Study On Strength And Deformation Characteristics Of Wheat Based On Local Measurement Of Triaxial Test

Accurate acquisition of the mechanical properties of grain in the silo is the key to the safe design of the silo structure.The silo needs to load and unload grain during the grain storage cycle.During unloading,under the influence factors of side pressure and wall friction of the grain in the silo,the pressure distribution of the grain piles at different positions in the silo will be uneven.And then the structure of the silo may be damaged.In this paper,wheat is selected as the research object.Digital image measurement technology is introduced in the triaxial test,which can eliminate the influence of the end restraint.The local strength and deformation characteristics of wheat samples can be directly tested by tests.At the same time,this experiment provides a theoretical basis for the storage and structural safety design of wheat silos.The main contents of this paper are as follows:(1)Using an improved triaxial instrument based on the digital image measurement system,the stress-strain development of local points and local areas on the surface of the wheat sample was obtained.Compare the difference in strength and deformation of the whole wheat and the local measurement.It is proved that under the influence of end restraint,the radial deformation and volume deformation of the specimen are underestimated,and the overall strength of the specimen is overestimated.Therefore,the average data during the overall measurement cannot fully reflect the actual strength and deformation characteristics of wheat.(2)The end restraint of the triaxial specimen leads to the uneven radial deformation of the specimen.During the shearing process,the radial deformation of the specimen is mainly concentrated in the middle 40 mm area.By introducing the coefficient of strain inhomogeneity,the non-uniformity of the deformation in different local areas of the triaxial test is quantified.It is found that the closer to the middle of the sample,the more uniform the deformation of the sample is.It is proposed to apply the force and deformation results within 40 mm of the middle of the sample,which can effectively eliminate the influence of the endmost constraint.The test result is more representative of the true force and deformation of the sample.(3)The measurement results of the 40 mm area in the middle are used to calculate the strength parameters cohesion,internal friction angle,deformation parameters dilatancy angle and Poisson’s ratio,and based on this,the stress-strain characteristics of wheat are analyzed.The Shen Zhu-jiang’s double-yield surface model was selected and revised,and a constitutive model which is suitable for wheat samples was obtained.Through local measurement data,the relevant model parameters are obtained.The prediction results showed that the model can be used to describe the stress-strain curve and the volume deformation-strain curve of wheat.(4)The following two types of shear failure while the process of shear failure of wheat usually appear: Bulging failure and shear band failure.For wheat specimens of low or medium ambient pressure,a bulging disruption usually occurred.Bulging failure was influenced by endmost restraint,forming a disrupted morphology with uniform bulging on the surface of the specimen.For wheat samples with high ambient pressure and density,shear band failure is easy to occur.Shear band failure is a typical non-uniform local deformation that develops from bulge failure and ultimately manifests as the formation of macro shear bands.