35 Shear testing

The shear modulus of metallic foams is most easily measured by torsion tests on waisted cylindrical specimens. The specimens should be machined to the shape of ASTM E8-96a, to avoid failure of the specimen in the neck region or at the grips. The minimum dimension of the specimen (the diameter of the cylinder) should be at least seven times the cell size to avoid specimen/cell size effects. Torque is measured from the load cell. Displacement is measured using two wires, separated by some gauge length. The wires are attached to the specimen at one end, drawn over a pulley and attached to a linear voltage displacement transducer (LVDT) at the other end. The motion of the LVDTs can be converted into the angle of twist of the specimen over the gauge length, allowing the shear modulus to be calculated. The shear modulus is again measured from the unloading portion of the stress-strain curve, as in uniaxial compression testing. The shear strength is taken as the maximum stress. The standard deviation in the shear strengths of aluminum foams are similar those for the compressive and tensile strengths.

A alternative test for measurement of shear strength is ASTM C-273. A long thin specimen is bonded to two stiff plates and the specimen is loaded in tension along the diagonal using commercially available loading fixtures (Figure 3.5(a)). If the specimen is long relative to its thickness (ASTM C-273 specifies L/ti>i12) then the specimen is loaded in almost pure shear. Metallic

Figure 3.5 Measurement of shear strength of a foam (a) by the ASTM C-273 test method, and (b) by the double-lap shear test

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.

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