PSI - Issue 26

Costanzo Bellini et al. / Procedia Structural Integrity 26 (2020) 120–128 Bellini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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As concerns the ILSS, this parameter was calculated through the following relation: = 3 4 ℎ ⁄ That is quite different from the eq. 1, relevant to the flexural strength; in fact, in this relation the span length is not present, and the specimen thickness term is not quadratic. In this case, as visible from Fig. 5, the strongest laminate was the Type A, that one with only three layers bonded with adhesive, while the weakest was the same without resin; in fact, the former presented an ILSS of about 48 MPa, while the latter of about 40 MPa, so a decrement of about 17% was found. Moreover, the data scattering for the five specimens tested for each kind of laminate was quite low for the short beam too; in fact, the coefficient of variation obtained was less than 10% for all the types. It must be noted that the results of the laminate bonded with prepreg resin and constituted by five layers are not reported due to premature failure of this laminate under these load conditions. It can be concluded that the type of adhesive strongly influenced the resistance, while the number of layers was unaffecting. (2)

Fig. 5. ILSS of short beam specimens.

The flexural strength and the ILSS depend on two opposite factors: on one hand, the latter is enhanced by the adhesive presence, that improves the interfacial bonding; on the other hand, the adhesive diminishes the fibre volume content, and consequently the strength of the material is reduced too. Therefore, for the long beam specimen, in which the normal stress is prevailing, the best condition is the absence of the adhesive to obtain a higher fibre content, while for the short beam specimen, where the shear stress is predominant, the best solution is the attainment of a good interface connection between metal and composite, that is guaranteed by the structural adhesive. However, for a better comprehension of the failure mode, a deeper analysis of micrographies was carried out. The micrographies relevant to the breakage of the specimen with three layers bonded with structural adhesive, that is the type A, are presented in Fig. 6. It can be noted that the long beam specimen failure was due to the damage of the fibre, that resulted broken due to tensile stress, as visible in Fig. 6a, while the interface between the aluminium and the adhesive and that one between composite and adhesive remained undamaged (Fig. 6b). As concerns the short beam, it can be noted that also for this specimen the interfaces between the different materials remained intact, while the failure was due to intra-layer and inter-layer delaminations, as visible in Fig. 6c and 6d, respectively. As regards the type B laminate, that one with three layers bonded with prepreg resin, the micrographies are reported in Fig. 7. Also for this kind of laminate, the failure of the long beam specimen was determined by the fibre tensile failure in the composite materials, as visible in Fig. 7a. Moreover, the interface between composite and aluminium remained undamaged; in fact, in Fig. 7b it can be noted that a thin veil of composite resin was attached to the aluminium, while the delamination happened in the composite layer, just near the first fibres. As concerns the short beam specimen, in this case there was the delamination at the aluminium/composite interface, as witnessed in Fig. 7c, together with the intra-layer delamination, visible in Fig. 7d. The micrographies relevant to the type C