To examine the failure mode in the potato tuber, intact tubers were first tested under flat plate compression. Some potatoes were cut into two halves and then were tested diametrically and radially under flat plate compression. The fracture mode in most of the tests conducted was observed to be a splitting fracture at the center of the sample. To remove the effect of the shape of the tubers in their failure strength or mode, a peeling device was designed to reduce the tubers to spheres of 50 mm in diameter. To formulate a failure theory, it was necessary to find the stress distribution within the spherical tuber. A viscoelastic finite element program was adapted to analyze the stress distribution within a spherical sample compressed between two flat plates. The problem consisted of axisymmetric analysis and quasi-static loading. Since smaller elements in the finite element model produce a more accurate analysis, it was decided to use the rectangular coordinates to achieve the required fineness at the center of the sphere. The results of the stress analysis from the finite-element program indicated the existence of the normal tensile and compressive stresses at the center of the sphere, which is believed to be the cause of the splitting failure at that point. An additional set of failure tests was conducted under torsion. The specimen was a cylinder with square ends. This test was found to be useful in checking the failure theory which was developed for the spherical specimen. Most failure theories utilize the failure strength of the material under uniaxial tension and compression. Uniaxial tension tests were conducted on rectangular strips of the potato sections. The failure most often occurred in the middle section of the specimen. The failure plane was a flat surface normal to the loading axis. This type of failure is an indication of brittle failure. Uniaxial compression tests were conducted on cylindrical specimens. The failure most often occurred in planes inclined approximately 45(DEGREES) with the vertical axis. There were two distinct failure planes in this material, unlike isotropic brittle material in which there was only one such plane. This is believed to take place due to the directional properties of the potato. The Mohr-Coulomb fracture theory was found to be a suitable failure theory for the potato tissue. The failure stresses were predicted with a (+OR-) 5 percent difference from the experimental values. The analytical stresses at the center of a spherical potato under concentrated loading conditions were obtained using the correspondence principle.
