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

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 10 4 -10 5 /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.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. Reasons for this are lacking process knowledge and reliable process design strategies. When considering conventional forming processes, it is possible to assume a close approximation of quasi-static conditions, and numerical process design and optimization as state of the art. By contrast, the analysis and design of manufacturing processes with high forming speeds has hitherto been strongly experimental. 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 10 5 s −1 or even more. The identification of these parameters is complicated, because in addition to the influencing factors known from quasi-static conditions such as temperature and strain, the plastic flow and failure behavior of many materials strongly depends on the strain rate. Therefore, neglecting forming heat, as well as a strain rate-dependent hardening is no longer acceptable. Providing a method for determining reliable material and failure characteristics for the simulation of high-velocity forming, cutting, and joining processes will therefore contribute to making technological, economic, and ecological advantages of these processes exploitable in industrial production.

Local Temperature Development in the Fracture Zone during Uniaxial Tensile Testing at High Strain Rate: Experimental and Numerical Investigations

“…This study uses commercial DC06 (1.0873) steel sheets which were cold rolled to 1 mm thickness. This material has a wide industrial application and features significant strain rate dependency [15]. Table 1 shows the chemical composition (in wt %).…”

“…The fracture parameters listed above were revealed by a combination of experimental studies of different sample geometries (in order to cover different triaxial states) and inverse modeling performed in LS-OPT. In more detail, these calculations are given in [15] but the basic fracture curves as well as the m and n parameters are presented in Figure 4. and inverse modeling performed in LS-OPT.…”

“…As part of previous work [15], a method was developed for the determination of material and failure characteristics for processes with high forming speeds including extremely high strain rates in the range of more than 10 4 /s, using DC06 steel and AW5754 aluminum alloy as examples. The calculation of the values was performed by testing samples of different stress states, using an electromagnetically accelerated hammer and following inverse simulation of the process.…”

Adiabatic Blanking: Influence of Clearance, Impact Energy, and Velocity on the Blanked Surface

“…The required material data can be determined, e.g., via Split-Hopkinson pressure bar tests for strain rates of up to 10 3 s −1 and temperatures of up to 1000 • C [21]. An alternative approach allowing strain rates of up to 10 4 s −1 uses a test setup with an electromagnetically accelerated punch (up to 50 m/s) and subsequent inverse numerical simulation [22]. However, experimental tests with varying process parameters are indispensable for validating the numerical simulation.…”

Numerical Estimation of Material Properties in the Electrohydraulic Forming Process Based on a Kriging Surrogate Model

“…In general, material properties under high-strain-rate conditions (strain ranges 10 2 -10 4 s −1 ) are acquired in a Split-Hopkinson pressure bar test [11,12]. Besides, many experimental studies were conducted to obtain dynamic material properties [13][14][15][16][17]. Although the experimental approach directly obtains the material properties, it is both costly and time consuming.…”