Effect of moisture content on compression mechanical properties and frictional characteristics of millet grain

Abstract: As the first grain of "five cereals", millet is one of the favorite grain crops and has high nutritional value and high market demand. Millet grains can be mechanically damaged under compressive load and friction in the processes of seeding, threshing, storage, and transportation. Moisture content plays an important role to mechanical properties and frictional characteristics of millet grain. The previous researches on grains' mechanical properties were mostly about big grain kernels, but small grain kernels like millet grain have been rarely studied. In this research, a typical millet grain in Shanxi Province, Jingu-21, was selected as test material. Compression tests were carried out using a texture analyzer. Millet grains were quasi-statically loaded in vertical orientation with 7 moisture content treatments i.e. 11.4%, 12.7%, 14.2%, 17.1%, 18.7%, 21.4% and 23.7% and 25 replicates per treatment. Compression mechanical properties of millet grain were measured in terms of damage force, deformation, and damage energy, and meanwhile elastic modulus and allowable compressive stress were calculated according to Hertz theory. The damage process was analyzed, and the force-deformation curve and the variations of mechanical properties with the moisture content were obtained. The test results showed that the moisture content had a significant effect on the compression mechanical properties. The force required for initiating grain rupture decreased from 19.457 to 11.732 N as the moisture content increased, and changed slowly when moisture content was lower than 12.7% and higher than 21.4%, but decreased rapidly with moisture content increasing from 14.2% to 21.4%. Deformation and damage energy decreased initially and increased subsequently as the moisture content increased. They reached their minimums at moisture content of 17.1% and 21.4%, respectively. Elastic modulus and allowable compressive stress linearly decreased with the increase of the moisture content and the trend similar to big grain kernels was observed, while the variations of other compression mechanical properties with the moisture content were found to be more complex than big grain kernels. The study showed that with low moisture content, millet grain had higher hardness and therefore had higher resistance to deformation and capacity to bear load. As the moisture content increased, millet grain was softened and its elasticity increased, and then under the same load, its deformation, elliptic contact area, and damage energy increased. Subsequently, the elastic modulus and the allowable compressive stress decreased. Millet moisture content should be kept at a level lower than 17% during seeding and storage because millet grains with lower moisture content would have better compression mechanical properties. Based on this finding, it is reasonable that millet grains are typically stored with a moisture content of about 11% and threshed with a moisture level higher than 21.4%. At the same time, frictional characteristics of millet grains with different moisture contents were determined using friction tester with 2 treatments and 5 replicates. As the moisture content increased, the coefficient of sliding friction increased from 0.183 to 0.203and from 0.269 to 0.307 for steel and aluminium, respectively. Coefficients of sliding friction of millet against aluminium plate were higher than that against steel plate. In the terms of friction reduction, the mechanical parts contacting with millet, such as metering device, should use steel material. The above results were analyzed using statistical regression, and the functions between moisture content and mechanical performance parameters of millet grain on compression and friction were established respectively. The study can provide a theoretical basis for the design and optimization of appropriate equipment and parameters.