Pascale Kanouté

Residual stress determination in a shot‐peened nickel‐based single‐crystal superalloy using X‐ray diffraction

A residual stress depth profile up to 1 mm is determined with the Ortner method in a single crystal of a nickel-based superalloy which has been subjected to shot-peening. An optimization procedure is assessed to minimize uncertainties connected to Bragg angle, mosaic spread and numerical stability. The theoretical background is reviewed to highlight the connections between Bragg angle positions and the stress tensor components in different coordinate systems and also to obtain a mathematically consistent formulation. Transformation matrices required to express the strain components with respect to the initial state are provided for the general case. It is shown that, when a stress gradient occurs beneath the sample surface plane, the value of the σ33 component of the stress tensor determined from measurements is twice its true value. For a sample surface oriented along a 〈100〉 crystallographic direction, the data analysis shows that the compressive stresses which develop in the 150 µm-thick surface layer are compensated for by small tensile stresses developing at long scale rather than a specific layer of finite size featuring high tensile stresses. At least 17 Bragg angles are required to have stable solutions with standard deviations close to 30 MPa. Maximum compressive stresses of 1000 or 1400 MPa depending on the assumption used to describe the initial state occur at a 30 µm depth.

A high temperature fatigue damage model for single crystal superalloys

This work presents a new formalism for a time-dependent (or incremental) fatigue damage model developed for single-crystal nickel-based superalloys. The proposed coupled damage viscoplastic formulation accounts for the damage associated with microcracks lying on planes parallel to those of the crystallographic slip systems, and for the effect of oxidation on microcrack closure. The latter is introduced in the microcrack damage evolutionary behaviour through an additional oxidation-related damage variable. A simplified approach is relied upon to enable an efficient numerical integration of the single crystal formulation within a cycle, by treating the viscoplasticity and oxidation-related damage in an uncoupled manner, and an extrapolating approach is proposed to deal with periodic fatigue loading conditions. Fatigue tests on the commercial superalloy AM1 are performed at several load amplitudes, frequencies and crystallographic orientations to calibrate the model and to study the effect of fatigue loading on the crack initiation process. Fatigue torsion tests on perforated specimens are then carried out to validate the proposed formulation under local multiaxial mechanical fields. Finally, the accuracy of the proposed model to estimate crack initiation behaviour in the superalloy and its potential for component life prediction are discussed.

Impact of Residual Stresses on the Fatigue Behavior of a Nickel-Based Superalloy

Aircraft engine components are subjected, voluntarily or not, to the influence of residual stresses (RS). These RS may evolve in service conditions and may have an influence on fatigue life of the component. This paper presents a method to take into account the RS and their relaxation in a finite element calculation to obtain the fatigue life. This method is applied to a representative high-pressure turbine disk specimen made of N18 Nickel-based superalloy. Firstly, residual stresses are measured using X-Ray diffraction technique on the surface and the thickness of specimens. The influence of different surface finishing processes on the intensity and distribution of RS is compared to as-received specimen. Then, using the experimental profile as an initial state, a fatigue life analysis is performed (on fatigue specimen) by applying a multiaxial extension of the Smith-Watson-Topper model. Numerical and experimental results are discussed in detail and it appears that residual compressive stresses have almost no influence for high strain range but they improve the fatigue life for lower ranges.

Multiscale Modelling of Non-Linear Behaviour of Heterogeneous Materials: Comparison of Recent Homogeneisation Methods

This paper aims at evaluating recent homogenisation methods for scale changes in the micro-to-macro elastoplastic analysis of composites as well as polycrystalline aggregate. The corresponding localisation rules are recalled including, the T.F.A scheme, the incremental tangent approach of Hill, and the more recent affine method. These different schemes are finally applied to predict the overall behaviour of metal-matrix composites. With the help of simulations performed by the Finite Element method, we will discuss the limitations and the advantages of these procedures.