Issue 33

J. Toribio et alii, Frattura ed Integrità Strutturale, 33 (2015) 434-443; DOI: 10.3221/IGF-ESIS.33.48 439 For completing the analysis of hydrogen diffusion and accumulation on the rod during life in service, the information obtained from the estimation of hydrogen concentration in the radial direction [2,3] can be completed by a discussion of the implications of diffusion in the circumferential direction. To do so, the circumferential distribution of the variables affecting the hydrogen diffusion assisted by stress and strain is plotted in Fig. 5, considering diverse layers within the plastic zone. The circumferential distribution of hydrostatic stress shown in Fig. 7a reveals a local stress concentration in the vicinity of the contacting plane  = 0º where the maximum hydrostatic stress is placed. Within a range of planes around 5º, the hydrostatic stress progressively decreases, becoming almost constant for other values of  . This behaviour is observed for the distributions corresponding to depths around half size of the plastic zone (173  m approximately) with compressive stresses out of the affected zone. As the depth from the rod surface increases, the maximum value of the stress is suddenly decreased (a 90% for the depth around the size of the plastic zone x = 300  m, a 60% for the depth around half size of the plastic zone x = 173  m and a 25% just for a depth of 86  m). Beyond this depth the stress continuously decreases up to becoming almost null for deeper points. This way, in hoop direction hydrogen will be pumped out of the contact plane by means of a huge gradient of plastic strains. With regard to circumferential plastic strains, a minimum is placed close to the contact section (  = 0º) where a slight local maximum appears, thereby creating a gradient of plastic strains. This gradient drives hydrogen out of the contact plane to planes with a higher  . This effect is vanished with depth resulting almost null for depths from surface of 216  m and null for depths from the rod surface out of the plastic zone ( x > 315 μ m) observed in Fig. 5. Plastic strain slowly increases with the hoop coordinate  reaching a maximum value at   = 45º. So, hydrogen will be pumped out suddenly from the contact plane and lately is dragged slowly for points placed at higher hoop coordinates (due to slower gradient far from the contact plane). -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 -40 -20 0 20 40 x=0 x=86  m x=173  m x=216  m x=300  m x=700  m  (MPa)  º  0 0.005 0.01 0.015 0.02 -40 -20 0 20 40 x=0 x=86  m x=129  m x=173  m x=216  m x=300  m  P  (º) (a) (b) Figure 7 : Circumferential distribution of (a) hydrostatic stress and (b) equivalent plastic strain at diverse layers of the rod between the contacting balls. Finally, Fig. 8 shows the axial distribution of both hydrostatic stress and equivalent plastic strain for diverse values of depth from the rod surface ( x ). In the axial direction, a very located distribution of both hydrostatic stress and plastic strains near to contact plane is obtained. With regard to the hydrostatic stress distribution, the high compressive stress at the contact plane is progressively decreased as the distance from the contact plane ( z ) is increased, obtaining a null distribution of such a variable for z > 1.5 mm. As the depth from the rod surface increases, the hydrostatic stress at the contacting plane ( z = 0 mm) progressively decreases and, consequently, the inwards gradient of hydrostatic stress in the axial direction is reduced as the depth from the rod surface is increased. Thus, hydrogen placed close to the contact between ball and bar is also pumped in the axial direction due to the positive inwards gradient of hydrostatic stress. This effect is progressively reduced with the depth x becoming almost negligible for depths x > 600 μ m. Finally, the axial distribution of plastic strains appears through a narrow zone becoming null for axial distances z > 500 μ m.