Finite element analysis on large deformation of compressed cornstalk powder

Abstract: Molded tray that was manufactured by crop stalks is a new type of green packaging material. In order to provide a basic date for design and technology for molded tray, it is necessary to understand the forming laws of cornstalk powder in the course of molded compression. Since previous research to study the principle of compression and forming for cornstalk used rheology theory, it could not solve the problem of the plastic deformation in the compression process. In addition, the molding process of stalk powder is the problem of double nonlinear that contains material nonlinear behavior and geometric nonlinear behavior. To analyze the relationship between the deformation and load histories for cornstalk powder, the uni-axial compression tests of cornstalk powder, which was with about 12% of moisture and 0.4-1.0 mm of granularity, were studied in this paper. The elastoplastic characteristics of cornstalk powder in molded compression process were investigated. Then the relationships between load and displacement were discovered after analyzing the results of the experiments. Moreover, the constitutive equation of cornstalk powder was established using the non-linear fitting method. Using plasticity theory, the Eulerian method of large deformation for the compression analysis of powder was introduced. Based on finite deformation theory, this paper studied a finite element model of axial symmetry for cornstalk powder during compaction, and a FEM program was proposed. Three main formulae, including tangent modulus、tangent stiffness matrix、and unbalanced force, were discussed in the model of the large deformation. The deformation principles were revealed during the forming process. Because of the friction between the corn stalk powder and the die wall, the distance of deformation curve nearby the axial line was obviously greater than the deformation curve nearby the die wall. The paper also obtained the equivalent strain and load-displacement curve. The maximum axial strain was located at the upper portion of the compaction block where there was contact with the die wall. The strain increased along with the height of the die wall. The total load increased very quickly in the process of the plastic deformation, and the smaller deformation was needed to exert greater load. The relationship between the numerical results and the experimental results was analyzed using paired t-test and regression analysis. Paired t-test gave that P-value (Bilateral) of the large strain was greater than 0.05 and agreed well with the experimental results. This paper put forward the finite element model of large strain which can better describe the molding process of compression for cornstalk powder. The correlation coefficient between the results of the large strain and the experimental results was R2=0.9855, but the correlation coefficient between the results of the small deformation and the experimental results was R2=0.9398. The study indicated that the large deformation method has a higher calculating precision. The large deformation computation can adopt large step (0.01) and the step of the small deformation is 0.0001. The result showed that the computation efficiency is improved. The paper provides a reference for further study on the technology of cornstalk powder's molded compression, and the optimization design of techniques and equipment.