Issue 48

J.P.S.M.B. Ribeiro et alii, Frattura ed Integrità Strutturale, 48 (2019) 332-347; DOI: 10.3221/IGF-ESIS.48.32 340 Figure 6: Traction-separation law with linear softening law available in ABAQUS ® . G IC and G IIC calculation by the DCB and ENF tests G IC and G IIC were calculated by the CBBM data reduction method using the DCB and ENF tests, respectively. The P -  curves obtained for both tests were all consistent between specimens joined with each adhesive. Regarding damage growth of the specimens bonded with Araldite ® AV138, all attained a brittle failure, which grows abruptly throughout the bondline. On the other hand, both Araldite ® 2015 and Sikaforce ® 7752 suffered a gradual failure accompanied by a reduction of P . The R -curves depicted the expected steady-state crack growth for both loading modes, which represent the G IC and G IIC values [38]. Nevertheless, crack propagation presented few oscillations mainly owing to small fabrication defects and imperfections, adhesive mixing variations and crack arrest phenomena [39]. In the ENF test, G IIC can be measured up to the crack reaching the proximity of the loading cylinder, because at this zone the almost pure-shear stress state is cancelled due to the compression effect. Tab. 4 presents the average G IC and G IIC and respective deviation of all specimens bonded with the three adhesives. The highest standard deviation, normalized over the average fracture toughness, was 10.4% for the Araldite ® AV138 ( G IIC ), depicting the repeatability of the experimental tests. Comparing these values with the ones presented in Tab. 3, the following deviations were found for G IC : -30.0, +25.6 and +60.4% for the adhesives Araldite ® AV138, Araldite ® 2015 and the Sikaforce ® 7752, respectively. Regarding the deviations for G IIC , the values were: -7.4, -37.1 and +2.6% in the same order. These differences are justified by the dependence of G IC and G IIC with the specimens’ geometry, more specifically with h and t A , given that these parameters influence FPZ’ size in the vicinity of the crack tip [40]. Adhesive Araldite ® AV138 Araldite ® 2015 Sikaforce ® 7752 Fracture toughness G IC G IIC G IC Average 0.140 0.352 0.540 Deviation 0.012 0.037 0.041 Table 4: Average values of G IC and G IIC and respective deviation [in N/mm] obtained by the DCB and ENF tests, respectively. G I and G II calculation by the SLB tests The P -  curves of the SLB specimens bonded with the Araldite ® AV138 are depicted in Fig. 7 as an example of the repeatability between specimens of each adhesive. Generally, a good reproducibility was found between specimens bonded with same adhesive regarding the elastic stiffness (up to P m ). The only exception occurs with an Araldite ® AV138 specimen due to a higher a 0 value, nonetheless, this does not affect the measurements. Crack growth was usually stable, apart from few Araldite ® AV138 specimens in which, after P m is attained, the load falls abruptly due to crack growth instability associated to the brittleness of this adhesive. This can also be indicative that this particular adhesive is affected by the presence of small defects that trigger this behaviour [32]. The crack growth behaviour was also similar between specimens bonded with the same adhesive. On the other hand, the ductility of the Araldite ® 2015 and the Sikaforce ® 7752 led to a softening near P m . Fig. 8 shows example tensile (a) and shear (b) experimental R -curves for an SLB specimen with the adhesive Araldite ® AV138. One may notice that crack propagates at a fairly accurate steady-state value of G I or G II (for 130≤ a eq ≤180). This constant crack growth region was the one considered to estimate both G I or G II values. In fact, when the adhesives’ FPZ spreads to the loading cylinder, the toughness artificially increases owing to the compression effects of the applied load. The R -curves for the joints bonded with the Araldite ® 2015, compared to the ones showed in Fig. 8,