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Ultrasonic surface rolling process (USRP) is one of the effective mechanical surface enhancement techniques. During the USRP, unstable static force will easily do harm to the surface quality. In order to achieve a higher surface quality on the part with a curved surface, an active and passive compliant USRP system has been developed. The compliant USRP tool can produce the natural obedience deformation along the part surface. Force control based on the fuzzy Proportional-integral-derivative (PID) method is then designed to maintain the static force during the USRP. Experiments have been performed on a real aero-engine blade with curved surface. It is proved that the deigned active and passive compliant USRP system can significantly reduce the force variation from 42.2 N to 4.2 N, and achieve a uniform surface quality after processing.

The thermal stability and mechanical properties of a gradient-nanograined structure (GNS) CoCrNi medium entropy alloy (MEA) processed by ultrasonic surface rolling were studied by using isothermal/isochronal annealing tests combined with quasi-in-situ electron backscatter diffraction (EBSD) characterization and Vickers micro-hardness (HV) measurements. A layer by layer high-throughput investigation method was used to quantitatively study the grain growth kinetics and grain boundary evolution with different initial grain sizes, which could effectively save specimen and time costs. The grain nucleation and growth, as well as shrink and disappearance process through Σ3 coincidence site lattice boundary migration with slightly lattice rotation during annealing were directly revealed. The layer by layer grain growth kinetics and calculated activation energy indicate that the thermal stability of nano-grained top surface layer is relatively higher than that of nano-twined subsurface layer for the gradient CoCrNi MEA processed by ultrasonic surface rolling. Further analysis show that the grain boundary relaxation and dynamic recrystallization of the topmost nano-grains led to the decrease of grain boundary energy, thus improving their thermal stability. The present work provided theoretical basis for the application of CoCrNi MEA at high temperatures. Moreover, the high-throughput method on the investigation of grain stability by using gradient structure can be easily extended to other materials and it is of great significance for understanding the microstructural evolution of gradient materials.

Ultrasonic surface deep rolling (USDR), oxygen boost diffusion (OBD), and their combination (USDR-OBD) were all used to improve the surface hardening of pure titanium. The microstructure, microhardness, and fatigue life of pure titanium treated by USDR, OBD, and USDR-OBD methods were analyzed. USDR treatment induced a severe deformation area, while OBD treatment produced a brittle oxygen diffusion zone. The USDR-OBD treated samples approached the highest hardness in comparison with other treated samples. The fatigue lives of USDR treated samples were improved, which was due to the high compressive residual stress and refined grains. However, the fatigue lives of both OBD treated samples and USDR-OBD treated samples were decreased due to premature crack initiation and rapid propagation in the oxygen diffusion zone. Finally, the fatigue fracture mechanisms of different samples were proposed.

The microstructural evolution and mechanical properties of ultrafine-grained (UFG) CP-Ti after an innovative large-volume equal channel angular pressing (L-ECAP) and multi-directional forging (MDF) were systematically examined using monotonic tensile tests combined with transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) techniques. Substantially refined and homogeneous microstructures were achieved after L-ECAP (8-pass and 12-pass) and MDF (2-cycle and 3-cycle), respectively, where the grain size distribution conformed to lognormal distribution. The grain refinement of 450 °C L-ECAP is dominated by dynamic recrystallization (DRX) and dynamic recovery (DRV), while that of MDF is dominated by DRX. The iron impurities promote recrystallization by pinning-induced dislocation accumulation so that DRX is prone to occur at iron segregation regions during L-ECAP. The monotonic tensile results show that the strain hardening rate of CP-Ti increases with the decrease of grain size, while the continuous strain hardening ability decreases. The relationship between the average grain size and yield strength is in accordance with Hall-Petch relationship. Meanwhile, the individual strengthening mechanisms were quantitatively examined by the modified model. The results indicate that the strengthening contribution of dislocation accumulation to yield strength is greater than that of grain refinement.

