利用管状试样测试各向异性材料双向应力状态力学性能的新方法

A New Method for Directly Testing the Mechanical Properties of Anisotropic Materials in Bi-Axial Stress State by Tube Bulging Test

为解决各向异性材料双向加载性能测试理论模型存在的测试物理量过多且实测困难的问题,提出了一种采用管状试样胀形直接测试双向加载力学性能的新方法:一点法。利用圆几何轮廓线为显性函数表达式的特征,推导了胀形过程中最高点轴向曲率半径和壁厚理论模型。仅需在胀形过程中测量最高点胀形高度,即可获得材料双向加载下的力学性能,为建立一个简单可靠且能在线实时测量的材料力学性能测试方法奠定了基础。并利用所建立的测试方法进行了AA6061铝合金挤压管坯的胀形实验。结果表明:管坯自由胀形时,其最高点实时壁厚和曲率半径均可表示为最高点胀形高度的显示函数。轮廓形状理论模型的预测精度随膨胀率的增大先提高后降低,膨胀率约为13%时预测精度最高,当膨胀率超过20%后,预测精度开始下降,但最大误差不超过±0.9%。最高点实时壁厚理论模型的预测精度基本不受试件几何尺寸的影响,长径比和径厚比改变时,差异很小,预测误差均不超过0.8%,这对保证双向加载条件下的力学性能测试精度是非常有益的。一点法可同时测得环向和轴向的应力应变分量,这为进一步分析各向异性对复杂应力状态下材料的流动及后继屈服奠定了基础。

Due to the increasing demands for lightweight parts in various fields, such as bicycle, auto- motive, aircraft and aerospace industries, hydroforming processes have become popular in recent years.Since tubular materials during tube hydroforming are under a bi-axial even tri-axial stress state, which is different from that in the tensile test, it is necessary to test the mechanical properties of the material un- der bi-axial stress state. Tube bulging test is an advanced method for characterizing the mechanical prop- erties of tubular materials under bi-axial stress state. But there are excessive physical quantities in the theoretical model of tube bulging test for testing the mechanical properties of tubes under bi-axial stress state which are difficult to be obtained during the experiment. In order to solve the problems, a method for directly testing the mechanical properties of tubes under bi-axial stress state was proposed in this work, which will be referred to as ＂one point method＂. Because of circular model is characterized by a dominant function expression, theoretical models of both the pole axial curvature radius and the pole thickness during bulging test are derived under supposing the geometrical models for bulging zone as cir- cular. Thus, the mechanical properties of tubes under bi-axial stress state can be obtained only through measuring the bulging height at the pole point during the bulging test, which laid the foundation for the es- tablishment of a simple and reliable method for testing the mechanical properties of the tube online. Based on the above proposed method, the extruded aluminum alloy tubes AA6061 were tested. The re- sults showed that both the pole axial curvature radius and the pole thickness during bulging test can be expressed as display functions pertaining to the bulging height at the pole point. For the theoretical mod- el of the pole axial curvature radius, as the bulging rate increases, the prediction accuracy increases at beginning, and decreases at the end when using circular as the theoretical geometrical models for bulg- ing zone. The prediction accuracy is the highest as the bulging rate is about 13%, the prediction accuracy decreases after the bulging rate is more than 20%. Fortunately, the overall prediction error is small. The maximum error does not exceed ＋0.9%. The prediction accuracy of the pole thickness using the theoreti- cal model is almost unaffected by the specimen geometry. When the ratios of length to diameter and di- ameter to thickness change, the difference is very small, the prediction error is not more than 0.8%. This is very helpful to ensure the accuracy of mechanical testing under bi-axial loading conditions. Using the ＂one point method＂, the stress and strain components along the circumferential and axial directions can be simultaneously measured, this laid the foundation for further analysis of the anisotropic property im- pacting on the flow and subsequent yield under complex stress state.