High-temperature Low-cycle Fatigue Life Research Articles

A machine learning method for HTLCF life prediction of titanium aluminum alloys with consideration of manufacturing processes

Most conventional methods only consider the effects of materials and loading conditions when predicting the high-temperature low-cycle fatigue life of titanium aluminum alloys. However, the effects of manufacturing processes are not considered. To address this limitation, this paper proposes a machine learning method. First, the loading conditions are preliminarily filtered by physics knowledge. Second, the manufacturing processes and loading conditions are analyzed by using the average value of multiple mean impact values. Then, the variables that have little contribution to the output are filtered out, and the remaining variables are used as inputs. Finally, an optimized genetic algorithm-back propagation artificial neural network is established to predict the low-cycle fatigue life of the materials. The optimization methods are Levy flight and an adaptive adjustment mechanism of probability. Min-Max normalization is used to normalize and denormalize the data in the proposed method. A set of experimental data for Ti-685 is used to validate the proposed method. Moreover, the results show that the proposed method has better prediction accuracy than other methods.

Effects of Machined Surface Integrity on High-Temperature Low-Cycle Fatigue Life and Process Parameters Optimization of Turning Superalloy Inconel 718

Machined surface integrity characteristics, including surface stresses, physical-mechanical properties and metallographic structures, play important roles in the fatigue performance of machined components. This work aimed at investigating the effects of machined surface integrity on high-temperature low-cycle fatigue life. The process parameters were optimized to obtain required surface integrity and fatigue life of the turning superalloy Inconel 718. The relationships between low-cycle fatigue life and machined surface integrity characterization parameters were established based on the low-cycle fatigue tests at a high temperature (650 °C). The sensitivities of turning process parameters to high-temperature low-cycle fatigue life were analyzed, and the optimization parameters were proposed with the goal of antifatigue manufacturing. Experimental results indicated that the impact order of the characterization parameters of machined surface integrity on the high-temperature low-cycle fatigue life were the degree of work hardening RHV, the residual stress in the cutting speed direction S22, the fatigue stress concentration factor Kf, the degree of grain refinement RD and the residual stress in the feed direction S33. In the range of turning parameters of the experiments in this research, the cutting speeds could be 80~110 m/min, and the feed rate could be 0.10~0.12 mm/rev to achieve a longer high-temperature low-cycle fatigue life. The results can be used for guiding the fatigue-resistant manufacturing research of aeroengine superalloy turbine disks.

Low Cycle Fatigue Life Prediction Model of 800H Alloy Based on the Total Strain Energy Density Method.

In this paper, a high-temperature low-cycle fatigue life prediction model, based on the total strain energy density method, was established. Considering the influence of the Masing and non-Masing behavior of materials on life prediction, a new life prediction model was obtained by modifying the existing prediction model. With an 800H alloy of the heat transfer tube of a steam generator as the research object, the high-temperature and low-cycle fatigue test was carried out at two temperatures. The results show that the predicted and experimental results are in good agreement, proving the validity of the life prediction model.

High-Temperature Low Cycle Fatigue Life Prediction and Experimental Research of Pre-Tightened Bolts

Bolted connections are widely used in various mechanical structures due to their superior fastening properties. However, vibration and fatigue loads in the structure are likely to cause fatigue failure of the bolted joints, especially those under high temperature, such as in aero-engines. This paper mainly studies the low-cycle fatigue life of the pre-tightened bolts working at a high temperature. A novel test fixture is designed for fatigue tests, and low cycle fatigue tests of pre-tightened bolts are conducted at the temperatures of 550 °C and 650 °C, respectively. Furthermore, a new low cycle fatigue model that is based on the Von Mises equivalent stress/strain criterion is proposed. Meanwhile, the proposed model is used to predict the high-temperature low cycle fatigue life of pre-tightened bolts according to the stress/strain results obtained by finite element analysis. There is good agreement between the experimental results and those obtained by theoretical prediction, which validates the accuracy of the proposed fatigue model. Research results will provide a theoretical basis for the low cycle fatigue life prediction of pre-tightened bolts.

