PSI - Issue 13

Mehul Lukhi et al. / Procedia Structural Integrity 13 (2018) 607–612 M. Lukhi et al. / Structural Integrity Procedia 00 (2018) 000–000

608

2

results by Komotori et al. (1989) show that the fracture mode (surface or internal) is depended on the applied strain amplitude. For higher strain amplitude, the fracture process initiates in the bulk.

Fig. 2: Fracture surface in GJS-400 after LCF test (private communication from T. Mottitschka)

Fig. 1: Failure process in ferritic ductile iron (Komotori et al., 1998)

Mottischka et al. (2012) have performed low cycle fatigue (LCF) experiments in ferritic cast iron GJS-400. A fractography of the failed specimen is shown in Fig. 2. The dimple structure indicates severe plastic deformation and deformed ligaments between the particles. These observations indicate that NCI experiences nucleation, growth and coalescence of voids under cyclic loading and it is suggested that the void ratchetting can be relevant for the LCF of NCI. In the past, a number of researchers have performed cell model simulations which show void ratchetting in di ff erent materials including Gilles et al. (1992), Leblond et al. (1995), Devaux et al. (1997), Ristinmaa (1997), Rabold et al. (2005), Steglich et al. (2005), Mbiakop et al. (2015). But, these studies did not correlate the void ratchetting mechanism with LCF of NCI. In the present work, the void ratchetting is simulated until final failure using the cell model approach. Strain-life curves are extracted from the simulations and compared with the experimental results. The load sequence e ff ect is investigated by simulations and compared to the Miner-Palmgren hypothesis. Furthermore, the e ff ect of strength of material and temperature on the strain-life curves are studied using the cell model simulations.

2. Model

The microstructure of NCI can be represented in an idealized way as a periodic arrangement of hexagonal cells where the graphite particle are situated at the center of each cell. For simplicity, hexagonal prism is considered as a cylinder and graphite particle is considered as an ellipsoidal void as sketched in Fig. 3. Equal axis of the ellipsoid, i. e. a spherical void, are considered, if not stated otherwise. The matrix material is modelled as elastic–plastic with isotropic hardening of Ramberg-Osgood type. Thus, the yield curve σ y ( ¯ ε pl ) in terms of the equivalent plastic strain ¯ ε pl is given in implicit form as

¯ ε pl

=

n

E σ 0

σ y σ 0

σ y σ 0

(1)

.

+

The values of hardening exponent, initial yield stress and Young’s modulus, respectively, of the ferrite matrix are adapted from (Lukhi et al., 2018): n = 0 . 2 σ 0 = 230 MPa, E = 210 GPa, if not stated otherwise. Note that only the ratio σ 0 / E is relevant for the predicted lifetime under strain-controlled loading for dimensional reasons. The axisymmetric cell model is subjected to uniaxial cyclic straining at the macroscale which is imposed via displacement boundary conditions at the microscale. Cycle by cycle simulations are carried out using the commercial finite element software Abaqus / Standard.