Issue 49

C. Bellini et alii, Frattura ed Integrità Strutturale, 49 (2019) 739-747; DOI: 10.3221/IGF-ESIS.49.66 745 interaction between factors was less than 0.5% and it can be neglected. However, for a more significant comparison the p- value was calculated too for each factor; the analysis confirmed that both the considered factors had an effect on the material flexural strength, since their value was less than 0.05, the commonly chosen a-level, instead the interaction term p-value was 0.688 and so it can be disregarded. Source DF Seq SS Contribution Adj SS Adj MS F-Value P-Value n. sheets 1 42087.7 53.41% 42087.7 42087.7 21.55 0.001 adhesive 1 23937.6 16.42% 12937.6 12937.6 6.62 0.024 n. sheet * adhesive 1 330.9 0.42% 330.9 330.9 0.17 0.688 error 12 23439.7 29.75% 23439.7 1953.3 total 15 78795.9 100.00% Table 3 : ANOVA analysis of experimental data. The flexural stress-strain curves of the tested specimens are reported in Fig. 6; in particular, a representative curve is shown for each specimen type since the data scatter was quite limited. The mechanical behaviour of all the four specimens was quite similar; in fact, they presented a first stress increment, almost linear till the maximum stress value was reached, followed by a pseudo-ductile behaviour, with a fluctuating trend characterized by stress increment and decrement. However, there are substantial differences, especially in the second pseudo-ductile phase and, as abovementioned, the maximum stress level obtained. On the contrary, in the first elastic phase the slope of the stress-strain curve, that represents Young’s modulus of the material, was quite the same for all the laminates, even if that slope was slightly steeper for the laminate without the adhesive. This conclusion can be justified by the fact that the adhesive is less stiff than the other materials and consequently it made the module of the whole laminate decrease. As concerns the post-first stress peak behaviour, the type A and type D laminates presented the highest stress drop, while for the type B and type C the drop was more reduced. In fact, the type A laminate lost about the 55% of its load capacity, while the type D about the 45%; this loss was equal to 20% for the type B and 2% for the type C specimen. After the stress drop there is a stress recovery for all the laminates, but the value of the second peak is quite different depending on the specimen type; in fact, for the type C the second peak value is quite similar to the first peak one, for the type B it corresponded to the 90% of the first peak while for the remaining laminates it was slightly higher than the drop value. These trends show that the type C laminate is the safest one, although it had the lowest flexural strength, since the stress drop after the first stress peak is low and the value of the second peak stress is like the first one. Finally, it must be noted that the residual load capacity at the end of the test is negligible for the all the specimen except the type D. C ONCLUSIONS ML materials, that consist of composite layers alternated to metal sheets, are more and more adopted in several industrial fields due to their very high mechanical characteristics features. In this work, the mechanical behaviour of different types of FML was analysed, investigating the effect of the staking sequence and the adhesive presence at the interface between metal and composite material. In this study, aluminium was considered for the metal part of the FML, while the composite material consisted of an epoxy resin reinforced by carbon fabric. The flexural behaviour of the produced laminates was analysed, paying attention not only to the maximum strength but also to the post-peak stress behaviour. The produced laminates had the same thickness and the same metal/composite material ratio, but the material distribution along the thickness was dissimilar in order to obtain different layer thickness without changing the surface density of the panels. From the test results it can be concluded that the highest strength was reached by the laminate with a single metal sheet and without adhesive, while the worst condition was represented by that one with two metal sheets and the presence of the adhesive; in fact, passing from the latter to the former there is a strength increment of about 30%. As concerns the specimens F