Determination of elastic-plastic properties of Alporas foam at the cell-wall level using microscale-cantilever bending tests

Abstract

The presented paper is focused on determining the mechanical properties of the Alporas closed-cell aluminium foam. To utilise the favourable properties of cellular metals (e.g., high strength-weight ratio, energy absorption or insulation capabilities) a detailed description of the mechanical properties is required. Cellular metals exhibit heterogeneity at several scale levels. The contribution of the internal structure to the overall mechanical properties may not be in detail evaluated utilizing only the macroscopic testing. On the other hand, the compact material of cell walls is influenced by its composition (titanium-and calcium-rich regions are present in aluminium). Therefore, localised testing techniques with a small region of interest (e.g., indentation methods) may neglect the inhomogeneity along the cell walls. Hence, a testing of isolated cell walls was performed. A custom-developed modular loading device (based on precise linear bearing stages) was assembled to enable cantilever bending tests. The load was applied with a stepper motor and the loading force was measured with a micro-scale load cell with a loading capacity of 2.25 N. Displacements of the samples were measured optically. Several points along the longitudinal axis of a sample were tracked using the Lucas-Kanade tracking algorithm and the obtained displacements were compared to the analytically prescribed deflection curve. Based on the obtained deflections and measured forces, a stress-strain diagram was constructed and the constants of the elastic-plastic material model were evaluated. Keywords: aluminium foam, cantilever bending, micromechanics, optical strain measurement lanek obravnava dolo~anje mehanskih lastnosti aluminijaste pene Alporas z zaprtimi porami. Za uporabo ugodnih lastnosti celi~nih kovinskih materialov (npr. visoko razmerje med trdnostjo in maso, absorpcija energije ali izolacijske zmo`nosti), je potrebeno poznati mehanske lastnosti. Celi~ni materiali ka`ejo heterogenost,~e jih opazujemo pri razli~nih merilih velikosti. Podrobna ocena notranje zgradbe in njen prispevek k mehanskim lastnostim ni mogo~zgolj z uporabo makroskopskega preizku{anja. Po drugi strani pa zgradba vpliva na kompakten material celi~nih sten (v aluminiju je mogo~e najti podro~ja, ki so bogata s titanom in kalcijem). Zato je mogo~e, da preve~lokalizirano preizku{anje (npr. metoda za dolo~anje trdote) zanemari nehomogena podro~ja na stenah celic. Zato je bilo izvr{eno preizku{anje posamezne celi~ne stene. Sestavljena je bila modularna obremenitvena naprava, ki omogo~a natan~ne stopnje linearne obremenitve, da so mogo~i upogibni preizkusi z mikroskopsko iglo. Vzorec je bil obremenjen s kora~nim motorjem, pri~emer je bila sila obremenitve izmerjena z mikroskopsko merilno celico z nosilnostjo 2,25 N. Premik vzorcev je bil izmerjen opti~no. Nekaj to~k ob vzdol`ni osi vzorca je bilo spremljano s sledilnim algoritmom Lucas-Kanade, ugotovljeni premiki pa so bili primerjani z analiti~no dolo~eno krivuljo deformacije. Na podlagi ugotovljenih deformacij in izmer...