Preferred surface integrity around the hole wall is one of the key parameters to ensure the optimized performance of hole components for nickel-based superalloy. The novel hole cold expansion technique introduced in this work involves the laser texturing process (LTP) followed by the Hertz contact rotary expansion process (HCREP), where the cylindrical sleeve is the critical component connecting the above-mentioned two processes. The purpose of LTP is to obtain the most optimized strengthened cylindrical sleeve surface, preparing for the following HCREP. Hereafter, the HCREP acts on the nickel-based hole components by the rotary extruding movements of the strengthened sleeve and conical mandrel tools. As compared to the as-received GH4169 material, the surface integrity characterization for the strengthened hole shows that a plastic deformation layer with finer grains, higher micro-hardness, deeper compressive residual stress (CRS) distribution and lower surface roughness is formed at the hole wall. In addition, transmission electron microscope (TEM) observations reveal the microstructure evolution mechanism in the strengthened hole. Grain refinement near the hole wall is regarded as the fundamental reason for improving the surface integrity, where the aggregated dislocations and recombined dislocation walls can be clearly observed.

The machinability of 7055 aluminum alloy with wide temperature range is examined, with focus on the three cutting forces, surface quality and work hardening of the material under low, medium, and high temperatures. The results demonstrate that, under low temperature, the work hardening depth of 7055 aluminum alloy is almost insensitive to the cutting speed, whereas at a higher cutting speed, the work hardening degree of the material first decreases and then increases; both the work hardening degree and hardening depth are significantly positively correlative to the cutting depth: the work hardening degree is positively correlative, though not so significantly, to the feed rate, while the work hardening depth is insensitive to the feed rate and remains at 100 μm in all cases. Under high temperature, the work hardening degree of 7055 aluminum alloy is positively correlative to the cutting speed; at depths smaller than 80 μm below the machined surface, the work hardening degree is negatively correlative to the cutting depth; at depths larger than 80 μm below the cutting surface, the work hardening degree of the material becomes significantly positively correlative to the cutting depth. A mathematical model of three cutting forces in dry cutting with wide temperature range is established based on wide temperature-range dynamic impact experimental results and the orthogonal cutting model, and modified using the LMSE (least mean square error) principle. The errors between the predicted and experimental three cutting forces, after modification, are all smaller than 10%, which is within the permissible limit of error. This verifies that the modified three cutting force prediction model can predict cutting forces accurately.

As crucial parts of an aeroengine, blades are vulnerable to damage from long-term operation in harsh environments. The ultrasonic surface rolling process (USRP) is a novel surface treatment technique that can highly improve the mechanical behavior of blades. During secondary machining, the nominal blade model cannot be used for secondary machining path generation due to the deviation between the actual and nominal blades. The clamping error of the blade also affects the precision of secondary machining. This study presents a two-sided USRP (TS-USRP) machining for aeroengine blades on the basis of on-machine noncontact measurement. First, a TS-USRP machining system for blade is developed. Second, a 3D scanning system is used to obtain the point cloud of the blade, and a series of point cloud processing steps is performed. A local point cloud automatic extraction algorithm is introduced to extract the point cloud of the strengthened region of the blade. Then, the tool path is designed on the basis of the extracted point cloud. Finally, an experiment is conducted on an actual blade, with results showing that the proposed method is effective and efficient.

A crystal plasticity model is developed to predict the cyclic plasticity during the low-cycle fatigue of GH4169 superalloy. Accumulated plastic slip and energy dissipation as fatigue indicator parameters (FIPs) are used to predict fatigue crack initiation and the fatigue life until failure. Results show that fatigue damage is most likely to initiate at triple points and grain boundaries where severe plastic slip and energy dissipation are present. The predicted fatigue life until failure is within the scatter band of factor 2 when compared with experimental data for the total strain amplitudes ranging from 0.8% to 2.4%. Microscopically, the adjacent grain arrangements and their interactions account for the stress concentration. In addition, different sets of grain orientations with the same total grain numbers of 150 were generated using the present model. Results show that different sets have significant influence on the distribution of stresses between each individual grain at the meso-scale, although little effect is found on the macroscopic length-scale.

