Sandwich Construction with Carbon Foam Core Materials

To study suitability of the emerging ultralightweight carbon foams in load-carrying structures, the state-of-the-art carbon foams from various manufacturers were evaluated as core material in a sandwich construction. The carbon foams were firstly characterized by measuring compressive and shear mechanical properties. The carbon foam possessed highly anisotropic properties between in-plane and through-the-thickness directions. The foams aged at 316°C (600°F) for 100 h in air lost 0.6% of their weight and showed little degradation in the properties within the scatter range of test data. The carbon foams tested at an elevated temperature of 316°C (600°F) showed no degradation in the compressive modulus and strength as compared to the properties measured at a room temperature of 21°C (70°F). Sandwich beams with laminated composite facesheets were fabricated with a carbon foam core and tested under static and fatigue four-point bending loads. The beams under the static loadings showed nearly linear elastic behavior until the maximum failure loads, and then postfailed either in yielding or in brittle mode following the postfailure behavior of the carbon foam core. The failure occurred in the core material in a shear mode. Sandwich beams with carbon foam cores survived after a few hundred or even a few hundred thousands of cyclic loads. Both the moduli and strength of the beams remained nearly unchanged after the fatigue loads. Central displacement and strain measurements at the gauge locations matched well with the analytical solution from a sandwich-beam theory. The shear moduli and strength of the carbon foams calculated from the four-point bending tests were in good agreement with those from torsional tests. The sandwich beam test technique is useful for the determination of the foam shear properties, especially for uncoupled shear properties of only one plane. The sandwich beams with the carbon foam cores tiled together in the middle of the beam exhibited nearly identical load-displacement behavior as well as the shear failure mode to those of the beam made with an intact core. From the modulus point of view, the hole-drilling with the high-density foams is a better way of achieving the weight reduction in the sandwich construction than using the low-density foams. If the materials were removed in the noncritically loaded area (e.g., central section of the beams under the four-point bending), the weight reduction can be achieved without sacrificing much strength property of the core materials as well.