A crystal plasticity study of cyclic constitutive behaviour, crack-tip deformation and crack-growth path for a polycrystalline nickel-based superalloy

Abstract

Crystal plasticity has been applied to model the cyclic constitutive behaviour of a polycrystalline nickel-based superalloy at elevated temperature using finite element analyses.A representative volume element, consisting of randomly oriented grains, was considered for the finite element analyses under periodic boundary constraints. Strain-controlled cyclic test data at 650°C were used to determine the model parameters from a fitting process, where three loading rates were considered. Model simulations are in good agreement with the experimental results for stress-strain loops, cyclic hardening behaviour and stress relaxation behaviour. Stress and strain distributions within the representative volume element are of heterogeneous nature due to the orientation mismatch between neighboring grains. Stress concentrations tend to occur within "hard" grains while strain concentrations tend to locate within "soft" grains, depending on the orientation of grains with respect to the loading direction. The model was further applied to study the near-tip deformation of a transgranular crack in a compact tension specimen using a submodelling technique. Grain microstructure is shown to have an influence on the von Mises stress distribution near the crack tip, and the gain texture heterogeneity disturbs the well-known butterfly shape obtained from the viscoplasticity analysis at continuum level. The stress-strain response near the crack tip, as well as the accumulated shear deformation along slip system, is influenced by the orientation of the grain at the crack tip, which might dictate the subsequent crack growth through grains.Individual slip systems near the crack tip tend to have different amounts of accumulated shear deformation, which was utilised as a criterion to predict the crack growth path.

In Situ Experimental Study of Near-Tip Strain Evolution of Fatigue Cracks

“…Progressive accumulation of strain was found in a compact tension (CT) specimen under cyclic loading conditions. Further extensive modelling work has since been carried out using a number of constitutive modelling tools, including visco-plasticity [8][9][10], crystal-plasticity [10,11] and Discrete Dislocation Dynamics [12], in both two and three dimensions, selected materials (nickel alloys, titanium alloy), specimen geometries (CT, single edge notch tension and 3D aerofoil specimen) and temperatures (room temperature, 300, 650°C) and at variable loading conditions. Encouragingly, strain ratchetting was found common in all the cases examined.…”

Oxygen diffusion and crack growth for a nickel-based superalloy under fatigue-oxidation conditions

“…For crystal plasticity, a three-dimensional representative volume element (Figure 2a), consisting of randomly oriented grains, was built for the finite element analyses under periodic boundary constraints [50,51]. Strain-controlled cyclic test data at 650C were used to determine the model parameters from a fitting process, where three loading rates were considered.…”

Integrated experiment and modelling of microstructurally-sensitive crack growth

“…Plastic strain field development around crack tips has consequently led some authors [46,47] to assess and utilise accumulated plastic strain as the critical parameter for crack growth development in their studies. The study in [48] incorporated crystallographic slip in their modelling, and crack tip stresses were found to remain largely independent of crystal orientation and the local slip established, but plastic zone size and shape did depend upon it.…”