Dwell-fatigue Tests Research Articles

Anisotropy effects during dwell-fatigue caused by δ-phase orientation in forged Inconel 718

Inconel 718 is a commonly used superalloy for turbine discs in the gas turbine industry. Turbine discs are often subjected to dwell-fatigue as a result of long constant load cycles. The effect of anisotropy on dwell-fatigue cracking in forged turbine discs have not yet been thoroughly investigated. Crack propagation behaviour was characterised using compact tension (CT) samples cut in different orientations from a real turbine disc forging. Samples were also cut in two different thicknesses in order to investigate the influence of plane strain and plane stress condition on the crack propagation rates. The samples were subjected to dwell-fatigue tests at 550°C with 90s or 2160s dwell-times at maximum load. Microstructure characterisation was done using scanning electron microscopy (SEM) techniques such as electron channelling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and light optical microscopy (LOM). The forged alloy exhibits strong anisotropic behaviour caused by the non-random δ-phase orientation. When δ-phases were oriented perpendicular compared to parallel to the loading direction, the crack growth rates were approximately ten times faster. Crack growth occurred preferably in the interface between the γ-matrix and the δ-phase.

The influence of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires

Dwell sensitivity in titanium alloys is generally attributed to the phenomenon of load shedding, which is a time dependent redistribution of stress from weak grains, with their c-axis perpendicular to the loading direction, to strong grains, with their c-axis approximately parallel to the loading direction. This leads to the formation of internal quasi-cleavage facets on the basal planes of strong grains. In this paper, the effect of load holds on the fatigue behaviour of drawn Ti-6Al-4V wires is investigated, because these wires do not contain strong α grains. It has been found that this leads to a different dwell fatigue behaviour compared to what has been described in literature. In the case of drawn wires, introducing load holds promoted crack initiation at the surface, through the formation of a facet on a prismatic plane of a surface grain that was oriented for very easy prismatic slip. This was confirmed by sectioning facets, using focused ion beam milling, and electron backscatter diffraction measurements. Because of the specific crystallographic texture of drawn wires, the phenomenon of load shedding is less pronounced, and there is no formation of internal facets. The amount of cycles to failure was reduced by two to three orders of magnitude compared to fatigue tests without load holds. The time to failure remained similar, and was even higher for some dwell fatigue tests. There was a significant amount of strain accumulation during dwell fatigue tests. The maximum strain increased more rapidly in tests with 30s load holds compared to tests with 120s load holds, due to creep recovery during the periods in between load holds. During these periods, the strain decreased, even though a small tensile load was still applied.

Strengthening behavior in non-isothermal monotonic and cyclic loading in a Ni-based single crystal superalloy

Dwell-fatigue tests and variable strain rate tensile tests followed by cycling tests were performed using 〈001〉-oriented specimens made of a first-generation Ni-based single crystal superalloy. A short thermal jump from the nominal temperature of 1050 to 1200°C was introduced along the lifetime of dwell-fatigue experiments and at the beginning of tensile tests. A fine γ′ precipitation occurred in the γ matrix upon such event which induced a large transient strengthening effect on the mechanical properties. In fact, a transient decrease of the plastic strain rate, corresponding to a reduced magnitude in the hysteresis loops, was measured after the temperature peak during the dwell-fatigue experiments. In addition, a temperature peak produced a large hardening effect and a mechanical behavior change during the tensile tests since a hardening of 160MPa was recorded and a perfectly plastic behavior was obtained. These transient phenomena were due to a temporary additional strengthening provided by fine γ′ precipitates lasting for the time necessary to not hinder dislocation motions anymore. Furthermore, the experimental results were also used to determine how fine γ′ precipitates modified the magnitude and the saturation speed of the isotropic and kinematic hardenings.

