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Microstructure based modeling of fatigue life in Inconel 718

By Sergio Lucarini (IMDEA Materials)
Co-authors: Javier Segurado (Universidad Politecnica de Madrid/IMDEA Materials)

The fatigue performance of Ni-based polycrystalline superalloys is closely related to their microstructure. Computational homogenization by means of Finite Elements (FE) and Fast Fourier Transform based solvers (FFT) is used to connect fatigue life with microstructure by simulations using periodic representative volume elements (RVE) of the polycrystalline microstructure. The RVEs are synthetically generated to reproduce the statistical distributions of grain size and shape [1] experimentally measured. The behavior of each grain follows a crystal plasticity (CP) model [2] that accounts for the elasto-visco-plastic behavior of the crystal including all the specific features of this alloy: Bausching effect, cyclic softening, mean stress relaxation and ratcheting. This approach has been applied to predict the low cycle fatigue (LCF) of Inconel 718 alloy. The fatigue life estimation was based on simulating the polycrystalline cyclic plastic behavior until the stable cycle to extract from the microfields Fatigue Indicator Parameters (FIP) that link cyclic response with fatigue crack initiation. The FFT solver used is based on the finite deformation variational method proposed in [3] that has been implemented and adapted to use general constitutive equations defined as Abaqus UMATs. Therefore, the same crystal plasticity model and RVE can be used in the two methods allowing direct comparisons between FE and FFT. The microfields and FIPs obtained by FFT are then validated with the FE results and FFT has been used for sizes exceeding the FE capabilities. Using simulation results, a power-law was proposed to relate FIP and number of cycles to initiation and the two parameters of the law were obtained using two experimental fatigue test. The model was used to predict a set of LCF tests covering a wide range of strain amplitudes and ratios and was able to accurately predict the experimental results, including the characteristic dual slope Coffin Manson response of this alloy. [1] A. Cruzado, B. Gan, M. Jimenez, D. Barba, K. Ostolaza, A. Linaza, J.M. Molina-Aldareguia, J. Llorca, J. Segurado, Acta Materialia, 98, 242-253, 2015 [2] A. Cruzado, J. Llorca , J. Segurado, IJSS 2017 [3] T.W.J. de Geus, J. Vond?ejc, J. Zeman, R.H.J. Peerlings, M.G.D. Geers. Computer Methods in Applied Mechanics and Engineering 318, 412-430, (2017)

Ⓒ Photos:Toerisme Leuven