法国国营航空发动机研究制造公司

High pressure turbine disks are critical components of aeroengines. The failure of such component is strictly forbidden for safety reasons. This component must meet the certification requirements in terms of fatigue life to initiation as well as crack growth life via damage tolerance approach. Fastener holes are highly stressed areas. The cold expansion process is used in aeronautical industry on fastener holes in order to introduce compressive residual stress around the hole up to 4 mm beneath the surface. This residual stress will prevent early crack initiation and reduce the crack growth rates. The residual stress field must be characterized and modeled in order to take into account the beneficial effect of cold expansion on the crack growth rates. Two different models including a three dimensional finite element model and a reconstructed residual stress model by the eigenstrain theory have been created. Indeed, these models aim to study the distribution of the residual stress field around an Inconel718 cold expanded hole. The three dimensional finite element model was established to simulate the actual split-sleeve cold expansion process and the reaming/chamfering operations. The results reveal that the residual stress varies through the thickness at different cross sections perpendicular to the hole axis of the sample. The eigenstrain study was applied to the same sample, subject to cold expansion treatment. The eigenstrain distribution was calculated using limited experimental strain measurement by neutron diffraction. The results reveal that the distribution of the residual stress field along the radial direction of the expanded hole is in good agreement with the numerical model, however, this simplified model is, by essence, unable to predict the residual stress variation through the thickness of the sample. Finally, the experimental data obtained by neutron diffraction were compared with those derived from simulation approaches.

Full conformal prediction is a framework that implicitly formulates distribution-free confidence prediction regions for a wide range of estimators. However, a classical limitation of the full conformal framework is the computation of the confidence prediction regions, which is usually impossible since it requires training infinitely many estimators (for real-valued prediction for instance). The main purpose of the present work is to describe a generic strategy for designing a tight approximation to the full conformal prediction region that can be efficiently computed. Along with this approximate confidence region, a theoretical quantification of the tightness of this approximation is developed, depending on the smoothness assumptions on the loss and score functions. The new notion of thickness is introduced for quantifying the discrepancy between the approximate confidence region and the full conformal one.

This study examines the Portevin-Le Chatelier (PLC) effect in the nickel-based superalloy Inconel 718 through a combined experimental-computational approach using statistical indicators from nonlinear dynamical systems theory. We develop a finite element model incorporating the Kubin-Estrin-McCormick constitutive law to capture Dynamic Strain Ageing effects, accounting for machine stiffness influence, and reproduce various dynamics under both hard and soft loading conditions. Statistical analysis reveals that machine stiffness significantly affects serration morphology and dynamics, influencing mean amplitudes, stress drop periods, and dynamical indicators such as the correlation dimension and Lyapunov exponents. Comparison of simulated and experimental stress time series demonstrates chaotic behaviour for type B and C serrations across all strain rates, with no evidence of self-organised criticality in experimental data. However, simulations predict self-organised criticality at high strain rates corresponding to type A bands, consistent with literature references. Statistical indicators reveal power-law behaviour for stress drop amplitudes as a function of strain rate, with critical exponents dependent on band type. Multifractal analysis shows that simulations overestimate complexity relative to experimental observations, suggesting the need for additional internal variables and finer-scale dynamics in modelling. Additionally, multifractal analysis of spatio-temporal diagrams reveals power-law distributions of plastic strain rates with consistent critical exponents across all strain rates, demonstrating its potential for characterising PLC spatio-temporal dynamics. Statistical, dynamical, and multifractal indicators show consistent correlations, collectively capturing transitions between serration regimes and serving as reliable quantitative metrics for characterising PLC dynamics. The analysis is finally applied to the spatio-temporal strain fields measured by digital image correlation. The results demonstrate the value of multi-indicator analysis for assessing the agreement between experiment and simulation and subsequently improving constitutive model parameter identification.

The hot corrosion behavior of the DS200 +Hf superalloy was studied at 900 degrees C in air (1 atm.) with the presence of Na2SO4 deposits for different exposure times. Carbides are proven to be preferred sites for initiation of Type I hot corrosion due to their high Hf, Nb, W and Ti content. While the formation of NaNbO3 is generally recognized to inhibit degradation, the formation of Na2WO4 reveals accelerated acidic fluxing leading to the formation of a thick, mixed and porous layer of corrosion products on the surface. Likewise, differences in the volumes of the different oxides formed induce marked local cracking.

A series of experiments investigating the recrystallization (RX) mechanisms of the single-crystalline superalloy AM1 during its manufacture have been carried out. High and very high-temperature tensile tests were conducted to investigate the effect of the level of plastic deformation and the temperature at which the strain was applied. These parameters were then analyzed for their influence on RX mechanisms during the subsequent solution heat treatment. The strain threshold for RX under various temperature ranges has been determined, and it has been shown that RX is more likely to occur within the 900 °C to 1150 °C deformation temperature range. Some correlations between deformation mechanisms and volume fraction of γ′ in this type of alloy over these temperature ranges have been discussed in order to explain these trends. In addition, non-isothermal tensile tests were conducted to reproduce as closely as possible the thermomechanical path experienced during the manufacturing of single-crystalline parts using investment casting. A strong correlation has been demonstrated between the thermomechanical path followed during non-isothermal tests that lead to the appearance of recrystallized grains and the “recrystallization zone” identified from pure isothermal tests. This correlation becomes evident when the thermomechanical path crosses this zone. The effect of non-isothermal thermomechanical loading on microstructure was studied by EBSD analysis. Significant local misorientation was observed around microstructural inhomogeneities such as casting pores and eutectic/casting pore pairs. These local rotations suggest that microstructural inhomogeneities act as stress concentrators leading to the first RX nuclei once a super-solvus solution heat treatment has been applied.