Issue 48

V. P. Berardi et alii, Frattura ed Integrità Strutturale, 48 (2019) 222-229; DOI: 10.3221/IGF-ESIS.48.23 227 properly determining the slips over extended areas respect to punctual displacement evaluation provided by traditional LVDTs. In such a way, it is possible to model a more accurate adhesive interface behavior and to track the debonding propagation. The strains provided by DIC technique, obtained by the displacements of 16 th -17 th ROIs, resulted in good agreement with those measured by the strain gage #2, except for the final debonding stage, close to the failure occurrence (Fig. 10). This is probably due to the formation of out of plane displacements that affect the accuracy and effectiveness of bidimensional DIC technique. The mean shear stresses at the adhesive interface, τ i,i+1 , acting between the cross-sections corresponding to the i-th and (i+1)-th GFRP strain gauges, were evaluated by means of strains variation measured along the loading x axis, according to the method proposed in literature [13-14]: τ i,i+1 =- E GFRP ·A GFRP · ሺ ε i+1 -ε i ሻ B GFRP · ሺ x i+1 -x i ሻ (1) being:  A GFRP =t GFRP ·B GFRP the cross sectional area of the composite laminate;  GFRP GFRP GFRP t , B and E the thickness, the width and the Young’s modulus along the fiber direction of the composite laminate, respectively. The shear stress vs. tangential slip curves are depicted in Fig. 11. These curves are representative of experimental evaluations of cohesive shear acting at the concrete-GFRP interface in a form coherent with a Cohesive Zone (CZM) approach and with recent literature results [21-32]. Figure 10 : Strains vs. time curve by DIC. Figure 11 : Shear stress vs. tangential slip curves. The value of the observed debonding force falls within the range of experimental outcomes obtained by the authors through a previous testing system on similar specimens made of the same GFRP lamina and epoxy resin [18]. Moreover, further experimental tests were carried out in order to validate the proposed testing system. The outcomes, in terms of debonding force, are reported in Tab. 2, thus assessing the reproducibility of the results. Specimen # Debonding force [kN] Mean value [kN] Standard deviation [kN] Variance [kN 2 ] 1 13.04 13.05 0.902 0.814 2 14.16 3 11.95 Table 2 : Debonding force values. 0 0.001 0.002 0.003 0.004 0.005 0 50 100 150 200 250 300 350 Strain [  ] Time [s] DIC strain Strain Gauge # 2 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Shear stres [MPa] Tangential slip [mm]  