High Temperature Fatigue Tests Research Articles

Mechanism of Fatigue-Life Extension Due to Dynamic Strain Aging in Low-Carbon Steel at High Temperature.

An enhancement in fatigue life for ferrite-pearlite low-carbon steel (LCS) at high temperature (HT) has been discovered, where it increased from 190,873 cycles at room temperature (RT) to 10,000,000 cycles at 400 °C under the same stress conditions. To understand the mechanism behind this phenomenon, the evolution of microstructure and dislocation density during fatigue tests was comprehensively investigated. High-power X-ray diffraction (XRD) was employed to analyze the evolution of total dislocation density, while Electron Backscatter Diffraction (EBSD) and High-Resolution EBSD (HR-EBSD) were conducted to reveal the evolutions of kernel average misorientation (KAM), geometrically necessary dislocations (GND) and elastic strains. Results indicate that the enhancement was attributed to the dynamic strain aging (DSA) effect above the upper temperature limit, where serration and jerky flow disappeared but hindrance of dislocations persisted. Due to the DSA effect, periods of increase and decrease in the total dislocations were observed during HT fatigue tests, and the fraction of screw dislocations increased continuously, caused by viscous movement of the screw dislocations. Furthermore, the increased fraction of screw dislocations resulted in a lower energy configuration, reducing slip traces on sample surfaces and preventing fatigue-crack initiation.

Applicability analysis of induction bending process to P91 piping of PGSFR by high-temperature fatigue test

The application of the induction bending process in pipe fabrication is expanding across industries, significantly reducing leakage by minimizing welded sections in curved pipes. In this study, the applicability of the induction bending process to P91 bent pipes of PGSFR was analyzed, focusing on both material fatigue tests and structural fatigue tests for induction bent pipe at high temperatures. First, both high-cycle and low-cycle fatigue tests on specimens from the bent pipe were carried out at 550 °C to confirm that the fatigue properties meet the ASME Code's fatigue requirements. Second, material constants for a Chaboche combined hardening model were identified by using the material test results and an inelastic finite element analysis of the P91 bent pipe were performed to determine the fatigue test load for structural test effectively. Lastly, a high-temperature fatigue test on the bent pipe structure was performed to assess its structural integrity and post-test non-destructive examination confirmed that no fatigue cracks developed, and thereby affirming the applicability of the P91 bent pipe.

High-Temperature Fatigue Testing of Turbine Blades

Abstract This paper evaluates the efficacy of a patented grip for high-temperature fatigue testing by establishing the S-N curve for full-scale nickel-based turbine blades under simulated environmental conditions. Initially, a bending test assessed the stress-displacement characteristics of the component. This was followed by a series of fatigue tests at 950°C, using cyclic bending with force amplitudes from 5.2 kN to 6.6 kN and a constant frequency of 10 Hz. The setup, integrating the grip into a standard testing machine, proved effective for high-temperature tests and successfully determined the service life of full-scale components.

Stress redistribution and stress triaxiality effect on the fatigue life of notched Ni-based single crystal superalloy at 760℃

The multiaxial stress state caused by geometric discontinuities will result in significant differences in the fatigue performance compared to the uniaxial stress state. It is crucial importance to research the notch fatigue behaviors of nickel-based single crystal (Ni-SX) superalloys at high temperature. This study investigated the impact of stress redistribution and stress triaxiality on the fatigue life of notched Ni-based single crystal superalloy at 760℃. The high temperature fatigue tests were carried out on the specimens under different stress ratios and the stress amplitudes under the same life conditions were obtained. A novel anisotropic damage model, incorporating the influence of stress triaxiality is proposed coupled with crystal plasticity theory. Based on experimental findings, an evaluation of the test parameters was conducted.The results showed that Mises stress, maximum principal stress, normal stress of principal stress along the loading direction, maximum principal strain, and strain along the loading direction were inadequate in reflecting the significant variations of samples observed among different stress ratios. However, the stress triaxiality at the unloading valley exhibited a high sensitivity to changes in fatigue life. To evaluate the fatigue life of notched specimens under different stress ratios, an improved Basquin’s type life prediction model was proposed, utilizing the stress triaxiality at the unloading valley. The proposed model demonstrates a favorable normalization trend for the fatigue life of notched specimens subjected to various stress concentration factor (SCF) conditions.

