PSI - Issue 16

Grzegorz Lesiuk et al. / Procedia Structural Integrity 16 (2019) 51–58 Grzegorz Lesiuk et al. / Structural Integrity Procedia 00 (2019) 000 – 000

53

3

period prediction, phase I and III are also often used in kinetic fatigue fracture modelling. On the other hand, the nonlinear nature and various factors (mean stress effect, crack closure, microstructure, geometric constraints etc.) caused several uncertainties in fatigue crack growth lifetime predictions including linear KFFDs dependancy. Therefore, the simplest Paris law is still used as a power-law function for modeling of the KFFD. One of the major topics in fatigue fracture mechanics is still the dependence of the KFFDs from mean stress effect (Vosikovsky O., (1979), Noroozi et al. (2007), Szata and Lesiuk (2009), Lesiuk et al. (2018)). The schematic influence of R-ratio on fatigue crack growth curves is shown in Fig. 2. According to above, the main goal of the paper is the presentation of the new energy approach for a fatigue crack.

Fig. 2. Schematic influence of R-ratio on the kinetic fatigue fracture diagram.

2. Material properties For experimental investigations, two mild steels were chosen – P355NL1 and 18G2A (S355 grade). The P355NL steel is a fine grain low-alloy carbon steel certified for pressure vessels and pipes, with special requirements for low temperatures. P355NL1 is the EU-designation of the steel, according to the EN10028-3 standard. S355 is typical structural steel, commonly used in various engineering fields. The chemical composition of the tested steel is presented in Table 1. Table 1. Chemical composition (in % by weight) of tested materials, based on De Jesus et al. (2006), Rozumek and Macha (2006) Material C Mn Si P S Cr Ni Cu Fe P355NL1 0.133 1.38 0.35 0.014 0.0016 0.025 0.148 0.137 bal. 18G2A 0.200 1.49 0.33 0.023 0.0240 0.010 0.01 0.035 bal.

Static and cyclic mechanical properties for selected materials are collected in Tables 2 and 3 correspondingly.

Table 2. Static mechanical properties of analysed metallic materials, based on De Jesus et al. (2006), Rozumek and Macha (2006)

E (GPa)

J IC (MPa  m)

 (-)

 pl /  0.2 (MPa)

UTS (MPa)

material

P355NL1

568 535

418 357

205 210

0.275 0.300

n/a

18G2A

0.320