Security and performance of aircraft engines are extremely affected by the blades’ surface quality. Ultrasonic surface rolling process (USRP) can improve material surface properties effectively and increase the alloys’ service life. However, applying USRP onto blades remains a challenge due to the free-form surface with thin-walled leading/trailing edges and the pre-load pressure on the blade during the USRP process. In this paper, a coordinated bilateral method of USRP on the blade is proposed. A machine tool is first designed and integrated with two USRP devices. A method of coordinated bilateral path generation is then proposed, so that the blade surface can be processed by the two USRP devices simultaneously. Finally, we perform experiments on a real aero-engine blade. The results demonstrate that by using the proposed method, USRP can be applied on the bilateral surfaces of the aero-engine blade coordinately.

GH4169 at 650 °C in atmosphere was investigated by using single edge notch tensile specimens. The number of main cracks and crack initiation mechanisms at the notch surface strongly depended on the grain size. The crack initiation life accounted for more percentages of the total fatigue life for the alloy with smaller grain size. The fatigue life generally increased with increasing crack initiation life. The small crack transited to long crack when its length reached ˜10 times the grain size.

The aim of this paper was to investigate the effect of thermal annealing on the microstructure, mechanical properties, and residual stress relaxation of deep rolled pure titanium. The microstructure and mechanical properties of the surface modified layer were analyzed by metallographic microscopy, transmission electron microscope and in-situ tensile testing. The results showed that the annealed near-surface layer with fine recrystallized grains had increased ductility but decreased strength after annealing below the recrystallization temperature, where the tensile strength was still higher than that of the substrate. After annealing at the recrystallization temperature, the recrystallized near-surface layer had smaller grain size, similar tensile strength, and higher proportional limit, comparable to those of the substrate. Moreover, the residual stress relaxation showed evidently different mechanisms at three different temperature regions: low temperature (T ≤ 0.2 Tm), medium temperature (T ≈ (0.2‒0.3) Tm), and high temperature (T ≥ 0.3 Tm). Furthermore, a prediction model was proposed in terms of modification of Zener-Wert-Avrami model, which showed promise in characterizing the residual stress relaxation in commercial pure Ti during deep rolling at elevated temperature.

In this study, the cyclic mechanical characters of 316L stainless steel at elevated temperature are extensively investigated by the experimental and cyclic constitutive models. The experiments include the monotonic tensile tests with different loading rates and the low cycle fatigue tests considering the effect of strain amplitudes, strain rates and loading sequences. The evolution of cyclic stress amplitudes, hysteresis loops and elastic modulus under various loading conditions are comprehensively analyzed. The experimental results show that the 316L steel at elevated temperature performs a typical three-stage cyclic mechanical response, i.e., initial hardening, subsequent saturation and final accelerated softening. The cyclic softening in both stiffness and flow stress is mainly caused by the nucleation of micro-voids or micro-cracks, and the subsequent coalesce and propagation. Furthermore, although the nearly rate-independent mechanical behavior is observed at monotonic tensile and first several fatigue cycles due to the DSA effect, the cyclic hardening/softening behavior shows a significant strain-rate and loading history dependence. Finally, inspired by the experimental observations and analyses, a damage-coupled cyclic elastic-viscoplastic constitutive model involving strain-range, strain-rate and loading history dependence is proposed to predict the complex cyclic behaviors of the material at elevated temperature. A hardening factor is incorporated into the Chaboche kinematic hardening equations to model the kinematic-induced hardening behavior. And the plastic strain memory surface and the maximum plastic strain rate are introduced to model the strain-range, strain-rate and loading history dependence of cyclic behavior. The proposed model is proved to effectively describe the complex evolution of not only cyclic stress amplitude but also hysteresis loops for the 316L steel at elevated temperature.

The low cycle fatigue (LCF) and creep-fatigue behaviors of Ni-based GH4169 superalloy are investigated by uniaxial strain-controlled fully-reversed testing at 650 °C. Compared with LCF tests, the effects of tensile and compressive strain hold times on creep-fatigue lifetimes are experimentally explored with varying total strain ranges in the present work. In order to elucidate the damage mechanisms under complex loading waveforms, an additional series of tests with both tensile and compressive hold times are carried out at a given total strain range of 2.0%. Posterior to the cyclic tests, main-crack-failure modes and secondary cracking modes are studied via optical microscopy (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) techniques. Main-crack failure mechanisms are examined by the fracture appearance observations. Cracking modes are explored through quantitative characterization on the distributions of secondary cracks in the longitudinal cross sections under different loading waveforms. Moreover, a generalized life model based on linear damage summation (LDS) framework and energy dissipation criterion (EDC) is elaborated to estimate the damage mechanisms of fatigue, creep and oxidation. The prediction results can well establish the correlations between the reductions of numbers of cycles to failure and the presences of different damage mechanisms under respective loading waveforms.

