Issue 33

J. Toribio et alii, Frattura ed Integrità Strutturale, 33 (2015) 434-443; DOI: 10.3221/IGF-ESIS.33.48 440 -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 0 0.5 1 1.5 2 2.5 3 x=0  m x=86  m x=130  m x=210  m x=430  m x=600  m  (MPa) z (mm) 0 0.01 0.02 0.03 0.04 0.05 0 0.5 1 1.5 2 2.5 3 x=0  m x=86  m x=130  m x=210  m x=430  m x=600  m  P z (mm) (a) (b) Figure 8 : Axial distribution of hydrostatic stress for diverse depths ( x ): (a) general plot and (b) detail plot near the rod surface (zone with strong gradients). As in the case of the hydrostatic stress distribution, the plastic strain at the contact plane ( z = 0 mm) decreases with depth from the rod surface ( x ), and consequently the inwards gradient is progressively reduced as the variable x is increased becoming null for depths x > 600  m. However, the inwards gradient of equivalent plastic strains is negative, thereby; the hydrogen diffusion is not enhanced. This opposition is progressively annulled as the depth from rod surface is increased. So, two competitive factors are involved in the diffusion of hydrogen placed near to the contact between ball and bar. On one hand, the inwards gradient of hydrostatic stress enhances the diffusion of hydrogen out of the contact plane whereas; on the other hand, the inwards gradient of equivalent plastic strains is opposite, impeding the aforesaid diffusion. This effect is only noticeable near the contact zone and, therefore, the diffusion of hydrogen placed at deeper points ( x > 600  m) can be considered only driven by the gradient of hydrogen concentration in axial direction. C HEMICAL ANALYSIS : HYDROGEN TRANSPORT BY DIFFUSION or assessing the HA-RC-MF behaviour of the rolling rod, it is interesting to analyse the long-time behaviour of the component under hydrogen exposure. To this end, the steady state distribution of hydrogen concentration through the rod radius was obtained (Fig. 9) using eq. (3) and taking into account both hydrostatic stress and equivalent plastic strain. Plot is associated with infinite time (steady state solution from the mathematical point of view) or with thermodynamical equilibrium of the hydrogen-metal system (from the physical view point). 0 0.2 0.4 0.6 0.8 1 1.2 0 1 2 3 4  =60º  =20º  =10º  =5º  =2º  =0º C eq /C 0 r (mm) 0 0.2 0.4 0.6 0.8 1 1.2 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6  =60º  =20º  =10º  =5º  =2º  =0º C eq /C 0 r (mm) (a) (b) Figure 9 : Radial distribution of the hydrogen concentration for diverse circumferential coordinate  : (a) general plot and (b) detail plot near the rod surface. F