Automatized Kinetic and Strainfield Based Calibration for a Thermoplastic Material Model using High Speed Tensile Tests

Current and future automotive development cycles are driven by the needs for lightweight designs, cost reductions, comfort- and safety improvements and the reduction of time-to-market. One way to cope with the listed challenges is the usage of thermoplastic materials for integrative designs of components. Among the challenges for passive safety supplier Autoliv to design thermoplastic components, which are placed in the load path of seatbelt components, is the strong dependency on loading velocity of the components. As crash situations are the most dominant load cases for design and functionality, a strong demand for predictive strain rate dependent material models is given. Strain rate effects are next to temperature- and humidity effects the major challenge concerning thermoplastics. As an industrial demand for a comprehensive material database, it needs to be fast, efficient, economical and accurate. Also, the need for a fully automatized material model calibration process is expressed. To fulfill these demands a two-stage reverse engineering process fits test results to analytical approaches for a quasi-static and a strain rate dependent stress-strain response along with an analytic approach for modelling of visco-elasticity and strain rate dependent damage. The needed test results, to which the analytical parameters are fitted, consist of force-displacement as well as strainfield characteristics and were measured using a newly developed high-speed tensile testing device. This device is designed to get close to constant loading velocity of specimen resulting in strain rates up to 𝜀𝜀̇ = 320 𝑠𝑠−1. The accuracy of the test results is ensured by a wedge-to-wedge, self-locking coupling mechanism, a start-up length for acceleration travel of the tensile testing machine as well as a local force gauge. Especially by the local force gauge, consisting of strain gauges arranged as Wheatstone bridge, it is realized that oscillations in force signals of dynamic testing are minimized. The automatized material model calibration routine fed with accurate test results from the high-speed tensile testing device shows promising results to further enhance simulation quality and predictability for the design of thermoplastic components in crash load cases.