STUDY ON SURFACE INTEGRITY AND ITS INFLUENCE ON HIGH TEMPERATURE FATIGUE BEHAVIOR WHEN TURNING POWDER METALLURGY NI-BASED SUPERALLOY

As a member of the latest generation of powder metallurgy Ni-based superalloy, FGH96 has already been widely applied in the manufacturing of aero-engines because of its distinguished mechanical performances. The surface integrity plays an essential role in the final fatigue life of the machined parts. However, surface modification induced by the machining process is inevitable, which profoundly affects the high temperature fatigue behavior of the manufactured part during the service lifetime. To cover the gap, the present work was carried out to specially study the surface integrity and its influence on high temperature fatigue behavior of powder metallurgy Ni-based superalloy when subjected to the mechanical turning process. The surface integrity, including surface morphology, micro-hardness, and depth-dependent residual stress, was initially characterized under different input parameters. High temperature fatigue tests were then conducted on the processed specimens to correlate the surface integrity and its fatigue behavior. Results showed that input parameters greatly influence the surface condition and thus the high temperature fatigue behavior. Poor integrity on machined surface leads to poor fatigue performance with typical fatigue fracture morphology, including crack initiation, propagation, and final fracture. While fine integrity on machined surface contributes to better fatigue performance. More interestingly, the fracture morphology in this situation shows similarity to the static tensile fracture, which presents shear failure along the direction of maximum shear stress.

Research on low‐cycle fatigue behavior of Ni‐based single crystal superalloy with shaped film cooling holes at high temperature

AbstractFilm cooling holes (FCHs) geometric structure significantly affects low‐cycle fatigue (LCF) of the single crystal turbine blade. This study investigates the LCF behavior of Ni‐based single crystal plate specimens with two different structure FCHs, including elliptical and fan‐shaped FCHs, by high temperature fatigue tests and crystal plasticity simulation. The results show that the LCF life of the fan‐shaped FCHs is shorter than those of the elliptical FCHs. The fracture characteristics and microstructure evolution of different structure FCHs are dissimilar. Oxidation affects the fatigue crack nucleation process at the FCH edge. The resolved shear stress (RSS) around fan‐shaped FCHs is greater than elliptical FCHs under one cyclic stress, and the area with larger RSS is consistent with fatigue crack nucleation position. The early LCF failure is mainly attributed to the high temperature oxidation of the local slip deformation induced by RSS concentration around the FCHs.

Research on multi-scale failure mechanism of gradient nanostructured 316L steel under strain-controlled fatigue at 650 °C

Gradient nanostructured materials have excellent fatigue resistance. At present, there are few research on the fatigue properties of gradient nanostructured (GNS) materials at high temperature. Herein, the multi-scale failure mechanism of GNS 316L steel at 650 °C is studied, and the high temperature fatigue tests under different strain amplitudes are conducted. Then, post-test microstructure observations were carried out to reveal the damage mechanisms, and the fatigue behavior was numerically simulated by crystal plasticity model. The experimental results show that the strength of the GNS 316L steel is still higher than that of the coarse-grained (CG) 316L steel at 650 °C, and the fatigue life of the GNS 316L steel is also higher than that of the CG one under strain fatigue loadings. The fatigue fracture results show that the fatigue crack presents a mixed propagation mode, and the crack source of GNS 316L transfers from surface to subsurface under low strain amplitude. The numerical results directly reflect the influence of the GNS surface layer on the plastic slip behavior. The fatigue failure mechanism of GNS 316L steel under strain-controlled cyclic loading at 650 °C was explained from a multi-scale perspective. This study provides data and method support for the application of GNS 316L steel at high temperature.

Creep-Fatigue Life Property of P91 Welded Piping Subjected to Bending and Torsional Moments at High Temperature

Abstract In recent years, the role of thermal power plants has shifted from providing a baseload to providing supplemental supply to compensate for fluctuations in the output of renewable energy sources. Thus, the operation of these plants involves frequent startup and shutdown cycles, which lead to extensive damage caused by creep and fatigue interactions. In addition, the piping utilized in thermal plants is subjected to a combined stress state composed of bending and torsional moments. In this study, a high-temperature fatigue testing machine capable of generating such a bending-torsional loading was developed. Creep-fatigue tests were conducted on P91 steel piping with weldment. The results clarified that the creep-fatigue life was reduced by the superposition of the torsional and bending moments and that it was further reduced by a holding load. It was shown that the creep-fatigue life of piping with weldment can be estimated accurately using the equivalent bending moment, which is composed of the torsional and bending moments. It was also confirmed that crack occurred in the heat-affected zone (HAZ) of the welded part, which has been often observed in actual thermal power equipment. From the finite element analysis, it was identified that cracking was initiated in the HAZ due to the accumulation of creep strain and increase in the hydrostatic pressure component during a holding load.

