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Harvesting wheat is carried out by cutting the stem and threshed. When the stem (straw) bends due to pest and weather, losses are incurred especially during harvesting. Solid stem wheat varieties have been bred to resist pest like wheat stem sawfly and lodging. Solid stem varieties may lead to higher straw strength and energy which consequently impacts harvesting and collection. Also, farmers are faced with the challenge of increased cost of transporting the straw outside the farm due to their high volume. Previous research investigations have been done on increasing the straw bulk density and have led to producing more dense straws (double-compressed bales, pellets, cube, and briquette) but the cost of processing them and their physical quality is still a challenge. There has been a report that it takes low capital producing bale than other dense products. This means that if the bulk density of bales can be further increased through compression, it will be more economical using the wheat straw in a dense bale form. The research project investigated the mechanical properties of stems of twelve varieties of wheat (solid and hollow stem) at different moisture levels and internode positions. Aside from the compression test that was carried out on single moisture (14% w.b), samples were conditioned to three moisture content levels (14, 18, and 22% w.b) before testing was carried out. Shearing, cutting, tensile, and compression tests were done using different tools mounted on the InstronTM universal tester while the texture analyzer and a three point tool were used for bending test. The shear box apparatus was employed in determining the coefficient of internal friction. The stem diameters were determined by individually imaging the stems to be tested. Compression and relaxation models were fitted to the compression test data to determine their applicability to wheat straw compression and relaxation experimental data, respectively. Different orientations of fibers were obtained across varieties for studies on stem imaging with varying stem areas. Data analysis revealed that moisture had significant effect on coefficient of internal friction while moisture and internode position had positive correlation on shearing, cutting, and tensile strength as well as shearing and cutting energy but a negative effect on bending strength and modulus of elasticity for all varieties (P< 0.05). The coefficient of internal friction ranged from 0.095-0.669. Average shearing, tensile, and cutting strength varied between 4.9-23.0 MPa, 14.3- 114.7 MPa, and 1.4- 10.2 MPa, respectively, while the average shearing and cutting energy ranged from 62.4-270.0 mJ and 27.0-133.3 mJ, respectively. Mean bending strength and modulus of elasticity varied between 43.9-4.2 MPa and 3.5-0.1 GPa, respectively. Different trends were found across varieties when the mechanical properties were compared with respect to the internode position. Solid stem varieties had much lower shearing, cutting, and tensile strength than hollow stem wheat varieties while there was no difference between both stem types in relation to coefficient of internal friction, shearing, and cutting energy as well as bending strength and modulus of elasticity. The compression and relaxation models fitted accurately to the compression and relaxation test data, respectively, for all wheat varieties. The k4 values obtained from fitting the Peleg and Moreyra model to the relaxation data were greater than one (k4 > 1). Average percentage relaxation and asymptotic modulus range from 38.6 to 42.4% and 10.57 to 11.49 MPa, respectively, with no difference between the average percentage relaxation and asymptotic modulus of solid and hollow stem varieties. Models developed relating moisture content to shearing strength and energy, cutting strength and energy, bending strength, modulus of elasticity, and coefficient of internal friction, respectively, had varying R2 values.



wheat, mechanical properties, harvesting, models, lodging



Master of Science (M.Sc.)


Chemical and Biological Engineering


Biological Engineering



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