不同含水率对谷子籽粒压缩力学性质与摩擦特性的影响

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

为了探明不同含水率谷子籽粒的物理机械性质,减少谷子籽粒在播种、碾米加工及储运等过程中受到压缩载荷及摩擦而产生的机械损伤,该文针对不同含水率的谷子籽粒进行压缩力学性质与摩擦特性试验。研究了谷子籽粒的挤压破碎过程,获得不同含水率谷子籽粒的力—位移(变形)曲线,破坏力、变形量及破坏能。随着含水率升高,破坏力减小,变形量和破坏能呈现先降低后升高的变化规律。同时采用赫兹接触理论,得到谷子籽粒单向表观弹性模量和许用挤压应力,结果表明二者都随含水率升高线性降低。分别测定了谷子籽粒与钢板和铝板间的滑动摩擦系数,随含水率升高,谷子与该2种材料的摩擦系数均增大,且与铝板的摩擦系数要高于钢板。根据试验结果,分别拟合得到了压缩和摩擦力学性能指标与谷子含水率的关系方程,为谷子播种、仓储、加工等装备设计及参数优化提供了基础依据。

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.