Issue 31

R.D.S.G. Campilho et alii, Frattura ed Integrità Strutturale, 31 (2015) 1-12; DOI: 10.3221/IGF-ESIS.31.01 9 function of h . The deviation is somehow large, and whose justification lies on the experimental process to obtain G n c , which relies on a number of measured parameters and approximation functions, which are difficult to adjust to the experimental data [24]. While for the specimens with h =1 mm, a value of G n c =0.781±0.146 N/mm was obtained, improvements of 12.6, 37.7 and 40.2% were attained by increasing h up to 4 mm. These results show the stabilization of G n c for a given value of h (in this case between G n c =1.075±0.226 N/mm for h =3 mm and G n c =1.095±0.195 N/mm for h =4 mm a stabilization of G n c was found). y = 4.6496E+07x 6 - 1.1350E+07x 5 + 1.0422E+06x 4 - 5.1126E+04x 3 + 1.5765E+03x 2 - 5.6343E+00x + 4.8466E-03 R² = 9.9994E-01 0 0.4 0.8 1.2 1.6 0 0.02 0.04 0.06 0.08 G I [N/mm]  n [mm] Experimental law Polinomial (Experimental law) 0.02 . . . . 0.4 0 y ial ( eri e tal la ) Figure 9 : Experimental G n -  n law for one test specimen with h =3 mm and polynomial fitting curve (configuration 2). 0 0,4 0,8 1,2 1,6 0 1 2 3 4 G n c [N/mm] h [mm] . . .8 . Figure 10 : Average values and deviation of G n c as a function of h by the J -integral. This increase of G n c is reported in the literature because of the stress field variations ahead of the crack tip being dependent on the joint geometry, which highly influences the shape and size of the damage zone, and the local yield stress as well [25]. As it was discussed in previous works [26], thicker adherends provide an elevation of peel stresses further within the joint, shifting the loading conditions from peeling to cleavage, and giving a larger length for the damage zone. These findings are corroborated in the work of Azari et al. [27], regarding the adherend stiffness influence on the fatigue failure of bonded joints, which proved by Finite Elements that the plastic zone in adhesive joints between steel adherends was consistently higher than identical joints between aluminium adherends during the entire damage uptake process up to crack initiation. Mangalgiri et al. [28] justified this tendency with the plastic zone and stress distributions ahead of the debond tip. Actually, the plastic zone was bigger in length across the adhesive layer with increasing number of composite plies (and thus, increasing h ). Also, thicker adherends used a larger amount of the input energy to the specimen to develop a lengthier plastic zone, thus leaving less available energy for damage growth [29]. On account of this, higher values of G n c can be expected for joints with higher degrees of restraint (i.e., stiffer or thicker adherends).