Elastic Properties of Cork. I. Stress Relaxation of Compressed Cork

Abstract

The stress relaxation of corky materials is of theoretical significance for the understanding of such two-phase (corky material+air) systems, and is of great practical interest, since these materials are commonly used as gaskets, seals, etc. Stress relaxation was studied at various degrees of compression and at temperatures ranging from 30°C to 200°C. The experimental data were obtained from an automatic stress-relaxation machine which employed the principle of a chainomatic balance. It was found that the stress-time curves for various compressions could be obtained from each other by multiplication. This experimental fact is generalized by the statement that the stress is a product of two functions. The first factor is essentially identical with the S-shaped load-compression curve of cork and is independent of time. The second factor is shown to be a linear function of log t and is independent of compression. The linear stress-log time curve is found to persist over a wide range of times and to continue until the stress has decayed to zero at the higher temperatures. The decay time as obtained either by actual experiment or by extrapolation is shown as a function of temperature. This gives a fairly complete picture of the stress-time-temperature behavior of cork under compression.