PSI - Issue 13

Stanislav Seitl et al. / Procedia Structural Integrity 13 (2018) 1494–1501 Seitl et al./ Structural Integrity Procedia 00 (2018) 000–000

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These elements are often damaged under load values below those corresponding to the elastic stress limit; see e.g. Kruml et al. (2011), Kuběna et al. (2016), Plachý et al. (2017), Dinas et al. (2017), Wysokowski (2018). Generally, the materials used for manufacture of these structural elements contain defects and flaws (Sakagami et al. 2017), which have effect on durability, i.e. remaining service life. Therefore, it is important to know the fatigue properties of the steels used in complex design analysis among them the S355 steel grade, traditionally used in construction the fracture and fatigue properties of which are partially known, see De Jesus et al. (2012), Ribeiro et al. (2016), Bozkurt et al. (2017), Rozumek et al. (2017), Seitl et al. (2018) etc. In this paper, the fatigue properties (Wöhler’s field and crack propagation rate curve) of the S355 J0 steel grade are analyzed using traditional and probabilistic models. In the latter case, the ProFatigue and ProPagation software programs are applied in the assessment of experimental fatigue data of S355 J0 for derivation of the probabilistic S– N field and fatigue crack growth rate curve, respectively. The data consist of results for different number of various investigated specimens subject to low cycle and high cycle fatigue with focus on the rolling direction, see Fig. 1. . The results obtained are compared with the customary Basquin’s formula for the Wöhler-curve and Paris‘ law for the crack growth rate curve, both represented as straight lines in a double-logarithmic scale.

Nomenclature a

crack length

initial crack length

a 0 d a d N

increment of crack length

increment of lifetime as number of cycles

specimen thickness specimen width specimen’s height

B

W H

minimum applied stress intensity factor maximum applied stress intensity factor threshold stress intensity factor range limit stress intensity factor range

K min K max  K th

 K up

fracture toughness number of cycles

K IC

N p

probability

material constants of the Basquin‘s model S-N field parameters of the Castillo- Canteli model

A, B

 ,  , β ,B, C

Paris’ law constants index of dispersion

C, m

R 2

stress ratio

R

 , 

crack propagation rate parameters of the Castillo et al. model

2. Studied material, S355 J0 steel, and specimens’ geometries The chemical composition of the steel used for the experimental tests, as verified by producer and in agreement with the standard EN 10025-2:2004, is shown in Table 1. No heat treatment was applied. The yield strength is 355 MPa, the ultimate tensile strength ranges between 470 and 630 MPa, the density is 7850 kg/m 3 like any mild steel and the fracture toughness is around 40 MPam 1/2 , see e.g. Bozkurt & Schmidova (2017). The microstructure of the S355 J0 steel grade, consisting in equiaxed grains ferrite – pearlite, is shown in Fig. 2 where the rolling direction can be observed. The use of S355 J0 steel is particularly suitable because it offers a unique combination of good welding properties with guaranteed strength. As indicated in Fig. 1, the specimens of the S355 J0 steel were manufactured in the two rolling directions marked as ‘A’ and ‘B’. The geometry and dimensions of the compact tension specimens used for measurement of the crack pro pagation rate were L = 62.5 mm, W = 50 mm, B = 10 mm, a n = 12.5 mm, H /2 = 30 mm and the angle  1 = 60°, according to ASTM647, see Fig. 3 (a), whereas those of the round bars used in the S-N fatigue tests are shown in Fig. 3 (b).