Syntactic Iron Foams – On Deformation Mechanisms and Strain‐Rate Dependence of Compressive Properties

Abstract

Using hollow particles as reinforcement, syntactic foams are a unique class of materials with features of conventional foams as well as composites. Usually, the main role of the filler is to introduce porosity, and thus also change specifically the compressive response to that of a typical foam or cellular material. However, depending on which of the many variants of such syntactic foams is considered, hollow particles can also have a strengthening effect. In the way this is achieved, however, they are distinguished from the many kinds of metal foams that use ceramic particles first for stabilization during processing and secondly for some strengthening effect: [1][2][3] In syntactic foams, the reinforcement provides a stable inner lining of each pore, while in conventional particle-reinforced metal foams, the distribution of ceramic particles in the matrix effectively allows them to be understood as cellular MMCs in which matrix ductility is adversely affected by the otherwise strengthening addition. In syntactic foams of the type discussed here, the matrix is unaltered and can thus maintain considerable levels of ductility.So far, numerous materials differing in the size of structural elements, nature of filler, etc. have been realized, featuring both polymer [4,5] and metal [6] matrices. In the case of metals, reinforcing particles can be millimeter sized steel hollow spheres, [7] or, on another, micrometer length scale, cenospheres [8] as well as glass microspheres. [9] The present study concentrates on the latter variant. Main production methods for such materials are melt stirring, melt infiltration [9][10][11][12] or powder metallurgy processes. [13,14] When contrasted to metal foams based on the powder compact melting or the APM process [15][16][17] iron matrix syntactic foams combine lower maximum porosity and thus higher density with greatly improved performance under both quasi-static compressive and tensile load while showing the typical deformation characteristics of foams. [14] In comparison to other types of iron-based foams, such as the so-called lotus-type porous metals recently discussed in detail by Song et al. and Tane et al.,[18][19][20] the present materials combine still higher strength with isotropic behavior and greater ease in manufacture of theSyntactic iron foams are produced by metal injection moulding from pure Fe powder and two grades of commercial glass microspheres. Mechanical performance of samples containing 5/10/13 wt% of microspheres is compared to unfilled reference material properties at strain-rates covering 6 orders of magnitude, including Split Hopkinson Pressure Bar (SHPB) experiments. Complex mechanical behavior including strengthening effects of microspheres leading to a plateau strength level which is nearly independent of porosity as well as strain-rate sensitivity of compressive properties are observed. Typical plateau onset stress levels exceed equivalent characteristics of most comparable cellular metallic materials, reaching between approximately 220 and 270 MPa...

Compressive behaviour of aluminium matrix syntactic foams reinforced by iron hollow spheres

“…In particular, the strain-rate sensitivity response at three different strain rate levels was studied by taking into account the influence of type and volume fraction of glass spheres. The materials' behaviour was found to be very similar to that of the metal matrix [2,[6][7][8]. Castro and Nutt produced steel based MMSFs, filled with ceramic hollow spheres by gravity fed and mechanical pressure infiltration.…”

Characteristic compressive properties of hybrid metal matrix syntactic foams

“…Peroni et al [59,60] investigated the mechanical behaviour of MMSFs made of hollow glass spheres mixed in iron matrix. The produced materials offered greatly increased quasi-static compressive strength, though at higher density.…”

X‐Ray Microtomographic Characterization and Quantification of the Strain Rate Dependent Failure Mechanism in Cenosphere Epoxy Syntactic Foams

“…[10,[12][13][14][15][16][17][18][19][20] Dynamic properties of syntactic foams are affected by the geometry and volume fraction of hollow microspheres. [13][14][15][16] The treatment of hollow microspheres can improve their interfacial adhesion to the epoxy matrix and thus the impact properties of the bulk foam, especially the strength. [10] Pellegrino et al reported the glass microballoon polyurethane syntactic foam possesses higher strain rate sensitivity in tension compared to compression.…”