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Some mechanical and rheological properties of the netted gem potato Timbers, Gordon Ernest

Abstract

In order to be able to predict the behaviour of the potato tuber under conditions of applied load, a description of the mechanical and rheological properties in terms of engineering parameters is necessary. The behaviour of the potato was studied under several conditions of external load. Equipment to facilitate the application of various types of loads to the material was purchased or constructed. Electronic sensing and recording equipment was used to observe the response of the material to the applied loads. One-half potato tubers were used to study the response of the potato to the application of a constant stress. Whole tubers were subjected to conditions of constant strain to observe stress relaxation within the potato tuber. One inch, and two inch cores as well as half tubers were used when studying the response of the potato to compression over a range of strain rates and under four loading areas. Under conditions of external loading it was found that the potato behaves as a viscoelastic material and that the methods for the analysis of linear viscoelastic behaviour were applicable. When held under a constant stress the potato was found to exhibit the property of creep. The creep rate was dependent on the applied stress and the total creep deformation was dependent on the stress and the duration of loading. The potato tubers possess the property of stress relaxation. The stress or force relaxation curve may be represented by a three term exponential equation for periods up to ten minutes. For periods longer than ten minutes a four term exponential equation would be required to adequately describe the force relaxation curve. When subjected to compressive loading the potato tuber flesh was found to become more rigid with increasing rates of strain. The area of the test plunger used to load the sample was found to have a marked influence on the apparent modulus of elasticity, when this value was calculated from the slope of the stress-strain curve. While the loading under small diameter plungers is of a very complex nature, a reasonable approximation of the apparent modulus of elasticity may be obtained using the analysis for a rigid die acting on a semi-infinite elastic solid. The apparent modulus of elasticity for-the unsupported potato flesh was found to be 579 ± 76 psi. The degree of elasticity of the potato tuber was found to be relatively constant up to a load of 70 lbs. with a mean value of 0.820 ± .041. An abrupt decrease was observed between the loads of 70 and 80 lbs. with a much less marked decrease occurring with loads greater than 80 lbs. The mean degree of elasticity for loads from 80 to 120 lbs., was O.691 ± .026. The overall mean value for the degree of elasticity was O.765 ± .074. The energy required to produce failure in the potato tuber under compression was found to increase with a decrease in the rate of strain. The increase in energy required at low rates of strain is attributed to the property of stress relaxation exhibited by the potato. The potato was found to absorb a large portion of the energy used in deforming the tuber, as indicated by a large hysteresis loop. The hysteresis loss on the first loading cycle was found to increase with an increase in the applied force. For the subsequent two cycles there was no apparent change in the loss with an increase in the applied force. Hysteresis loss in potato tuber cores was found to be considerably higher than with whole tubers.

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