Date of Award
Master of Science
Alvin R. Womac
Michael J. Buschermohle, Paul D. Ayers
Switchgrass stem cross-sections were failed with ultimate shearing stresses that were one-fifth the magnitude of ultimate tensile stresses, thereby indicating that shear- dominant size reduction equipment would be much more efficient than tensile-dominant size reduction processes. Individual tensile strength measures ranged from 28 to 205 MPa and shear strength ranged from 6.9 to 38 MPa for all test conditions. Tests were conducted throughout a cropping season to obtain a range of switchgrass properties representative of those available for typical biomass processing. Representative processing conditions were organized and statistically tested based on switchgrass variety, maturity, elapsed time after harvest, moisture content, stem diameter, and stem thickness. Significant differences were detected among tensile and among shear strength measures for switchgrass lowland varieties of Alamo and Kanlow. Mean tensile strength increased as mean moisture content decreased from about 60 to 10 % wet basis, and tensile strength increased two-fold with a corresponding increase in elapsed time after harvest ranging from 2 to 386 h. This indicated that tensile-dominant size reduction should be conducted early in the harvest process and at a high moisture content to minimize energy consumption for grinding. Mean shear strength was relatively unaffected by moisture content and elapsed time after harvest.
Data analyses also provided indicators of consistency and validity of the test method. For example, desiccation of stem samples for tensile and shear tests occurred at the same statistical rate even though a slightly offset testing schedule was a necessary strategy to accommodate coincident measures. Mean stem moisture was statistically consistent throughout the study. Diameter and thickness of stems slightly decreased as time after harvest increased, possibly due to a stem shrinkage phenomena associated with desiccation. Switchgrass stem diameter and thickness were directly proportional to moisture content, with Pearson correlation coefficients of 0.24 (P = 0.0) and 0.30 (P = 0.0), respectively. Test cross section widths of samples were consistent among varieties and maturity class, thereby indicating uniform sample preparation throughout the experiment.
Yu, Manlu, "Ultimate Strength Characteristics of Switchgrass Stem Cross-Sections at Representative Processing Conditions. " Master's Thesis, University of Tennessee, 2004.