Issue 12

S. Marfia et alii, Frattura ed Integrità Strutturale, 12 (2010) 13-20; DOI: 10.3221/IGF-ESIS.12.02 15 often characterized by the peeling of a thin layer from the external surface of the body 1  . This observation leads to the consequence that the interface damage depends on the degradation state of the material constituting 1  . Thus, a coupling between the interface and body damage variables, D  and D  , respectively, arises. On the other hand, the independent evolution of the two damage variables can lead to physically unacceptable results. The mechanical system illustrated in Fig. 2, representing the scheme of a possible decohesion test, is considered. A damage evolutive model is adopted for the material constituting the body 1  ; regarding the interface, a mode II damage model is assumed. In particular, limiting the analysis to the tangential effect, i.e. neglecting the normal stress, the interface constitutive relationship is written in the form: (1) The damage parameter is function of the history of relative displacement as follows: (2) where the parameter D   is expressed by the relationship : (3) with 0 0 / T T T s K   the first cracking relative displacement and 0 2 / f T cT T s G   the full damage relative displacement, being 0 T  the peak stress on the first cracking relative displacement and cT G the specific fracture energy in mode II. The properties of the materials constituting the considered scheme are the following: 1 1 2 2 3 0 3 16000 MPa 0.2 230000 MPa 0.0 1500 N/mm 3 MPa 0.3 N/mm T T cT E E K G           (4) Figure 2 : Scheme for decohesion test. Finite element analyses are performed in order to evaluate the maximum value of the carried force max F before the decohesion of the body 2  from the body 1  , for different lengths of adhesion. Computations are developed considering the damage state of the body 1  assigned and constant during the whole analysis, while the interface damage evolves during the loading history. Thus, the body and interface damages are uncoupled. In Fig. 3, the value of max F is plotted versus the adhesion length. Note that I D denotes the initial value of the interface damage. It can be remarked that increasing the adhesion length, initially the value of max F grows, till the optimal adhesion length e  is reached, after which max F remains constant. Moreover, it can be emphasized that for higher values of the damage state of the body 1    1 T T T D K s         max min 1, history D D     0 0 f T T T f T T T s s s D s s s             