ESF project: Micromechanical Simulation of Fatigue of Ductile Cast Iron

Project Manager

PD Dr.-Ing. habil. Geralf Hütter , Prof. Dr. rer. nat. habil. Meinhard Kuna

Ph.D. Student

M.Sc. Mehul Lukhi

Motivation

Fatigue of materials under cyclic loading is the most relevant damage mechanism of machines, vehicles and plants. Fatigue of metals is a complex mechanism. In ductile cast iron (DCI), the embedded graphite particles form heterogeneities, which possess an essential influence on the fatigue behavior. Empirical studies show, that the mean stress sensitivity of the endurance limit depends on the microstructure of DCI. Also for fatigue crack propagation, the graphite particles play an essential role since they debond from the metallic matrix. The voids formed thereby induce (compared to steels anormally) crack acceleration effects. However, the inhomogeneous microstructure has the positive effect that DCI is less prone to notches than steels of comparable strength. Furthermore, DCI has a higher threshold against fatigue crack propagation than comparable steels.

Aim

The aim of the project is to investigate the mentioned effects of fatigue of ductile cast iron by means of micromechanical FEM simulations. On the one hand, the simulations shall contribute to the qualitative understanding of the manifold empirical observations regarding fatigue of DCI. Furthermore, the simulations shall provide quantitative correlations between macroscopic quantities and microstructure parameters. Such correlations allow the optimisation of cast components towards strength and lifetime under efficient usage of the required resources.

Results

To understand low-cycle fatigue (LCF) and extremely low-cycle fatigue (ELCF) in DCI, a unit cellFigure 1: Cyclic necking model is prepared and cycle by cycle simulations till the final failure of the model are carried out under strain-controlled loading (strain-life approach). The increase in volume fraction of the void from cycle to cycle called void ratchetting is observed from the simulations. Furthermore, the void deforms during cycling and finally cyclic necking is observed, see Figure 1. In a real material with many graphite particles (voids), this cyclic necking leads to void coalescence and ultimately incipient macrocrack formation. Thus, void ratchetting can be identified as an LCF and ELCF mechanism in DCI. This behavior is consistent with the experimental observations. The strain-life data the simulations are shown in Figure 2 for different values of the shape factor S in comparison with experimental data from literature. The model predicts LCF behaviour in NCI quite accurately for S=0.7, a value which is allowed for DCI in common standards. The graphite particles whose shape deviates mostly from the sphere S=1, seem to be responsible for the lifetime of DCI components. For this prediction, only four parameters are required defining hardening behaviour of the matrix and geometrical quantities of the microstructure. Furthermore, the effects of hardening type and graphite particle – matrix interaction on strain-life curves have been studied, cf. [1].

While the type of hardening (isotropic/kinematic) under strain-controlled loading hardly affects the predicted lifetime, it has a very significant influence under stress-controlled loading [5]. In addition, crack propagation simulations under cyclic loading were performed with plane boundary layer models, which predict the three stages of the da/dN - dK curve qualitatively correct [6].

Figure 2: Strain-Life data (from [1])

 

Funding

European Social Fund ESF 2016-2019

Publications

[1] M. Lukhi, M. Kuna, G. Hütter: Numerical investigation of low cycle fatigue mechanism in nodular cast iron, International Journal of Fatigue 113 (2018), pp. 290-298, preprint

[2] M. Lukhi, M. Kuna, G. Hütter: A Novel Micromechanics Approach for Understanding of Fatigue in Nodular Cast Iron, Procedia Structural Integrity 13 (2018), pp. 607-612

[3] M. Lukhi, M. Kuna, G. Hütter: Characterising Fatigue Behaviour of Nodular Cast Iron Using Micromechanical Simulations, MATEC Web of Conferences 300 (2019), 13002

[4] M. Lukhi, G. Hütter, M. Kuna: A Numerical Simulation of Fatigue Crack Growth in Nodular Cast Iron by Plastic Collapse of the Intervoid Ligaments, DVM-Bericht 252 (2020), pp. 107-116

[5] M. Lukhi, M. Kuna, G. Hütter: Micromechanical Simulation of Fatigue in Nodular Cast Iron under Stress‐Controlled Loading, Material Design & Processing Communications

[6] M. Lukhi: Micromechanical Simulation of Fatigue in Nodular Cast Iron, Dissertation, TU Bergakademie Freiberg, 2020