Issue 33

C. Montebello et alii, Frattura ed Integrità Strutturale, 33 (2015) 159-166; DOI: 10.3221/IGF-ESIS.33.20 162 Figure 3 : radial and tangential evolution of d a . The projection of the reference fields on the velocity field allows us to extract I s and I a as follows:   ˙ ,            Ω Ω  N s s s i t i s i t N s s s s i i i v d v d I t I d d d d (4)   ˙ ,            Ω Ω  N a a a i t i a i t N a a a a i i i v d v d I t I d d d d (5) A first approximation of the velocity field is therefore obtained:           ,     e s s a a v x t I t d x I t d x (6) v e is the elastic approximation of the velocity field. Since the friction between the contacting bodies introduces a nonlinearity in the system a third term, v c , has to be added to the approximation presented in Eq. (6),       , , ,   c e v x t v x t v x t (7) Here, the Karhumen-Loeve decomposition is used twice: the first time to partition the residual velocity field in a product between two functions depending separately on time and space,             ,      c c c c c c v x t I t d x I t f r g (7) and the second time to analyze the evolution of dc with respect to r and θ, as presented in Fig. 4. Also in this case the field is normalized in order to assure that fc(r=0) is equal to 1. By contrast with the elastic reference fields, here the intensity decreases quickly far from the contact confirming the extremely localized effect of friction. The error introduced by the approximation can be defined as follows:         2 2 , , ,         Ω Ω e e v x t v x t ξ t v x t (8)           2 2 , , , ,          Ω Ω e c t v x t v x t v x t ξ t v x t (9) where  e is the error introduced considering just the elastic approximation while in  t all the terms presented in Eq. (1) are taken into account. In Fig. 5 the evolution of the error during a fretting-simulation is shown. It is worth noting that the total error falls below 5 %.