Metal Foams: A Design Guide

Characterization methods 31
foam specimens can be bonded to the plates using a structural adhesive (e.g.
FM300 Cytec, Havre de Grace, MD). The load is measured using the load
cell while displacement is measured from LVDTs attached to the plates.
The double-lap configureation, shown in Figure 3.5(b), produces a more
uniform stress state in the specimen and is preferred for measurement of shear
strength, but it is difficult to design plates that are sufficiently stiff to measure
the shear modulus reliably. Data for the shear modulus and strength of metallic
foams are given in Chapter 4.
3.6 Multi-axial testing of metal foams
A brief description of an established test procedure used to measure the multiaxial
properties of metal foams is given below. Details are given in Deshpande
and Fleck (2000) and Gioux et al. (2000).
Apparatus
A high-pressure triaxial system is used to measure the axisymmetric compressive
stress–strain curves and to probe the yield surface. It consists of a
pressure cell and a piston rod for the application of axial force, pressurized
with hydraulic fluid. A pressure p gives compressive axial and radial stresses
of magnitude p. Additional axial load is applied by the piston rod, driven
by a screw-driven test frame, such that the total axial stress is p C .The
axial load is measured using a load cell internal to the triaxial cell, and the
axial displacement is measured with a LVDT on the test machine cross-head
and recorded using a computerized data logger. The cylindrical test samples
must be large enough to ensure that the specimens have at least seven cells in
each direction. The specimens are wrapped in aluminum shim (25 µm thick),
encased in a rubber membrane and then sealed using a wedge arrangement
as shown in Figure 3.6. This elaborate arrangement is required in order to
achieve satisfactory sealing at pressures in excess of 5 MPa.
With this arrangement, the mean stress m and the von Mises effective stress
e follow as
and
m D
� �
p C
3
⊲3.1⊳
e D j j ⊲3.2⊳
respectively. Note that the magnitude of the radial Cauchy stress on the specimen
equals the fluid pressure p while the contribution to the axial Cauchy
stress is evaluated from the applied axial force and the current cross-sectional
area of the specimen.