DFG-SFB 799: Mechanical modelling of the behaviour of austenitic cast TRIP-steels under deformation and failure

Project Manager

Prof. Dr.rer.nat.habil. Meinhard Kuna, i.R.

Consultant

Dr.-Ing. Andreas Burgold

Motivation

The martensitic phase transformation is the essential process for hardening and heat treatment of ferrous materials. Here, the microstructural component austenite is transformed into hard martensite by rapid cooling (thermally induced). A mechanically-induced martensitic phase transformation is observed in TRIP-steels. It occurs during loading and does not require rapid cooling. The phase transformation contributes to work hardening and causes additional deformation (TRIP - TRansformation Induced Plasticity). According to this, TRIP-steels offer high work hardening capability and high ductility. Low-alloyed TRIP-steels contain up to 20% retained austenite and are used as metal sheets in the automotive industry for crash-relevant applications. High-alloyed TRIP-steels can have either higher content of austenite or a fully austenitic initial microstructure and are widely used as stainless steels. High-alloyed CrMnNi-steels offering TRIP-effect are developed in the Collaborative Research Center 799 (CRC 799) in order to use them for innovative metal-ceramic-composites. Finite element simulations of those compound materials require material models for both the TRIP-steel and the zirconia ceramic that were developed during the first period of the CRC. In the second and third period the fracture mechanical properties of the TRIP-steels are investigated.

Objective

The objective of the project C5/1 is the exploration of the influence of martensitic phase transformation on crack initiation and propagation behaviour under static, dynamic and cyclic loading.

Methods

Plot of the martensite volume fractionThe stress state at a crack tip and the zone of phase transformation can be calculated using the material model of the TRIP-steel. Due to the high ductility of the material crack tip blunting has to be taken into account. A detailed stress analysis is carried out in order to investigate the influence of phase transformation on the mechanical fields at the crack tip [2,3,8,9]. The zone of martensitic phase transformation is shown in Fig. 1.

 

 

Furthermore, a modified J-Integral is formulated that takes into account the dissipated work due to the phase transformation [1,4,8,9]. Hereby, the material force method is used, which computes generalised forces acting on defects (e.g. crack tip, voids, inhomogeneities ...) in the material. In the case of TRIP-steel different defects exist in the region around the crack tip (Fig. 2): the crack tip itself (red dot), plastic and transformation zone (inhomogeneities). In fracture mechanics the generalised force acting at the crack tip Fmat is of interest, which represents the crack driving force.

Sketch of material forces

 

 

 

 

 

 

 

Simulations including cohesive zone models enable to distinguish between dissipated work spent for inelastic deformation and crack extension (opening of the cohesive zone), too. Therefore, the fracture process in austenitic steels and TRIP steels is modeled using a cohesive zone approach [5-9]. The parameters of the cohesive zone have to be identified based on experimental data and characterize the fracture toughness of the material.

In order to investigate cyclic crack growth a material model for the TRIP steel is developed, which takes into account the mechanical behaviour of TRIP steel under cyclic loading conditions [8,10]. A two-surface model is developed, which contains a yield surface and a transformation surface. Furthermore, nonlinear kinematic hardening is considered. The model parameters are found by parameter identification procedures.

Financial Support

Deutsche Forschungsgemeinschaft (DFG) until 2020

Publications

  1. Burgold, A., Kuna, M., Prüger, S.: Material forces in consideration of phase transformation in TRIP-steel, Procedia Materials Science 3 (2014) pp. 461-466, DOI: 10.1016/j.mspro.2014.06.077
  2. Burgold, A., Kuna, M.: Beanspruchungsanalyse von Rissen in TRIP-Stählen und der Einfluß der Phasenumwandlung, 47. DVM Arbeitskreis Bruchvorgänge, 19.-11.02.2015, DVM-Bericht 247 (2015) pp. 209-218, ISSN 1616-4687
  3. Burgold, A., Kuna, M., Prüger, S.: Crack tip fields in ductile materials with martensitic phase transformation - A numerical 2D study, Engineering Fracture Mechanics 138 (2015) pp. 169-184, DOI: 10.1016/j.engfracmech.2015.03.002
  4. Kuna, M., Burgold, A., Prüger, S.: Stress analysis and configurational forces for cracks in TRIP-steels, International Journal of Fracture 193 (2015) pp. 171-187, DOI: 10.1007/s10704-015-0027-0
  5. Burgold, A., Henkel, S., Roth, St., Kuna, M., Biermann, H.: Experimentelle und numerische Untersuchung eines hochduktilen austenitischen Gussstahles, 50. DVM Arbeitskreis Bruchvorgänge, DVM-Bericht 250 (2018) pp. 217-226, ISSN 2366-4797
  6. Burgold, A., Henkel, S., Roth, St., Kuna, M., Biermann, H.: Fracture mechanics testing and crack growth simulation of highly ductile austenitic steel, Materials Testing 60 (2018) pp. 341-348, DOI: 10.3139/120.111156
  7. Burgold, A., Roth. St., Kuna, M.: Cohesive Zone Modeling of Stable Crack Propagation in Highly Ductile Steel, Key Engineering Materials 774 (2018) pp. 167-172, DOI: 10.4028/www.scientific.net/KEM.774.167
  8. Burgold, A.: Modellierung des Bruchverhaltens austenitischer TRIP-Stähle, Dissertation,TU Bergakademie Freiberg, Berichtsband IMFD Heft 32 (2019), ISBN: 978-3-86012-617-2, https://nbn-resolving.org/urn:nbn:de:bsz:105-qucosa2-353185
  9. Seupel, A., Burgold, A., Prüger, S., Budnitzki, M., Kuna, M.: Modeling of the Thermomechanical Behavior, Damage, and Fracture of High Alloy TRIP-Steel, In: Biermann, H., Aneziris C. G. (Editors) Austenitic TRIP/TWIP Steels and Steel-Zirconia Composites -- Design of Tough, Transformation-Strengthened Composites and Structures, Springer Series in Materials Science, vol. 298-22 (2020)  pp. 723-769, ISBN 978-3-030-42602-6 (Hardcover), ISBN 978-3-030-42603-3 (eBook), DOI: 10.1007/978-3-030-42603-3\_22
  10. Burgold, A., Droste, M., Seupel, A., Budnitzki, M., Biermann, H., Kuna, M.: Modeling of the cyclic deformation behavior of austenitic TRIP-steels, International Journal of Plasticity 133 (2020) 102792, DOI: 10.1016/j.ijplas.2020.102792