Fatigue and cyclic creep of replicated microcellular aluminium

Frédéric Diologent, Andreas Mortensen, Sébastien Philippe Soubielle
Elsevier, 2011
Article

Résumé

The cyclic behaviour of 400 mu m pore size replicated aluminium foam is assessed in tension-tension fatigue with a stress ratio equal to 0.1, keeping the load amplitude constant, for relative density values comprised between 0.175 and 0.220. The number of cycles to failure ranges from 6 x 10(2) (lowest relative density) to 5 x 10(6) (highest relative density). The foams display cyclic creep coupled with a strong influence of relative density on their general fatigue performance. Data analysis shows that the foam fatigue behaviour is dominated by cyclic creep, which governs both the deformation and the fatigue life of the cycled specimens, yielding characteristics globally in line with what is expected knowing the metal making the foam. (C) 2010 Elsevier B.V. All rights reserved.

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Fracture toughness of Al replicated foam

Etienne Combaz, Andreas Mortensen

Elastoplastic toughness testing of pure aluminium replicated foam is conducted following a J-procedure adapted from the ASTM E1820-08a standard using disc-shaped compact tension specimens. Tests cover a wide range of foam relative density V-m (10-24%). Resulting data show pronounced R-curve behaviour, computed J values increasing steadily beyond the crack blunting line before reaching a plateau, corresponding to a "steady-state" J value. Fractography reveals that the crack propagates via the rupture of struts normal to the crack plane, this being accompanied by limited plastic deformation of the struts near the crack plane, the extent of which, made visible by slip markings along the strut surface, increases in extent with V-m. Intact and fractured struts coexist over a significant portion of the crack plane, indicating that the strut rupture is stochastic. Measured crack initiation and steady-state propagation J values vary roughly as V-m(3); this is a stronger dependence than has been observed in commercial (closed-cell) metal foams. A simple model is proposed to describe the initiation toughness of these open-cell foams, which are characterized by a greater matrix ductility than other metal foams characterized in the literature. The model is based on an estimate of the crack tip opening displacement at the instant when a strut aligned in the loading direction fails by ductile rupture just ahead of the crack tip. The model accurately predicts the scaling law observed here between the toughness and the relative density of the foams, and provides a relatively good predictor of the absolute value of fracture toughness up to a relative density near 20%, while at higher densities measured toughness values exceed predictions. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Elsevier2010

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We study the response to age hardening of Al-4.5%Cu replicated foams with 75 and 400 pm cell sizes and with varying relative densities. The 400 mu m cell size foams have a similar behavior to the bulk alloy, whereas the 75 pm cell size foams with relative densities below 20% show no increase in strength with age hardening; this effect is interpreted as due to damage accumulation. Overall, the present study shows that, by age hardening the metal making the foam, open-celled replicated aluminium foams can be made as strong as commercial closed-celled aluminium foams, despite their inferior mesostructure.

Destech Publications, Inc, 439 Duke Street, Lancaster, Pa 17602-4967 Usa2008

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It has been proposed in the literature, based on theoretical considerations and on finite-element calculations, that all three stress tensor invariants govern the yield surface of cellular materials. Recent experiments on 75 gm pore size aluminium replicated foams (Combaz E, Bacciarini C, Charvet R, Dufour W, Mortensen A. Multiaxial yield behaviour of AI replicated foam, submitted for publication) showed such a dependence of the yield surface in axisymmetric tests. This study explores the yield behaviour of 400 gm pore size aluminium replicated foams: experiments confirm the influence of the third invariant on the yield surface shape, together with the observations that (i) the yield surface shape does not depend on relative density and (ii) measured flow vectors conform with normality. A simple parabolic model fitting data in the previous study also captures well the present data under all tested stress states (biaxial, axisymmetric and Pi-planes in stress space). Biaxial and axisymmetric tests are also performed on 400 pm pore size polyurethane (PU) replicated foams with a similar mesostructure. Results show yield to occur at a value lower than predicted by micromechanical models for both matrix materials (aluminium and PU). This suggests that the "knock-down" factor usually observed between predicted and observed stress values probably cannot be explained by a lowered yield stress in the material making the foam. The data also suggest an influence of the matrix nature on the yield surface geometry. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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