| Most of the coarse cereal crops are planted in the hilly and mountainous areas,mainly by manual labor,and the degree of mechanization is very low.Because of the high nutritional value of coarse cereal,it has been widely favored by people in recent years.The development of coarse cereal industry has become the theme of readjustment of agricultural industry structure in coarse cereal plating areas of our country.The mechanization of coarse cereal production has also been focused on and it has been included in the key research and development contents of the 13 th five-year plan.In the design and parameter optimization of coarse cereal combine harvester,coarse cereal threshing machine and related processing machinery,it is necessary to know the biomechanical properties and damage mechanism of coarse cereal grain.Therefore,under the special support of the Ministry of Science and Technology,this paper systematically studies the basic physical parameters,conventional mechanical properties and viscoelastic mechanical properties friction mechanical properties and dynamic load collision damage of four coarse cereal grains in different moisture content,which including millet,buckwheat,oat and highland barley.The main research contents and conclusions are as follows:1.The basic physical parameters of millet,buckwheat,oat and highland barley grain were studied.The basic physical parameters including length,width,height,bulk density,mean diameter of arithmetic,average diameter of geometry and quality of 1000 grains were measured under different moisture content.The results showed that moisture content had a significant effect on basic physical properties(P<0.0001).In a certain range of moisture content,the grain length,width,height and 1000-grain quality increased with the increase of moisture content,while the bulk density decreased with the increase of moisture content.2.The conventional mechanical properties including compression,shear and friction of millet,buckwheat,oat and highland barley grain were studied.Experiments were carried out on the compression,shear and friction mechanical properties of millet,buckwheat,oat and highland barley grains with different moisture content.The results showed that moisture content and variety had significant effects on mechanical properties such as compression,shear,friction and so on.For the same millet and buckwheat,with moisture content increased,the yield load and apparent elastic modulus of grain decreased,the deformation and damage energy of millet first decreased and then increased,the deformation of buckwheat increased gradually,and the damage energy decreased.With moisture content increased,the shear failure power of millet,buckwheat,oat and highland barley grain decreased gradually,and the shear failure energy first decreased and then increased,and the shear failure stress decreased gradually.With moisture content increased,the friction coefficient between grain and 304 stainless steel plate,aluminum alloy plate and Q235 steel plate increased gradually,and the rest angle increased gradually.The the friction coefficient decreased on Q235 steel plate,7075 aluminum alloy plate and 304 stainless steel plate in turn.The mathematical model between compression mechanical properties and moisture content of millet and buckwheat were obtained by fitting.And mathematical model of shear,friction mechanical properties and moisture content of coarse cereal grain were obtained by fitting.The coefficient of determination R2 is greater than 0.90.The results of finite element simulation for grain of millet and buckwheat showed that: under three compression forms,the maximum stress in Y-axis compression is greater than that in X-axis compression,and the minimum in Z-axis compression.The maximum stress of buckwheat shell was about 8.27 MPa,which is close to the experimental results.3.The formation process and damage mechanism of compressive damage crack in millet and buckwheat grain were studied.During compression in X,Y,Z axis,there was no crack in grain before yield point,the grain began to produce visible cracks in naked eye after reached yield point,the crack gradually growth and extension as the load increased,there were several cracks that caused rupture eventually.The formation process,mechanism,propagation form and location of damage crack were different.About crack formation of the millet: on the X axial,a crack started from the crest of the grain along the 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.When buckwheat grain was compressed,the inner nutlet produced cracks because of the different strength of buckwheat shell and inner nutlet,and cracks appeared on the edge of the buckwheat shell due to the low compression strength and the stress concentration.The compression damage of buckwheat shell was similar to that of natural damage.It was a longitudinal crack on a shell edge.4.Based on the microscopic analysis of grain section morphology and cracks by scanning electron microscope(SEM)and stereomicroscope,Griffith energy balance theory,Cottrell dislocation plug model and fractal geometry theory were used to analyze the crack formation in static compression experiment and the microcosmic mechanism of internal mechanical cracks formation and propagation during collision are studied.The results showed that microscopic crack along the coupling between the starch granules and the perforating starch granules were most likely to occur,while microscopic cracks with the starch granules and bifurcation propagation were more difficult to occur.In the same propagation model,the larger the diameter of starch granules in millet grain,the faster the propagation rate of microscopic crack.In the same diameter of starch granules,the rate of coupling propagation was the fastest,the rate of propagation along starch granules was the second,and the rate of bifurcation propagation was the lowest.On the basis of Boltzmann principle and Volterra principle,the basic constitutive equation of anisotropic viscoelastic body was established,and the stress and displacement fields near the crack tip of viscoelastic body were derived.5.The viscoelastic mechanical properties of buckwheat inner nutlet powder and highland barley grain were studied.The viscoelastic properties such as the storage modulus,loss modulus,loss tangent of buckwheat inner nutlet powder and the stress relaxation and creep property of highland barley grain under different moisture content were obtained.The storage modulus decreased as the moisture content increased,and the loss modulus and loss tangent increased with the moisture content increased.For the same buckwheat,the lower the moisture content,the smaller the loss modulus,the bigger the storage modulus,the greater the elasticity and the smaller the viscosity.When the moisture content was between 9.0% and 22.9%,at the same time,the strain value of highland barley grain increased with the increased of moisture content,and the stress value decreased with the increased of moisture content.The Burgers model can describe the creep property of the highland barley grain well.The three-element Maxwell model can describe the stress relaxation property of the highland barley grain.6.The impact and collision experiment device were developed to study the impact and collision properties of millet and buckwheat grain.The results showed that: at the same moisture content,the higher the impact load,the higher the crushing rate of millet and buckwheat;under the same impact load,the crushing rate of the millet grain decreased with the moisture content decreased,and the crushing rate of the buckwheat grain decreased first and then increased with the moisture content decreased.The effect of falling height and material thickness on the recovery coefficient of four kinds of grain were significant at the level of 0.05(P<0.05),while the effect of moisture content was not significant(P>0.05).The higher the speed of the rotary table,the higher the collision force,the higher the breaking rate and the crushing degree of the millet and buckwheat grain;at the same speed,there was a quadratic polynomial relation between the crushing rate and the moisture content. |
