13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- and fracture mechanics methods. Taking into consideration that the application of cohesive damage models in sandwich structures for the numerical simulation of the crack propagation is very limited [11-13]; a cohesive model in terms of cohesive parameters will also propose in this study in order to simulate crack propagation and kinking into the core of sandwich structures. Thus the scope of this study is to numerically investigate the crack propagation and crack kinking into the core of lightweight structures under mixed mode loading conditions and to present a cohesive damage model appropriate to simulate crack propagation into the core. Results from the computational analysis predict the crack growth and kinking under flexural loading. 2. Specimens and Loading Material Testing. The geometric and the loading conditions for the sandwich are shown in Fig. 1. The sandwich is composed of PVC-core and face sheets from isotropic glass (Table 1). The dimensions of the test specimen were L=228.6 mm (support span) and b=63.5 mm (width). The core thickness was 2t =12.7 mm and the face sheet thickness was 1t =2.28 mm. The overall thickness was 17.26 mm. Static tests were first conducted to generate ultimate strength data [2]. Flexural fatigue tests on sandwich beams were performed at room temperature under load control at a stress ratio of 0.1, using a sinusoidal wave form. Fatigue data was generated for a minimum of three specimens at stress levels of 90%, 85%, 80%, 75%, 70%, 65% and 60% of the ultimate flexural strength. Three distinct damage events take place before the failure of the specimen. At first crack initiation and propagation was observed on the compression side just below the top face sheet/core interface. This delamination crack was about 1-1.5mm below the interface. The crack runs parallel to the beam axis from the point of initiation towards the end support (Fig. 1). This first damage event occupies about 85% of the fatigue life. The propagated crack kinks at a certain distance and shears through the core thickness. The crack reaches the bottom face sheet/core interface. Finally, delamination takes place at bottom face/core interface using the separation of the core from the face sheet. This is also a rapid event and occupies the remaining 7-8% of fatigue life. Table 1. Specimen properties Material E ( )2 N mm ν Face sheets (isotropic glass) 16300 0.3 PVC foam core (R75 by DIAB [5]) 80 0.4 From the plot of the crack lengths via N (number of cycles) at different stress levels in [2], it is observed that the length of the crack depends on the stress level. Lower is the stress level; the longer is the crack in damage event-1. Thus, at lower stress levels ( ) 0.65, 0.70, 0.75 r = the first damage event dominates the fatigue life and consequently the crack propagation near the top face core-skin interface, may be used to develop a failure model for sandwich composites.
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