304不锈钢棒气动式可控旋弯下料工艺及试验

针对金属棒料传统下料工艺存在断面质量差、材料利用率低、能耗高等问题，提出一种综合利用预制环状V型槽缺口效应、起裂微裂纹及扩展宏观裂纹裂尖应力集中效应的新型气动式棒料可控旋弯下料工艺，详细介绍了该下料工艺的工作原理。基于该工作原理研制了金属棒料气动式旋弯下料试验装置，利用材料力学强度理论与线弹性断裂力学理论，建立了弯矩载荷作用下金属棒料下料过程的力学模型，获得了环状“V”型槽根部微裂纹萌生载荷、扩展裂纹起裂载荷及瞬断载荷的理论计算公式。采用三种不同恒应力强度因子幅水平载荷控制曲线对φ30 mm直径的304不锈钢棒料进行下料试验研究，结合所提出的坯料断面评价方法及其评价指标，对下料所得坯料质量进行了量化评价。结果表明，保持裂尖处于恒应力强度因子幅水平，可以实现金属棒料的可控精密下料，断面质量和下料时间随幅值水平提高而降低，当裂尖处△K=0.7K_c时，坯料最终瞬断区最小。该下料工艺利用断裂力学理论建立旋弯下料机理，可以指导实际下料试验，为后续下料工艺数据库的建立提供理论支撑。

Process and Experimental Research of Controllable Pneumatic Rotary Bending Cropping of 304 Stainless Steel Bar

A new controllable pneumatic rotary bending cropping technology is proposed aiming to reduce the disadvantages of the traditional bar cropping methods such as bad cross-section quality, low material utilization and high energy consumption. It skillfully utilizes the notch effect of the V-groove, the crack-tip stress concentration effect of the initiated micro-crack and the propagating macro-crack. The working principle of the cropping method is introduced. The experimental setup is successfully designed and manufactured according to the principle. The mechanical model of the bar cropping process, which is loaded by the bending moment, is established by using the theory of material strength and linear elastic fracture mechanics. The theoretical formulas of the V-groove notch-root micro-crack initiation load, the propagating crack initiation load and the transient-breaking load are obtained. The cropping experiments of the 304 stainless steel metal bars with 30 mm diameter are studied under three different load control curves with different levels of the constant stress intensity factor range. Moreover, the cross-section quality of the cropped-billets are quantitatively assessed by the new proposed cross-section quality assessment method. The results show that the cropping process can be accurately controlled and implemented by keeping the crack-tip under a constant stress intensity factor range level. The cross-section quality of the cropped-billet and the cropping time decrease with increasing the range level. The ideal load control curve is △K=0.7K_c. The cropping process uses the fracture mechanics theory to establish the rotating bending cropping mechanism, which can not only guide the actual cropping experiments, but also provide theoretical support for the establishment of the subsequent cropping process database.