In-situ SEM study on fatigue crack behavior of a nickel-based single crystal alloy at 950 °C and 1050 °C

As the blade material of engine, nickel-base single crystal superalloy has been subjected to complex service environment such as high temperature and alternating load for a long time. In this work, the mechanism of fatigue microcrack initiation and propagation in a nickel-based single crystal at 950 °C and 1050 °C was studied by using the in-situ scanning electron microscope high temperature fatigue test system. At 950 °C, the alloy deformation was controlled by slip and the crack shear γ/γ’ phase cracking, showing Stage I mode. At 1050 °C, the alloy crack tearing occurred at the γ phase parallel to the loading direction and γ/γ' interface perpendicular to the loading direction and presents a Stage II mode perpendicular to the loading axis, then changes into Stage I in the accelerating stage. The deformation mechanism of fatigue cracking at two temperatures was compared. Besides, the influence of the microstructure defects on the fatigue crack behavior was discussed. The results show that the propagation of the microcrack below 100 μm shows evident fluctuations, which is closely related to the microstructure and the development stage of the fatigue crack.

Correction Method for the Bending Characteristics of Aero Ball Joints under High Temperature and High Pressure.

The aero ball joint is pivotal in aircraft duct systems due to its favorable properties, including displacement compensation and flexibility. In the stress assessment of air ducts, ball joints are usually simplified by using “Joints” connections to reduce the convergence problems caused by non-linearity, which requires a high degree of accuracy in the characteristic parameters of the ball joint. Accordingly, this paper builds a high temperature and pressure fatigue test platform to investigate the bending characteristics of the ball joint at different temperatures and pressures and points out the limitations of the current method. Then, a method combining finite element analysis (FEA) and the BP neural network is proposed to obtain the characteristic parameters of the ball joint. The results showed that the bending process of the ball joint tended to have two typically different stiffness properties, which were high rigidity and low rigidity. The bending characteristics were strongly influenced by pressure, but less influenced by temperature. The existing test platform increased the force reaction at the contact areas of the ball joint, resulting in errors in the measurement of characteristic parameters. The BP neural network prediction method could effectively alter the ball joint properties and reduce errors.

A Comparison of Two Methods Modeling High-Temperature Fatigue Crack Initiation in Ferrite–Pearlite Steel

High-temperature fatigue tests are carried out to investigate the fatigue crack initiation behavior of ferrite–pearlite steel. Two approaches to modeling fatigue crack initiation under high temperatures in ferrite–pearlite steel are developed and compared in this study. In the first approach, basal energy is introduced as the criterion for crack initiation. Various initial basal energy values are assigned to each potential location. The increments in basal energy after each cycle can be calculated. In the second approach, a crystal plasticity model is developed, combined with the cyclic hardening/softening effect and three fatigue indicator parameters (two of them are adopted from previous works and one is developed in this work). Results indicate that both approaches are capable of predicting the fatigue crack initiation location and the corresponding life. The first approach requires a lower computational cost but has many limitations, and the second approach is suitable for calculating mesoscale stress and strain distributions but with a higher computational cost.

Fatigue behavior and damage evolution of SiC/SiC composites under high-temperature anaerobic cyclic loading

In the present study, the fatigue behavior and damage evolution of SiC/SiC minicomposites at elevated temperatures in oxygen-free environment are investigated which are important for their application and are still unclear. The high-temperature fatigue test platform is developed and the fatigue stress-life curve and the stress-strain response are obtained. The test result shows that the life of the material at elevated temperature is shorter than that at room temperature under the same stress level. Moreover, the hysteresis loop width and the residual strain increase with the increasing of the cycles while the hysteresis modulus decreases during the fatigue cycling. The evolution process of matrix cracks is observed using the real-time remote detection system. It is found that matrix cracking is insensitive to the cyclic loading which is similar to room temperature and is due to that the degeneration of the interfacial shear stress reduces the area of high stress in matrix. The fiber/matrix interfacial shear stress under different cycles is determined based on the fatigue modulus of each hysteresis loop. The result shows a fatigue enhancement phenomenon for the interface which is not observed at room temperature.

Effect of shot peening and vibration finishing on the fatigue behavior of TC17 titanium alloy at room and high temperature

The effect of shot peening (SP) and vibration finishing (VF) on the fatigue performance of TC17 alloy at room-temperature was investigated. Meanwhile, to match the high-temperature service environment of the blade, the thermal exposure test and high-temperature fatigue test were also performed. The improvement in fatigue performance at room-temperature by SP + VF process was more than that by SP alone. After thermal exposure at 300 °C for 100 h, the high-temperature (300 °C) fatigue strength of the composite-treated sample remained 18.2% higher than that of the substrate. This is mainly because that SP + VF can induce lower surface roughness and remaining relatively stable gradient microstructure and compressive stress residual layer compared with SP.

