Structural And Mechanical Characteristics Of Corn Stubble And Its Tribological Properties Against Soil

Corn is one of the main crops in three provinces of northeast China. The plantingarea of corn is 42.43% in the three provinces, 64.62% in Jilin province. Corn stubbleis a potential natural resource, which accounts for 12% to 15% of the whole corn crop.The corn stubbles after harvested are often threw away as waste, because it take alarge manpower and material resources to dig them up. The structure of corn stubbleis complicated, combined with the around soil closely. To find the best diggingposition and make root and soil separated are two problems urgently to be solvedbefore the design and manufacture of the corresponding machine. The morphologystructure and material mechanics of corn stubble, interaction with the soil for thesetwo problems were examined.The morphology of the corn stubble of'Nongda 108'was examined usingstatistics through method of the half section method in field. It was found that a cornstubble displayed a cone-like multi-layer cage shape structure with around soil; thewidth of the root was 300-450 mm with a average width of the root of 376±73 mm;the depth of the root was 200-350 mm with a average depth of the root of 275±62mm; and the mass of the root mainly ranged within the depth of 150 mm. Themoisture content of the root was increased with the depth where the root was.The microstructure of the surface and the cross-section of the corn root fibrilswere examined by stereomicroscopy and scanning electron microscopy (SEM). It wasfound that the root fibril of corn stubble was composed of epidermis, cortex and stele.The root fibers was supported only by cell wall after cell death. The cortexparenchyma and phloem formed a comby structure. The pith of stele became chip orirregular floccus after loss of the water, which made root fibers became hollow tubularstructure, weaken the joint of the epidermis and vascular cylinder to separate easily.The structure of vascular cylinder outside surface and inside surface of the cortexhave difference considerably, the former is rule corrugated shape structure with thefeatures of compact structure and close texture; the latter is rough, porous andsponge-like, with lax texture, as a result, it is easy damaged.The tensile and shear properties of root fibrils of corn stubble were examinedusing electronic universal testing machine. The results showed that diameter andwater content of the corn root fibrils were important factors affecting them. Thetensile force was proportional to diameter of root fibrils and increased with the watercontent, and the smaller the diameter was, the greater the impact of water content ofroot fibrils was. A linear relationship existed between ultimate elongation and diameter of root fibrils. The required shearing force for air dried root fibrils was smaller than that for the root fibrils with high water content and the shearing force was larger than the tensile force, so it is better to use shearing combined with drawing in digging up corn stubble at a lower moisture content of soil. Because moisture content and compactness of soil increased with the depth and corn stubble mass mainly is in soil within the depth of 150 mm, it was appropriate to dig corn stubble up at 150 mm depth.Interface friction characteristics of corn stubble against soil was examined using friction direct shear tester (ZJ-Quad strain control direct shear apparatus). It was found that the vertical pressure, moisture content and dry density of soil were three important affecting factors. When vertical pressure was increased, soil particles are pressed and the contact of root with soil closer, as a result, shear strength of root-soil interface was enhanced. The adhesive force of corn stubble root with soil increased with moisture content of soil, but it was reduced with soil density decreased,. In the same moisture content of soil, there was no apparent influence on cohesive force of corn stubble root-soil interface by soil density. Friction coefficient decrease with moisture content of soil and the decreased degree was different for different soil density, the higher the soil density was, the more decrease the friction coefficient was.Shear strength of corn stubble root-soil complex was examined by unconsolidated undrained triaxial test. It was showed that corn fiber roots in soil body had no influence on its internal friction angle when the density and moisture content of it was unchaged, however the cohesive force and ultimate principal stress difference increased considerably. That is to say, corn root fibrils enhanced the shear strength of soil body. The improvement degree of the cohesive force and the ultimate deviator stress were different for different arrange of the root fibrils, the order of influence was the complex>the vertical>the horizontal. When moisture content of soil range from 10 %db to 20 %db, the soil cohesion and internal friction angle reduced with the moisture content of soil increased, shearing strength of soil reduced as well. The orthogonal test results showed that the major factor affecting soil cohesion was the arranging mode of root fibrils, the soil density was smaller, the moisture content of soil had lowest impact. The leading factor affecting soil internal friction angle was moisture content of soil, the arranging mode of root almost had no impact to it. It was suggested that the corn stubble harvest should dig at shallow soil which has less root fibrils (with the depth of 15cm) and higher moisture content (20 %db).