TOOLKIT for Damage Tolerant Microstructure Design

  Toolkit IEHK Figure 1: “TOOLKIT” – A multiscale approach for the optimization of microstructure and processing parameters to meet desired component performance improvement.

For the optimization of steel mechanical properties, the conventional measures have been widely exploited for many steel grades, so that tailoring the microstructure is the most promising measure for future steel developments. Therefore, the project aims to provide a simulation toolkit for the computer-assisted design of damage tolerant microstructures, as shown in Figure 1. It presents an approach to translate mechanical property requirements into optimized microstructural configurations and identifies the suitable processing routes for them.

Two component-level tests, crash box test for a steel sheet (DP1000) and Battelle drop-weight tear test (BDWTT) for a pipeline steel (X70) are addressed. To transfer the component performance to mechanical profiles, experimental and numerical investigations at both lab and component scales are performed in this study. An extensive experimental program (Figure 2) is designed involving various sample geometries that cover a wide range of stress states and tests are performed under quasi-static and high strain rate conditions up to 2500 s-1 to obtain the plasticity and fracture description of the material. For the macro scale numerical modelling, the modified Bai-Wierzbicki (MBW) damage model (Figure 3) describing both cleavage and ductile fracture under various strain rates and temperatures is extended to a non-local formulation to cope with the simulations for both lab and component levels.

 
 

Two component-level tests, crash box test for a steel sheet (DP1000) and Battelle drop-weight tear test (BDWTT) for a pipeline steel (X70) are addressed. To transfer the component performance to mechanical profiles, experimental and numerical investigations at both lab and component scales are performed in this study. An extensive experimental program (Figure 2) is designed involving various sample geometries that cover a wide range of stress states and tests are performed under quasi-static and high strain rate conditions up to 2500 s-1 to obtain the plasticity and fracture description of the material. For the macro scale numerical modelling, the modified Bai-Wierzbicki (MBW) damage model (Figure 3) describing both cleavage and ductile fracture under various strain rates and temperatures is extended to a non-local formulation to cope with the simulations for both lab and component levels.

  Characteristic stress states in the space of stress triaxiality and Lode angle parameter IEHK Figure 2: Experimental program of the lab tests and their characteristic stress states in the space of stress triaxiality and Lode angle parameter [1].
 
  : Schematic illustration of modified Bai-Wierzbicki IEHK Figure 3: Schematic illustration of modified Bai-Wierzbicki damage model for cleavage and ductile fracture [2].
 
  Volume-Elements IEHK Figure 4: Representative volume elements with statistical distributions for DP1000.

For the linking between the microstructure features and the mechanical properties, the representative microstructure model is employed allowing consideration of the microstructure parameters and at the same time bridging the equivalent quantities from microstructure to macroscopic level by incorporating a crystal plasticity material model. In the microstructure model, various features, such as phase fraction, the distributions of grain size, grain shape, crystallographic orientation and misorientation are considered, as shown in Figure 4. The up- and down-scaling between the models at different levels are powered by the virtual experiments and the entire approach is validated by lab and component scale experiments.

 

[1] Lian J., A generalised hybrid damage mechanics model for high-strength steel sheets and heavy plates, PhD Thesis, RWTH Aachen University, 2015, DOI: 10.2370/9783844040630.

[2] Lian J., Wu J., Münstermann S., Evaluation of the cold formability of high-strength low-alloy steel plates with the modified Bai-Wierzbicki damage model, Int. J. Damage Mech. 2015; 24(3):383-417.