Defect sensitivity of a 3D truss material

doi:10.1016/S1359-6462(01)01073-9

Scripta Materialia

Volume 45, Issue 6, 28 September 2001, Pages 639-644

https://doi.org/10.1016/S1359-6462(01)01073-9 Get rights and content

Abstract

The effect of randomly removing members of a three-dimensional truss material on the Young's modulus and compressive strength has been calculated numerically. The results indicate that this structure is more tolerant to this type of defect than open-cell foams.

Introduction

A typical three-dimensional (3D) truss material and its unit cell are shown in Fig. 1. The triangulation of the members of the material induces axial forces in the individual members, giving it a high specific stiffness and strength in all three orthogonal directions. Recent manufacturing developments allow production of such structures with member lengths on the order of hundreds of microns to tens of centimeters [1]. The open architecture can be exploited in multi-functional applications. For instance, the material could be simultaneously be used as a structural and a heat transfer device. The elastic moduli and normal and shear strengths of the truss material shown in Fig. 1 have been described by Wallach and Gibson [2]; those of a similar, so-called “octet” truss material have been described by Deshpande et al. [3].

Conventional cellular materials, such as foams, are sensitive to defects in their structure. For instance, the stiffness and strength of closed cell aluminum alloy foams have been shown to be lower than expected from analysis of models as a result of defects such as cell wall curvature and highly elongated cells [4], [5], [6], [7], [8]. Randomly removing cell walls has a dramatic effect on the stiffness and strength of honeycombs and foams: for example, removal of 10% of the struts reduces the modulus and strength of a honeycomb by about 60–70% and reduces those of an open-cell foam by about 40–50% [8], [9], [10]. In this study we analyze the effect of randomly removing members of the truss material shown in Fig. 1 on its Young's modulus and compressive strength.

Section snippets

Analysis

The model analyzed in this study was six unit cells in the x- and y-directions and a single unit cell in the z-direction. The aspect ratio, h/b was 0.62 and the ratio of the member diameter to the edge length b was 0.0833. The vertices of the interior members are offset to the inside of the upper and lower faces by 11% of the cell height h. This offset is a consequence of the manufacturing process and is included in the model to facilitate experimental verification [2]. The stress–strain

Results and discussion

A typical stress–strain curve for a model with 5% reduction in density is shown in Fig. 3. Young's modulus decreases linearly with increasing fraction of missing members, with a roughly 17% decrease in modulus for every 10% reduction in density (Fig. 4). The 0.2% offset yield strength in compression as well as the peak compressive strength also decrease linearly with increasing fraction of missing struts (Fig. 5). The 0.2% offset yield strength decreases with the reduction in density at about

Conclusions

The effect of randomly removing members on the Young's modulus and the compressive strength of a 3D truss material has been calculated. The modulus and strength decrease linearly with the fraction of members removed, at a rate about 1.7 times that for an equivalent density reduction from uniform thinning of the members. The truss material is more tolerant to randomly removing members than open cell foams.

Acknowledgements

We are grateful for the support of ONR grant N00014-96-1-1028 and of the Fannie and John Hertz Foundation (JCW).

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