Joost Van Slycken

The effect of silicon, aluminium and phosphor on the dynamic behavior and phenomenological modelling of multiphase TRIP steels

Multiphase TRansformation Induced Plasticity (TRIP) steels combine excellent ductility and high strength, making them ideally suited for shock absorbing parts in the automotive industry. When designing structures for impact, an understanding of the mechanical properties of materials under high strain rate conditions is essential. An extensive experimental program using a split Hopkinson tensile bar set-up was established in an effort to investigate the dynamic properties of various TRIP steel grades. Four different TRIP steels are described with varying contents of the alloying elements silicon, aluminium and phosphor. Moreover, several phenomenological models describing the strain rate and temperature-dependent mechanical behaviour are validated. TRIP steel grades in which aluminium is the main alloying element show high elongation values, whereas a high silicon content results in an increase in strength. The widely used Johnson-Cook model can describe the behaviour of TRIP steels and provides the opportunity to study its material and structural response.

Physically Based Model for Static and Dynamic Behaviour of TRIP Steel

Low alloy multiphase TRansformation Induced Plasticity (TRIP) steels offer an excellent combination of a large uniform elongation and a high strength. This results from the composite behaviour of the different phases that are present in these steels: polygonal ferrite, bainitic ferrite and a martensite/austenite constituent. In order to obtain a clear understanding of the behaviour of the different constituents within the multiphase steel, they were prepared separately. The stress-strain relationship of the different single phase and multiphase steels were simulated with physically based micromechanical models. The model used to describe the stress-strain curves of the separate phases is based on the Mecking-Kocks and Seeger-Kocks theories and uses physical properties such as the microstructural properties and the chemical composition of the different phases. Strain-induced transformation kinetics, based on a generalized form of the Olson-Cohen law, were used to include the influence of the transformation of the metastable austenite. Static stress-strain properties of multiphase steels were modelled by the successive application of a Gladman type mixture law for two-phase steels. The model yields detailed information of stress and strain partitioning between the different phases during a static tensile test. A model for the dynamic stress-strain properties of ferritic steels is also proposed.

Static and impact-dynamic characterization of multiphase TRIP steels

In this study, results are presented of an extensive experimental program to investigate the strain rate dependent mechanical properties of various Transformation Induced Plasticity (TRIP) steel grades. A split Hopkinson tensile bar setup was used for the high strain rate experiments and microstructural observation techniques such as LOM, SEM and EBSD revealed the mechanisms governing the observed behavior. With elevated testing temperatures and interrupted tensile experiments the material behavior and the austenite to martensite transformation is investigated. In dynamic conditions, the strain rate has limited influence on the material properties. Yet an important increase is noticed when comparing static to dynamic conditions. The differences in strength, elongation and energy absorption levels observed between the investigated materials can be attributed to their chemical composition. Adiabatic heating during high strain rate deformation tends to slow down the strain induced martensitic deformation. The elongation of the ferritic and austenite constituents is found to be strain rate dependent and the strain induced martensitic transformation occurs gradually in the material.

Advanced Use of a Split Hopkinson Bar Setup for the Extended Characterization of Multiphase Steel Sheets

In this paper some highlights are presented of an integrated numerical and experimental approach to obtain an in-depth understanding of the high strain rate behavior of materials. This is illustrated by an investigation of the multiphase TRansformation Induced Plasticity (TRIP) steel. ‘Classic’ high strain rate tensile experiments using a split Hopkinson tensile bar setup are complemented with strain rate jump tests, tensile tests at elevated temperatures and interrupted experiments. High strain rate compression and three-point bending experiments are performed on the steel sheets as well. The results reveal the excellent energy-absorption properties in dynamic circumstances of TRIP steels. Advanced experimental setups using the Hopkinson principle provide indeed tools for validation of the material and structural properties of TRIP steels.