Development of experimental techniques and material models at high strain rates with verifications for vehicle crash simulation

• Applicant: Professor Dr.-Ing. Xiangfan Fang
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2013 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 238688643

The automotive lightweight design requires numerical material description with improved accuracy in terms of large plastic deformation with strain rate up to 1000 /s and failure strain at various stress triaxiality for FEM crashsimulation. Unfortunately, the state of art experimental techniques cannot provide satisfactory data quality in the strain rate range from 200 /s to 1000 /s. Therefore, the material behaviors, such as strain hardening, strain rate hardening, adiabatic heating and material failure, are not well understood in this strain rate range and cannot be correctly described in numerical crash simulations.The first step of this project is to deal with the system ringing effect at high strain rates for servo-hydraulic tensile test systems. We plan to construct a comprehensive understanding of the initialization and propagation of the stress wave within the load frame based on physical stress wave analysis. With this theoretical framework, practical solutions to combat the system ringing effect will be further studied and optimized by means of numerical simulations. Experimental validations are also planned using modal analysis to identify the source of ringing effect. This kind of analysis has not been conducted previously and may generate new solutions.Failure strain determination at various stress triaxiality is the experimental basics for numerical failure prediction in crash simulation. So far, the influences of stress triaxiality and lode angle have been studied quasi-statically. We plan to extend the scientific knowledge to the high strain rate range and therefore improve the accuracy of the numerical simulation. A bi-axial tensile test apparatus has been designed and will be optimized and installed as an extension to the servo-hydraulic system.Adiabatic heating is another topic of this project, as the material properties will be affected when strain rate is high enough to cause large temperature increase locally. The temperature will be experimentally determined with spatial resolution by means of high speed infrared photography. The results above will be used to modify and improve the existing material models within the project.At the last step, drop tower test, both in axial crashing and bending configurations, will be carried out to validate the accuracy of the material data obtained from previous steps. Modification to the material mode will be carried out if necessary and the verified results from this process will ensure high reliability for crash simulations so that they can be used for engineers to assess the lightweight potential of different new materials, such as ultra-high strength steel and high strength aluminum.

• DFG Programme: Research Grants