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Lightweight cellular materials such as foams exhibit excellent energy absorption characteristics and are widely used for impact mitigation in a variety of applications. In this study a modeling framework is developed in order to investigate the crushing behavior of Al-alloy open-cell foams under quasi-static and dynamic loadings. Quasi-static crushing produces a response that exhibits a relatively stiff linearly elastic regime that terminates into a load maximum; it is followed by an extended load plateau during which localized cell crushing initiates and gradually spreads throughout the specimen. When most of the cells are crushed the densified material stiffens again. Quasi-static compression is simulated using micromechanically accurate foam models. Skeletal random models are generated from soap froth using the Surface Evolver software. The linear edges of the skeletal microstructure are then dressed with appropriate distributions of solid to match those of ligaments in the actual foams and their relative density. The ligaments are modeled as shear-deformable beams with variable cross sections discretized with beam elements in LS-DYNA, while the Al-alloy is modeled as a finitely deforming elastic-plastic material. Utilization of the beam-to-beam contact algorithm of the code is an essential component of the simulation of crushing. Such models are shown to reproduce all aspects of quasi-static crushing faithfully. Dynamic crushing experiments on the same foam have shown that specimens impacted at velocities of 60 m/s and above develop nearly planar shocks that propagate at well-defined velocities crushing the specimen. The same modeling framework is used to simulate these impact experiments. It is demonstrated that random foam models reproduce essentially all aspects of the dynamic crushing behavior observed experimentally. This includes the formation and propagation of shocks, the stresses at both ends, the Hugoniot strain, and the linear relationship of shock front vs. impact velocities. The same models are also used to examine the transition from quasi-static to shock front type crushing. In addition, a detailed parametric analysis is performed to examine the effect of relative density on the crushing response, from the quasi-static initiation and plateau stresses to the formation of shocks and the associated Hugoniot. / text

Thesis (Ph. D.)--Lehigh University, 2005. / Includes vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 1996. / Vita. Includes bibliographical references (leaves [152]-161).

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Metallic foams have become widely available and have unique properties that make them attractive for use in a variety of engineering applications. Due to their complex structure, the behavior of foams under complex loading conditions is a subject of continued research. Digital volume correlation is a technique wherein full-field strains in three dimensions can be measured from high resolution x-ray CT image data. This technique was employed to measured strains in two commercially available aluminum foams, one each of open and closed-cell morphology, under two complex loading scenarios: rigid spherical indention, and uniaxial compression of a sample with a central hole. In addition to comparing the behavior of the two foams, results are also compared to strain fields analytically predicted by a third-party constitutive model implemented in finite element analysis. Under indention loading, the two examined foams showed a distinct difference in deformation and strain field, however the foams behaved similarly under uniaxial compression of rectangular samples with central holes. The constitutive model was found to be unsuitable for modeling the experimentally measured foams. / Graduation date: 2004

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2002. / Includes bibliographical references (leaves 154-166). Also available in electronic version. Access restricted to campus users.

Title from PDF of title page (University of Missouri--Columbia, viewed on Feb 25, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Dissertation advisor: Dr. Fu-hung Hsieh. Vita. Includes bibliographical references.

This study was designed to examine the effect of impact velocity on the crushing behavior of open-cell aluminum foam over a range of velocities similar to what would be encountered for impact mitigation and blast protection applications. An experimental set-up was designed, fabricated and validated for studying the crushing response of cellular materials at high velocities. It consists of a gas gun, a pressure bar, high-speed data acquisition and high-speed imaging. The facility uses high-speed video images of the crushing event synchronized to force measurements with a pressure bar at one end of the foam to examine the dynamic stress and deformation history of foam specimens. Ten pores per inch open-cell Al-6101-T6 Doucel foam cylindrical specimens with a relative density of about 0.085 were impacted in the rise direction at velocities ranging from 21.6 to 127 m/s. The experimental results show that for impact speeds greater than about 40 m/s crushing of the foam occurred through a shock front. Furthermore, the experiments show an increase in the densification strain, average stress in the crushed region and shock velocity with increasing impact velocity, whereas the stress in the uncrushed region appears to be insensitive to velocity. A method of determining the states across a shock front was derived from shock equations by enforcing conservation of mass and momentum. This was verified through a combination of experiments and direct measurements. The use of high-speed imaging and pressure bar measurements allowed this derivation to be independent of any constitutive model and showed that the assumptions in the commonly used rigid-perfectly-plastic-locking model are not applicable for dynamic impacts. A shock Hugoniot was generated from the test data to characterize the impact response of the foam. / text

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