Tensile failure observations in sintered steel foam struts revealed by sub-micron contrast-enhanced microtomography

• Triangular hollow steel foam struts contain macropores and numerous micropores.
• Re-entrant corners form stress-raisers that lead to complex unzipping in tension.
• During fracture unzipping precedes uncoordinated failure of isolated strut walls.
• Cracks propagate through interfacial pores, deflected by higher density particles.
• Significant amounts of Mn-Si-Cr oxides were found within micropores in the steel.

Powder metallurgical open-cell steel foams made of hollow struts are of great interest for impact resistance. Here we report experimental results of high-resolution characterisation of single struts, extracted from a commercially available steel foam. Synchrotron radiation X-ray microtomography reveals that the hollow struts are made of triplets of lens shaped rods, connected to each other along the long axis, appearing triangular in cross-sections. The struts have a non-uniform geometry and they include macro and micropores as well as high-density inclusion powder particles, poorly fused with the matrix. Micro-tensile testing showed that tensile failure is accompanied by longitudinal unzipping due to shear along the thinner material found near the hollow corners. Included, higher-density powder particles deflect crack propagation along the interface with the matrix, where much porosity is observed. Results of finite element simulations well match our mechanical tests, revealing plastic deformation due to bending and significant shear stresses arising between neighbouring rods within the same strut. The failure behaviour and the mechanical response of sintered struts in steel foams produced using non-uniformly coated polyurethane templates is the result of an interplay between the triplet geometry, sub-micron pores, precipitates and high-density inclusions.

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