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FACTORS INFLUENCING DYNAMIC MECHANICAL PROPERTIES OF RED ‘DELICIOUS’APPLE TISSUE
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Transactions of the ASAE. VOL. 43(6): 1725-1731 . (doi: 10.13031/2013.3074) @2000
Authors: R. W. Bajema, A. L. Baritelle, G. M. Hyde, M. J. Pitts
Keywords: Apples, Impact failure stress, Failure strain, Strain rate, Size, Homogeneity, Isotropy
This research measured tissue failure stress, failure strain, and shock wave speed in dynamic axial
compression tests to assess, with respect to these properties: (1) the homogeneity of Red Delicious apple tissue samples
(10 mm dia., 15.2 mm long) taken around the apple equator; (2) the isotropy between tissue samples taken radially and
parallel to the stem-calyx axis; and (3) the effects of strain rate and fruit size on dynamic (impact) properties. Results
were compared with Magness-Taylor penetrometer measurements.
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At strain rates from 20 to 150 s 1 (equivalent to drop heights of 5 to 265 mm, respectively), the measured tissue
properties were homogeneous around the periphery of the apple, but not isotropic for radial versus parallel sample
orientation. Radial samples had lower failure strain but slightly higher failure stress than parallel samples. Smaller apple
tissue had lower failure strain, higher failure stress, higher elastic modulus, and higher shock wave speed than that of
larger apples. Failure stress and strain both increased as strain rate increased from 20 to 150 s 1 . However, the failure
strain was largest at the quasi-static strain rate (0.027 s 1 ), likely due to a bio-yield occurring at the quasi-static strain
rate, but not at dynamic strain rates. As expected, very poor linear correlations occurred between the Magness-Taylor
penetrometer force and dynamic failure properties. Generally, correlations were better at the lower than at the higher
strain rates, which is logical considering the viscoelastic nature of apples and the quasi-static loading rates used in
Magness-Taylor measurements. Important conclusions are that measurement of dynamic failure properties must take into
consideration fruit size and strain rate, and that Magness -Taylor tests cannot predict such properties, at least in the
apples we measured.