Improvement of fracture characterisation of high-strength steel sheet metals by means of coupling of optical strain analysis systems with acoustic emission techniques

高强度钢在汽车工业中起着重要作用。基于仿真的成形过程设计可以帮助利用它们的高潜力。这就要求对材料的成形性有一个准确的描述。为此，通常采用成形极限曲线。然而，高强度钢成形极限曲线的预测质量并不尽如人意。在板料成形有限元分析中，考虑应力状态的起裂模型可用于断裂预测。借助光学测量系统来确定局部变形和数值模拟来识别裂纹起裂位置的应力状态，可以通过测试不同几何形状的试样来获得断裂应变。然而，最近的结果表明，目前用于表征高强度板材断裂行为的许多试样，首先在试样内部表现出临界损伤和裂纹萌生。根据材料的进一步应变局部化、损伤积累率和抗断裂性，材料破坏或多或少延迟到达试样表面，在那里它可以被光学测量系统检测到。该项目的主要目标是开发一种方法，通过声发射技术与成像技术相结合，能够及时检测高强度钢板断裂特征的临界损伤累积率和裂纹起裂。在耦合测量系统的帮助下，可以准确地检测和记录临界应变，该临界应变导致试样内部的临界损伤积累率或裂纹起裂，并因此伴随着弹性变形中存储的能量释放。对于许多试样和测试安排，应该能够比仅使用光学测量系统更早地检测材料断裂，光学测量系统只能检测试样表面上足够大小的裂纹。通过更准确地确定试样内部临界损伤积累率或断裂的时间点，这对于光学测量系统是不可见的，应该提高材料失效表征的准确性。这将提高基于模拟的高强度钢成形过程设计的质量，并允许更好的材料利用率和更高的制造过程稳定性。此外，预计在项目期间将获得有关微观结构水平上材料损伤的新见解，这可以为进一步开发或优化高强度钢板提供基础。

High-strength steels play an important role in the automotive industry. Simulation-based design of forming processes can help utilise their high potential. This requires an accurate description of the material formability. For that, forming limit curves are usually used. However, the prediction quality of forming limit curves for high-strength steels not always suffice. Alternatively, fracture initiation models, which consider the stress state, can be used for fracture prediction in the FEA of sheet metal forming. With the help of an optical measurement system for determination of local deformations and numerical simulations for identification of the stress state at the location of crack initiation, it is possible to obtain fracture strains via testing of specimens of different geometries. Resent results show, however, that many specimens which are currently used for characterisation of the fracture behaviour of high-strength sheet materials, exhibit critical damage and crack initiation first in the specimen interior. Depending on the further strain localisation, damage accumulation rate, and fracture resistance of the material, the material failure reaches the specimen surface more or less delayed, where it can be detected by the optical measurement system.The main objective of the project is to develop a method, which enables in-time detection of critical damage accumulation rate and crack initiation for the fracture characterisation of high-strength steel sheet metals by means of acoustic emission technology coupled with imaging techniques. With the help of the coupled measuring systems, critical strains, which lead to a critical damage accumulation rate or crack initiation in the specimen interior and, as a results, go along with a release of energy stored in elastic deformation, are to be accurately detected and recorded. For many specimens and test arrangements, it should be possible to detect material fracture earlier than with an optical measurement system only, which can only detect cracks of a sufficient size on the specimen surface. Via a more accurate determination of the time point of the critical damage accumulation rate or fracture inside the specimen, which is invisible for optical measurement systems, the accuracy of the material failure characterisation should be improved. This should lead to an increase of the quality of the simulation-based design of forming processes with high-strength steels and allow a better material utilisation and higher manufacturing process stability. Moreover, new insights regarding material damage on the microstructure level are expected to be gained during the project, which can provide a basis for a further development or optimisation of high-strength sheet steels.