Effects of moisture content on elastic-plastic properties of bulk wheat

The total annual output of grain is about 600 million tons in China. With the development of grain storage technology, grain bins with diameter larger than 30 m and capacity exceed tens of thousand tons emerged in recent years. As the climate in different regions vary greatly in China, grain moisture content varied significant in bins accordingly, for instance, grain moisture content is high in bins in southeast of China, while it is substantially low in northwest of China. The effect of moisture content on elastic-plastic properties of bulk wheat was investigated in this study. Wheat grain produced in Zhengzhou, China was utilized in this study, the equivalent diameter of kernel was 4.5 mm. The wheat was dried in oven under 105℃, wetted to moisture content of 4.90%, 8.56%, 10.64% and 13.45%, and sealed in plastic bag and placed in refrigerator under 10℃ to equilibrate for 7 days to a stable state. Then bulk wheat sample was tested in stress path triaxial apparatus. The bulk density was 0.8 g/cm3, the height of sample was 124 mm, and diameter was 61.8 mm which was about 13.7 times of equivalent kernel diameter, and diameter ratio conformed to the requirement of triaxial test. Cell pressure in the test was set to be 50, 100, 150, 200, 250 and 300 kPa, the maximum cell pressure of 300 kPa covered most situation in grain bins. The wheat samples were sheared by increase axial stress, while keep cell pressure constant, until axial strain was large than 25%, which was correspond to the critical state. Test results showed that peak shear strength and residual shear strength followed nonlinear Mohr-Coulomb criterion for granular material, in that the cohesion was zero, friction angle was a nonlinear parameter depending on cell pressure. The friction angle decreased with the increase of logarithmic value of cell pressure. Peak friction angle and residual friction angle under reference pressure (100 kPa) increased with the increase of moisture content, as moisture content increased from 4.90% to 13.45%, peak friction angle increased from 25.4° to 27.4° and residual friction angle increased from 24.1° to 26.7°. Peak friction angle and residual friction angle under reference pressure were 24.03° and 22.31° when moisture content was zero. As moisture content increased for 1%, peak friction angle and residual friction angle under reference pressure increased by 0.22° and 0.30° respectively. In deviator stress - mean normal stress plane, the peak result and critical state result followed Cambridge elastic-plastic theory, i.e. deviator stress increased linear with mean normal stress. Peak stress ratio and critical state stress ratio increased linear with moisture content. As moisture content increased from 4.90% to 13.45%, peak stress ratio increased from 0.905 to 1.008 and critical state stress ratio increased from 0.863 to 0.981. When moisture content was zero, peak stress ratio and critical state stress ratio was 0.835 and 0.776 respectively, and moisture content increased for 1%, peak stress ratio and critical state stress ratio increased by 0.012 and 0.014 respectively. Initial modulus was the modulus under axial strain of 0.05%, secant modulus was the modulus under half of peak deviator stress. The relationship between initial modulus, secant modulus and cell pressure could be expressed through power function model. Initial modulus and secant modulus under reference pressure decreased linear with moisture content. Initial modulus and secant modulus under reference pressure was 28.9 and 6.86 MPa when moisture content was zero, and moisture content increased for 1%, initial modulus and secant modulus under reference pressure decreased for 0.98 MPa and 0.25 MPa respectively. As moisture content ranged from 4.90% to 13.45%, the exponent in initial modulus model was between 0.482 and 0.866, the exponent in secant modulus model was between 0.759 and 0.881. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.