Experiment on stress relaxation characteristics of intact rice straw during rotary compression

Abstract: In recent years, rice straw as a kind of renewable material is being used widely in China. The rheologic characteristics of crop straw have great effect on the power consumption, productivity and compression process of compressing equipment, as well as on the bale durability. At present, a lot of researches have been done by many domestic and foreign scholars on the compression rheologic characteristics, but the researches have been aimed at the rectangular baler instead of the steel-roll round baler, and the test materials are mainly concentrated on corn straw, wheat straw and alfalfa, which limit the development and application of the round baler for crop straw in China. Stress relaxation is an important phenomenon in the rheologic characteristics of straw. In order to deeply research stress relaxation phenomenon of intact rice straw during rotary compression under the round baler, a steel-roll round baler was used for rotary compression test in this paper. To timely monitor the force during rotary compression, the force testing device was developed. In experiments, moisture content of rice straw (12%-68%), dry material quantity of bale (15-23 kg), steel roll speed (222-318 r/min) and feeding speed of rice straw (0.4-1.6 kg/s) were chosen as experimental factors, which were at 5 levels. Stress relaxation time and balance elastic modulus were decided as the evaluation indices, and quadratic orthogonal rotatory combination design was used to analyze the effect of different factors. Through analyzing experimental data by Matlab 8.0, the stress relaxation model of intact rice straw during rotary compression could be expressed by three-element equation which was available through the parallel connection of one Maxwell unit and one spring. The regression equations of stress relaxation time and balance elastic modulus had been established respectively by using Design Expert software. Through variance analysis of regression equations for stress relaxation time and balance elastic modulus, the coefficients of determination (R2) for the models were 0.96 and 0.97 respectively, which indicated that the proposed model was agreeable with the practical condition, thus the proposed model could be used to predict the change of the stress relaxation time and balance elastic modulus during rotary compression. In addition, the relations between evaluation index and each single factor were shown in response surface. The results showed that: with the increase of moisture content of rice straw, both of stress relaxation time and balance elastic modulus decreased; with the increase of dry material quantity of bale, stress relaxation time decreased and balance elastic modulus increased; with the increase of steel roll speed, stress relaxation time and balance elastic modulus decreased and then increased; and with the increase of feeding speed of rice straw, stress relaxation time decreased and balance elastic modulus increased gradually. The contribution rate of each factor on stress relaxation time was as follows: moisture content of rice straw > dry material quantity of bale > feeding speed of rice straw > steel roll speed. The contribution rate of each factor on balance elastic modulus was as follows: dry material quantity of bale > moisture content of rice straw > feeding speed of rice straw > steel roll speed. When moisture content of rice straw was 65%, dry material quantity of bale was 17 kg, steel roll speed was 257 r/min and feeding speed of rice straw was 1.6 kg/s, stress relaxation time was 17.08 s and the balance elastic modulus was 4.01 kPa, which were the best results. The research provides theoretical and technical support for the process optimization of round baler, as well as the analysis of its compression mechanism.