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Development of fatigue crack growth testing under thermo-mechanical fatigue conditions

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Abstract

As the need for the prediction of component life and maintenance interval schedules becomes more demanding, there is an increasing requirement for thermo-mechanical fatigue (TMF) test data including fatigue crack growth rates under such conditions. The test equipment requirements to meet this challenge are discussed and finally a conventional servo-electric load frame is utilised in combination with a radiant lamp furnace to generate the desired thermal cycles. The radiant lamp furnace enables reasonably consistent temperature gradients to be achieved with notched test pieces. The temperature calibration method to achieve the desired thermal cycle will be briefly described, along with some considerations for ensuring reproducibility in the thermal cycles applied during tests. The measurement of crack growth under TMF conditions will also be considered. Such measurements are challenging because of the changing thermal conditions and the effect on conventional potential difference (PD) electrical methods. These effects make it difficult to continuously monitor crack size during the thermal/load cycles. Thus, convenient dwells within the TMF cycles, where both the load and temperature are held constant for a brief period, have been utilised to make time-averaged crack size measurements using PD methods. The performance of these experimental methods has been demonstrated with some trials on an advanced nickel base superalloy, RR1000.

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... Ni-based superalloys [119][120][121], which is supported by the higher FCG rates associated with intergranular fractography under the in-phase (IP) loading conditions compared with out-of-phase (OP) testing associated with transgranular fractography, due to the increased temperature at peak stress and therefore increased time-dependent FCG. A few reviews of the interaction between environmental damage and fatigue in Ni-based superalloys can be found in [4,21,109]. ...
... However, in general, studies with regards to TMF crack growth mechanisms in PM Ni-based superalloys are quite limited, and further studies are needed. This is key in providing quasi in-service condition FCG data so that more accurate predictions can be made using appropriate phenomenological and mechanistic FCG models for PM superalloy turbine disc lifing assessment [120,156]. ...
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Powder metallurgy (PM) Ni-based superalloys are widely used for aeroengine turbine disc applications due to their excellent mechanical properties and good corrosion resistance at elevated temperatures. Understanding the fatigue crack growth (FCG) mechanisms of PM Ni-based superalloys is important for both disc alloy development and life prediction of disc components in these advanced aeroengines where damage tolerance design prevails. FCG in PM Ni-based superalloys is a complicated function of microstructure, temperature, loading conditions and environment and is usually a consequence of the synergistic effects of fatigue, creep and environmental damage. In this review, the mechanisms controlled by microstructural features including grain size, grain misorientation, γ′ size and distribution on short and long FCG behaviour in PM Ni-based superalloys are discussed. The contribution of creep and environmental damage to FCG has been critically assessed. The competing effects of mechanical damage (i.e. fatigue and creep) and environmental damage at the crack tip are microstructure-sensitive, and usually results in transition between transgranular, mixed-trans-intergranular and intergranular FCG depending on the contribution of environmental damage to FCG processes.
... Thus, to achieve accurate life predictions models within the crack growth phase, TMF crack propagation testing is needed. Several studies have been carried out to increase the knowledge and establish guidelines related to TMF crack growth testing [8][9][10][11]. The type of the TMF loading cycle used in testing generally depends on the target component being investigated. ...
Article
Exploring crack growth behaviour is needed to establish accurate fatigue life predictions. Cracked specimens were tested under strain-controlled out-of-phase thermomechanical fatigue conditions. The tests included dwell times and three different minimum temperatures. Higher minimum temperature gave faster crack growth rates while the additions of dwell times showed no effects. Crack closure was observed in all the tests where the addition of dwell times and change in minimum temperature displayed little to no effect on crack closure stresses. Finite element models with a sharp stationary crack and material parameters switching provided acceptable predictions for the maximum, minimum, and crack closure stresses.
... Nevertheless, TMF crack growth testing has emerged as an important topic when studying critical components. Thus, several researches have been focusing on investigating and laying guidelines for TMF crack propagation testing [4][5][6]. ...
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Understanding of crack growth behaviour is necessary to predict accurate fatigue lives. Out-of-phase thermomechanical fatigue crack propagation tests were performed on FB2 steel used in high-temperature steam turbine sections. Testing results showed crack closure where the compressive part of the fatigue cycle affected crack growth rate. Crack closing stress was observed to be different, and had more influence on the growth rate, than crack opening stress. Crack growth rate was largely controlled by the minimum temperature of the cycle, which agreed with an isothermal crack propagation test. Finite element models with stationary sharp cracks captured the crack closure behaviour.
... Seven locations were selected for thermocouples, similar to those presented by Jacques et al. [25], to monitor the thermal profiles across the specimen. Figure 3 shows the locations used for the current work, showing five thermocouples around the notch plane on the faces and corners, with a set 2 mm above the plane at the notch and a set 2 mm below. ...
Article
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Article
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Article
The fatigue crack growth rate and fracture behaviour of a nickel–base superalloy UDIMET 720 Li was investigated at 700°C in vacuum and air environments using corner crack specimens. The effects of load ratio at a frequency of 0.25 Hz were examined while the effects of loading frequency from 5 Hz to 0.008 Hz were also examined for a constant load ratio. The mode of fracture was intergranular at all load ratios at a frequency of 0.25 Hz in an air environment. Two-parameter models were proposed to describe separately the effects of load ratio and frequency. The model prediction was combined with data from vacuum tests to form a fracture mechanism map showing limited contribution of creep, while oxidation controls the fatigue crack growth rate as the frequency decreases.
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