Characterisation of auxetic meta-materials for modelling and Simulation of new lightweight structures

在本项目中，将为轻量化设计开发新的、可伸缩的微结构材料的方法，以取代传统的金属材料。为此，必须对这些伸展结构进行表征，因此必须为有限元模拟定义新的材料模型。这个项目的重点是作为超材料的2D金属薄板结构。对于组件的真实模拟，必须准确地知道这些结构的材料属性。为此，必须对潜在材料的力学性能进行实验分析和理论描述。因此，本项目的第一个目标是利用新的无损检测方法来表征具有不同微观结构的金属材料的力学行为。在第一步中，我们将研究二维金属薄板结构的力学行为。新材料模型的开发将基于广泛的实验研究和数值均化策略。结果表明，将宏观材料性能定义为延伸性，微观结构的哪些因素是重要的。在此基础上，建立了微结构超材料的弹塑性材料模型，该模型可以更好地描述盘状伸展轻质结构的力学行为，因此所建议的模型将远远超越以往的模型，并且除了考虑塑性效应外，还将考虑典型伸展材料的各向异性。此外，材料模型还必须考虑大变形。将这些模型应用到有限元程序中，可以通过数值模拟的方法可靠地预测拉伸金属板材结构的力学行为。对二维金属薄板结构的试验测试将用LLB的非破坏性方法进行监测。为此，将加强和发展热成像技术，以将温度变化与弹塑性行为的应力和应变条件相关联。使用数字图像相关(DIC)将测量组件的变形。通过这两种方法的具体组合，应利用每种方法的特定优势，在微观和宏观层面上对样品进行改进的现场表征。此外，还将使用电磁声换能器(EMAT)进行导波检测，以检测实验期间厚度的变化。这种方法的另一个优点是在力学测试过程中对材料性能进行原位表征。

Within this project methods for new, auxetic microstructured materials shall be developed for lightweight design in order to replace conventional metallic materials. For this purpose, these auxetic structures have to be characterized and as a consequence new material models have to be defined for a FEM simulation. This project focusses on 2D-metal sheet structures as meta-materials. For a realistic simulation of the component material properties of these structures have to be known exactly. For this reason the mechanical properties of potential materials have to be experimentally analysed and described theoretically. Therefore a first aim of this project is to characterize the mechanical behavior of metallic materials with different microstructures by new adapted non-destructive methods.In a first step the mechanical behavior of 2D-metal sheet structures will be investigated. The development of the new material models will be based on extensive experimental investigations and on numeric homogenization strategies. As a result it will be shown which factors of the microstructure are significant to define the macroscopic material properties as auxetic. Based on this, elastic-plastic material models are developed for microstructured meta-materials, which can be used to describe the mechanical behavior of tray-like auxetic lightweight structures much better than previously.For this reason the suggested modelling will go far beyond the previous models and will include besides plastic effects additionally the anisotropy of typically auxetic materials. Furthermore, large deformations have to be considered for the material model. The implementation of the models into a finite-elements code allows a reliably predicting the mechanical behavior of auxetic metal sheet structures by means of numeric simulations. The experiments tests with 2D-metal sheet structures will be monitored with non-destructive methods of the LLB. For this purpose thermography will be enhanced and developed to correlate temperature variations to stress- and strain-conditions for elastic-plastic behavior. Using the digital Image Correlation (DIC) deformations of the component will be measured. By a specific combination of these two methods the specific advantages of each shall be used to reach an improved in situ characterization of the sample on micro and macro level. Furthermore, guided wave inspection with electromagnetic acoustic transducers (EMAT) will be applied in order to detect changes of the thickness during experiments. A further advantage of this method is the in situ characterization of materials properties during mechanical testing.