Temperature-Dependent Dwell-Fatigue Behavior of Nanosilver Sintered Lap Shear Joint

A series of dwell-fatigue tests were conducted on nanosilver sintered lap shear joint at elevated temperatures from 125 °C to 325 °C. The effects of temperature and loading conditions on dwell-fatigue behavior of nanosilver sintered lap shear joint were systematically studied. With higher temperature and longer dwell time, creep played a more important part in dwell-fatigue tests. Creep strain accumulated during maximum shear stress hold was found partly recovered by the subsequent cyclic unloading. Both the fracture mode and silver particle growth pattern were characterized by X-ray tomography system and scanning electron microscope (SEM). The mean shear strain rate γ˙m synthesized the effects of various factors, such as temperature, shear stress amplitude, mean shear stress, and dwell time, by which the fatigue and dwell-fatigue life of nanosilver sintered lap shear joint could be well predicted within a factor of two.

Relationships between Microstructural Parameters and Time-Dependent Mechanical Properties of a New Nickel-Based Superalloy AD730™

High temperature creep and dwell-fatigue properties of the new nickel-based superalloy AD730™ have been investigated. Three microstructures have been studied in creep (850 °C and 700 °C) and dwell-fatigue (700 °C stress control with trapezoidal signals, and dwell times ranging from 1 s to 3600 s): a coarse grains microstructure, a fine grains one, and single crystalline samples. The aim of this study is to assess the influence of the grain size on creep and creep-fatigue properties. It is demonstrated that fine and coarse grains microstructures perform similarly in creep at 700 °C, showing that the creep properties at this temperature are controlled by the intragranular precipitation. Moreover, both the coarse grains and the fine grains microstructures show changes in creep deformation mechanisms depending on the applied stress in creep at 700 °C. At higher creep temperatures, the coarse grains microstructure performs better and almost no effect is observed by suppressing grain boundaries. During dwell-fatigue tests at 700 °C, a clear effect of the mechanical cycling has been evidenced on the time to failure on both the coarse and the fine grains microstructures. At high applied stresses, a beneficial effect of the cyclic unloading to the lifetime has been observed whereas at lower applied stresses, mechanical cycling is detrimental compared to the pure creep lifetime due to the development of a fatigue damage. Complex creep-fatigue interactions are hence clearly evidenced and they depend on the pure creep behavior reference.

Effects of microstructure on high temperature dwell fatigue crack growth in a coarse grain PM nickel based superalloy

The influence of microstructure on the dwell fatigue crack growth behaviour of an advanced nickel-based superalloy was investigated at a temperature of 700°C. Microstructural variations were induced by heat treatment variables: different cooling rates of quenching from super-solvus solution heat treatment, 0.7 and 1.8°Cs−1, and an addition of a high temperature stabilisation heat treatment (2h at 857°C) between the solution treatment and the final ageing treatment. With a one hour dwell introduced at the peak load of the fatigue cycle, such different microstructural conditions can lead to a difference of up to two orders of magnitude in crack growth rates in air, when compared to those obtained under baseline fatigue loading. By performing such dwell fatigue and baseline fatigue tests in vacuum, it is confirmed that such increases in crack growth rates under dwell fatigue loading in air are mainly environmentally related. Transmission electron microscopy (TEM) was utilised to analyse both crack tip oxides and associated deformation mechanisms in the matrix. A novel mechanism taking into account competing interactions of crack tip oxidation (leading to increases in crack growth rates) and stress relaxation (leading to decreases in crack growth rates) is outlined.

Fatigue and dwell-fatigue behavior of nano-silver sintered lap-shear joint at elevated temperature

Load-controlled fatigue and dwell-fatigue tests were conducted at elevated temperature to describe the high temperature mechanics behavior of nano-silver sintered lap-shear joints. The results showed that the shear strength of nano-silver sintered lap-shear joints was strongly temperature dependent, and almost halved at the temperature of 325°C. The Basquin model was used to assess the fatigue life of the joints at elevated temperature and the constants in the model were figured out, which yielded good prediction for experimental data. In dwell-fatigue tests, at the temperature of 325°C, creep was found the dominant factor that resulted in failure acceleration and cyclic life reduction. With the temperature decreasing to 225°C, the creep played a less important role to the total deformation.

