Issue 48

C. E. Cruz Gonzalez et alii, Frattura ed Integrità Strutturale, 48 (2019) 530-544; DOI: 10.3221/IGF-ESIS.48.51 536 those reported by Cruz et al [23]. Finally, according to Akhavan et al [24]. the MPP5420 adhesive falls within the very ductile adhesive classification while the other two in the intermediate strength classification. As can be noted from Fig. 3, the mechanical behavior was different among the three adhesives even at first sight. The stress-strain graph for the DC-80 adhesive, reveals a straight line and small area under it, whereas the Betamate 120 depict a small transition between elastic-plastic behaviors but without significant difference between elongation values. In contrast, the MP55420 showed a transition between elastic-plastic behaviors, however its elongation was higher in comparison to the other two adhesives. The highest slope (expressed as Young modulus) was depicted by DC-80 adhesive and its Young Modulus is almost twice the Betamate 120 modulus and five times higher than MP5520 modulus. Single lap shear testing results The results, expressed as load-displacement graph, are depicted in the Fig. 4. The SLS for the 12.7 mm overlap length DC-80 joint was 5980±320 N, 5743±130 N for the 12.7mm overlap length Betamate 120 adhesive joint and 4269±280 N for the 12.7mm overlap length MP55420 adhesive joint. On the other hand, the 50.0 mm overlap length joint made with Betamate 120 adhesive revealed a SLS of 20980±320 N (3.50 higher than the 12.7 mm), in the case of the 50.0 overlap length joint made with DC-80 adhesive revealed a SLS of 17911±570 N (3.12 higher than the 12.7 mm) and finally 9250±176 N (2.20 higher than the 12.7 mm) for MP55420 joint. The displacement was 0.65 mm for the 12.7 mm overlap length group of adhesives, contrasting a 1.2 to 1.8 mm for the case of the 50.0 mm group. It worth mention, that the highest displacement was noted for the Betamate 120 adhesive, followed by the DC-80 adhesive and finally the MP55420. The DC-80 adhesive, reveals a broken glass like sound before break approximately at 60% of the maximum load. The MP55420, reveal some striations upon the spew fillet at 50% of its maximum load, because its ductility is higher than the other two adhesives. The 50.0 mm overlap group reveals some bending in the adherents being more pronounced in the steel side at approximately 80% of their maximum load. Figure 4: Load-displacement derived from SLS testing for: a) 12.7mm overlap length and b) 50.0 mm overlap length. The strain measurement revealed that the HSLA steel reached a maximum micro-strain of -300×10 -6 and -508×10 -6 at maximum load for 12.7 and 50.0 mm of overlap respectively, then according to Hooke law the stress at this point is below its yield stress. However, in the case of aluminum the higher strains were noted (see Tab. 2), however the Betamate 120 reaches a strain near to the aluminum yield stress (the value was 233 MPa in comparison to the aluminum yield stress of 250 MPa) at 90% of its maximum load. The above, suggests that maximum strain was withstand by the adhesive itself, since its mechanical properties are small in comparison to the adherends. After mechanical characterization, loads for the fatigue testing were selected according to ASTM procedures (50 % of the maximum shear load as initial) and some spectra to analyze the fatigue behavior in loads higher that 50 % and loads lesser than 50 %. However, for Betamate 120 adhesive, loads near to 90 % of its maximum could generate yielding and bending would act as stress riser. In that sense, 70 % of its maximum was selected and finally 30 % of the maximum load in order to obtain a point of comparison in a wide range of loading. Fatigue results Fatigue testing results are presented in Figs. 5, 6 and 7. Fig. 5 shows the S-N curves for a bonding with DC-80 adhesive for both 12.7 and 50 mm overlapping. Fig. 6 depicts the S-N curves for bonding with Betamate 120 for both 12.7 and 50