The behavior of the graded cellular material under impact
Date of Issue2014
School of Mechanical and Aerospace Engineering
Nanyang Technological University
Due to their excellent properties, including low weight, high stiffness and strength and heat insulation, cellular materials are widely used in engineering applications, especially in the aerospace and defense industries, as energy absorption devices. Introducing a gradient in material parameters into the cellular material will significantly influence the behavior of the cellular material under impact loadings. Current research aims to investigate the influence of the gradient on the behavior of the graded cellular material under impact conditions, including the deformation mode, underlying mechanism, energy absorption capacity and the transmitted force on the protected structure. At first, the honeycomb, composing of regular two dimensional cells, is modeled in the Finite Element software (ABAQUS/EXPILCIT). The yielding stress of the parent material linearly increases or decreases along the loading direction, introducing a gradient in the quasi-static plateau stress. Constant velocities are applied to the impact plate. Deformation modes are identified from the observation of the deformation process. Empirical equations for critical velocities, at which the deformation mode transmits, are summarized in terms of the thickness ratio, yielding stress and the gradient. The analytical study, based on the finite element result of the one-dimensional cellular chain under impact, is conducted by using the one-dimensional shock theory. A uniform cellular rod with a gradient in the material’s yielding stress, is proposed in two impact scenarios. Double shock (DS) mode and Single shock (SS) mode are proposed to describe deformation processes in the graded cellular rod with negative and positive gradient, respectively. The analysis for both the scenarios shows that the gradient significantly influences the dynamic energy-absorbing capacity of the graded cellular rod when the mass ratio is small. To achieve higher energy absorption, the weakest part of the graded cellular material is suggested to be placed at the proximal end. Further analytical investigation focuses on a more practical problem with a gradient in the initial density gradient, since the quasi-static plateau stress and the locking strain are all related to the initial density. Analytical derivation is also conducted based on the one-dimensional shock theory. Similarly, the two basic deformation modes exist in such case with density gradient. The FE result shows similar trends to the analytical result and derivation is found at the densification stage due to the simplification of the rigid-perfectly plastic-locking material model. Finally, an experimental study is conducted by using a gas gun, impinging on a stationary foam block with a gradient in its cross-section. The experimental result confirms the existence of the two basic deformation modes in the graded cellular block. Due to the limitation of the equipment, the initial velocity only varies from 21 to 40m/s. Finite element simulations are employed to extend the study to cases with higher impact velocities. Similar to previous investigations, analytical modeling with one-dimensional shock theory is also implemented.
DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics