Issue 47

M.F. Funari et alii, Frattura ed Integrità Strutturale, 47 (2019) 277-293; DOI: 10.3221/IGF-ESIS.47.21 290 Numerical Results Figs. 11 (a) and (b) show the load / opening displacement curves numerically obtained with reference to both the schemes here considered. As shown, the constitutive behavior shown is initially linear and stable. Once the crack function criterion is satisfied, the curves show a sudden descending trend. A clearly different trend can be observed in the two descending branches: the point loading causes a snap-back, a phenomenon that is not evidenced when a uniformly distributed opening load is applied to the structure. In Figs. 12 and 13, the Von Mises stress maps at four subsequent load steps are shown together with crack propagation pattern for both loading configurations. Although in both cases a mixed mode fracture arises, the crack propagation paths exhibited by the two specimens are visibly distinguishable. Whereas the point load activates a mode II dominated fracture process I, the distributed load tends to cause a mode I dominated fracture mode. In sandwich structures, loads are typically applied to the face-sheets and these are transmitted to the core by the adhesive interface. The core behavior and its sensitivity to crack propagation is, therefore, strongly affected by the interfacial stress, which are, in turn, related to the external loads, the materials stiffness’s, and the cohesive law. In this view, the interfacial traction forces detected at the upper skin-to-core interface are shown in Figs. 14 and 15 for both load configurations analyzed. Fig. 14(a) depicts the distribution of the interfacial forces corresponding to the peak load of the linear elastic branch (A) in the distributed load case. The distribution of stresses is quite uniform, which suggest the cohesive interface is not threatened by any possible debonding phenomenon. The propagation of the crack in the core is able to slightly affect the distribution of the interfacial traction forces (Figs. 14(b), (c), and (d)), whilst it does not substantially modify the quality of interface response. (a) (b) (c) (d) Figure 12: Uniformly distributed opening force: Von Mises contour plots and crack evolution for different loading steps. (a) (b) (c) (d) Figure 13 : Point opening force: Von Mises contour plots and crack evolution for different loading steps. A similar analysis is illustrated in Fig. 15 (a-d) with reference to the point opening force scheme. As shown in Fig. 15(a), high values of the interfacial traction arises at the sandwich panel edge, in correspondence of the applied load. The onset condition could have been easily activated at the interface region but it is prevented, in this case, by the crack propagation in the core. This phenomenon tremendously affects the interfacial stresses, whose peak tends to move congruently with the crack tip position. However, a more even distribution of stresses suggests that an interface debonding is unluckily to occur at this stage. The results show how the interfacial stresses distribution can affect the crack propagation in the core and highlights the usefulness of a numerical model able to couple the two effects in an effective manner.