Effects of vertical pressure and shear velocity on direct shear strength and dilatancy properties of wheat

Abstract: Grain storage facility is an important infrastructure of guaranteeing food security and concerning people's livelihood as well. Grain heap in bins will undergo filling and discharging process after a certain period. During the process, change of vertical and horizontal load will lead to generation of shear stress and slip band in heap, which then acts on grain bin wall, and will finally influence the stability of bin structure. Therefore, shear strength and dilatancy properties of wheat grain heap are important indicators in design of grain bins. A series of researches were conducted with Henan wheat (variety is Zhengmai 113) through direct shear tests on the strength and dilatancy characteristics under different vertical pressure and shear rate. According to grain heap parameters in bins, vertical pressure was set at 6 levels: 50, 100, 150, 200, 250 and 300 kPa; shear rate was set at 3 levels: 0.78, 1.55 and 2.33 mm/min. Direct shear test terminated at displacement-diameter ratio of about 0.2, and in the process, shear area decreased by 14%. By revising shear area, the research tried to improve the veracity of shear stress result. Research results revealed that, according to the relation between shear stress and shear displacement, volumetric strain and shear displacement, the shear deformation of wheat grain bulk could be divided into 3 stages: elastic stage, plastic deformation stage and kernel compression stage. In elastic stage, shear stress and shear displacement were small; grain skeleton was in elastic state, and with grain skeleton contracting, grain volume contracted as well; the relationship between shear stress and shear displacement was linear. In plastic deformation stage, large plastic deformation was generated in grain skeleton, and the relation between shear stress and shear displacement was nonlinear; as shear stress increased, huge plastic deformation occurred in grain skeleton, and grain was upraised on shear band, which thus led to the volume expansion. In kernel compression stage, grain kernel was compressed and volume contracted to a certain extent, and as the shear displacement rose, shear strength kept stable or decreased slightly. Wheat heap shear strength included 2 components: dilatancy and frication. Shear strength of wheat grain stack accorded with the Mohr-Coulomb strength principle. Results showed that shear rate influenced interlock stress, and as shear rate increased from 0.78 to 2.33 mm/min, interlock stress increased from 7.5 to 12.9 kPa, and internal friction angle varied from 38.2° to 35.0°, and dilatancy angle ranged from 5.1° to 4.8°. Volume of grain heap contracted in elastic stage, and the maximum contracted volumetric strain was less than 0.4%. Volume of grain heap dilated in plastic deformation stage, and the maximum dilated volumetric strain was generally greater than the maximum contracted volumetric strain. In plastic deformation stage, the maximum dilated volumetric strain decreased with the increase of vertical pressure, and the maximum volumetric strain rate decreased with the increase of the shear rate. This study provides a scientific basis for stress and strain calculation of grain heap and grain bin design.