Mechanical shear and tensile properties of selected biomass stems

Perennial grasses and corn stalks can supply abundant lignocellulosic feedstock in the northern Great Plains of the U.S. There is a need to understand the mechanical properties of these crops for better handling and processing of biomass feedstocks in bioprocessing industries. The objectives of this research included determination of tensile and shear stresses of big bluestem, corn stalk, intermediate wheatgrass, and switchgrass stems and measurement of specific tensile and shear energies during tensile and shear failure. A high-capacity MTI-100K universal testing machine equipped with adapted tensile clamps and a specially designed and fabricated double-shear device were used for tensile and shear testing. Ultimate shear stresses were not statistically different for big bluestem, corn stalk, and intermediate wheatgrass, with values of 7.33, 8.53 and 6.23 MPa, respectively, which were less than switchgrass at 13.39 MPa (p < 0.05). Corn stalk had the greatest ultimate tensile stress of 69.30 MPa, followed by switchgrass, big bluestem, and intermediate wheatgrass. The ultimate tensile stress and shear stress were statistically different (p < 0.05) from each other for all four feedstocks. Both ultimate shear energy and specific shear energy for corn stalk were statistically different (p < 0.05) from big bluestem, intermediate wheatgrass, and switchgrass. The evaluated shear to tensile stress ratios, expressed as percentages, were 29.3%, 12.3%, 30.5%, and 31.9%, whereas the shear to tensile energy ratios were greater, with values of 42.2%, 67.2%, 61.2%, 50.4% for big bluestem, corn stalk, intermediate wheatgrass, and switchgrass, respectively. Based on these results, shear-dominant size-reduction devices (e.g., knife mills and chippers) should be more energy efficient, and this energy efficiency should be taken advantage of when designing size-reduction devices. Further research efforts are necessary to determine the effect of moisture, crop variety, and maturity stage, as these factors are known to affect the mechanical characteristics of biomass feedstocks.