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 shotpeening. 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 33 component of the stress tensor determined from measurements is twice its true value. For a sample surface oriented along a h100i crystallographic direction, the data analysis shows that the compressive stresses which develop in the 150 mm-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 mm depth.
In this work, X-ray diffraction measurements and finite elements calculations are combined to investigate the effect of the shot-peening process on the fatigue lifetime of the AMI nickel-based single crystal superalloy. The Ortner method is used to determine residual elastic stress depth profiles in plane-parallel samples. They exhibit a I30-I60 µm thick hardened layer where there are compressive stresses up to 1000-I400 MPa. The tensile stresses which ensure the mechanical equilibrium of the samples are not localized in a specific layer but rather distributed in a few millimeters thick layer. The eigenstrain theory is then used to incorporate measured stresses in the elasto-viscoplatic modeling of shot-peened fatigue test specimens. A numerical method is proposed to initialize hardening variables in the shot-peened layer independently of the complexity of the constitutive law or measurements in calibration samples. Finally, a fatigue analysis at 650 °C is performed in samples with a stress-concentration. The effect of shot-peening on the fatigue lifetime is studied using both modeling and measurements. Results are in good agreement in the investigated range of applied stresses. However, measurements show that residual stresses from shot peening are not always beneficial.
In this work, X-ray diffraction measurements and finite elements calculations are combined to investigate the effect of the shot-peening process on the fatigue lifetime of the AM1 nickel-based single crystal superalloy. The Ortner method is used to determine residual elastic stress depth profiles in plane-parallel samples. They exhibit a 130-160 μm thick hardened layer where there are compressive stresses up to 1000-1400 MPa. The tensile stresses which ensure the mechanical equilibrium of the samples are not localized in a specific layer but rather distributed in a few millimeters thick layer. The eigenstrain theory is then used to incorporate measured stresses in the elasto-viscoplatic modeling of shot-peened fatigue test specimens. A numerical method is proposed to initialize hardening variables in the shot-peened layer independently of the complexity of the constitutive law or measurements in calibration samples. Finally, a fatigue analysis at 650 • C is performed in samples with a stress-concentration. The effect of shot-peening on the fatigue lifetime is studied using both modeling and measurements. Results are in good agreement in the investigated range of applied stresses. However, measurements show that residual stresses from shot peening are not always beneficial.
Abstract. Shot-peening is used to improve the lifetime of mechanical components through the introduction of compressive residual stresses (RS) in a surface layer. In this study, we investigate the impact of such a pre-stressing treatment on a single crystal nickel-based superalloy for high pressure turbine blades of engine aircrafts. In addition to conventional metallographic tools used to characterize the alloy microstructure and the zone affected by shot-peening, X-ray measurements have been performed in order to determine residual stress depth profiles.
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.
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