Determination of Material and Failure Characteristics for High-Speed Forming via High-Speed Testing and Inverse Numerical Simulation

Verena Psyk,Christian Scheffler,Christina Guilleaume,Marc Tulke,Alexander Brosius,Sven Winter

doi:10.3390/jmmp4020031

Abstract

In conventional forming processes, quasi-static conditions are a good approximation and numerical process optimization is the state of the art in industrial practice. Nevertheless, there is still a substantial need for research in the field of identification of material parameters. In production technologies with high forming velocities, it is no longer acceptable to neglect the dependency of the hardening on the forming speed. Therefore, a method for determining material characteristics in processes with high forming speeds was developed by designing and implementing a test setup and an inverse parameter identification. Two acceleration concepts were realized: a pneumatically driven one and an electromagnetically driven one. The method was verified for a mild steel and an aluminum alloy proving that the identified material parameters allow numerical modeling of high-speed processes with good accuracy. The determined material parameters for steel show significant differences for different stress states. For specimen geometries with predominantly uniaxial tensile strain at forming speeds in the order of 104–105/s the determined yield stress was nearly twice as high compared to shear samples; an effect which does not occur under quasi-static loading. This trend suggests a triaxiality-dependent rate dependence, which might be attributed to shear band induced strain localization and adiabatic heating.

Highlights

A method for the determining material characteristics in processes with high forming speeds was successfully developed. It was demonstrated by verification tests based on the principle of free electromagnetic forming that the application of this method allows numerical modeling of high-speed processes with good accuracy
The material parameters determined for steel DC06 show clear differences for different stress states
It is remarkable that the strain at failure and the strain rate dependency seems to be a function of the stress state

Summary

Introduction

Realizing challenging manufacturing tasks and processing complex materials including modern lightweight materials such as aluminum or magnesium becomes possible via special techniques using these velocity effects This allows for new products with improved quality and reduced product weight. It is possible to save resources and reduce emissions, especially in the transport sector, and in other applications including moving masses such as the manufacturing sector, energy generation sector and many more Despite these advantages, the corresponding technologies, which are often referred to as high-speed forming, impulse forming or high-energy-rate-forming (HERF) [2] have not yet achieved a great industrial breakthrough. Numerical considerations are helpful to investigate fundamental relationships, but often only qualitative interpretation is possible One reason for this is that material parameters are hardly available at the process-specific high strain rates in the range of 10 s−1 up to 105 s−1 or even more. Providing a method for determining reliable material and failure characteristics for the simulation of high-velocity forming, cutting, and joining processes will contribute to making technological, economic, and ecological advantages of these processes exploitable in industrial production

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Discussion

Conclusion