法国国营航空发动机研究制造公司

Understanding dislocation evolution during viscoplastic deformation is essential to accurately predict hightemperature superalloy behavior. While creep and monotonic deformation are well studied, the evolution of the dislocation substructures during stress relaxation, particularly following prior plastic straining, remains unclear. This study investigates a single crystal gamma/gamma' AD730TM superalloy deformed at 700 degrees C, focusing on dislocation evolution and redistribution. Transmission electron microscopy reveals that stacking faults formed during plastic straining progressively disappear during relaxation, suggesting thermally activated mechanisms such as dislocation re-association and recovery. After re-association, perfect dislocations spread into the matrix, forming a homogeneous dislocation network. These microstructural evolutions are believed to underlie the macroscopic stress relaxation behavior, which exhibits two regimes in the Norton diagram: an initial high-stress exponent regime linked to rapid dislocation rearrangement, followed by a low-stress exponent regime associated with broader dislocation activity.

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.

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.

This article presents an open virtual test case of an Open Fan geometry. The Open Fan is the newest and most promising unducted aero-engine concept. It is expected to be quieter and mechanically less complex than a counter-rotating open rotor (CROR), while delivering a similar level of fuel burn. The fuel burn and emission improvements with respect to an ultra-high bypass ratio (UHBR) ducted fan are expected to be very significant. A challenge will be to minimize the acoustic impact. Currently, there is no publicly available state-of-the-art Open Fan design, or even a state-of-the-art CROR geometry, that researchers can use to conduct independent simulations. A realistic industrially driven Open Fan virtual test case has been released by the PANDORA project consortium. The key outcome of the PANDORA project is an estimate of the improvements delivered by the Open Fan configuration with respect to a CROR in terms of noise, performance, and emissions. The added value of having a realistic design is that it naturally drives the research in the correct direction and provides the range of parameters in which the physics must be properly solved and understood. In this work, a new Open Fan geometry will be presented along with the pitch settings at various operating conditions. The aerodynamics of an Open Fan is more demanding than that of a CROR. In fact, the Open Fan presents a higher aerodynamic loading and Mach number for the same thrust. The aerodynamic performance of the Open Fan configuration is obtained using RANS simulations. This type of simulation is also helpful in assessing the influence of design changes on the Open Fan performance. RANS simulations have been successfully used to compute the aerodynamic loading, thrust, and efficiency at different operating conditions.