Modelling the area expansion ratio on uniaxial compression of cylindrical potato samples

Abstract

Uniaxial compression tests were performed on potato flesh. Cylindrical samples of heights (5, 10, 15 and 20 mm) and diameters (25.4 and 19.05 mm) were tested under lubricated (mineral oil), non-lubricated (natural) and increased friction (emery paper) conditions. Sample flatness (force free area/loaded area) ranged between 0.79 and 4.20. Deformation rate effect was also examined by performing compression tests at 50, 100, 200 and 400 mm min −1. In this study, parameter derived from compression to failure was failure strain ( ε r %). It was established a method for determining the area expansion ratio, AF/ A 0 (actual maximum area in contact with the loading platens/original area) at several deformation levels ( ε %). For that, loaded area was determined at deformation levels (20%, 40% and 80%) by planimetring of a graphic mark, and then adjusted by regression with respect to deformation level. From this regression, area expansion ratio at failure, AF r/ A 0 was obtained by replacing at each equation corresponding failure strain value. Cylindrical potato samples compression was accompanied by a significant cross-sectional area expansion, evidencing a far-ideal area expansion. For lubricated and non-lubricated friction conditions, increasing deformation rate and sample diameter increased AF r/ A 0, whereas increasing sample height decreased the ratio, mainly under non-lubricated condition. Lubrication causes an increase of AF r/ A 0 average value, near 5% with respect to that obtained with non-lubricated friction condition, meaning a lower influence of deformation rate and sample height, but a higher flatness influence. Effect due to surface lubrication was smaller than in previous compression tests on cheeses, which could be attributed to release of fluid from the damaged tissue of the potato flesh which relatively reduced lubricant effectiveness.