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International audienceThe concept of ratchetting strain as a crack driving force in controlling crack growth has previously been explored at Portsmouth using numerical approaches for nickel-based superalloys. In this paper, we report the first quantitative experimental evidence of near-tip strain ratchetting with cycles, as captured in situ by digital image correlation (DIC) technique on a compact tension specimen of stainless steel 316L, using both Stereo and SEM systems. The evolution of the near-tip strains with loading cycles was monitored whilst the crack tip was kept stationary. The strains normal to the crack plane were examined over selected distances from 6 to 57 lm to the crack tip for a number of cycles. The results show that strain ratchetting occurs with loading cycles, and is particularly evident close to the crack tip and under higher loads. 3D finite element models have also been developed to simulate the experiments and the results from the simulation are compared with those from the DIC measurements. This is the first time that near-tip strain ratchetting has been captured in situ at the peak loads during cyclic loading

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.

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Advanced microscopy characterisation and numerical modelling have been carried out to investigate oxygen diffusion and crack growth in a nickel-based superalloy under fatigue-oxidation conditions. Penetration of oxygen into the material and the associated internal oxidation, which leads to material embrittlement and failure, have been found from Focused Ion Beam (FIB) examinations. Applied fatigue loading tends to enhance the extent of internal oxidation for temperatures at 750°C and above. Using a submodelling technique, finite element analyses of oxygen penetration at grain level have been carried out to quantify the fatigue-oxidation damage and calibrate the diffusion parameters based on the measurements of maximum depth of internal oxidation. The grain microstructure was considered explicitly in the finite element model, where the grain boundary was taken as the primary path for oxygen diffusion. A sequentially coupled mechanical-diffusion analysis was adopted to account for the effects of deformation on diffusion during fatigue loading, for which the material constitutive behaviour was described by a crystal plasticity model at grain level. Prediction of oxidation-assisted crack growth has also been carried out at elevated temperature from the finite 2 element analyses of oxygen diffusion near a fatigue crack tip. A failure curve for crack growth has been constructed based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The predictions from the fatigue-oxidation failure curve compared well with the experimental results for triangular and dwell loading waveforms, with significant improvement achieved over those predicted from the viscoplastic model alone.