Study of Deformation Behavior of Tubes in Tube Hydroforming

首先，对管材进行了拉伸试验和液压胀形试验，明确了管材的基本变形行为。我们研究的管子是由铝合金、铜、钢和钛制成的。在液压胀形试验中，控制管材试件的面内应力比。研究结果摘要如下。1.其应变硬化行为可以用n次方硬化定律来描述。2.应变硬化特性随应变量的不同而变化。它大致分为两个区域，小应变区和大应变区。3.钛管材的应变硬化与面内应力比有关。对于其他材料，实验结果中没有出现面内应力比的影响。在管材液压成形中，管材的内压和轴向进给都是加载到管材上的。当内部压力相对大于轴向进料时，管子容易爆裂。然而，当轴向进给比内压大时，管子会发生屈曲。根据我们的实验检验，明确了有两种足够的加载方法可以实现无故障的大管径膨胀。一种是线性加载路径，其中轴向进给与内部压力成正比。另一种是双线性加载路径，其中轴向进给在恒定的内压下施加。在线性加载路径中，有可能在内压和轴向进给之间找到合适的比率。另一方面，在双线性加载路径上存在足够的内压值，建立了有限元模拟模型，并制作了一个方形截面的成形模具来检验有限元模拟结果。通过有限元模拟和实验，证实了管材与成形模具之间的摩擦对管壁厚度分布有明显的影响。

First of all, tensile test and hydraulic bulging test were carried out to clarify basic deformation behavior of tubes. The tubes in our study were made of aluminum alloy, copper, steel and titanium. In the hydraulic bulging tests, in-plane stress ratio in the tube specimen was controlled. Summaries of the results are as follows. 1. It is possible to describe their strain hardening behavior using n-th power hardening law. 2. The strain hardening characteristics change depending on the amount of strain. It is roughly divided into two region, small strain region and large strain region. 3. Strain hardening of the titanium tube depends on in-plane stress ratio. The effect of in-plane stress ratio did not appeared in the experimental results with the other materials.In tube hydroforming, internal pressure and axial feeding are loaded to tube. Tube easily bursts when internal pressure is relatively larger than axial feeding. However, tube buckles when axial feeding is relatively larger than internal pressure. According to our experimental examination, it was clarified that there were two adequate loading methods to achieve large tube expansion without failures. One is the linear loading path where axial feeding is proportional to internal pressure. The other is the bilinear loading path where axial feeding is applied under a constant internal pressure. In the linear loading path, it is possible to find adequate ratio between internal pressure and axial feeding. On the other hand, there is an adequate value of internal pressure in the bilinear loading path.A model for FEM simulation was constructed and a forming die with square cross section was manufactured to examine the FEM simulation results. Through the FEM simulation and the experiment, it was confirmed that effect of friction between tube and the forming die clearly appeared in distribution of tube wall thickness.