Micro-inertia Effects Research Articles

Dynamic failure of metallic pyramidal truss core materials – Experiments and modeling

The quasi-static and dynamic compressive behavior of pyramidal truss cores made of 304 stainless steel were investigated using a combination of experimental techniques. Quasi-static tests were performed using a miniature loading stage while a Kolsky bar apparatus was used to investigate intermediate deformation rates. High deformation rates were examined using a light gas gun. Optical imaging of the sample deformation was performed in real time by means of high-speed photography. In this article, we provide a quantification of load-deformation response and associated failure modes across the sample as captured by high-speed photography. A finite element model is formulated and thorough simulations performed to understand the roles of material strain rate hardening and structural microinertia. Deformation modes were identified from acquired images, force-deformation histories and numerical modeling. Comparison between force-deformation histories under quasi-static and Kolsky bar loading reveals a moderate microinertia effect as manifested by a small increase in peak compressive stress. At high deformation rates, gas gun experiments, a totally different deformation mode is manifested with a major increase in peak compressive stress. In this case, the inertia associated to the bending and buckling of truss struts played a significant role. This effect appears to dominate the early truss core response because of two effects: (i) the propagation of a plastic wave along the truss members; (ii) buckling induced lateral motion. These findings are consistent with prior theoretical and computational work carried out by Vaughn et al. (2005) [Vaughn, D. Canning, M., Hutchinson, J.W., 2005. Coupled plastic wave propagation and column buckling. Journal of Applied Mechanics 72 (1), 1–8]. At larger deformations, the material strain rate hardening contribution to the total energy is as pronounced as the contribution arising from microinertia effect.

An experimental study on the behaviour under impact loading of metallic cellular materials

This paper presents an experimental study on the impact response of metallic cellular materials, i.e. aluminium honeycombs of various cell sizes and wall thicknesses, aluminium foams made from two different manufacturing processes (IFAM and Cymat), as well as hollow sphere agglomerates (nickel and iron). A 60 mm diameter nylon Hopkinson pressure bar is used to improve the signal/noise ratio and to host larger samples containing a sufficient number of cells. Quasi-static and classical Split Hopkinson Pressure Bar (SHPB) tests as well as direct impact Hopkinson bar tests (higher speeds up to 50 m/s) are performed. Significant rate sensitivities are observed for most of the cellular materials studied. Analyses of the potential causes of this macroscopic rate sensitivity show that the microinertia effect in the successive folding process could be an important factor.

Micro-Inertia Effects in Laminar Thin-Film Flow Past a Sinusoidal Boundary

Micro-inertia effects of surface roughness on hydrodynamic lubrication are analyzed in the light of similitude principles, viz. a newly conceived reduced Reynolds number and the classical parameter of relative roughness. In particular, the dynamic properties of laminar sheet flow in a two-dimensional channel between a sinusoidal wall and a flat wall are studied. FEM solutions of the Navier-Stokes equation are compared with corresponding experimental findings. The latter are gathered in an especially designed laminar-flow wind tunnel. Conclusions are drawn concerning the roughness sensitivity of laminar thin-film flows.

On balance laws for fluid-saturated porous media

The space-averaging formalism of Anderson and Jackson is employed to develop a set of balance laws for a multicomponent mixture. The interactions between the mixture components appear as surface integrals in the averaged (or macroscopic) balance laws. These balance laws are used to study the motion of a porous solid; it is shown that the formulations of Herrmann and of Morland are only approximate in that both of these authors neglected the effects of microinertia. The motion of an incompressible Newtonian fluid through a rigid porous matrix is considered, and the assumptions required to recover Darcy's law are discussed. Finally the averaged balance laws are used to derive a first order theory for the motion of fluid-saturated porous media; it is shown that reasonable approximations lead to the balance laws previously derived by Garg, et al. on heuristic grounds.