Issue 51

A. Chiozzi et alii, Frattura ed Integrità Strutturale, 51 (2020) 9-23; DOI: 10.3221/IGF-ESIS.51.02 15 where     [ , , ] nn ns nt σ is the stress vector in the local reference system at k P , ( ) f σ is a suitable yield function and  λ is the (unknown) vector of plastic multipliers. In Eqn. (7),   u represents the relative velocities at points k P in the local reference system. The same quantity  u in the global reference system is defined as:      i i P P u u u (8) being  i P u and  i P u are the velocity vectors at k P on the two sides of the interface. Furthermore, the following relation must hold:      u R u (9) where  R is a rotation matrix. The yield surface ( ) f σ can be obtained by means of a homogenization technique based on the so-called Method of Cells (MoC), first introduced in [42] for unidirectional composites and recently applied to in- plane loaded masonry walls [43]. The reader is addressed to [26] for further details. Be the m -th plane linearizing ( ) f σ defined by the equation       1 m nn m ns m nt A B C . Thus, Eqn. (9) reduces to the equation:                                                   1 1 1 [ ] pl pl pl N M M m m m k s N k M M k t m m m k n N M M m m m A u u B u C u (10) for each collocation point k P , where    M M m is the plastic multiplier corresponding to the m -th linearizing plane and  M M PL N is the total number of linearization planes. On each interface j , internal dissipation rate in the local reference system reads:       int, j M M j S P dS σ u (11) FRP NURBS elements are assumed rigid, as well. Therefore, dissipation is allowed along interfaces between adjacent elements only, and is related to longitudinal stresses in the fibers direction. Again, with the aim of imposing plastic compatibility along FRP-FRP interfaces and correctly evaluating the internal dissipation rate, the mid-line of each interface has discretized into an assigned number  ( 1) F sd N of collocation points k P (see Fig. 5). Figure 5: FRP-masonry interface and local reference system. On each point k P , the local reference system ( , , ) F F F n s t is introduced, F n being the outward unit vector normal to the interface, F s is the longitudinal tangential unit vector and F t the transversal tangential unit vector. Velocity jumps are 3 3 