Static and Dynamic Material Behavior of Syntactic Foam

Abstract

The use of foams or porous solids in a variety of applications such as energy absorbers. buoyancy materials, etc. has renewed interest in the accurate representation of the properties of these materials. In particular, a “compressive constitutive law” or equation of state is needed in the calculation of the dynamic response of the material to suddenly applied loads. Static testing to provide such data is appealing because of its simplicity; however, the importance of rate effects cannot be determined bv this one method alone. Therefore, additional but numerically limited elevated strain-rate tests must be run for this purpose.

In the present paper, results of uniaxial strain static and gas gun compression tests on syntactic foam are reported. The foam is buoyant and is composed of hollow glass microspheres (average diameter 100 microns) embedded in an epoxy resin. Static testing consists of compressing a 0.25 cm × 2.5 cm dia. wafer between carefully aligned 2.5 cm dia. steel pistons. Lateral expansion of the wafer is suppressed by mounting it in a thick-walled (10 cm OD, 2.5 cm ID) cylinder. The degree of expansion is monitored by a circumferential strain gage mounted on the outside of the cylinder, coplanar with the foam wafer. Loading is increased until crushing of the microspheres, as indicated by a flattening of the stress-strain curve and by photomicrograph, is complete.

Dynamic testing is conducted using the LMSC plane wave generator (“gas gun”). Impact pressure and specimen particle velocity are recorded using a quartz transducer as impactor. Dimensions of the foam specimen for this test are equivalent to those of the static test. The data reduction technique for the conversion of stress-strain relation to impact stress-particle velocity behavior is based on an analysis of Courant and Friedrichs [1] which can be adapted to a rate-independent crushable-locking material (i.e., foam). Comparison between static and dynamic tests shows reasonable agreement at higher stress levels, with discrepancies at lower stress levels interpreted in terms of rate effects.