Issue 47

M.F. Funari et alii, Frattura ed Integrità Strutturale, 47 (2019) 277-293; DOI: 10.3221/IGF-ESIS.47.21 288 (a) (b) Figure 9 : MMB test: (a) influence of the core typology in terms on the loading-displacement curve; (b) Influence of the core typology on the nominal crack tip speed. Fig. 8(a) depicts the first set of results obtained in terms of dimensionless resistance curves. Whereas static and dynamic solutions are substantially overlapped for low value of v 0 , a remarkable increment of the peak load and a slightly unstable post-peak behavior can be observed when the value of v 0 is increased. Furthermore, the relationship between nominal crack tip speed and applied displacement is shown in Fig. 8(b). The crack speed tends to be larger in the initiation phase while subsequently tends to decrease according to a damped oscillation, during the process of delamination. The second set of results pertains the influence of the core mechanical properties (on the interfacial debonding under the dynamic loading rate described above and adopting a constant value of v 0 equal to 10 ms -1 . As shown in Tab. 4, the previously analyzed core (Divinycell H100) is used as reference material in comparison to a high- density core (Divinycell H250) and a low-density one (Divinycell H35). Fig. 9(a) shows that peak loads observed in different cores are generally similar, whereas marked differences in terms of initial stiffness are visible. In particular, the higher density specimen (H250) produces a slight increment in terms of stiffness and almost same peak load as the medium-density one. On the other hand, the lower density core (H35) can guarantee relevant deformation capacity during the crack propagation, whilst its performance in terms of stiffness and peak load are less remarkable. Finally, Fig. 9(b) presents an investigation on the nominal crack tip speed in function of the applied displacement. The results show that an increment in the core density does not produce relevant amplifications in the nominal crack speed. It is also evident that more deformable cores are associated with a delayed crack onset, and consequently a higher fracture toughness. C RACK PROPAGATION IN A SANDWICH STRUCTURE CORE n this set of numerical results aims to validate the capability of model to describe the crack propagation in 2D solids. With this purpose, a sandwich structure fixed at bottom skin with an initial horizontal crack is taken into consideration as reference case study (Fig. 10). Although the proposed model is able to describe both the skin/core interface delamination and the crack propagation in the core, the aim of these simulations is specifically to describe the crack path occurred in the core of the sandwich structures and verify its effects on the interfacial traction forces. Geometry, load conditions, and material properties The reference specimen has a 150 mm length and width equal to 35 mm. The core and skin thickness are 75 mm and 2 mm, respectively. The initial crack has a length to 25 mm and it is located at the core mid-depth. Two different loading conditions are investigated: I