Dynamic Characteristics Of Solid Granules By Superimposing High Frequency Vibrations

Solid granules medium forming technology(abbreviated as SGMF)is a technology in which granules are used as a pressure transmission medium instead of a punch(or a concave die)to form a sheet.It is necessary to increase the overall pressure of the granules when forming a tubular member having a local boss,and an excessively high internal pressure has an adverse effect on the flowability and force transmission characteristics of the granules.In order to improve the above-mentioned problems encountered in the forming process,SGMF and ultrasonic vibration machining technology are combined to form a vibration granules medium forming process(abbreviated as VGMF).The force transmission characteristics of granular media under vibration excitation in VGMF directly determine whether the plate can be shaped and its forming quality.Therefore,it is very necessary to study the changes of internal structure and force transmission characteristics of granular media under external loads.Granular system has the characteristics of multi-scale structure: granular element is micro scale;the force chain formed by the contact forces between granules is mesoscopic scale;granule system is macro scale.The force chains and granules jointly determine the macroscopic properties of the granular system.The dynamic characteristics of the granules subjected to vibration loading are analyzed through the combination of theory,experiment and discrete element simulation.(1)Through the friction test between the granules and the steel plate under vibration excitation,the relationship between the friction coefficient and the normal pressure of the granule and the plate under vibration was obtained.Based on the method of powder mechanics,the model of granule medium transmission under vibration excitation is established.The integral solution of granule transfer stress is obtained based on the modified nonlocal friction theory.(2)A two-dimensional discrete element model for the force transfer of granular media was established.The mesoscopic parameters required for the simulation were determined based on the lateral pressure test under static conditions.The change law of contact force,force chain,porosity and coordination number of granular media after vibration was analyzed.(3)Through the test of force transmission of solid granules under ultrasonic vibration excitation,single variable method was used to study the influence of amplitude,force transmission distance,granule size and material on the pressure at the bottom of granules.The results show that the vibration can enhance the fluidity of the granules and reduce the friction between the granules and between the granules and the bearing cylinder wall.The pressure at the bottom of the granules increases with the amplitude.