A generalized frequency separation–strain energy damage function model for low cycle fatigue–creep life prediction

ABSTRACTFatigue–creep interaction is a key factor for the failures of many engineering components and structures under high temperature and cyclic loading. These fatigue–creep life prediction issues are significant in selection, design and safety assessments of those components. Based on the frequency‐modified Manson–Coffin equation and Ostergren's model, a new model for high temperature low cycle fatigue (HTLCF), a generalized frequency separation–strain energy damage function model is developed. The approach used in this model to reflect the effects of time‐dependent damaging mechanisms on HTLCF life is different from those used in all the earlier models. A new strain energy damage function is used to reduce the difference between the approximate strain energy and real strain energy absorbed during the damage process. This proposed model can describe the effects of different time‐dependent damaging mechanisms on HTLCF life more accurately than others. Comparing traditional frequency separation technique (FS) and strain energy frequency‐modified approach (SEFS), the proposed model is widely applicable and more precise in predicting the life of fatigue–creep interaction. Experimental data from existing literature are used to demonstrate the feasibility and applicability of the proposed model. A good agreement is found between the predicted results and experimental data.

Effects of compressive strain hold on low-cycle fatigue life at elevated temperatures

The effects of compressive strain holding on the high-temperature low-cycle fatigue life of austenitic, martensitic and bainitic steels were investigated. The effects were scarcely observed in austentic steels, but were clearly observed in martensitic and bainitic steels. We conclude that life reduction following compressive strain holding is attributable to oxidation effects and the stress ratio that results from the asymmetric wave shape.

A generalized frequency modified damage function model for high temperature low cycle fatigue life prediction

The models for predicting high temperature low cycle fatigue (HTLCF) life are significant in the design of engineering components and structures working under high temperature and cyclic loading. In this paper, a review of the earlier models is given first, and then a new model for the HTLCF life prediction, a generalized frequency modified damage function (GFMDF) model, is set up based on the energy principle. By using time as the modifying factor and introducing the optimization approach to estimate the waveshape coefficients in the factor, the model can describe the effects of different time-dependent damaging mechanisms on the HTLCF life more accurately than previous ones. The model is evaluated and compared with the four well-known ones by the HTLCF test data of a nickel-base superalloy Rene'95 used for turbine disks at 650°C. It has been proved that the predictability of the model is better than the other four.

Damage Development Under Creep-Fatigue in a Titanium and a Superalloy

A titanium alloy, IMI 829 and a superalloy MAR M 002 tested under high temperature low cycle fatigue (HTLCF) at the University of Bristol, U.K., were examined for metallographic and fractographic features and the results are shown in this paper. From these observations a new five stage damage development model was developed for the HTLCF of IMI 829 and MAR M 002. It was shown that under creep-fatigue test conditions oxidation occurs which contributes to the development of cracks and their propagation for which, in general, no quantitative tool exists that can be accounted in HTLCF life prediction.

