New experimental approach for yield loci determination using a modified Nakajima setup

在现有技术中，Nakajima试验用于确定板材的模具成形极限。在由球形冲头、模具和压边机组成的通用测试装置中，只测量板材中的应变。该研究项目的目的是开发一种通过使用改进的中岛试验来确定应力状态的实验方法。为此，通过模拟和校准试验台，开发了不同的测量概念来检测冲头顶端区域和不同定制试件之间的法向压力。试验材料为高强度钢DP600。在实验和不同宽度试件的虚拟实验的基础上，设计了一种将测得的法向压力区分为膜应力的数学方法。该方法考虑了完整的冲压行程，从而考虑了测试板的整个成形历史。通过所得到的知识，可以确定板材在应力空间第一象限内不同应力状态下的流动曲线或屈服轨迹，以及变形历史的每种应变状态，直到确定局部颈缩。通过可测量压力的预分标箔和鼓包试验验证了该方法的有效性。最后，实验屈服轨迹被确定为示范性的。研究项目中所需的中岛试验是在DFG大型仪器应用范围内采购的BUP1000通用板材试验机上确定的。这种确定屈服轨迹的新方法为更好地理解塑性材料的行为、将数值材料模型与实际材料参数进行比较以及开发新的屈服函数方法提供了可能。在一台试验机和一台已被广泛应用于确定成形极限的试验装置上进行所有必要的试验，最大限度地减少了确定屈服极限所需的大量工作。研究项目的主要目的包括：1.建立测量概念和校准试验装置；2.根据不同的试件宽度推导校准函数；3.发展一种分解膜应力中法向压力的数学方法；4.利用膜方程确定流动曲线和屈服轨迹

In the state of the art the Nakajima test is used to determine die forming limit of sheet metal. In the common test setup, consisting of a spherical punch, a die and a blank holder, only the strains in the sheet are measured. The aim of the research project is the development of a method for the experimental determination of stress states by use of a modification of the Nakajima test. For this purpose, different measurement concepts are developed for the detection of the normal pressure between the apex area of the punch and different tailored specimens by means of a simulation and a calibrated test rig. The high strength steel DP600 is used as a test material. On the basis of the experiments as well as the virtual experiments with specimens, which have different widths, a mathematical method is designed for differentiation of the measured normal pressure into the membrane stresses. This approach takes into account the complete punch stroke and thus the entire forming history of the test sheet. By means of the resulting knowledge, it is possible to determine flow curves or yield loci for sheet metal, at different stress states in the first quadrant of the stress space, as well as for each strain state of the deformation history up to the determination of the local necking. The methodology is validated by using of prescale foil, which can measure the pressure, and the bulge test. Finally, experimental yield loci are determined exemplary. The Nakajima tests, which are required in the research project, are determined on the BUP1000 universal sheet metal testing machine procured within the scope of the DFG large instrument application. The new approach to determine the yield locus offers the possibility to understand the plastic material behavior better, to compare numerical material models with actual material parameter, and to develop new yield function approaches. Performing all the necessary tests on one test machine and one experimental setup, which already finds a wide application for the determination of the forming limit, minimizes the extensive effort required for the determination of yield loci.The main objectives of the research project include:1.Construction of measuring concepts and a calibrated test rig2.Derivation of calibration function depending on the different specimen widths3.Development of a mathematical method for the decomposition of the normal pressures in membrane stresses4.Determination of flow curves and yield loci by means of the membrane equation