PSI - Issue 20

Valeriy Lepov et al. / Procedia Structural Integrity 20 (2019) 24–29 Valeriy Lepov et al / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction The damage accumulation concept in fracture mechanics allows explaining the structural scale effect and mechanical properties degradation, and exhaustion of the equipment lifetime, proposed by Kachanov (1974) and Bolotin (1990), during operation also. The physical interpretation of such the damage permits the probability interpretation by Freudenthal (1975), but the point is that multiple factors should be taking into account so the process of the measure of distribution s revealing couldn’t be successful an ytime. After all, except the fluctuation nature of point lattice defects by Bartenev and Zuev (1964), and the dislocations that form at the boundary of grains under the influence of internal and external stress, there are many other flaws in the metal such as pores and inclusions, and surface imperfections stipulated by fabrication and processing operations. All that flaws could grow due to corrosion and degradation processes described by Mikhailov and Lepov (1999) and Botvina (2008). But the mechanism of delayed fracture or hydrogen embrittlement (HE) has a primary role in stress corrosion cracking and pitting of structural steels. The interest to HE phenomenon increase again last years because of increased application and temperature range of high-strength steels and alloys, as shown by Gonzáleza et al (2018), and in Envir.-Ind. Crack. Mater. (2008) and Stress Corr. Crack. (2011). The analysis of damage accumulation mechanism in structure members under the combined action of hydrogen and elastoplastic strain has been presented in this paper. The interconnection and lack of clarity of the processes of HE phenomenon make such a problem very complex for direct numerically solution.

Nomenclature c

Rayleigh wave velocity, m/s hydrogen content, sm 3 /100 g

C

DBT

low-temperature ductile-brittle transition

k Boltzmann constant, eV/K KCV impact toughness, J/sm 2 HE hydrogen embrittlement t time, s T temperature, K U 0

activation energy of bound spontaneous rapture in unstressed crystal lattice, eV

scalar parameter of damage



structure sensitive parameter, eV  MPa

 μ

shear modulus, MPa specific density hydrostatic stress, MPa

 

durability at given stress level, s thermal vibration period, s

  0

2. A mechanism of brittle fracture under the hydrogen influence The significant progress in understanding of the mechanism of low-temperature ductile-brittle transition (DBT), hydrogen transfer and kinetics of delayed fracture has been made in recent years, particularly, for bcc structural steels and alloys, by Achikasova and Lepov (2015), Nagornyy et al (2015), Chernov et al (2016). Thus, first authors confirm the phonon-dislocation mechanism of DBT owing to the value of internal friction activation energy for armco iron and low-carbon high-strength steels. The study of fracture mechanism and internal friction for metals with different types of crystal lattice came to the same results. Suddenly the last authors had been found that the dislocation mobility and the cold resistance of steels improve by neutron activation of defect structure. The study of hydrogen influence on strength for static and impact loading of steel probes remains highly actual in the field of brittle strength of metals at low climatic temperature research. Even for high impact strength steels as is well known the significant decrease of the cold brittleness threshold has been observed at low temperature. It has