Closed cell aluminum foams exhibit uniaxial compressive behavior similar to other porous and/or cellular materials: the initially near-linear stress – strain curve leads to a stress peak and stress drop, followed by a long stress plateau. However, surface strain maps, produced using digital image correlation, reveal the formation of bands of localized axial compaction, oriented approximately perpendicular to the loading direction. These “compaction bands” appear to initiate near the stress peak, and during the subsequent stress drop, the band becomes clearly defined. Detailed examination of the surface strain maps, during the stress drop, reveals that the band absorbs nearly all of the specimen shortening, while some material outside the band unloads slightly. The bands are roughly planar, and the angle between the band normal and the axial direction is small, typically less than 20 degrees. Using the theoretical framework developed by Rudnicki and Rice for shear localization in low porosity rock (later employed by others to predict compaction band initiation in high porosity sandstone), band formation is modeled as a bifurcation from uniform deformation, due to an instability in the constitutive relation for continued inelastic deformation. Using this approach, the predicted band orientations agree reasonably well with the observed band angles. Observations and theoretical predictions of compaction bands in diverse porous materials, such as aluminum foams and porous sandstone, suggest that compaction band formation may be a common deformation mode in compacting porous materials.
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