A Ceramic Damage Model for Analyses of Multi-Layered Ceramic-Core Sandwich Panels Under Blast Wave Pressure Loading

A damage model for ceramic materials is developed and incorporated into the geometrically nonlinear solid shell element formulation for dynamic analyses of multi-layered ceramic armor panels under blast wave pressure loading. The damage model takes into account material behaviors observed from multi-axial dynamic tests on Aluminum Nitride (AlN) ceramic. The ceramic fails in a brittle or gradual fashion, depending upon the hydrostatic pressure and applied strain-rate. In the model, the gradual failure is represented by two states: the initial and final failure states. These states are described by two separate failure surfaces that are pressure-dependent and strain-rate-dependent. A scalar damage parameter is defined via using the two failure surfaces, based on the assumption that the local stress state determines material damage and its level. In addition, the damage model accounts for the effect of existing material damage on the new damage. The multi-layered armor panel of interest is comprised of an AlN-core sandwich with unidirectional composite skins and a woven composite back-plate. To accommodate the material damage effect of composite layers, a composite failure model in the open literature is adopted and modified into two separate failure models to address different failure mechanisms of the unidirectional and woven composites. In addition, the effect of strain-rates on the material strengths is incorporated into the composite failure models. For finite element modeling, multiple eighteen-node elements are used in the thickness direction to properly describe mechanics of the multi-layered panel. Dynamic analyses of a multi-layered armor panel are conducted under blast wave pressure loadings. The resulting dynamic responses of the panel demonstrate that dynamic analyses that do not take into account material damage and failure significantly under-predict the peak displacement. The under-prediction becomes more pronounced as the blast load level increases. Numerical analyses also indicate that the multi-layered armor design, while tailored for penetration resistance, performs poorly against blast shock wave. An alternative design is proposed and its performance is compared with the original design. Computational modeling of the fundamental material behaviors of ceramics would help expanding the use of ceramics to other structural applications, via enabling designers to efficiently explore design options

Design of Organic Tandem Solar Cells Using PCPDTBT: PC61 BM and P3HT: PC71BM

We conducted optical and electrical simulations with the goal of determining the optimal design for conjugated polymer-fullerene tandem solar cells using poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H-cyclopenta[2,1- b;3,4- b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT): [6,6]-phenyl C61 butyric acid methyl ester (PC61 BM) as a bottom cell and poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as a top cell. The effects of photon density, absorption, balanced and unbalanced charge carrier transport, and bimolecular recombination in the two subcells were incorporated into the simulations. We found that the maximum energy conversion efficiency (η) is 9% when charge carrier mobilities in both top and bottom cells are balanced. However, the efficiency drops significantly if the carrier mobilities are unbalanced in either the top or bottom cell. In addition, we found that unbalanced carrier mobilities in the top cell require a reduction in the thickness of the bottom cell whereas unbalanced bottom cell mobilities require an increase in the thickness of the bottom cell to compensate for the reduced current. © 2010 American Institute of Physics

An Instantaneous Impact Point Guidance for Rocket with Aerodynamics Control

This paper aims to propose a new guidance algorithm for a rocket with aerodynamics control for launch operations, based on the concept of the instantaneous impact point (IIP). In this study, the rocket with aerodynamics control is considered with the purpose of reducing dispersion of the impact point after separation of the rocket for safety reasons. Since a very limited aerodynamic maneuverability is typically allowed for the rocket due to the structural limit, a guidance algorithm producing a huge acceleration demand is not desirable. Based on this aspect, the proposed guidance algorithm is derived directly from the underlying principle of the guidance process: forming the collision geometry towards a target point. To be more specific, the collision-ballistic-trajectory where the instantaneous impact point becomes the target point, and the corresponding heading error are first determined using a rapid ballistic trajectory prediction technique. Here, the trajectory prediction method is based on the partial closed-form solutions of the ballistic trajectory equations considering aerodynamic drag and gravity. And then, the proposed guidance algorithm works to nullify the heading error in a finite time, governed by the optimal error dynamics. The key feature of the proposed guidance algorithm lies in its simple implementation and exact collision geometry nature. Hence, the proposed method allows achieving the collision course with minimal guidance command, and it is a desirable property for the guidance algorithm of the rocket with the aerodynamics control. Finally, numerical simulations are conducted to demonstrate the effectiveness of the proposed guidance algorithm

Booster Dispersion Area Management through Aerodynamic Guidance and Control

The disruptive evolution of rockets design observed during the last decade is pushing agencies and companies to re-plan their strategy with the ultimate goal of partially or totally reusing boosters. This paper describes the first steps made in the study of guidance and control strategies aiming at reducing the dispersion area of the first stage of a rocket through aerodynamic steering, and compares the results for a low L/D body. Numerical results show that the introduction of such a mechanism reduces effectively the dispersion area compared to passively stabilized boosters

A multi-level depiction method for painterly rendering based on visual perception cue

International audienceIncreasing the level of detail (LOD) in brushstrokes within areas of interest improved the realism of painterly rendering. Using a modified quad-tree, we segmented an image into areas with similar levels of saliency; each of these segments was then used to control the brush strokes during rendering. We could also simulate real oil painting steps based on saliency information. Our method runs in a reasonable fine and produces results that are visually appealing and competitive with previous techniques

A multi-level depiction method for painterly rendering based on visual perception cue

International audienceIncreasing the level of detail (LOD) in brushstrokes within areas of interest improved the realism of painterly rendering. Using a modified quad-tree, we segmented an image into areas with similar levels of saliency; each of these segments was then used to control the brush strokes during rendering. We could also simulate real oil painting steps based on saliency information. Our method runs in a reasonable fine and produces results that are visually appealing and competitive with previous techniques

Numerical Derivation of Buckling Knockdown Factors for Isogrid-Stiffened Cylinders with Various Shell Thickness Ratios

This study is aimed at providing a numerical derivation of the shell knockdown factors of isogrid-stiffened cylinders under axial compressive loads. The present work uses two different analysis models such as the detailed model with modeling of numerous stiffeners and the equivalent model without modeling of stiffeners for isogrid-stiffened cylinders. The single perturbation load approach is used to represent the geometrically initial imperfection of the cylinder. Postbuckling analyses using the displacement control method are conducted to calculate the global buckling loads of a cylinder. The shell knockdown factor is numerically derived using the obtained global buckling loads without and with the initial imperfection of the isogrid-stiffened cylinder. The equivalent model is more efficient than the detailed model in terms of modeling time and computation time. The present knockdown factor function in terms of the shell thickness ratio (radius to thickness) for the isogrid-stiffened cylinder is significantly higher than NASA’s knockdown factor function; therefore, it is believed that the present knockdown factor function can facilitate in developing lightweight launch vehicle structures using isogrid-stiffened cylinders

A multi-level depiction method for painterly rendering based on visual perception cue

International audienceIncreasing the level of detail (LOD) in brushstrokes within areas of interest improved the realism of painterly rendering. Using a modified quad-tree, we segmented an image into areas with similar levels of saliency; each of these segments was then used to control the brush strokes during rendering. We could also simulate real oil painting steps based on saliency information. Our method runs in a reasonable fine and produces results that are visually appealing and competitive with previous techniques