Co-authors​ :

 Fabrizio QUADRINI (ITALY), Alice PROIETTI (ITALY), Leandro IORIO , Denise BELLISARIO , Loredana SANTO  

Hybrid composite laminates have been compression molded with elastomeric interlayer. Composite samples consisted of 2-ply carbon fiber plies, laminated with a commercial uncured rubber sheet in between. The final thickness of the rubber interlayer ranged between 70 and 900 µm, whereas the total thickness of the hybrid composite ranged between 400 µm (in absence of elastomeric interlayer) and 1293 µm. Figure 1 shows the 7 final carbon fiber laminates in the end of compression molding, whereas Figure 2 shows a stereoscopic image of the section of the thickest sample. An optimal adhesion was achieved because of co-curing of prepreg skins and elastomeric sheet. Bending tests have been carried out to measure the stiffness of all the samples. By increasing the thickness of the elastomeric interlayer, more soft materials is inserted in the composite laminate and rigid composite skins increase their distance. This two mechanisms have an opposite effect on the sample stiffness and, as a result, a maximum in the stiffness is measured close to 500 µm of interlayer thickness. In order to deepen this structural interaction between the different layers of the hybrid laminate, a finite element model (FEM) has been implemented. From the bending test of the neat laminate, the elastic properties of the composite skins were inferred. By using this properties and the experimental stiffness of the hybrid laminate with the thinnest elastomeric interlayer, a bulk elastic modulus of 40 MPa is inferred as well. However, if this modulus is used for all the other hybrid laminates, a linear correlation between the interlayer thickness and the laminate stiffness is found, whereas experimental data showed a maximum. It is evident that the elastomeric interlayer provides different elastic contributions in the different samples. Furthermore, if the elastic modulus of the elastic interlayer is inferred from each single experimental stiffness in analogy with the numerical procedure for the thinnest laminate, it is found that it decreases by increasing the interlayer thickness. As a general conclusion, the rubber interlayer in the different hybrid samples behave differently. If the thickness decreases, the inferred modulus increases, by 20 times in the experimented range. These results show the presence of a proximity effect for the elastic behavior of the rubber interlayer, which becomes stiffer when the contribution of its interfaces become stronger. In particular, passing from 70 µm to 120 µm, the inferred modulus halves. It is reasonable that further increases could be achieved if the technology would be able to reduce the rubber interlayer. At very low thickness, unpredictable results could be obtained, opening the route to a new class of multi-functional materials. In fact rubber interlayers provide high damping and impact resilience but affect the laminate stiffness. This limitation could be overcome by this proximity effect.