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O.Ševeček et alii, Frattura ed Integrità Strutturale, 34 (2015) 362-370; DOI: 10.3221/IGF-ESIS.34.40 365 thickness of the inner AMZ layer is set to values between 30  m and 350  m; the length (depth) of the edge crack into the AMZ layer can be set in the interval 0  m to 400  m. For every thickness of the AMZ layer, a large number of simulations with different edge crack lengths was done. In the first step, when the edge crack length is equal to 0  m (no edge crack) the stresses  yy along the prospective crack path and potential energy of the uncracked body are calculated and saved for further postprocessing. In the next simulations, the edge crack of a given length is introduced in the model and the actual potential energy of the cracked body (for each crack length) is calculated again and used later for calculation of the incremental energy release rate G inc ( a ) and the energy release rate (ERR) G ( a ) at the crack tip as follows:        ( ) (0) ( ) / ( ) ( )/ inc G a W W a a G a dW a da , (1) where W is the potential energy which depends on the crack length – see [38]. For verification purposes, the ERR G (a) at the crack tip was also calculated in ANSYS simulation using the implemented CINT function (employing J-integral). It was found that it leads to practically same curves of ERRs as the energy approach based on Eq. (1). The biggest advantage of the energy approach is that we can directly calculate the incremental ERR G inc ( a ) as well as the ERR at the crack tip G ( a ). If the contour integral method is to be used, more care about the used mesh is necessary and also a recalculation of G ( a ) to G inc ( a ) has to be made in the postprocessing. Figure 3. Schematic of the fully parametric FE model, developed to study the propagation of edge cracks in the compressive AMZ layers. Material E [GPa]  [-]  x10 6 [K -1 ]  c [MPa] K Ic [MPa.m 1/2 ] G c [J/m 2 ] ATZ 390  10 0.22 9.8  0.2 422±30 3.2  0.1 25  2 AMZ 280  10 0.22 8  0.2 90±20 2.6  0.1 23  2 Table 1 : Material properties of the ATZ and AMZ layers [40]. Coupled stress-energy criterion In order to determine the suitable conditions for the onset of the edge crack, the coupled stress-energy criterion was employed – see [39, 41]. This criterion states that a crack originates if two conditions (i.e. stress and energy conditions) are fulfilled simultaneously – namely G inc ( a )≥ G c (AMZ) and  yy≥  c . The first condition means that there is enough energy x z y Laminate cross-section Circumferential edge cracks H=const.=3mm 2D FE model (1/4 symmetry) t 1 t 1 t 2 a W=4mm FE parametric study:  t 2 =30-350  m (step 10  m)  a=0-400  m (step 1  m) t 2 =100  m t 2 =350  m Element size 0.5  m t 1 = t (ATZ) t 2 = t (AMZ)