Research On The Mechanism Of Bar Fracture In High-speed Centrifugal Cropping

Low-stress cropping technology is a new type and green cropping method that utilizes crack technique to cut and separate materials.Compared with conventional cropping technologies,it has many advantages,such as saving material and energy,efficient and having high-qulity cropping sections.However,because the mechanism of bar fracture is not clear,the existing low-stress cropping methods fail to control the bar crack initiation and propagation process concisely,which impedes the widespread diffusion of low-stress in industry.This paper adopts the method of theoretical analysis,numerical simulation and experiments to obtain a deep understanding of the bar fracture mechanism in high-speed centrifugal and low-stress cropping.?1?To investigate the crack initiation mechanism of bars,the factors that influence the crack initiation location,crack initiation direction and critical load at the tip of V-shaped notch are studied with the Extended Finite Element Method?XFEM?,without preexisting cracks.The study results show that the clamping arm L₁,the loading arm L₂ and the diameter D of bars play a less significant role on the crack initiation location,and the parameter H which represents the crack initiation location is approximately equal to the half of the corner radius of the V-shaped notch;when L₁ is greater,L₂ is smaller and D is smaller,the crack initiation angle is smaller;the proper clamping position parameters with L₁ equaling to 15mm and L₂ equaling to 30mm are proposed.?2?In order to solve the sudden change of critical loading force before and after the crack initiation at the tip of V-shaped notch,it is proposed that the fracture process of the bar is divided into crack initiation and propagation stages.Then the calculation formulas of the critical loading force in the two stages are established by means of XFEM and FRANC3D,respectively.The concept of mutation coefficient F_p0.2/F_ci is also put forward.The research results show that the influence of bottom corner radius s of V-shaped notch on F_p0.2/F_ci is the largest;when s is between 0.12mm and 0.3mm,F_p0.2/F_ci decreases sharply;when s is equal to 0.12mm,there is no force mutation;while the radius b of the bar and notch depth d have little influence on Fp0.2/Fci.?3?With the help of XFEM and Zheng-Hirt formula of fatigue crack propagation rate,the influence of notch eccentricity e on the stress concentration effect,the crack propagation trajectory and life is studied.The results show that,at the crack initiation stage,the notch eccentricity produces secondary stress concentration effect,which is more beneficial for crack initiation;at the crack propagation stage,notch eccentricity can reduce the fatigue crack growth life of bars;meanwhile,when the nomarlized eccentricity e/b=0.09,the fatigue crack growth life of bars decreases approximately 1/4.?4?Based on the theoretical analysis of bar fracture mechanism,the high-speed centrifugal cropping machine is improved by designing a movable clamping mechanism which can adjust the bar clamping arm and loading arm dynamically,as well as the corresponding hydraulic control system.Then,the experimental platform and control system of the cropping machine is constructed to conduct bar cropping tests,in order to verify the proposed theories about crack initation,critical loading force curves and eccentric notch.This study reveals the bar fracture mechanism further and,thereby,lays a theorectical foundation for the development of low-stress precision cropping.