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Mechanicalproperties of Foam Sandwich Composites with Low-Velocity Impact Damage

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Tutor: WangHai ChenXiuHua
School: Shanghai Jiaotong University
Course: Aircraft design
Keywords: composite,foam sandwich,impact damage,residual strength,numerical simulation
CLC: V214.8
Type: Master's thesis
Year:  2012
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Due to light quality, high specific strength and stiffness, smooth surface, lower hygroscopicity, good stability and fatigue resistance, composite sandwich structures have been widely used in the field of aeronautics and astronautics. However, property of inter-laminate and low-velocity impacted resistance is weak, the sandwich panels easily cause matrix cracking, fiber breakage, deboning between face-sheets and foam core and many other kinds of damage, which make the ultimate bearing capacity of overall structures to drop, badly destroyed the integrity of the composites sandwich structure, and a sharp drop in its compression strength, bending strength and bending stiffness drastically.This paper is based on the composites laminates gradually accumulated damage theory and the foam gradually crushed mechanism, the low-velocity impacted finite element model of composite laminated plate foam sandwich is established. The cohesive elements were involved between the composite face-sheets and the foam core. The Hashin¡¯s failure criteria were used to predict the in-plane failure of the composite face-sheets, and the foam crushable elastic-plastic constitutive and nonlinear finite element method were used to study the low-velocity impacted damage characteristics. The numerical results agree well with the experimental results, and it can be a very good method to predict the residual dent depth of the sandwich face-sheets and the cavity crushed by the foam core. Parametric studies were conducted to study the effects of the energy and core thickness parameters on the impacted damage and the residual compressive strength of the foam sandwich structures, and some practical valuable conclusions were obtained.Under the impact of 50J energy, composite foam sandwich structures mainly caused tensile and compression matrix cracking, fiber tensile fracture and shear damage, no serious fiber breakage and interlayer damage. At the same time, foam core crushed an obvious cavity and a residual indent on the upper facesheet.The simulation results are very good with the experimental values in a reasonable error scope, which illustrated that the finite element analysis method is reasonable and effective.Based on the right finite element methods, the in-plane mechanics properties of composite sandwich structures with foam core are further studied under different impact energy. With the increasing impact energy, the impact force is growing and the happened time of the maximum impacted force point is ahead, and the residual kinetic energy is increasing, and the indent depth of the upper face-sheet and the cavity of the foam core are also increasing.At the same time, the change rules of the in-plane mechanical properties are further studied under 50 J energy impact for composite sandwich panels with foam core during the 50 mm, 40 mm, 30 mm, 20 mm and 0 mm thickness. Along with the increase of foam thickness, the time happened earlier but the biggest impact-force value decreased, and the last remaining kinetic energy were reduced which illustrated that thicker sandwich can absorb more energy and the transformed kinetic energy is less at last. In the same impact energy, the types of damage are same for laminated foam sandwich structure with different foam thickness, and it is no significant effect on the impact damage. The foam sandwich structure of 0 mm thickness is just the corresponding composite laminates, and the laminate can cause delamination in the layers in the 50 J energy impact.A full numerical analysis was presented to simulate the failure process of compression after low-velocity impact for foam sandwich composites, which simulated the whole process of the impact of foam sandwich structure and damage propagation of the compressive loading of sandwich structure with impact-damage. During the Analysis, it omitted the artificial assumption of the damage state after the impact on the sandwich structure made by previous scholars, and directly transferred the impact damage of sandwich structure to predict the residual strength. As a result, it enhanced the prediction precision for the ultimate failure load and residual strength.Numerical simulated results confirmed the overall failure mechanism hypothesis of foam sandwich structure under compressive failure process after low-velocity impact: that the overall damage of sandwich structure is happened due to tensile fracture between the faceplates and foam core, lead to the faceplates completely deboned from the foam core. After deboned, the faceplates occurred the permanent bending deformation and elastic buckling in the vertical main direction of compression load, and the foam core happened the shear failure and permanent plastic deformation.
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