Experiment on stress relaxation of corn stover during compression with assisted vibration

Abstract: Corn stalk is abundant in China, which is usually burnt in the field, resulting in environmental pollution and resource waste. Therefore, the recycling of corn stalks is urgent and necessary. However, the cost of transportation and storage is very high because of its loose structure and low bulk density. The best solution for this problem is to compress it into briquettes, pellets, or logs. Due to the rheological properties of corn stalk during the compressing process, the present compressing equipment commonly has the problems of high energy consumption, low efficiency, and quick wear of the key components, which are the bottleneck of its application and extension. At present, a lot of researches on the compression rheological properties of agricultural materials have been done by many domestic and foreign scholars, and stress relaxation has been analyzed by ideal mechanical components and mechanical models. But the most researches are based on the way of traditional compression. There is no relevant research about the influence of assisted vibration on stress relaxation during compression. In order to solve the problems of compressing equipment, assisted vibration was introduced into the compression process of corn stalks in this paper. By using a vibration-assisted densification test system, the stress relaxation experiments for without and with assistive vibration at supplied pressure of 0.4 MPa were performed in the same conditions: the moisture content of corn stalks at 18%, the particle sizes between 1 and 4 mm, the compression speed of 4.6 mm/s, the feeding rate of 15 g, and 10 s relaxation time. Through fitting the experimental data in MATLAB, the stress relaxation of corn stalk in the compression process with and without assistive vibration was expressed by a 2-order Maxwell model, and the coefficients of determination for the models were both greater than 0.9, which indicated that the fitting model was agreeable with the practical conditions. Based on the rheological model, the parameters of rheological model, such as the velocity of stress relaxation, the stress relaxation time, and the elastic restoring force were analyzed. And the results were as follows: from the curves of stress relaxation rate, it could be seen that the stress relaxation velocity obviously slowed down after 5 s without assistive vibration and slowed down after 3 s with assistive vibration during the compression. According to the models of stress relaxation, the stress relaxation time of the first and second order Maxwell models was respectively 0.96 and 99.4 s under the compression without vibration, and respectively 0.57 and 86.96 s under the compression with vibration. The proportion of residual elastic restoring force under the compression without and with vibration was respectively 77.2% and 76.5%. The results show that the vibration can increase the stress relaxation rate, lower the stress relaxation duration and internal residual stress of the block, and thus reduce the elastic deformation recovery after the block is extruded out of the die, improve the quality of product, and reduce the additional resistance when compression again; at the same time, the vibration can greatly save the time of stress relaxation when the block is compressed to the same density. The research provides a new way of lowering energy and raising production efficiency for the compression equipment.