Characterization of nonlinear behavior of apple flesh under stress relaxation

A simplified three-dimensional nonlinear viscoelastic constitutive equation, based on the Green-Rivlin theory, with four material property functions, was used to characterize the relaxation behavior of apple flesh. Unconfined uniaxial stress relaxation tests showed that apple flesh exhibited nonlinear behavior under small deformation. The nonlinear behavior was affected by strain level and initial deformation rate. Under hydrostatic creep loading, there were no statistically significant differences in bulk creep compliance at applied stress levels ranging from 200 to 350 kPa at the 0.05 level of significance. Therefore, apple flesh was linearly compressible in bulk loading. The four material property functions in the constitutive equation were determined from confined uniaxial relaxation and hydrostatic creep tests. The nonlinear constitutive equation was then used to predict experimental data from one-step and three-step confined uniaxial loading tests. The average predicted errors with respect to the experimental results were 6.0% for the one-step loading and 4.4%, 11.2%, and 15.3% for the first, second, and third step, respectively, of the three-step loading tests.