PSI - Issue 17

Jan Klusák et al. / Procedia Structural Integrity 17 (2019) 576–581 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

577

2

The fatigue resistance of HSS is a major concern since the structures are often cyclically loaded and it is well known that the fatigue resistance does not increase proportionally to the static strength of such materials, de Jesus et al. (2012). HSS used for this purposes has been less investigated than the more frequently used mild steels. This means relatively limited data for the safe design and partially deficient understanding of the fatigue behaviour of these materials. Most of the studies performed on HSS are focused on fatigue resistance of whole welded joints rather than investigation of the plain material, while the material microstructure of the component is strongly changed due to the processing, e.g. rolling . Steel S355 is one of the HSS often used in civil engineering industry. However, also in the case of S355 steel, most of the studies offer only analysis of the fatigue behaviour of welded components: Lewandowski and Rozumek (2016) and Gao et al. (2016). Pawliczek and Prazmowski (2015) analyzed fatigue properties of S355 steel under bending fatigue loading. They subjected the S355 steel specimens to block bending loads, with varying mean load value. The contribution by Rozumek et al. (2016) showed crack growth characteristics for S355 steel specimens under tension and bending with torsion loading, mixed modes I+II and I+III. Seitl et al. (2018) compared experimentally long fatigue crack growth rates in two grades of HSS (S355 J0 and S355 J2) and studied the influence of chemical composition and structure texture on the behaviour of long fatigue cracks. The advanced evaluation of experimentally obtain results for steel grade S355 J0 is published in Seitl et al. (2018a) for the stress ratio R = 0.1. The experimentally obtained results were discussed with results already published in de Jesus et al. (2012) and Adedipe et al. (2016).

2. Material tested and specimen preparation

The chemical composition of both steel grades is specified in EN 10025-2:2004 standard (2004) and is presented in Tab. 1.

Table 1. Chemical composition in weight percentage (wt. %) of the used steels according to EN 10025-2:2004.

Steel grade

C (max. %)

Mn (max. %)

Si (max. %)

P (max. %)

S (max. %)

N (max. %)

Cu (max. %)

CEV (max. %)

S355 J0 S355 J2

0.2 0.2

1.6 1.6

0.55 0.55

0.035

0.035

0.012

0.55 0.55

0.47 0.47

0.03

0.03

-

These two grades of material differ in chemical composition. Moreover, each steel grade was tested in two directions. Particularly, the specimens for fatigue testing were manufactured from a plate of material regarding the direction of rolling, see Fig. 1. As testing in the range of very high cycle fatigue (VHCF) is based on high frequency resonance, we have measured dynamic modulus and the mass density of both steel grades. The material characteristics were E d = 213.036  0.033 GPa,  = 7823.97 kg/m 3 for the steel S355 J0 and E d = 213.277  0.074 GPa,  = 7816.83 kg/m 3 for the steel S355 J2. Dynamic modulus was measured by non-destructive impulse excitation method. For these E d values specimens for VHCF were designed in order to reach intrinsic frequency 20  0.1 kHz. The geometry of the specimens is shown in Fig. 2. Specimens were loaded in fully reversible mode ( R = -1), while the level of loading was governed by the amplitude of displacements of the specimen’s terminations. The relation between displacement and stress level is expressed by a stress factor S f . The factors S f for the specimens designed as in Fig. 2 were S f (S355 J0) = 27.8862 MPa/  m and S f (S355 J2) = 27.91797 MPa/  m. The crack surface of the central part of the sample, where the crack is expected, was polished with sandpaper with 2400 grains/cm 2 . Due to high frequency loading, the effective cooling of the samples was realized by a closed circuit of flowing distilled water.