Study of monotonic and cyclic mechanical behavior and microstructural evolution at a high temperature of ASTM A297 HP steel modified with niobium

The present work studies the effects of niobium on the mechanical properties and the elastoplastic/viscoelastic behavior at 927 °C of ASTM A297 grade, HP cast stainless steel in the as-cast material, to simulate the actual work condition. Tensile tests were carried out at different temperatures, differential scanning calorimetry (DSC), relaxation tests, the fatigue of continuous cycles under the triangular waveform, and fatigue with a trapezoidal waveform of the type “Dwell” with dwell times of 120 and 300 s as specified in the E2714-13 standard and hot torsion tests, to analyze the variations in the mechanical properties. The results of the mechanical tests and studies of the microstructural evolution, as well as of its later analyzes, provided data, not only in the form of data sheet's for the application of the values obtained in projects but concurrently after comparing property and microstructure, to deepen the phenomenology of mechanical stress behavior versus monotonic and cyclic deformation, high-temperature deformation mechanisms, dynamic recovery, of the type of microstructure resulting after the tests and also of the fracture mechanisms associated with the types of mechanical stresses imposed on the material, obtained by through the mechanical tests mentioned above. The high-temperature fatigue tests showed expected stress versus strain behavior, where cycles without dwell times showed lower hysteresis area values, from where the energy consumed in a strain cycle is calculated. The increase in the consumed energy values is due to the insertion of the dwell times, but it is important to note that for the tests that contain dwell times of 120 s and 300 s they did not show significant differences between the consumed energy, showing a saturation effect of tension that does not promote more significant inelastic strains. The cyclic mechanical fatigue stresses with dwell times and the low deformation rate led to intense precipitation of secondary carbides Cr23C6 chromium alloys added to the intrinsic properties of stainless steel dislocation slip were factors that resulted in the dynamic recovery phenomenon. Fractographic analyzes performed on monotonically fractured specimens and specimens after fatigue tests revealed that the material is prone to fracture in intergranular regions.

A Novel Method for High Temperature Fatigue Testing of Nickel Superalloy Turbine Blades with Additional NDT Diagnostics.

In this paper, a novel method for high temperature fatigue strength assessment of nickel superalloy turbine blades after operation at different times (303 and 473 h) was presented. The studies included destructive testing (fatigue testing at temperature 950 °C under cyclic bending load), non-destructive testing (Fluorescent Penetrant Inspection and Eddy Current method), and finite element modelling. High temperature fatigue tests were performed within load range from 5200 to 6600 N using a special self-designed blade grip attached to the conventional testing machine. The experimental results were compared with the finite element model generated from the ANSYS software. It was found that failure of turbine blades occurred in the area with the highest stress concertation, which was accurately predicted by the finite element (FE) model.

Influence of laser peening on the high-temperature fatigue life and fracture of Inconel 718 nickel-based alloy

High-temperature fatigue test was performed to study the effects of laser peening (LP) on the fatigue behavior of Inconel 718 nickel-based alloy at high temperature. Compressive residual stress (CRS) induced by LP was investigated at both room and high temperature. Microstructural evolution was also studied before and after LP process. The macro- and micro-fracture morphologies in fatigue crack initiation (FCI) region, fatigue crack growth (FCG) region and finial rupture (FR) region of LPed samples with different laser power densities at different temperatures were systematically analyzed. The results showed that LP process can enhance the fatigue life of IN718 nickel-based alloy at both room and high temperature. Excessively high temperature (e.g. 800 °C) may result in a sudden decrease in the fatigue life of LPed samples due to the weakening of grain boundaries and oxidation damage. The losing of coherent strengthen between γ″ phase and matrix was another reason for the rapid reducing of fatigue life. Compared to the untreated samples, the crack source transferred from the surface to deeper layer indicated that LP can retard the crack initiation on the metal surface, which is conducive to enhance the final fatigue life. Some direct evidence, such as the decreased distance between the parallel fatigue striations, the tortuous crack growth path in the LPed specimen confirmed the retarding effects of LP on the FCG. The mechanism of microcracking and the dynamic plastic behavior of LPed specimens during high-temperature fatigue test of IN718 nickel-based alloy were also discussed.

