Elongational creep test is carried out on printing paper and cellophane with varied applied loads and contained amounts of moisture. For paper, anisotropy of creep is studied.
Three types of elongation are observed: instantaneous elongation J_i, retarded elongation J, and plastic flow J_ƒ. J_i is proportional to the applied stress and is perfectly recoverable. J_r increases first rapidly, then slowly and reaches a steady value after a sufficiently long time. This steady value is approximately proportional to the applied stress. J_r is observed only when the specimen contains moisture more than the critical amount (ca. 3% for paper and 5% for cellophane). J_r takes place only under the stress beyond the yield value and increases proportionally to logarithm of time. Cellophane does not exhibit J_ƒ within the stress range used so far.
Some presumptions are made concerning possible mechanisms of elongation of these types. Absorption isotherm and moisture dependence of dielectric constant of the samples are presented for comparison.