液压胀形薄壁管材料流动应力方程的构建

提出了一种采用管材液压胀形（THB）来构建管材流动应力方程的方法．基于薄壁管材在液压胀形中的试验数据，用曲线拟合方法、塑性薄膜理论及静力平衡方程通过计算来确定管材流动应力方程，避免了变形管材轮廓子午向曲率半径、变形区应力与应变的直接测量．采用模具内部增压方式，开发了一套可在单动压力机上使用的自然胀形试验装置，并在此装置上对不锈钢1Cr18Ni9Ti管及碳素钢20钢管进行了管端自由的液压自然胀形试验，获得了2种管材的THB材料参数并构建了流动应力方程．研究结果表明：与单向拉伸试验相比可以得到更大应变范围内的流动应力；在等效应变较小时，所得到的流动应力曲线与单向拉伸试验得到的流动应力曲线基本接近，但随着应变的增大，两者差距增大；从液压胀形试验法中得到的管材硬化指数、强度系数及流动应力均比相应板材的单向拉伸试验数值要大得多。

Stress Strain Relationship of Tubular Metal in Hydraulic Bulging Test

Based on the plastic membrane theory and static equilibrium equations,a novel hydro-bulging approach is proposed with curve-fitting of the experimental data to establish the flow stress equation of a thin-walled tube in hydro-bulging(THB) process,where the site measurements of stress and strain distribution,as well as the profile curvature radii in meridian direction of deformed tube are eliminated.A simple hydro-bulging test tooling under a conventional single action press without any external hydraulic pressure,but internal one,is developed to obtain required material parameters for the flow stress equations.A series of free bulging experiments on stainless steel tube and low carbon steel tubes with two unrestrained ends are carried out,and the flow stress equations for the two tubular materials are then established in biaxial stress state.The results indicate that an extended flow stress curve in a much wider range of strain than that in tensile test can be obtained,the flow stress curves by either this approach or tensile test are close under small strain,but bifurcate greatly as strain increases,and the total flow stress level,hardening exponent n-value and strength coefficient K-value in the approach are obviously larger than those by tensile test of corresponding sheet blank.