Optimization of Anticlastic Core Structures for Lightweight Sandwich Panels

Abstract : This thesis investigates the mechanical behavior of all-metal sandwich panels with anticlastic core structures. Anticlastic core structures provide a cost-effective alternative to hexagonal honeycomb core structures as these can be produced in a mass production environment through progressive stamping and embossing. Fracture during manufacturing limits the space of possible microstructural configurations of anticlastic core structures. The optimization of the load carrying capacity of sandwich panels with anticlastic core structures is therefore conducted under the constraint of ductile fracture. The choice of a suitable ductile fracture model for predicting the fracture of the basis sheet materials plays a central role in this research. The first part of this thesis is entirely devoted to the experimental and numerical evaluation of the predictive capabilities of the original stress-based and the mixed stress-strain based damage indicator version of the Mohr-Coulomb failure model. Limiting our attention to tension-dominated stress states between uniaxial and equi-biaxial tension of thin low carbon steel, basic fracture tests such as notched tension and circular punch test are carried out to identify the fracture model parameters. A Hasek test and the recordings during the stamping of an anticlastic structure are used to assess the predictive capabilities of both fracture models. It is found that the damage indicator model with stress state dependent weighting function predicts the onset of fracture in the stamping test with greater accuracy. In the second part of this thesis, the Mohr-Coulomb damage indicator model is used to determine the maximum stamping depth during the making of anticlastic core layers as a function of the tool geometry. The large deformation behavior is described through a quadratic plasticity model with an isotropic hardening law and a non-associated flow rule. Design maps are developed that express the effective core shear stiffness, the core shear strength, the delamination strength and elastic face sheet buckling as a function of the core layer geometry. A method to optimize the performance of anticlastic sandwich panels is presented for simply supported rectangular panels subject to uniform lateral loading. The results from the four-point bending experiments demonstrate that the optimized anticlastic sandwich panels provide a higher strength and stiffness (at a lower weight) than foamed HDPE panels. The last part of this thesis investigates the puncture resistance of all-metal sandwich panels with a stamped anticlastic core structure. The effect of the parameters describing the core geometry and the choice of the substrate materials is studied. In particular, a low carbon steel, a DP780 dual phase steel and an ultra-high strength martensitic steel as candidate materials for the face sheets. A newly-developed damage indicator model with a weighting function based on Hosford's criterion is employed to model the fracture response of these materials. A parametric finite element study is performed on the puncture resistance of sandwich panels covering different cell sizes for the anticlastic core structure. The analysis of the simulation results reveals that the puncture resistance decreases with the cell size and core height respectively, i.e. the thinner the sandwich panel the higher the puncture resistance. Selected experiments are also presented to support this important finding.
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Submitted on : Tuesday, January 8, 2013 - 11:50:59 AM
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  • HAL Id : pastel-00771256, version 1

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Fabien Ebnöther. Optimization of Anticlastic Core Structures for Lightweight Sandwich Panels. Mechanics of the structures [physics.class-ph]. Ecole Polytechnique X, 2012. English. ⟨pastel-00771256⟩

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