Embrittlement of several nickel-base alloys after high-temperature exposure

High temperature turbine blades in gas turbines are manufactured mainly from 7'-strengthened Ni-base superalloys. As is characteristic of all complex alloy systems, prolonged exposures at elevated temperatures lead to microstructural and property changes. The most common structural changes in Ni-base superalloys consist of 7' agglomeration, carbide reactions, and formation of 7' network at the grain boundaries. These changes in representataive blade alloys have been reported in the literature (i). The possibility of subtle environmental effects, which are known to affect mechanical properties in metals or simple alloys at lower temperatures, appears to have received an increasing recognition in the case of turbine alloys (2-8). The first detailed study was reported for Rene 77,80 and 100, which were embrittled following relatively short exposures at 980°C in air (2). This embrittlement manifested itself in a drastic reduction in tensile properties in a narrow temperature range around 8600C. Comparatively ductile behavior was retained at both higher and lower temperatures. This early work showed the embrittlement to be related to the penetration of gaseous species along grain boundaries, and as all the alloys studied had formed 7' networks along their grain boundaries by the MC+7 4 M2~CK+7' reaction, it was concluded that this was the embrittled phase and a neCeSsary prerequisite for embrittlement. Recently, a number of studies have been carried out to cover a range of Ni-base superalloys, all of which were shown to be embrittled in creep testing (3-5). Oxygen was identified as the penetrating and damaging species (3,4). In those studies, air exposure was used to induce the Observed environmental effect. In the case of the wrought IN X-750 superalloy, high temperature air exposure was not effective in embrittling the material in creep testing, while the exposure in reduced air pressure environments was extremely damaging (6,7). This was explained on the basis of a mechanism which assumed that in reduced air pressure the flux of oxygen along grain boundaries in th~ alloy IN X-750 was faster than in air (6,7). Hence in vacuum of ~ 2,6x10 Pa, oxygen diffused along grain boundaries without forming any oxides of chromium and nickel and instead formed gas bubbles of CO~ which acted as cavity nuclei (6). Recently the effect of environmental damage caused by thermal exposure on the high temperature low cycle fatigue (HTLCF) behavior of Ni-base superalloy IN738LC at 850°C, has been reported (9). The degradation in the HTLCF life of IN738LC, after thermal exposure at 1000°C for 240h in air, was attributed to the change in the modes of crack initiation and propagation due to the environmental effect. The aim of this paper is to report the results of an extensive study of the effect of prior thermal exposure in air on the tensile properties, mainly tensile ductility at different temperatures, of conventionally cast (CC), directionally solidified (DS) and single crystal (SC) Ni-base superalloy IN738LC. In addition, oxide dispersion strengthened (ODS) Ni-base alloys MA6000 and MA754, were also included in this investigation.

On the Improvement of High-temperature Low-cycle Fatigue Property by Grain-boundary Reaction Precipitates

The effect of grain-boundary reaction (GBR) on high-temperature low-cycle fatigue life was investigated using an austenitic 21-4N heat-resisting steel at 973K in air. Grain boundary was considerably serrated by the occurrence of GBR during aging. The high-temperature fatigue with symmetrical triangular wave shape. The longest fatigue life was observed in the specimens with about 8% GBR in area fraction. The improvement of fatigue strength resulted from the inhibition of a brittle intergranular fracture by the serrated grain boundary. Therefore, it is important for the improvement of fatigue strength to reduce the extent of GBR to be small and to effectively use the strengthening effect of serrated grain boundary. The fatigue life was less improved by the GBR in the fatigue with asymmetrical triangular wave shape.

On the improvement of high-temperature low-cycle fatigue property by grain-boundary reaction precipitates.

The effect of grain-boundary reaction (GBR) on high-temperature low-cycle fatigue life was investigated for austenitic 21-4N heat-resisting steel at 973K. The grain-boundary was considerably serrated by GBR during aging. The high-temperature fatigue life was remarkably improved by the occurrence of GBR for fast-fast and slow-slow symmetrical triangular strain wave shapes. The fatigue life was not changed by GBR for the asymmetrical triangular wave shape. The longest fatigue life was observed on the specimen with GBR nodule was internally cracked by cyclic deformation. The improvement of fatigue life by a small amount of GBR was resulted from the retardation of brittle intergranular fracture by the serrated grain-boundary configuration.

Mechanical Properties of Heat-Resistant Alloys Exposed to Air and HTGR Helium at High Temperatures

The effect of a corroded surface layer on the tensile properties and the high-temperature low-cycle fatigue life was studied on Hastelloy-X and on Incoloy alloys 800 and 800H by comparing the properties between specimens exposed to air and high-temperature gas-cooled reactor helium at 1000/sup 0/C prior to testing and specimens aged under the same temperature/time conditions as those of exposed specimens. The ratio of the corroded surface layer to the total cross-sectional area was controlled at 1000/sup 0/C by environment, exposure time, and shape/size combinations of specimens. Tensile strength could be quantitatively expressed in terms of the intergranular oxidation, irrespective of the variation of materials and corrosive conditions. By comparing the low-cycle fatigue lives at 1000/sup 0/C between exposed and aged materials, it was clarified that lifetime was remarkably reduced by the formation of a corroded surface layer. However, fatigue life of aged material was less than that of solution-treated materials. These two opposing effects of corrosion and aging brought about a small difference in fatigue life between solution-treated and exposed materials.