13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- same quadratic equation can be used [5]. There are two inflection points A and B at the force-displacement curves of FMLs. Inflection point A was caused by the crack generation at non-impact side of aluminium alloy sheet, and point B was the ultimate load of FMLs under low-velocity impact which was closed to the ultimate load of monolithic 2024-T3 sheets. The FMLs exhibit aluminium dominated behavior before first crack generated and exhibit fiber dominated failure behavior after the crack generated. The solid line in figure 2 was acquired by fitting the force-displacement curves of FMLs using quadratic and liner segmented function. 3. Numerical simulation To clarify the relationship between the internal and external damages, dynamic explicit analysis was carried out using the finite element software ABAQUS. The impactor and each layer in three dimensions in the FMLs were modeled as separate parts, see figure 5. Interface layers were inserted into each layer to model interlaminar delamination. The boundary conditions of model FMLs were held four edges fixed. An initial velocity in the vertical direction is prescribed to the impactor, simulating the impact velocity measured during the tests. The Isotropic elasticity and metal plasticity with ductile damage initial criterion and linear fracture energy damage evolution were used to simulate aluminum alloy failure mechanisms. Orthotropic elasticity in the plane stress field was used for each of the prepreg layers and interfaces layers. The Hashin damage initiation criteria and fracture energy damage evolution implemented in ABAQUS was applied to each ply in the prepreg layers to simulate their failure mechanisms. While the failure of interface layers were modeled as cohesive layers with the max traction-separation damage initial criteria and linear fracture energy damage evolution laws [10]. Figure 5. Finite element model Figure 6. The failure mode of the FMLs subjected to 14.6J impact Figure 7. The F-d (a), v-T(b) and Ud -U(c) curves of experiment and finite element results The finite element model corresponds well with the test results reasonably. Figure 6 shows the (a) (b) (c)
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