Uniaxial stress relaxation of celluloid softened by heating was observed experimentally The experiments consisted of the constant strain (single step) and the gradually stepup strain tests. The representation of stress relaxation was evaluated by the single integral nonlinear viscoelastic model up to the third order of strain. The main features obtained are summarized as follows:
(1) For the constant strain test, though residual stress at a certain duration after application of strain increases in proportion to strain in the range of small strain, it decreases with an increase in strain above a certain level of strain. The prediction of stress relaxation by the model with third order of strain agrees with the observed data. The relaxation function in the equation is represented by a power function of time. An exponent of time in the function decreases with an increase in strain.
(2) For the gradually stepup strain test, the variation of stress which occurs instantaneously under the change of strain is almost constant for the number of steps. Residual stress at the end of each step increases with the number of steps, and the hardening of the material appears. For stress relaxation on stepup strain, the recovery of the material with the number of steps appears at the same time. The prediction of stress relaxation by the model with simple modification of super-position principle does not agree with the observed data.