In this work, a dislocation-based crystal plasticity finite element (CPFE) framework was implemented to investigate the effects of loading conditions on dwell-fatigue crack initiation life. Experimentally, a large number of strain-controlled dwell-fatigue tests were carried out at 650 ℃ in a nickel-based superalloy. The combinations of CPFE simulations and post-test examinations were used to reveal the dwell-fatigue crack initiation mechanisms. Then, a life prediction approach was presented on the basis of accumulated energy dissipation at half-life cycle. Good agreement between the experimental and simulated lives verifies the robustness as well as the accuracy of the present approach. Finally, a new three-dimensional (3-D) damage tolerance diagram was proposed by introducing a CP-based physical parameter to describe the degradation levels and evaluate the residual dwell-fatigue life.

Telerobotic system is a typical human-machine system. The operation results not only depend on the performance of machine, but also the status of teleoperators (SoT). However, existing telerobotic systems scarcely consider the impact of teleoperators. This paper proposes a method for the online identification of the SoT and incorporates it to a shared control telerobotic system. First, some mental indicators are obtained based on Electroencephalogram during teleoperations. The relationship between the SoT and mental indicators is then established by a neural network. The online SoT identification is further implemented on a mobile telerobotic system. Second, a SoT based shared control framework is proposed in telerobotic system. The SoT is designed to dynamically adjust the control weight of the shared controller. Finally, comparative experiments are performed between a sensor based shared control method and the SoT based shared control method. The result validates the effectiveness of the proposed SoT based shared control method in telerobotic system.

The strengthening effect of chemical short-range order (SRO) structure in CoCrNi medium-entropy alloy (MEA) was investigated using molecular dynamics (MD) simulations of nanoindentation. The quantitative correlation between SRO parameters and mechanical properties was established. Results show that the strength and hardness of CoCrNi MEA increase with increasing chemical SRO parameters and reach a stable value with steady SRO structure. Compared with random solid solution (RSS) state model, the average hardness increases 8.1 % in an intermediate SRO model and 13.7 % in a stable SRO model. The dislocation nucleation force of SRO model is 55 % larger than RSS model. Moreover, dislocation pinning induced by local Ni SRO structure, as well as the promoted unique dislocation interaction were observed during the nanoindentation process. Finally, results also show that as the temperature rises, the enhancement of hardness becomes more significant ( 11.4 % at 70 K , 17.24 % at 300 K , and 23.8 % at 800 K ).

Fracture toughness of X80 steel is investigated and characterized by the nanoindentation technique and a modified constitutive model. Firstly the load-displacement of the X80 steel is tested by nanoindentation technique. Based on the load-displacement, the relationship of the nanoindentation load-depth is established. Secondly, the elastic modulus calculation equation was modified by introducing the radial displacement correction factor γ considering the pile-up phenomenon. At last, based on the strain energy density and indentation fracture energy theory, the nanoindentation fracture toughness constitutive model of X80 is proposed combined with the modified elastic modulus and the critical plastic depth. The fracture toughness calculated by the proposed method locates in the dispersion band range tested by the standard three point bending specimen, while the result calculated by the original method is far away from the dispersion band range. The comparison results illustrate that the proposed method can well characterize the fracture toughness of the X80 steel. The present work is of great significant for ensuring the long term and safety operation of the petroleum and natural gas transporting pipeline.

