Fatigue crack growth under large-scale yielding condition is studied for high-temperature loading. The applied strains are so important that diffuse damage phenomena are visible as a network of micro-cracks in front of the major crack. The survey of a macroscopic cracked surface is nevertheless possible, and numerical simulations with explicit representation of this crack are carried out to evaluate crack driving forces. The proposed numerical scheme takes into account plastic wake in the course of crack growth in a 3D model. A non-local model of fatigue crack growth rate, based on partition of strain energy density into elastic and plastic terms, yields improved results as compared to classical assessment of ∆J by numerical methods.

This study aims at characterizing the role of strong anisotropy in fatigue crack growth for a superalloy processed by additive manufacturing. The choice has been made to investigate two specimens exhibiting significantly different crystallographic textures, representative of possible variations of microstructures within a real part. Both cyclic mechanical behavior and fatigue crack growth have been investigated at high temperature. As a consequence of the oriented microstructure, high levels of anisotropy of elastic and viscoplastic behaviors were observed. These behaviors have been modeled by the combination of orthotropic elasticity, a Hill’s criterion and linear and non-linear kinematic hardening laws. To assess fatigue crack growth driving forces, in the context of complex crack shape, finite element analysis was proceeded taking care of 3D crack shape description. J was derived from G-θ , this method being convenient for anisotropic material. In most cases, fatigue cracks were found to grow preferentially at the vicinity of grain boundaries, regardless of the crystallographic orientation. An intrinsic fatigue crack growth rate related to this intergranular mechanism has been determined. On the other hand, mostly transgranular crack growth were observed when the loading was applied in the processing direction. In this case, the crystallographic texture appears to be the main factor governing the fatigue crack growth rate.

High-pressure turbine disks are subjected to extreme mechanical and thermal loadings throughout their life-time. This is the reason why during the design of turboshaft engines, regulation rules impose on manufacturers to demonstrate the integrity of rotating parts, especially turbine disks, with over-speed experiments. To improve the prediction of rotation speed at burst, it is essential to study the viscoplastic behavior and the fracture of the material. The present work focuses on the approach used to calibrate accurately a viscoplastic behavior model at high temperature (500 degrees C) of nickel-based superalloy Inconel718. The experimental testing campaign includes standard tensile tests on axisymmetric and flat specimens with and without notches. The behavior is described using the McCormick model for dynamic strain aging. This model accounts for the viscoplastic behavior including the range of negative strain rate sensitivity and the Portevin-Le Chatelier (PLC) effect. In addition to global measurements, digital image correlation at high temperature using a speckle directly engraved on the specimen is used to detect and measure the strain carried by the PLC bands. This information is used in the parameter calibration. The consideration of the machine stiffness is shown to be essential in the analysis for a correct description of serrations and of the critical strain for PLC effects. 3D simulations on full specimen geometries samples are performed to validate the finite element model. Finally, a full 3D representative disk geometry simulation is performed to detect instabilities and their propagation due to geometrical singularities.

Thermal fatigue experiments were performed on a single-edge wedge specimen of Ni-resist D5S cast iron to reproduce the conditions to which exhaust manifolds are subjected during service. The leading edge temperature was cycled between 20°C and 850°C, and the temperature distribution on the specimen surface was measured with thermocouples during thermal cycling. Due to the complexity of the loads, a uncoupled thermomechanical computation of the problem was performed using the three-dimensional finite element code ABAQUS to evaluate the temperature, stress and strain distribution over a thermal fatigue specimen. The heat fluxes and forced convection coefficients generated by the thermal fatigue benchmark were optimized in both heating and cooling phases by reverse engineering to reproduce the experimental temperature-time profiles measured by thermocouples on the surface of the specimen. Then, an elastic-viscoplastic tow-layer model was used to simulate the mechanical behavior of the studied material. It was found that the maximum stress value was located in the center of the specimen at the edge of the wedge. This result was confirmed by experimental observations. The crack initiation zone is located in the edge zone and the direction of crack propagation is perpendicular to the edge line toward the thick part of the specimen. The saturation of the main crack propagation near the uniform specimen zone was mainly explained by the rapid decrease of the energy dissipated per cycle.

Cold-work stress-relieved Zircaloy-4 kinetics of recovery-recrystallization has been studied under dynamic conditions. A specific furnace dilatometer was used to heat Zircaloy-4 specimens up to 430.degrees C. s(-1). Comparison of dynamic results with data of isothermal recovery and recrystallization showed that the process is delayed when the heating rate is increased. A model was proposed to describe the evolution of the material unrecrystallized fraction during rapid anisothermal transients. Comparisons showed a good agreement between the experimental results and the model predictions. [GRAPHICS] .

