Techniques for evaluating the behavior of apple skin under tensile loading
Within the past decade there has become a dire need for equipment to mechanically harvest and handle apples. Apple growers have had trouble finding suitable labor to economically harvest their crop. Thus far, mechanical harvesting has not been very successful. One basic problem encountered by mechanical harvesting and handling equipment has been the damage incurred by the apple during the process.
In an effort to discover what physical phenomena actually are responsible for this apple injury, several investigators have employed an engineering approach to analysing the physical properties of the apple. Several of the investigations have revealed that a large percentage of the apple's resistance to rupture lies in the skin alone.
The objective of this project was to establish techniques for determining and evaluating the mechanical and rheological properties of the apple skin. Skin specimen from three apple varieties (Winesap, Red Delicious, and Golden Delicious) were tested. The Specimen were loaded at two constant displacement rates, yielding the skin's force deformation behavior. In addition, the skin was displaced to a constant deformation and held constant, the relaxation behavior being observed. The creep behavior was determined by subjecting the specimen to a constant force. By examining the results of these experiments it was observed that apple skin was a viscoelastic material; in certain instances, however, the akin also exhibited some apparent plastic behavior.
From the experiments it was possible to determine many mechanical properties of the three varieties investigated. Some of the more important properties obtained were initial modulus of elasticity, secant modulus, tensile strength at break, and Poisson's ratio. The effect of displacement rate and specimen orientation on the strength was investigated. The apple skin was found to be anisotropic, with the maximum strength in a direction parallel to the core. It was observed that Poisson's ratio was not time dependent, and could be assumed a constant for each variety.
Mechanical models were employed to simulate the behavior of the Winesap apple skin under several loading conditions. It was established that no one simple mechanical model could be used to predict the skin's behavior in all loading situations. TWo models, a three-element and a four-element, were formulated from the relaxation and creep results respectively. The behavior of the models was compared with the experimental data, and it was found that the formulated models could be used to predict the behavior of the actual skin.