PSI - Issue 5

D. Rozumek et al. / Procedia Structural Integrity 5 (2017) 896–903 D. Rozumek et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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bending with torsion (Gasiak, Robak, 2011), and others described by Pook (1985), Richard et al. (2005) or by Lesiuk et al. (2017). The aim of this paper is to describe fatigue crack growth of S355 structural steel for mixed modes I+II and I+III. Nomenclature a 0 notch length a crack length A 5 elongation da/dN fatigue crack growth rate E Young’s modulus N number of cycles crack growth R load ratio  Poisson’s ratio  u ultimate tensile stress  y yield stress  K stress intensity factor range

2. Experimental procedure

2.1. Material and specimen

Flat specimens made of a low-alloy higher-strength steel according to the Polish-European Standard (PN-EN 10155) were tested. Metallographic tests of the material show a fine grained ferritic-pearlitic structure. Table 1 contains some monotonic quasi-static tension properties of the tested steel.

Table 1. Monotonic quasi-static tension properties of the material. Materials  y (MPa)  u (MPa) E (GPa) 

A 5 (%)

S355

357

535

210

0.30

21

The compact tension shear (CTS) specimens (mixed mode I+II) were manufactured from plate (thickness t = 8 mm) in rolling direction and specimens for bending with torsion (mixed mode I+III) were cut of the drawn bar, 16 mm in diameter and their dimensions were: length l = 90 mm, width w = 10 mm, thickness t = 8 mm (see Fig. 1). The CTS according to Richard (1985) specimens have the main dimension w = 42 mm, t = 8 mm with the initial mechanical notch prepared by electro-discharging machine (EDM) with the root radius ρ = 0.16 mm. The initial notch length was equal a 0 = 20 mm. The specimens for mixed mode I+III had an external unilateral notch, a 0 = 2 mm deep and with the rounding radius ρ = 0.2 mm. The notches were milled and their surfaces were polished after grinding. Unilaterally restrained specimens were subjected to cyclic bending with torsion (mixed mode I+III) with the constant amplitude of moment M a = 17.19 N  m (M a is total moment for combined bending and torsion).