Thermal loading induces modifications of the precipitation microstructure of Al–Si–Cu–Mg alloys. This study focuses on the effect of deformation on precipitation microstructure during thermomechanical loadings. Several specimens were thermomechanically cycled while others were exposed to the same thermal cycles without any mechanical loading. The nature and morphological characteristics of the precipitation microstructure of the thermomechanically cycled specimens are compared to those of the thermally aged ones, using transmission electron microscopy (TEM), in order to assess the effect of deformation on the precipitation microstructure and especially on the kinetics of precipitate growth. The absence of any significant effect of superimposed straining during thermal cycling is discussed. Implications for the prevision of yield strength degradation during service operation are briefly presented.

By combining differential scanning calorimetry analysis and high-resolution transmission electron microscopy observations, the precipitation map for Al-Cu-Mg alloy (AA2024-T3) is obtained for a wide temperature range from 85 to 250 degrees C and ageing durations from 1 to 10,000 h. For the first time, the appearance of equilibrium S-Al2CuMg phase is observed at 85 degrees C from 1000 h of ageing. Hardness values superposed with the precipitation map reveal that the maximum hardness for all studied temperatures is located at the frontier between S-phase + Guinier-Preston-Bagaryatski (GPB) zones and S-phase regions. (C) 2020 Elsevier B.V. All rights reserved.

The aim of this paper is to test the influence of different crack path models, from oversimplification currently used in literature of a straight crack front orthogonal to the specimen sides to a realistic 3D crack path. On the basis of experimental features observed for Ti17 and Ti6242 alloys under multiaxial loading, a sensitivity analysis is proposed to address the impact of realistic 3D crack path on SIF and FCGR assessment.

Thermal barrier coating (TBC) systems are currently often tested by thermal cycling with or without temperature gradient on cylinder coupons. As a major drawback, edge effect associated with this geometry induces large scatter in TBC life at spallation. Thus, we promote the use of a laser shock to induce an artificial defect located at the top-coat/oxide interface. This method enables firstly to monitor damage evolution by means of non-destructive methods from this defect during thermal cycling at homogeneous temperature and for burner rig testing with superposed thermal gradient across the TBC. Secondly, by the knowledge of artificial defect location, an accurate 3D reconstruction of the crack tip was performed based on serial sectioning by focus ion beam and viewing by scanning electron microscopy. Founded on these observations, a sensitivity analysis of the measurement uncertainties with respect to the energy release rate of propagating cracks and to the process zone where damage elaborates is proposed.

This work presents a novel method of obtaining in situ strain measurements at high temperature by simultaneous digital image correlation (DIC), which provides the total strain on the specimen surface, and synchrotron x-ray diffraction (XRD), which provides lattice strains of crystalline materials. DIC at high temperature requires specialized techniques to overcome the effects of increased blackbody radiation that would otherwise overexpose the images. The technique presented herein is unique in that it can be used with a sample enclosed in an infrared heater, remotely and simultaneously with synchrotron XRD measurements. The heater included a window for camera access, and the light of the heater lamps is used as illumination. High-temperature paint is used to apply a random speckle pattern to the sample to allow the tracking of displacements and the calculation of the DIC strains. An inexpensive blue theatrical gel filter is used to block interfering visible and infrared light at high temperatures. This technique successfully produces properly exposed images at 870 °C and is expected to perform similarly at higher temperatures. The average strains measured by DIC were validated by an analytical calculation of the theoretical strain. Simultaneous DIC and XRD strain measurements of Inconel 718 (IN718) tensile test specimens were performed under thermal and mechanical loads and evaluated. This approach uses the fact that with DIC, the total strain is measured, including plastic strain, while with XRD, only elastic strain is captured. The observed differences were discussed with respect to the effective deformation mechanisms.

To improve the reliability of the power electronic modules for the high temperature applications, an innovative Thermal Bridge Composite (i-TBC) was designed. It has the architectured structure consisting of perforated FeNi36 sheet inserted between two Cu sheets. Due to simultaneous use of Cu and FeNi36, i-TBC possesses both a good thermal transverse conductivity and a limited longitudinal coefficient of thermal expansion. Different characterisations of i-TBC are required to understand the formation of its microstructure leading to the final properties. Therefore, the aim of this study was to analyse the integrity of Cu-Cu and Cu-FeNi36 interfaces as well as copper microstructure evolution throughout all elaboration steps: (i) first cold rolling, (ii) heat treatment and (iii) second cold rolling. First cold rolling did not lead to a bonding of Cu-Cu interfaces in the thermal bridge. Moreover, heterogeneity of Cu grain microstructure was observed with formation of ultra-fine grained structure close to junctions of Cu and FeNi36. The heat treatment led to a degradation of different interfaces adherence and to a complete copper recrystallization. Finally, the second cold rolling ensured an efficient solid welding of Cu-Cu interfaces and led to a heterogeneity of strain hardening of copper.

