Issue 33

J. Toribio et alii, Frattura ed Integrità Strutturale, 33 (2015) 434-443; DOI: 10.3221/IGF-ESIS.33.48 438 According to this, plastic strains are distributed only over a plastic zone with ring shape spreading over 315  m from the periphery of the rod, as shown in the same radial distribution of plastic strain obtained for diverse radial planes (different  angles, cf. Fig. 5b) even for those closest to the contacting plane (  = 0º). This is due to the fact that plastic strains are only generated at the contacting plane. Outside of this zone, the von Mises stress is always lower than the material yield strength and consequently no plastic strain are generated in this region. Thus, the plastic strain remains the same in all sections of the rod. A progressive decreasing distribution is obtained with a small plateau close to the rod cylindrical surface of 50  m width. 0 0.005 0.01 0.015 0.02 4.4 4.45 4.5 4.55 4.6 4.65 4.7 4.75  =60º  =20º  =10º  =5º  =2º  =0º  P r (mm) (a) (b) Figure 5 : Distribution of equivalent plastic strain after the sixth loading cycle: (a) 3D view at the contact of one of the balls and (b) radial distribution for diverse hoop coordinates  . The first driving force for hydrogen diffusion, the inwards gradient of equivalent plastic strains, is negative and only affects the plastic strain ring near to the rod surface (Fig. 5). With regard to the second driving force for hydrogen diffusion, the gradient of hydrostatic stress, at the contact plane (  = 0º), a distribution of compressive nature in radial direction of such a variable is obtained, it progressively decreasing with depth up to becoming null for a depth from the rod surface of about 1 mm (Fig. 6b). Outside the contact plane the hydrostatic stress distribution is notably reduced, so that, for angles  higher than 20º, it is almost independent of such an angle (as it happened with the distributions of radial, hoop and axial stresses). The typical profile consist of compressive stresses over 200 μ m, tensile stresses for deeper points, and, in the case of radial coordinate lower than 4 mm, a null value for hydrostatic stress is obtained. -3500 -3000 -2500 -2000 -1500 -1000 -500 0 500 3.6 3.8 4 4.2 4.4 4.6  =60º  =20º  =10º  =5º  =2º  =0º  (MPa) r (mm) (a) (b) Figure 6 : Distribution of hydrostatic stress: (a) 3D view of the contacting plane and (b) radial distribution for diverse circumferential coordinate  .