Papers by Keyword: Plateau Stress

Abstract: Copper foams by using CaCl₂ as space holder were successfully manufactured by sintering and dissolution process. The porosity ranges from 75% to 91%, and cell size from 0.3mm to 3.0m. The volume fraction of CaCl₂ and sintering temperature are the main factors that affect porosity of copper foam. The yield plateau stress of copper foams with porosity between 75.88% and 90.19% is in range of 12.1~1.2MPa. The yield plateau stress decreases with the increase of porosity. The energy absorption per unit volume (W) copper foams with porosity between 75.88% and 90.19% is in range of 6.17~0.63MJ/m3. Under the condition of identical porosity, the absorption energy per unit volume (W) of copper foam is about 43% higher than aluminum foam. The maximum ideal energy absorption efficiency of copper foam is about 0.74, it indicates that the copper foam can be used as a good absorbing material.

Abstract: A full scale finite element (FE) analysis has been conducted on aluminum hexagonal honeycomb to investigate its deformation mechanism and mechanical performance under combined compression-shear loads. ANSYS LS/DYNA software has been employed in this numerical study. The honeycomb FE model has the same dimensions (cell size and cell wall thickness) as those used in the previous experimental study. The FE model has been verified by the experimental results. A good agreement between the experimental and numerical results has been reached. This numerical analysis facilitates the measurement of the vertical and horizontal forces applied to honeycomb specimens. The effects of strain rate, plane orientation of cell walls and loading angle on the plateau stress and energy absorption of honeycomb specimens under combined compression-shear loads were investigated.

Abstract: Cenospheres are very cheap, and are reasonably strong and thermally stable upto 1200°C. In view of this attempt has been made to use these cenosphere for making Titanium syntactic foams with varying relative densities. Precautions were taken for selecting cold compaction pressure to minimize cenosphere crushing. The sintered samples were then characterized in terms of microstructures primarily to see the extent of cenosphere crushing, distribution of cenosphere, and extent of sintering. The foams made using optimized pressure and sintering parameters, exhibits uniform distribution of cenosphere without any significant crushing. The plateau stress, energy absorption and modulus of these foams are varying with the cenosphere content or the relative density, and these parameters can be engineered by varying cenosphere content in the foam. These foams exhibit considerably higher strength and stiffness than the conventional foam and show the possibility of using them for biomedical and engineering applications.

Abstract: 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.

Abstract: Experimental and Finite Element analysis was used for the investigation of the effect of cell size and thickness on the compressive properties of Aluminium honeycomb core. Honeycomb cores were compressed experimentally in in-plane and out of plane directions. The effect of sample size, cell size and thickness on the elastic modulus, yield strength and plateau stress was investigated through FEA. It was found that the mechanical response was independent upon the sample size in in-plane direction. The smallest cell size honeycomb core was deformed at higher yield stress. Similarly, with increase in cell wall thickness, the modulus of the core increased.

Abstract: Mechanical behaviors of square honeycombs cores (SHCs) are investigated by using the finite element (FE) simulations under the in-plane dynamic crushing loadings. With the increasing impact velocities, different deformation modes are observed. The force-displacement curves include four regimes with distinct characteristics. The plateau stresses are calculated for the SHCs with different configuration parameters. The dynamic plateau stress is the sum of the static plateau stress and the dynamic enhancement due to the inertia effect. The static plateau stress is proportional to the relative density of SHCs. The dynamic enhancement stress is proportional to the square of impact velocity and the relation coefficient depends on the configuration parameters. The empirical formulas of dynamic plateau stress in terms of configuration parameters and impact velocity are given.

Abstract: The dynamic crushing behavior of cellular metals is closely related to their microstructure. Two types of random defects by randomly thickening/removing cell walls are investigated in this paper. Their influences on the deformation modes and plateau stresses of honeycombs are studied by finite element simulation using ABAQUS/Explicit code. Three deformation modes, i.e. the Homogeneous Mode, the Transitional Mode and the Shock Mode, are used to distinguish the deformation patterns of honeycombs under different impact velocities. The critical impact velocity for mode transition between the Homogeneous and Transitional modes is quantitatively determined by evaluating a stress uniformity index, defined as the ratio between the plateau stresses on the support and impact surfaces. It is found that the critical impact velocity decreases with increasing thickening ratio but increases with increasing removing ratio. The plateau stress on the impact surface heavily depends on the impact velocity due to the inertia effect. The random defects lead to a weakening effect on the plateau stress. For the honeycombs with randomly removing cell walls, the weakening effect is especially obvious at a moderate impact velocity. For the honeycombs with randomly thickening cell walls, the weakening effect is particularly severe at a low impact velocity, but this effect almost disappears when the impact velocity is high enough.

Abstract: A finite element (FE) model of hexagonal honeycomb cores (HHCs) with single or double cell walls is framed based on the honeycomb cell array, to reproduce the compressive behavior under the quasi-static out-of-plane compression. The FE calculated deformation modes, deformation curves and values of quasi-static plateau stresses are presented in the forms of diagrams, compressive force-displacement curves and data tables, which are consistent with the existing experimental and theoretical calculated results. It is shown that the proposed FE analysis methodology is reliable.

Abstract: The in-plane dynamic crushing of hexagonal honeycombs was numerically studied by means of explicit dynamic finite element method using ANSYS/LS-DYNA. Under the assumption that the edge length and thickness were the same, the metal honeycomb models filled with convex and concave cells were established. And then the effects of expanding angle and impact velocity on the plateau stress and the energy absorption capacities of hexagonal honeycombs were discussed in detail. Numerical results show that the energy absorption capacities of convex hexagonal honeycombs are stronger than the concave ones. These results will provide some useful guides for the dynamic energy absorption design of cellular materials.

Abstract: Cu-2.2wt%Ni-0.5wt%Si alloy single crystals were grown by the Bridgman method and aged at 723 K for 10 h to form Ni2Si precipitates. Fully reversed tension-compression fatigue tests were conducted on the aged single crystals with a single slip orientation under constant plastic-strain amplitudes at room temperature. Cyclic softening occurred at plastic-strain amplitudes between 2.5x10-4 and 2.5x10-2. Using the maximum stress amplitude in each cyclic hardening/softening curve, a pseudo cyclic stress-strain curve (CSSC) was obtained. The CSSC was found to exhibit a plateau region with a stress level of about 167 MPa. Transmission electron microscopic observation revealed the formation of persistent slip bands (PSBs) in the plateau regime. It was found that the Ni2Si precipitate particles were intensively sheared by glide dislocations within the PSBs and were eventually re-dissolved into the Cu matrix. The macroscopic cyclic softening can be attributed to the local softening induced by the re-dissolution of the Ni2Si particles in the PSBs.