TENSILE AND COMPRESSIVE PROPERTIES OF FLEXIBLE GRAPHITE FOILS

Tensile and compressive properties of flexible graphite foils prepared from expanded graphite flakes are compared with like properties of bulk graphites, and a model relating mechanical properties of these foils to their structure is proposed. Plots of tensile stress, +94,versus strain, +69,in the foil plane can be represented by the relation epsilon equals A sigma plus B sigma **2 known for bulk graphites, although elastic compliances A and especially plastic compliances B exceed those of bulk graphites. Ultimate tensile stresses of graphite foils increase with density and are less than observed in bulk graphites, but values of work of extension to fracture, 10-60 mJ/g, are comparable. Plots of compressive stress versus strain normal to the foil plane, in contrast, are concave to the stress axis. The restoring force in compression is too large to be attributed to trapping of gas in the pores of the foil and is dependent on the misalignment of graphite basal planes with respect to the sheet plane. Irreversible work expended in compressing isolated expanded graphite flakes to foil, 5-10 J/g, is accompanied by reduction of surface area to approximately two-thirds its original value.