Discrepancy between fatigue and dwell-fatigue behavior of near alpha titanium alloys simulated by cellular automata

Cellular automata were used to simulate microstructure heterogeneities at local (one grain) and global (aggregate of grains) levels in an attempt to better understand the large discrepancies observed between fatigue and dwell-fatigue behaviors of some titanium alloys. Eshelby theory was used to estimate the local stresses and strains developed in the microstructure. In the case of simple fatigue tests, loading and unloading stages were used to calculate and describe the strain accumulation history. For dwell fatigue analysis, a thirty second steady state at maximum load was applied to simulate the dwell period. In the present study, the local stress, strain and creep rate in each grain are calculated as a function of the mechanical properties of the neighboring grains. The data are then compiled and the overall behavior of the aggregate is predicted. The results can reproduce and explain some specific features observed experimentally in fatigue and dwell-fatigue tests of near alpha titanium alloys.

Crack Growth Behavior in ATI 718Plus<sup>®</sup> Alloy

ATI 718Plus® alloy is a new, cast and wrought, Ni-base superalloy with a maximum use temperature approximately 55°C higher than alloy 718. The mechanical properties have been well characterized by turbine engine OEM’s and the alloy has been specified for use as static components and blades in gas turbine engines. Broader use of ATI 718Plus alloy in engine disk applications requires detailed understanding of the damage tolerance under creep and cyclic loading conditions. The results of a large testing program to evaluate the crack growth behavior of ATI 718Plus alloy at temperatures between 649°C and 704°C under conditions of fatigue, dwell-fatigue, and creep are presented. Crack growth rates in ATI 718Plus alloy in this temperature range are lower than alloy 718 and comparable to Waspaloy under non-dwell-fatigue conditions, and comparable to alloy 720 in dwell-fatigue tests.

Dwell-fatigue of a titanium alloy at room temperature under uniaxial or biaxial tension

Load-controlled fatigue and dwell-fatigue tests were run at room temperature on tubular specimens of Ti6Al4V under combined tension and internal pressure in a proportion producing nearly equibiaxial loading, or under internal pressure alone. The maximum stress ranged from 75% to 92% of the 0.2% yield stress (92–112% of the cyclic yield stress) and the R-ratio was zero, so that no cyclic plasticity occurred. Biaxial loading had a positive influence on fatigue lives. In dwell-fatigue, creep was observed in the longitudinal and transverse directions and was associated with a reduction in fatigue lives that increased with the loading range. SEM observations of damage mechanisms as well as finite element computations of stress and strain fields were performed to analyse and discuss these results.

Dwell-fatigue life dispersion of a near alpha titanium alloy

An experimental study was undertaken to examine the evolution of life, strain, strength and damage in titanium IMI 834 during dwell-fatigue loading conditions. The tests were conducted on smooth cylindrical specimens under load control with trapezoid wave form of 0.033 Hz frequency and zero load ratios. The specimens were cut from identical disk forgings and tested in the same conditions at room temperature, as it is normally done in the industry. Thirty seconds dwell time was imposed at the maximum load test, 824 MPa (90% of the yield strength). The total rigidity decrease during the test was plotted against the number of cycles for all specimens. Jumps were observed in the curves. They may be related to the nucleation and propagation of cracks in the material. Scanning electron microscopy observations show that all failures take place from a sub-surface nucleation site which seems to nucleate at random times during the dwell-fatigue tests.

Crystal plasticity, fatigue crack initiation and fatigue performance of advanced titanium alloys

The fatigue behaviour of a novel large grained variant of the near α titanium alloy Ti 685 is described. Load controlled low cycle fatigue fractures in plain cylindrical specimens demonstrate the highly crystallographic nature of the failure process. When compared to similar data for conventional grain size Ti 685 variants the LG685 material clearly offers reduced fatigue and static strength. An alternative flat plate specimen design was employed together with electronic speckle pattern interferometry and strain gauges to monitor the inhomogeneous strain accumulation in this large grained variant under load control conditions. Dwell fatigue tests indicated that the eventual location for crack initiation and subsequent failure could be identified as early as the first loading cycle. The precise crystallographic orientations of the surrounding microstructure were defined using electron back scattered diffraction. In contrast to previous models to describe facet formation and early fracture in this class of alloy, basal plane slip systems were not implicated. The use of EBSD to identify “effective structural units” of common orientation, which do not necessarily relate to colony size, will be demonstrated for both the large grained material and a conventional compressor disc alloy Timetal 834 in typical forged condition.