Issue 47

F. Cucinotta et alii, Frattura ed Integrità Strutturale, 47 (2019) 367-382; DOI: 10.3221/IGF-ESIS.47.27 376 increasing displacement at the upper skin). Thus, a fixed support constraint was applied to the lower punches, and an increasing displacement was imposed to the upper punches. Fig. 10 (a) represents this boundary conditions in the FE model: the core elements were hidden in the figure to better show the skin surfaces, which are geometrically overlapped to the core and would not be easily visible in the full model. The contacts between the punches and the skins were set to frictionless. Fig. 10 (b) shows the displacement map corresponding to the maximum displacement as an example. A more complete result presentation and comparison with experimental analysis is provided in the Section Results and discussion. (a) (b) Figure 10 : Bending test modelling: (a) boundary conditions and (b) vertical displacement map. The impact test, which is less trivial and more interesting, was finally simulated. A transient structural analysis was set up by updating the geometry of the specimen to reproduce the dimensions reported in Fig. 9. A further modelling effort was spent to properly reproduce the boundary conditions. In the actual test, the specimen is held between two bolted plates, which are tightened through four bolted joints with no preload. This assembly is then placed on the top of the testing machine, and no further constraints are applied. Preliminary simulations shown that the direct application of fixed constraints to the specimen surfaces would overestimate the equivalent stiffness, giving great discrepancies with respect to experimental results. To properly reproduce the actual boundary conditions, both upper and lower steel plates were added in the geometrical model, as surface bodies. Standard isotropic homogeneous steel properties were set for both plates. The connection between the plates was modelled through beam elements (Beam188) representing the screws diameter (steel properties). A further surface body was introduced, representing the machine top plane and modelled as an infinitely rigid body. Finally, the striker was modelled as a rigid surface tangent to the specimen skin. A mass element was attached to this surface to represent the actual striker mass. Since the striker in the machine is constrained to move vertically along a single translational degree of freedom, all the other inertia properties were neglected. The interaction between all the different entities was modelled exploiting different contact regions. The contact between the lower steel plate and the machine plane was set to be frictionless, thus allowing all the relative displacements except interpenetration. The contacts between both steel plates and the specimen, and between the striker and the specimen were set to be frictionless. Remote boundary conditions were applied to the striker to constraint any degree of freedom except the vertical displacement. A fixed constraint was applied to the surface representing the machine top. Finally, the actual loading conditions were determined by imposing an initial velocity to the striker corresponding to the simulated impact energy. Fig. 11 (a) shows a section view of the described FE model, reporting the main parts (the surface bodies are repented as thick bodies just for graphical purposes, while they are meshed with shell elements). Fig. 11 (b) shows the vertical displacement map corresponding to the maximum displacement as an example. A more complete results presentation and comparison with experimental analysis is provided in the next section. R ESULTS AND DISCUSSION he four-point bending tests have been carried out on three specimens for each sandwich typology. The experimental results for each kind of sandwich are reported in Tab. 10 (mean values). The considered quantities, derived from the load-displacement curve, are: the maximum load before the failure (i.e. the peak of the curve), the deflection at the maximum load and the specimen stiffness (i.e. the slope of the curve at the origin). The two specimens have different weight and lay-up sequence but both have to respect the same rules. This discrepancy in design idea leads to a different T Upper skin Lower skin Core (unshown) Imposed vertical displacement Fixed supports