Mechanical properties and stress strain model for bulk wheat based on stress path test

Abstract: Total amount of stored grain in China is about 200 million tons, which is of great importance to ensure the quality and safety of grain during storage period, and to modernization of agriculture engineering. When a storage cycle is ended, grain will be discharged from bins, and then loaded in for another storage cycle. Vertical stress increases in loading process, while decreases in discharging process, and different stress path will be generated in the bulk grain, resulting in complicated pressure and frictional force on bin wall, which will cause crack, failure or even overturn of bin structures. It is vital to study stress strain relation and strength properties of bulk grain under different stress paths. Geotechnical constitutive models have been used in grain bin simulation, however, the feasibility of these models for bulk grain has not been validated. In this study, stress path triaxial test was utilized to simulate complicated load path in grain bins. Wheat purchased from grain depot in Zhengzhou City, Henan Province was used in this study. Specimen had water content of 10.9%, bulk density of 0.85 g/cm3, diameter of 61.8 mm and height of 125 mm, the average axial length of wheat kernel was 4.5 mm, and the ratio of specimen diameter to kernel axial length was 13.7, which was larger than the minimum specified ratio for triaxial test. Three types of stress paths were investigated in this study: conventional triaxial compression (CTC), constant mean normal stress compression (CMS), and reduced triaxial compression (RTC). In CTC test, radial stress was set as constant, while axial stress increased simultaneously; in CMS test, radial stress decreased, while axial stress increased simultaneously to keep mean normal stress constant; in RTC test, axial stress was set as constant, and radial stress decreased simultaneously. Stress path triaxial test procedure was as follows: After bulk wheat specimen was mounted on triaxial apparatus, isotropic stress was applied from 0 to prescribed value (50, 100, 150, 200, 250, 300 kPa); then CTC, CMS or RTC stress path was applied on specimen, and stress strain result was recorded until axial strain reached 30%. Initial modulus, secant modulus and critical state properties were determined from stress strain curve, strength and stress strain parameters were determined, and finally new model was proposed to depict stress strain relation for bulk wheat under different stress paths. Test results show that, under the same radial stress, CTC test has the largest initial modulus, and RTC test has the lowest value. Under all stress paths, initial modulus and secant modulus are found to be in a power function growth with the ascent of radial stress. In CTC test and CMS test, secant modulus is significantly lower than initial modulus; while in RTC test, secant modulus is not significantly reduced compared with initial modulus. Under reference pressure (atmospheric pressure), CTC test has the largest initial modulus of 24.1 MPa, and RTC test has the lowest value of 7.9 MPa; while initial modulus of RTC test has the largest increasing rate, CTC test has the lowest increasing rate. Under reference pressure, CTC test has the lowest secant modulus of 7.7 MPa, and RTC test has the largest value of 9 MPa; secant modulus of RTC test has the largest increasing rate, and CTC test has the lowest increasing rate. Deviator stress increased during shearing process, and specimen failed at the peak point on q-p (deviator stress - mean normal stress) plane. Under all stress path and radial stress situations, failure point fell on the same critical state line. Critical state line for bulk wheat has straight line form, and critical state stress ratio is 0.976. New modified cubic curve model for bulk wheat grain under different stress path conditions was proposed. In the model, strength conforms to Mohr-Coulomb failure criterion; crest reduction coefficient has linear relation with residual strength ratio; peak axial strain has linear relation with radial stress. Model parameters comprised apparent cohesion, internal friction angle, residual apparent cohesion, residual internal friction angle, parameter a and k for crest reduction coefficient, and parameter b and m for peak axial strain. Calculated results show that the proposed model can simulate the results of all stress paths under different confining stress levels. The model can reflect strain softening and strain hardening properties of bulk wheat; peak shear strength and residual shear strength can be determined; and the simulated stress and strain curve coincide well with the test results. The result of this paper provides more accurate parameters for grain bin load calculation considering the stress path conditions, and the new model can be used to estimate stress and deformation of bulk wheat, and to improve the designing method of grain bins.