Mechanical Properties of the Leaf Sheath of Sugarcane

Abstract

<abstract><title><italic>Abstract.</italic></title> Leaf impurities that contaminate sugarcane stalk that is to be refined into sugar can reduce the quantity and quality of the refined product. To reduce this contamination, a mechanical device is needed to completely remove the leaf from the stalk during sugarcane harvesting. A sugarcane leaf consists mainly of a blade and sheath. In mechanical leaf-stripping processes, it is easy to remove the blade from its junction with the sheath; however, it is difficult to strip the leaf sheath from the stalk because it encases the stalk tightly, similar to a tube, and multiple sheaths overlap one other. To provide a theoretical basis for improving the design of machinery for stripping leaves from sugarcane, information about the mechanical strength of the sugarcane leaf sheath is required. In this study, fundamental experiments, including longitudinal and transverse tensile tests and punch-and-die tests, were conducted using a universal testing machine (WDE-500N), and custom fixtures were designed using the characteristics of sugarcane leaf sheaths. For sample collection, one complete representative leaf sheath was selected from each of six adjacent stalk sections, with each section consisting of two internodes. The stalk sections originated at the apical growing point and were numbered in sequence from 1 to 6 from the apex toward the root. Data on the longitudinal tensile strength (LTS), transverse tensile strength (TTS), and punch-and-die shear strength (PSS) of the sugarcane leaf sheath were analyzed as a function of the growth period, sheath position along the stalk, and sheath moisture content (w.b.). Based on the analysis of these data, several methods for mechanically removing leaves from sugarcane stalks are presented. The LTS, TTS, and PSS values of the leaf sheath, which were examined in groups 1 to 4 when the sheaths were young and displayed leaf greenness characteristic of moisture contents between 45% and 70%, did not vary significantly with position along the stalk from internodes 1 to 4. Similarly, the strength values did not vary significantly between internodes 5 and 6, which were older, appeared to be dried, and exhibited moisture contents ranging from 10% to 20%; however, these values were lower than those observed for internodes 1 to 4. Because the leaf sheath at internode 7 and below was older than the internode 5 and 6 samples, which exhibited reduced fracture resistance, there was no need to measure below internode 6. The values of the three strength measurements at moisture contents above 45% were significantly higher than those at moisture contents below 20%. The maximum values of the LTS, TTS, and PSS were 28.19, 0.90, and 7.13 MPa, respectively. The mechanical properties of the leaf sheaths at internodes 1 to 4 are valuable data for the development of mechanical leaf-stripping machinery. Based on the differences among the maximum values of LTS, TTS, and PSS, the process of removing leaf sheaths from sugarcane stalk in a leaf-stripping system as proposed should be reasonable. In a leaf-stripping system, a force should first be applied in the radial direction of the cane cross-section to a leaf sheath by some leaf-stripping component to punch the sheath. At the same time, the component should also apply a force in the tangential direction of the cane cross-section on the leaf sheath to tear the sheath along the transverse direction. Then, the component should apply a pulling force along the fiber direction of the sheath to remove the sheath from its stalk in tensile failure mode.