This study aims at describing fatigue crack growth in dissimilar welding of Ti based alloys under macroscopic multiaxial loading. The proposed methodology involves the experimental analysis of fatigue crack behavior under equibiaxial tension and macroscopic combination of mode I and II for Ti17, Ti6242 and laser welded specimen of both base metals. Based on these experiments, crack path, fatigue crack growth rate and crack interaction with microstructure have been addressed. The 3D finite element analysis of cracks shapes has enabled to derive stress intensity factor (SIF) investigated for opening, in-plane and out-of-plane shear modes based on linear elastic fracture mechanics assumptions. Finally, an equivalent SIF has been proposed to take into account the local mode mixity induced by both macroscopic shear and 3D crack shape. As a conclusion, the dissimilar welding of Ti based alloys increase the fatigue crack growth rate (FCGR) for any macroscopic loading with or without shear. Moreover, the microstructure of Ti6242 alloy, is well known to inhibit FCGR by multiples local bifurcation of crack path induced by the coarse microstructure of this alloy. This point was confirmed during equibiaxial tension but anomalous and very high FCGR was observed for macroscopic mode I + II loading. For the welded material, the fatigue crack to microstructure interactions have shown that the FCGR was clearly limited by coarse a needles inducing local bifurcation and conversely that in both fusion zone and heat affected zone, local refinement of a needles could not slow down the crack propagation.

The main reliability issue encountered during cylinder head development is the appearance of fatigue cracks during severe endurance tests. Some of these cracks can initiate in the coolant water jacket and lead to the complete part failure. They are mainly associated with the combination of combustion pressure alternate stresses and high cycle fatigue load in infinite life domain. However, Haigh or Dang Van analysis issued from 3D Finite Element Analysis reveals several issues: crack initiations are not correctly located and scatter of crack sizes are not predicted. To overcome these difficulties, a new fatigue model dedicated to cylinder heads in Aluminum - Silicon alloy has been developed in Renault powertrain division. The model is based on fatigue crack growth modelling in order to take into account various and combined load ratio in one hand, and typical characteristics of the microstructure of casting alloy in the other hand. This paper presents the theoretical basis of this model and the associated identification methodology for two different aluminum alloys.

Low Cycle Fatigue crack growth tests have been performed at 250 degrees C in order to study fatigue crack growth under large scale yielding conditions in a material widely used at high temperature by the automotive industry for cylinder head applications. The studied material was a cast aluminum alloy AlSi7Cu3Mg (close to A319) produced by Lost Foam Casting. Two different microstructures were investigated: one containing large natural pores and another where pores have been removed by Hot Isostatic Pressing (HIP). Fatigue Crack Growth Rates (FCGR) have been measured by in situ surface optical microscopy for different loading conditions all inducing generalized plasticity and compared to assess the influence of pores on the FCGR. In situ observations coupled to post mortem analysis revealed strong crack interactions with both pores and large hard particles on specimen surfaces and in the bulk. FCGR ranging between 10(-6) and 10(-4) m/cycle appear to be mainly sensitive to applied strain amplitudes. Although pores promoted secondary crack initiations and crack coalescences, they seemed to have a limited effect on steady-state FCGR which has been analytically modeled using energy densities.

Fatigue crack growth under large scale yielding condition is studied for a commercial base Co superalloys, Haynes® 188, for single edge notch (SENT) specimen. Due to very high temperature and applied strain values, it is observed that major crack interacts with micro-cracks pattern. These micro-cracks are found to be associated to strain localization and local high triaxiality stress ratio. Detailed analysis of strain localization and stress concentration was used to define an optimal characteristic length used in a non-local modeling of strain energy involved in crack growth process. To obtain robust and low time consuming crack growth modeling, a post-processing methodology is thus proposed for fatigue crack growth under large scale yielding. This point was successfully achieved for SENT specimen test series to describe crack growth rate. Finally, test driven for another geometry has enabled to validate the proposed modeling methodology.

Abstract The study of both the interface strength and residual stresses within a plasma sprayed ceramic coating is of great interest which main purpose is a better understanding of the mechanical properties of metal/ceramic systems. In this work, experiments involving a LASAT facility (LASAT: Laser Shock adhesion test) were implemented in order to analyse the adhesion and the damaging behaviour (debonding and buckling) of alumina coatings onto Co-based alloy. Similar alumina coatings were deposited using same plasma parameters with various surface preparations: smooth or severe grit blasting, with and without pre-oxidation. The non-destructive analyses (Optical and IR imaging) of the buckled region after LASAT have allowed to compare and discuss the interface strength of the studied coated samples. Further discussion was carried out by analysing the blister, resulting from the release of residual stresses within the coating after LASAT. It was thus evidenced that the residual stress state is a key parameter on resulting adhesive properties. This explorating work suggests using the LASAT method to analyse the adhesion and residual stresses within thermal sprayings.