Stress relaxation in sugar beet root under various mechanical load conditions

Identification and determination of the mechanical properties of biological materials, especially fruits, vegetables and industrial plants, can be made using commonly applied stress relaxation tests.They are of particular significance because their results make it possible to propose a mechanical model of studied material.The aim of the study was to determine the effects of initial deformation and deformation velocity on the parameters of generalised Maxwell model during stress relaxation in the sugar beet root.The tests were carried out by means of the texture analyser (model TA.HD plus, Stable Micro Systems, Goldaming, UK) at the three deformations: 2.0, 3.5 and 5.0 mm and the four deformation velocities: 1, 2, 10, and 20 mms -1 .The cut sugar beet samples used in the experiment were cylindrical in shape, with a 9.5 mm diameter and a 20 mm height.They were initially compressed along the vertical axis in a state of uniaxial stress and constant deformation was maintained while recording the force response for 35 seconds.The two-branched generalised Maxwell model with an additional elastic element was used to describe the experimental force response curves.The sample dimensions as well as the initial deformation velocity were taken into consideration in the model formula.Two relaxation times of the model decreased with the increasing deformation velocity and increased with the increasing deformation value.The dependences of the obtained relaxation times on the phenomena inside the tissue such as the flow of fluids and gases under the load were interpreted.Changes of model parameters as a function of deformation velocity could testify the appearance of internal micro damages in the material during deformation.The tendency of increase in the peak force response along with the increase of deformation velocity shows typical viscoelastic behaviour of sugar beet root flesh.The novelty of the paper is the application of the generalized Maxwell model with the stable element for prediction of mechanical response of the sugar beet whose morphological structure is different compared to such soft fruits as apples and pears.