Gradation Design and Performance Evaluation of High Viscosity Asphalt Mixtures

The property of asphalt binder plays an important role in the performance of asphalt mixture. The natural rubber additive (NRA) was recommended as a modifier in the preparation of high viscosity asphalt (HVA) binder. Then, the gradation named LASTIKA‐13 was adopted to design the high viscosity asphalt mixture (HVAM) according to the orthogonal test analysis and the optimized gradation. The performance of the proposed HVAM was evaluated through a series tests including the permanent deformation at high temperature, moisture stability, low‐temperature cracks, and bending fatigue test. The results indicate that the proposed modified asphalt binder shows a comparable property with the conventional modified asphalt binder. The proposed HVAM with the NRA modifier has a superior performance to other conventional mixtures in the temperature susceptibility and resistance to moisture damage. Furthermore, the proposed HVAM was less sensitive to the temperature change and has good performance in terms of resistance to low‐temperature cracks, bonding characteristics, and fatigue behaviors.

High-temperature fatigue life prediction method for rubber bushing of new-energy vehicles based on modified fatigue damage theory

Rubber bushing are important components in suspension of new-energy vehicles. It is of great practical engineering significance to conduct in-depth study of high temperature fatigue characteristics. On the high temperature fatigue test bench, the same batch of dumbbell rubber test pieces with the same material as rubber bushing and the same vulcanization process were subjected to fatigue test at different temperatures, then the arithmetic mean value of the fatigue life of each rubber test piece was taken as the fatigue life at given temperature. It was found that the fatigue life of the rubber test piece at each temperature can be well fitted when the peak of engineering strain is taken as the damage parameter. Moreover, on the basis of the fatigue damage theory, the Arrhenius high temperature aging factor was introduced to correct the fatigue damage formula considering the influence of temperature on fatigue life. The finite element analysis was used to determine the peak of engineering strain of the dangerous point of the rubber bushing at 70 °C, and the modified fatigue damage formula was employed. The prediction value of the rubber bushing fatigue life was calculated and compared with the bench test result. It is shown that the predicted fatigue life dispersion coefficient is less than 2, indicating the effectiveness of the proposed high temperature fatigue life prediction method. The research in this paper can provide reference for the durability design and fatigue life prediction of rubber parts in new-energy vehicles.

Effects of Different Melting Technologies on the Purity of Superalloy GH4738.

The choice of melting technique is crucial for controlling the purity of a superalloy, which is especially important because purity has come to limit progress in the superalloy field. In this study, double- and triple-melting techniques were used to refine the GH4738 superalloy. Elemental analyses, inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction analysis, scanning electron microscopy with energy-dispersive spectroscopy, high-temperature cupping machine, high-temperature fatigue testing machine, and Image-Pro Plus software were used to analyze and compare the contents of specific elements, the types and sizes of inclusions, the mechanical properties, and the probabilities of white spot formation using the two melting techniques. The effects of the different melting processes on the purity of the superalloy were systematically studied. In terms of controlling the presence of impurities, the triple-melting process resulted in lower levels of harmful N, S, and O impurities in the superalloy, the triple-melted superalloy also contained fewer types of inclusion of smaller sizes and in smaller amounts than the double-melted alloy. Triple melting also promotes tensile strength and fatigue life, and minimizes the probability of forming defects in the superalloy.

Effect of aging on high temperature fatigue properties of Ni-23Cr-7W alloy for boiler pipes and tubes

This study focuses on the effect of aging on fatigue behavior of nickel-based Ni-23Cr-7W alloy for boiler pipes and tubes to reveal the fatigue properties of the alloy after long-term exposure (1000–5000 h) to high temperature (700 °C). Tensile, Charpy impact, and high temperature fatigue tests were conducted for both aged and non-aged alloys. An increase in the tensile strength and a decrease in the tensile ductility and Charpy absorbed energy caused by the aging were observed. The fatigue life of the aged alloy is slightly shorter than that of the non-aged alloy under PP (fast-fast) type of strain waveform. Under CP (slow-fast) type of strain waveform, the fatigue life of the aged alloy is slightly shorter in the high strain range region and equal to or slightly longer in the low strain range region as compared with the non-aged alloy. The non-aged alloy shows strong cyclic hardening behavior. Therefore, the stress–strain response of the non-aged alloy after fatigue loading locates in the higher stress side than that of the aged alloy, although the stress–strain curve of the non-aged alloy obtained by the tensile test locates in the lower stress side. The observations of the morphology of precipitates and dislocations for the alloy before and after the fatigue test reveal that fine carbides precipitate uniformly in the non-aged alloy during the fatigue test. Furthermore, it is revealed that the cyclic hardening caused by aging has a significant influence on the generation of inelastic strain in the non-aged alloy under low strain range conditions. Although a large influence of aging at high temperature occurs on the cyclic hardening behavior, aging does not significantly affect the fatigue life of Ni-23Cr-7W alloy.