normal ones , in the critical plane. The selected components of the stress vector are calculated in the plane of the maximum shear stress, and the form of the criterion is: , + , ≤ ǡ (1) where material parameter is determined from the formula: = 2 − 1 Ǥ (2) Adapting the Matake criterion according to the revised concept of the substitution of fatigue limits , by the characteristics ( ), ( ) the following function is obtained , + (2 ( ) ( ) − 1) , − ( ) = 0 Ǥ (3) The solution of equation (3) relative to the number of cycles determines the computed lifetime .