L.L. Hu

Indentation resistance of the re-entrant hexagonal honeycombs with negative poisson's ratio

Abstract The influences of the cell-wall angle on the indentation resistance of the re-entrant hexagonal honeycombs with negative Poissons ratio are studied by employing both the numerical tests and the theoretical analysis. In the numerical tests, two groups of honeycombs are investigated, i.e. the same-density group and the same-cell-wall group. It is shown that the honeycombs indentation resistance increases with the cell-wall angle in the same-density group, while the cell-wall angle plays a minor role in the same-cell-wall group. The theoretical analysis is employed to explain the phenomena. The effect of the negative Poisons ratio is calculated to increase with the cell-wall angle. The average stress of the re-entrant honeycomb during compression is deduced as a function of the cell-wall angle and the cell-wall size or the honeycombs relative density. The analytical results are consistent with the numerical tests and give an explanation for the phenomena in tests.

Mechanical Behavior of Polycarbonate Circular Honeycombs under In-plane Biaxial Compression

In-plane equi-biaxial compression tests were conducted quasistatically on polycarbonate circular honeycombs in a conventional universal testing machine with a specially designed test rig. In view of the complex characteristics of the deformation in a honeycomb block under biaxial compression, the focus was put on the initiation and evolution of deformation inhomogeneity. An inhomogeneity index, Iinh, is defined as a function of the overall compression of a sample to quantify the severity of the deformation inhomogeneity of the honeycomb block; and it facilitates quantitative comparisons among the results under various loading conditions. To examine the role of the friction between the honeycomb sample and the test rig, and to explore the effect of dynamic loading, finite element analysis is carried out to simulate the collapse process of a circular honeycomb block under equi-biaxial compression. The numerical simulation also reveals the transition from a global collapse mode (although inhomogeneous) to a progressive collapse mode at a critical loading velocity, which is further studied by the wave trapping theory.

Dynamic Behaviors of Aluminium Foam Analyzed Using Digital Image Processing Technology

A numerical model of the aluminium foam with voronoi cells is built and uni-directionally crushed with various velocities from 1m/s to 110m/s. It is shown that the foam deforms homogeneously within the whole specimen and the stress in the foam increases gradually with the strain without an obvious plateau stage under the low-velocity compression, while the deformation is concentrated within a zone near the impact end and an obvious plateau stage can be found in the stress-strain curves of the foams under the high-velocity crushing. By analyzing the distribution of the density within the foams using the digital image processing technology, the densification strain of the foams under dynamic crushing can be determined. Then combining the foam’s stress-strain curve under the low-velocity compression, the dynamic plateau stress of the foams can be predicted. It is shown that both the densification strain and the plateau stress of the foams under the high-velocity crushing predicted by employing the digital image process technology are in good agreement with the numerical simulations. The results show that both the plateau stress and the densification strain of the foams increase with the impact velocity, which is essentially caused by the localization of the foam’s deformation under dynamic crushing.

Effects of cell-wall angle on the mechanical properties of hexagonal honeycombs

The y-directional mechanical properties of hexagonal honeycombs with various cell-wall angles are explored. The results of both the quasi-static experiments and the dynamic simulations show that the cell-wall angle has a significant influence on the honeycombs’ mechanical properties, although the latter is dominated by the honeycombs’ relative density. This influence is weakened by the increase of the impact velocity. With retaining the honeycombs’ relative density as constant, the honeycomb with the cell-wall angle of about 45o exhibits the optimal crushing strength and energy absorption capacity.

Crushing strength of honeycombs with various cell wall angles

Abstract The effects of cell wall angle on the in‐plane impact behaviours of hexagonal honeycombs are studied by finite element method simulations. The cell wall angle varies from 15 to 75° while a regular hexagonal honeycomb possesses a cell wall angle of 30°. Three different impact velocities 10, 60 and 100u2005mu2005s−1 are involved. The crushing strengths of honeycombs with various cell wall angles but the same density are compared. The results show that the crushing strength of honeycombs is enhanced with increasing impact velocities for both x directional impact and y directional impact. The crushing strength ratio of y directional impact to x directional impact is sensitive to cell wall angle at lower impact velocities, while it approaches 1 for various cell wall angles at a high impact velocity of 100u2005mu2005s−1. The cell shape has significant influence on the crushing strength of honeycombs in both x directional impact and y directional impact at low impact velocities, in which the crushing strength can be diminished to only 10% of that of the regular honeycomb. However, this influence is weakened sharply with increasing impact velocity. Comparatively, the influence of cell shape on the x directional crushing strength is more significant than that in the y direction.