PSI - Issue 59

Jesús Toribio et al. / Procedia Structural Integrity 59 (2024) 198–205 Jesús Toribio / Procedia Structural Integrity 00 ( 2024) 000 – 000

199

2

Keywords: hydrogen embrittlement (HE); round notched specimens; slow strain rate testing (SSRT); stress-assisted hydrogen diffusion; hydrostatic stress; stress triaxiality; hydrogen-assisted microdamage (HAMD); tearing topography surface (TTS). 1. Introduction Hydrogen embrittlement (HE), with loss of ductility and decrease of fracture resistance, plays an important role in environmentally assisted cracking (EAC), that is, in all fracture processes of metals subjected to aggressive environments. Actually, when a sample is tested in a corrosive medium with electrochemical techniques, hydrogen embrittlement is a phenomenon associated not only with cathodic potentials (and cathodic overprotection), but also with anodic ones. The hydrogen transport phenomena in iron and steel have been studied thoroughly, but the results are sometimes contradictory, and the problem is not yet fully understood, although excellent classical papers were published in the past, analyzing the very different aspects of the deleterious phenomenon of hydrogen degradation in metallic materials (Hirth, 1980). This paper provides theoretical and experimental bases to elucidate the role of hydrostatic stress in hydrogen diffusion and embrittlement of pearlitic steels in the presence of notches. To this end, partial differential equations for stress-assisted diffusion of hydrogen in metals are formulated, and used to analyze the results of an ample experimental program on HE of notched samples of pearlitic steel. Consequently, the role of hydrostatic stress in hydrogen embrittlement phenomena will be outlined. 2. Theoretical statement The hydrogen diffusion model used in this study uses the diffusion equation (assisted by the hydrostatic stress field) developed by Van Leeuwen (1974), in which a hydrostatic stress dependent term is added to the Fick’s second law of diffusion, as follows:

c

 

 =  −   −  D c M c Mc

(1)



,

t

where c is the concentration of hydrogen in the steel, t the time, D the diffusion coefficient of the hydrogen in the metal, M a second coefficient (function of the previous one) and σ the hydrostatic stress. In absence of body forces, the equilibrium requires that Δ σ = 0. Thus hydrogen diffuses not only towards the points of minimum concentration, but also towards the locations of maximum hydrostatic stress, so that both the gradients of concentration and hydrostatic stress should govern the phenomenon of hydrogen transport. The second coefficient M in equation (1) is calculated by means of the following expression:

H DV

M

(2)

=

,

RT

V H being the partial molar volume of hydrogen in the metal, R the constant of the ideal gases and T the absolute temperature. The boundary condition corresponds to the Boltzmann distribution as follows:

H RT      V

c c =

0 exp

(3)

,

r



