33 Uniaxial compression testing

Uniaxial compressive tests are best performed on prismatic or cylindrical specimens of foam with a height-to-thickness ratio exceeding 1.5. The minimum dimension of the specimen should be at least seven times the cell size to avoid size effects. Displacement can be measured from crosshead displacement, by external LVDTs placed between the loading platens, or by an exten-someter mounted directly on the specimen. The last gives the most accurate measurement, since it avoids end effects. In practice, measurements of Young's modulus made with an extensometer are about 5-10% higher than those made using the cross-head displacement.

A typical uniaxial compression stress-strain curve for an aluminum foam is shown in Figure 3.3. The slope of the initial loading portion of the curve is lower than that of the unloading curve. Surface strain measurements (Section 3.10) indicate that there is localized plasticity in the specimen at stresses well below the compressive strength of the foam, reducing the slope of the loading curve. As a result, measurements of Young's modulus should be

Strain

Figure 3.3 Stress-strain curve from a uniaxial compression test on a cubic specimen of a closed-cell aluminum foam (8% dense Alporas): (a) to 5% strain, (b) to 70% strain (from Andrews et al., 1999a)

Figure 3.3 Stress-strain curve from a uniaxial compression test on a cubic specimen of a closed-cell aluminum foam (8% dense Alporas): (a) to 5% strain, (b) to 70% strain (from Andrews et al., 1999a)

made from the slope of the unloading curve, as shown in Figure 3.3, unloading from about 75% of the compressive strength. The compressive strength of the foam is taken to be the initial peak stress if there is one; otherwise, it is taken to be the stress at the intersection of two slopes: that for the initial loading and that for the stress plateau. Greasing the faces of the specimen in contact with the loading platens reduces frictional effects and can give an apparent compressive strength that is up to 25% higher than that of a dry specimen.

Variations in the microstructure and cell wall properties of some present-day foams gives rise to variability in the measured mechanical properties. The standard deviation in the Young's modulus of aluminum foams is typically between 5% and 30% of the mean while that in the compressive strength is typically between 5% and 15%. Data for the compressive strength of metallic foams are presented in Chapter 4.

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