Anisotropy and strain localization in steel foam

https://doi.org/10.1016/S0921-5093(00)01418-0Get rights and content

Abstract

Steel foam fabricated by a powder metallurgical process was tested in uniaxial compression. The closed-cell foam samples exhibited anisotropy in compression, a phenomenon that was caused primarily by the ellipsoidal cell shapes within the foam. Yield strengths were 3× higher in the transverse direction than in the longitudinal direction. Yield strength also showed a power-law dependence on relative density (n≅1.8). Compressive strain was highly localized and occurred in discrete bands that extended transverse to the loading direction. The deformation bands were comprised of collapsed cells, and deformation occurred in a sequential manner with repeating cycles of yield, collapse and densification of cells. Foam densification commenced when all of the foam was converted into deformation bands.

Introduction

Metallic foams exhibit unusual mechanical and thermal properties, including energy absorption, vibrational and acoustic damping, and thermal insulation [1], [2]. Because of these unique properties, metallic foams may find applications in impact absorbers, ultralight sandwich structures, compact heat exchangers, and heat dissipation media. However, before these applications can be realized, two things are required, a mature and robust process technology for manufacturing metallic foams with consistent properties, and a knowledge amongst design engineers about the mechanical characteristics of metallic foams, their durability, and how to use them intelligently in structural applications. These requirements have not been met, although various molten metal and powder metallurgical processing methods have been reported [3], [4], [5], [6], [7], [8], and the mechanical behavior of foams has been reviewed in the treatise by Gibson and Ashby [9].

The basic mechanical properties of metallic foams are not unlike polymeric foams, and both generally conform to the theory of cellular materials [9]. However, there are notable deviations from theoretical predictions. For example, the elastic modulus and compressive strength of closed-cell aluminum foams is reportedly lower than predicted values [9], [10]. Insight into this discrepancy is provided by microstructural observations, which indicate that defects present in the cell structure may degrade mechanical properties [11]. Simple defects include cell wall curvature and non-equiaxed cell shapes. The latter defect can contribute to anisotropic mechanical properties, while the former can lead to localized deformation [12].

The present work focuses on mechanical anisotropy in steel foams synthesized by a powder metallurgical process. In this process, steel powder is blended with a granular foaming agent, compacted, then expanded to foam by heating the compact briefly above the melting point [13], [14]. Results from compression tests performed parallel and perpendicular to the foaming direction are compared with model predictions, and deformation mechanisms are observed and related to the measured response.

Section snippets

Experimental

A closed-cell steel foam material was synthesized by a P/M route and used for experimental testing [13]. Commercially available steel powder (Fe-2.5C blend) was blended with 0.2 wt.% of a granular foaming agent (MgCO3 or SrCO3). Blended powders were compacted by uniaxial cold pressing, yielding virtually non-porous, semi-finished steel samples. The resulting semi-finished samples were subsequently melted in an open graphite mold held in an air furnace at 1330°C to effect foam expansion. The

Results

Expanded foams were elongated in the foaming (vertical) direction, resulting in roughly ellipsoidal cells (Fig. 2(a)). During foam expansion, ellipsoidal cells evolve because of the microstructure of the pressed compact. During powder compaction by uniaxial cold-pressing, some flattening of the steel powder particles occurs, resulting in ellipsoidal shapes [12], [15]. As a result, prior particle boundaries (PPBs) tend to extend normal to the pressing direction, and foaming agent granules tend

Conclusion

Steel foams fabricated by a P/M route show distinct anisotropy when subjected to uniaxial compressive loading. The foams are ∼3× stronger in the transverse direction than in the longitudinal direction, and exhibit a more pronounced yield point drop. The anisotropic response is directly related to the foam structure, which is characterized by ellipsoidal cells with thick walls extending transverse to the foaming direction. In both orientations, behavior is described by a power-law with a similar

Acknowledgements

Financial support from the TRW Foundation and from the M.C. Gill Corporation is gratefully acknowledged.

References (22)

  • T.J Lu et al.

    Acta Mater.

    (1999)
  • Y Sugimura et al.

    Acta Mater.

    (1997)
  • C Park et al.

    Mater. Sci. Eng.

    (2000)
  • K.Y.G McCullough et al.

    Acta Mater.

    (1999)
  • G.J Davies et al.

    J. Mater. Sci.

    (1983)
  • W.W. Ruch, B. Kirkevag, PCT/WO Patent 91/01387,...
  • I. Jin, L.D. Kenny, H. Sang, US Patent 4 973 358,...
  • J. Baumeister, DE Patent 40 18 360,...
  • Idem., US Patent 5 151 246,...
  • J. Baumeister, H. Schrader, DE Patent 41 01 630,...
  • J. Baumeister, J. Banhart, M. Weber, DE Patent 43 25 538,...
  • Cited by (73)

    • Effect of cell structure on the uniaxial compression properties of closed-cell foam materials

      2021, Materials Today Communications
      Citation Excerpt :

      In recent decades, the components made of foam materials are being increasingly used in the aerospace, automotive and biomedical industries [1–3]. Much effort has been put in the study of different kinds of foam materials, from the conventional metallic foam materials [4–7] to the recently developed metal matrix syntactic foams [8–10]. In the previous studies, the mechanical behaviors of closed-cell foam materials and their relevant components under different loading conditions are studied experimentally [11–16], theoretically [17–19] and numerically [20–29].

    View all citing articles on Scopus
    View full text