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1. Effect of Laser Forming on the Energy Absorbing Behavior of Metal Foams

   https://doi.org/10.1115/1.4051285  

   Zhang, Tom; Liu, Yubin; Ashmore, Nathan; Li, Wayne; Yao, Y. Lawrence (January 2022, Journal of Manufacturing Science and Engineering)

   null (Ed.)

   Abstract Metal foam is light in weight and exhibits an excellent impact-absorbing capability. Laser forming has emerged as a promising process in shaping metal foam plates into desired geometry. While the feasibility and shaping mechanism has been studied, the effect of the laser forming process on the mechanical properties and the energy-absorbing behavior in particular of the formed foam parts has not been well understood. This study comparatively investigated such effect on as-received and laser-formed closed-cell aluminum alloy foam. In quasi-static compression tests, attention paid to the changes in the elastic region. Imperfections near the laser-irradiated surface were closely examined and used to help elucidate the similarities and differences in as-received and laser-formed specimens. Similarly, from the impact tests, differences in deformation and specific energy absorption were focused on, while relative density distribution and evolution of foam specimens were numerically investigated. 

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2. Full‐Field Analyses of Density‐Graded Elastomeric Foams Under Quasistatic and Impact Loadings

   https://doi.org/10.1002/adem.202300994  

   Smeets, Mark; Kauvaka, Paul; Uddin, Kazi; Koohbor, Behrad; Youssef, George (October 2023, Advanced Engineering Materials)

   Density‐graded elastomeric foams are emerging as effective protective structures to guard humans against mechanical loading. This research investigates the deformation of ungraded and graded foams under quasistatic and impact scenarios using digital image correlation (DIC). The graded samples are assembled using two interfacing strategies (seamless and adhered), leveraging the adhesiveness of the foam slurry and bulk polyurea, respectively. Deformation mechanisms, including the effect of the interface type on strain transduction and localization in density‐graded structures, are imperative for improving the impact efficacy of protective paddings. Cuboid foam plugs are subjected to quasistatic and impact loading while recording the corresponding deformation for DIC analysis. The DIC results are separated into three case studies based on the number of layers (1, 2, and 3). The interface effect on the overall mechanical performance of polyurea foam is revealed from the bilayer, monodensity samples, showing drastic differences between the deformations within each layer. Seamless interface samples exhibit greater compliance than their adhered counterparts in the bilayer density‐graded configurations. Trilayer‐graded foams broaden strain–time history, extend the impact duration, and reduce strains. This research substantiates the importance of interfacing and gradation strategies on the mechanical response of elastomeric foams as a function of strain rate. 

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3. Density‐Dependent Impact Resilience and Auxeticity of Elastomeric Polyurea Foams

   https://doi.org/10.1002/adem.202200578  

   Youssef, George; Kokash, Yazeed; Uddin, Kazi_Zahir; Koohbor, Behrad (September 2022, Advanced Engineering Materials)

   This research investigates the dynamic response of a novel polyurea foam with different densities by separately submitting samples to single and multiple impacts at different energies ranging from 1.77 to 7.09 J. The impact and transmitted force‐time histories are acquired during the impact events. Deformation of the samples is also recorded using high‐speed photography and analyzed using digital image correlation (DIC) to characterize density‐dependent strain rate and Poisson's ratio. The analyses of the force‐time histories highlight the interrelationship between the incoming impact energy and force characteristics, including amplitude and durations. The experimental results reveal that polyurea foams can absorb nearly 50% of the incoming impact energy irrespective of their density. The dynamic impact efficacy of the foam persists even after sequential impact events are imparted on the same samples, with only a 20% drop in the load‐bearing capacity after seven consecutive impacts. Furthermore, as verified via electron microscopy observations, the higher‐density foam does not exhibit any permanent damage. This high‐density polyurea foam shows reversible auxetic transition at all impact energies considered herein. The outcomes of this research indicate the suitability of polyurea foams for cushioning and impact mitigation applications, especially in repeated biomechanical impact scenarios. 

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4. Mechanics of Microspheres Reinforced Hollow Microcells

   https://doi.org/10.1115/1.4049329  

   Youssef, George; Nacy, Somer; Huynh, Nha Uyen (April 2021, Journal of Applied Mechanics)

   null (Ed.)

   Abstract Emerging polymeric foams exhibiting unique microstructure of microspherical shells with reinforcing dense microspheres creates a new opportunity for impact-tolerant foam paddings in sport gears applications. This paper describes the static response of reinforced microcell consisting of an outer spherical shell and uniformly distributed microspheres while quantifying the stiffening effect. The distribution of the microspheres is illustrated using the Fourier series, allowing tuning of the reinforcing strategy. Expressions of the external and internal works are derived, whereas the Ritz energy method is adopted to calculate the deformations due to a compressive load distributed over a range of areas. Emphasis is given to the effect of the geometrical attributes of the microcell and the reinforcing microspheres on the resulting deformation response and stiffening effect. The framework is used to investigate the response of several case studies to elucidate the effects of relative radii ratio, reinforcement density, microcell wall thickness, and loading configurations on the stiffness. A new normalized strain energy parameter is introduced to simplify and accelerate the analysis while providing insights on the underpinnings of the observed buckling response. The results strongly suggest the viability of the newly discovered foam microstructure in managing static loads while providing an opportunity to strategically tune the mechanical response using the analytical framework presented herein. 

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5. Mechanisms underlying rapid electronucleation and freezing of hydrates

   Acharya, P; Shahriari, A; Lin, D; Bahadur, V. (November 2017, Proceedings of ASME 2017 International Mechanical Engineering Congress & Exposition)

   Nucleation of hydrates is constrained by very long induction (wait) times, which can range from hours to days. Electronucleation (application of an electrical potential across the precursor solution) can significantly reduce the induction time for nucleation. This study shows that porous aluminum foams (open-cell) enable near-instantaneous electronucleation at very low voltages. Experiments with tetrahydrofuran hydrates reveal that aluminum foam electrodes enable voltage-dependent nucleation with induction times of only tens of seconds at voltages as low as 20 V. Foam-based electrodes can reduce the induction time by up to 150X when compared to non-foam electrodes. Furthermore, this study reveals that electronucleation can be attributed to two distinct phenomena, namely bubble generation (due to electrolysis), and the formation of metal-ion coordination compounds. These mechanisms affect the induction time to different extents and depend on electrode material and polarity. Overall, this work uncovers the benefits of using foams for formation of hydrates, with foams aiding nucleation as well as propagation of the hydrate formation front. 

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