Fracture behaviour of potato samples (cv. Desiree) under uniaxial compression

Uniaxial compression tests were performed on potato flesh. Cylindrical samples of heights (5, 10, 15 and 20 mm) and diameters (19.05 and 25.4 mm) were tested under non-lubricated, lubricated and increased friction conditions. Sample flatness (λ = 4height/diameter of sample) of specimens ranged between 0.79 and 4.20. Displacement rate effect was also examined by performing tests at 50, 100, 200 and 400 mm min−1. Measured fracture parameters were Cauchy strain (εC), engineering stress (F/A0), true stress (F/Af), work of fracture (Wf), and the apparent elastic modulus (E). Fracture parameters showed a dependence upon sample flatness when no lubricant was used, which decreased with lubrication and increased with increasing friction conditions. A lower flatness ratio led to an increase in stress at a given strain level, i.e. the material appeared to be stiffer than it truly is. Shorter samples and increased friction resulted in an observed dependence upon displacement rate; a higher displacement rate resulted in a lower fracture stresses for a given strain level. Lubrication increases significantly the Cauchy strain, and reduces the true stress, values. Cauchy strain was found to increase with displacement rate and decreased when the flatness ratio increased. Engineering stress, true stress, and work of fracture decreased as displacement rate and sample flatness are increased. Lubrication was found to reduce the effect of displacement rate on the Cauchy strain. Sample flatness influenced engineering stress, work of fracture and apparent elastic modulus, whereas displacement rate influenced true stress and apparent elastic modulus. Increased friction resulted in a higher sample flatness influence on all fracture parameters. Results show that for any compression test with potatoes, it is possible to determine optimum conditions that will minimize the effects of the test on fracture parameters.