谷子籽粒压缩力学性质及损伤裂纹形成机理.

Millet contains protein, vitamin B1 and other mineral elements needed for human health, which has functions of enhancing blood circulation, brain tonic, and sleep quality. In recent years, the market demand for millet is rising. However, the mechanization level of millet harvesting is low. Manual harvesting is very common, which requires high labor intensity, has low efficiency and results in insufficient supply of millet. In this paper, we systematically investigated effects of moisture content and varieties of millet on its physical parameters, effects of millet varieties and compression azimuth on the compression mechanical properties (including yield load, deformation, failure energy and apparent elastic modulus) of millet, and effects of compression azimuth on crack formation of millet. The study results are expected to provide the basis for the design and optimization of millet harvester and related processing machinery. In this paper, the 1000-grain weight was measured by an electronic balance, the triaxial sizes by a vernier caliper, and the moisture content by fast grain moisture analyzer for four millet varieties: Zhangza-10, Dungu, Jingu-21, Changgu. The sphericity of the millet, the arithmetic mean diameter and the geometric diameter were calculated using the triaxial sizes. The compression tests were carried out along X, Y, Z axis using a texture analyzer and the data were analyzed by SAS statistics software. The yield load, deformation, and failure energy were obtained from the force-distance curve measured by a texture analyzer. At the same time, the apparent elastic modulus was calculated according to the Hertz formula. The experimental results were summarized as follows in six categories. 1) For the physical parameters of the millet, with a moisture content between 15.2% and 25.1%，the length, width, height, 1 000-grain weight, average diameter and geometric mean diameter of millet grain decreased as the moisture content decreased. The millet length, width and thickness range from 2.028 to 2.112 mm, 1.818 to 1.931 mm, 1.413 to 1.513 mm, respectively. The sphericity of millet was more than 84%; the arithmetic mean diameter varied from 1.750 mm to 1.852 mm; and the geometric mean diameter 1.732 mm to 1.834 mm. Therefore, the shape of millet can be regarded as ellipsoid. There were significant differences (P<0.001) in the triaxial sizes, 1000-grain weight, sphericity, arithmetic mean diameter and geometric mean diameter among different varieties. When the moisture content of millet was about 25%, the 1000-grain weight of Jingu-21 was 3.473 g, which was the heaviest millet, while Zhangza-10 was 3.264 g, which was the lightest millet. The moisture content has a negligible effect on the sphericity. The effect of varieties on sphericity was significant (P<0.001). 2) For factors on the compression mechanical properties, the influence of moisture content, compression azimuth, and variety on the compression mechanical properties were significant (P<0.01). The significant factors affecting the compression mechanical properties were compression azimuth, moisture content, and variety in a sequence from the most significant to the least significant. 3) For the effects of the moisture content, for the same kind of millet, the yield load and apparent elastic modulus decreased as the moisture content increased during compression in X, Y, Z axis. The yield load decreased slowly when moisture content ranged from 20.9% to 25.1%. The deformation and failure energy first decreased and then increased as the moisture content increased. For the same azimuth compression, the better the quality of the millet, the more compact the internal structure, and the smaller the deformation. 4) For crack formation of the millet: on the X axial, a crack started from the crest of the grain along the junction of embryo and endosperm, and then extended throughout the X axis. On the Y axis, a crack started from the connection point of grain and probe and the connection point of grain and base throughout the Y axis because of stress concentration. On the Z axis, a crack started from the junction of embryo and endosperm on the back to the ventral hilum along the weakest part of endosperm resistance extending throughout half of the grain. 5) For the effects of compression azimuth, the yield load increased in turn on the X, Y, and Z axis and the deformation and failure energy decreased on the X, Z, and Y axis in turn. The apparent elastic modulus of the millet increased in turn on the X, Y, and Z axis. This result indicated that millet was not uniform and it was an anisotropic body. 6) For the effects of millet varieties, with the same moisture content, the compression mechanical properties of Jingu-21, Dungu, Changgu and Zhangza-10 varied from large to small. This indicated that Jingu-21 had a relatively good compression resistance and quality. [ABSTRACT FROM AUTHOR]