Stress Relaxation of Apples at Different Deformation Velocities and Temperatures

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Abstract
Stress relaxation tests on apples (‘Beni Shogun’ variety) were performed at different velocities in the range of 0.0002 to 1 m s-1 and at three temperatures (2°C, 10°C, and 20°C). The generalized Maxwell model was used to describe the experimental stress relaxation curves. The two relaxation times of the model decreased with an increase in the deformation velocity. The relaxation times were related to the processes of gas and liquid flows in the intercellular spaces. This research showed the critical velocity associated with the weakness of the apple structure to lie between deformation velocities of 0.0002 and 0.002 m s-1, where a rapid decrease in the two relaxation times occurred. An increase in the peak force response with increasing deformation velocity showed the viscoelastic behavior of apple flesh. The equilibrium modulus describing the material condition after deformation was much larger under the quasi-static loading condition than the impact loading condition at all deformation velocities. The temperature did not seem to affect the Maxwell model parameters and peak force response for all deformation velocities under both loading conditions. Keywords: Apple, Generalized Maxwell model, Impact loading, Quasi-static loading, Stress relaxation, Viscoelasticity.

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