Metal Foams: A Design Guide

32 Metal Foams: A Design Guide
foam specimen
rubber membrane
female wedge
φ 38 mm
Al shim
70 mm
insulating tape
male wedge
caphead screw
Figure 3.6 Specimen assembly for multiaxial testing
The stress–strain curves
Three types of stress versus strain curves are measured as follows:
ž Uniaxial compression tests are performed using a standard screw-driven test
machine. The load is measured by the load cell of the test machine and the
machine platen displacement is used to define the axial strain in the specimen.
The loading platens are lubricated with PTFE spray to reduce friction.
In order to determine the plastic Poisson’s ratio, an essential measurement
in establishing the constitutive law for the foam (Chapter 7), the specimens
are deformed in increments of approximately 5% axial plastic strain and
the diameter is measured at three points along the length of the specimen
using a micrometer. The plastic Poisson’s ratio is defined as the negative
ratio of the transverse to the axial logarithmic strain increment.
ž Hydrostatic compression tests are performed increasing the pressure in
increments of 0.1 MPa and recording the corresponding volumetric strain,
deduced from the axial displacement. The volumetric strain is assumed to
be three times the axial strain. A posteriori checks of specimen deformation
must be performed to confirm that the foams deform in an isotropic manner.