A series of strain-controlled creep-fatigue tests under different loading waveforms are carried out on GH4169 superalloy at 650 °C. In the macroscopic view, a unified viscoplastic constitutive framework is used to describe cyclic deformation process. Particularly, a modified kinematic hardening rule considering loading-dependent effect is developed to simulate the stress relaxation behavior during hold periods. Then, creep-fatigue assessment based on interaction diagram is quantitatively determined by the strain energy density exhaustion (SEDE) approach. In order to elucidate damage mechanisms under various loading conditions, the characterization of damage mechanism is observed from the post-test examination. Main-crack-failure modes from fracture appearance observations and cracking modes from longitudinal sections are studied via scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), respectively. Furthermore, image-based creep-fatigue diagram is proposed based on metallographic interpretation of mechanisms.

In this paper, a dual-scale modelling approach is developed to investigate creep-fatigue behavior and predict crack initiation life for holed structures under multi-axial stress state. The macro-scale simulation supplies local deformation histories to the dual-scale simulation as boundary conditions. In the dual-scale simulation process, the micro-mechanical behavior and damage evolution are described by using crystal plasticity. In order to validate the dual-scale simulation procedures, a series of creep-fatigue tests as well as the post-test characterizations were carried out for nickel-based Inconel 718 at 650 ℃. The detailed results of macro- and micro-scale simulations are presented in terms of stress–strain behavior, damage evolution and life prediction. Regarding the macro-scale simulations as the benchmark, it may provide an assistant support and precognition for the micro-scale damage calculation at higher cycles. The predicted cycle numbers to crack initiation are in agreement with the experimental ones. More advantages are manifested in the potential scientific and engineering significance for the dual-scale modelling approach.

Hole cold expansion is an effective anti-fatigue manufacturing technology, which is widely used in hole strengthening of aviation components. In this paper, a novel small-deep hole (with the diameter less than 2 mm and the depth greater than 10 mm) cold expansion process, called the multi-spherical bump rotating cold expansion process (MBR-CEP), was proposed and verified by experiments on the Inconel 718 superalloy. The MBR-CEP tool with multi-spherical bumps produced by the laser texturing process (LTP) was first designed, and the MBR-CEP with different expansion degrees (δ = 2.1%, 2.6%, and 3.1%) was conducted on small-deep hole specimens. The surface integrity of the small-deep holes before and after the MBR-CEP in the inlet, middle, and outlet areas was studied. Results showed that compared with the small-deep hole after reaming, the hole treated by the MBR-CEP with different δ formed a plastic deformation layer with a maximum depth of 32.73–85.54 μm, a minimum surface roughness of 0.115–0.138 μm in Ra, and a compressive residual stress (CRS) layer with a depth of 0.39–1.077 mm. In addition, the CRS increased by up to 84%, and the microhardness increased by up to 13%. The microstructure evolution on the top surface and sub-surface of the small-deep hole wall was observed by transmission electron microscopy (TEM), which showed that nanocrystals with the size of 48–90 nm and a large number of dislocations and deformation twins formed. Finally, the effectiveness of the proposed process was verified by fatigue tests with different δ at 400 ° C . The average fatigue life increased by 3.66 and 8.05 times under δ = 2.1% and 2.6%, respectively. The fatigue fracture surface was examined with scanning electron microscopy (SEM), which revealed that low surface roughness and existence of a plastic deformation layer could effectively reduce the probability of crack initiation.

This paper was concerned with an approach to predict fatigue life based on a multi-scale crack propagation model. The expressions of crack propagation rates in microstructurally small crack (MSC), physically small crack (PSC) and long crack (LC) stages were unified in the multi-scale crack propagation model. Its integral form presented a fatigue life prediction approach. The nickel-based GH4169 alloy was employed to validate the prediction capacity of present approach. The prediction results of lives of specimens with different initial defects sizes were compared with the experimental data.

Creep crack growth behaviors of the Inconel 625 superalloy at 650 °C are investigated through experimental and numerical methods. The simulated data agree well with the experimental results, reflecting that the multi-axial creep performance parameter α obtained by present paper can reasonably predict the creep crack growth behaviors of Inconel 625 superalloy. The crack initiation time takes up the most proportion of the whole life for all the load levels, and intergranular fracture is the dominated failure mechanism. Creep constraint effect is not obvious for the C∗ to characterize the creep crack growth of Inconel 625 superalloy.

In the present study, a computational-experimental framework is developed for fatigue reliability assessment of turbine bladed disks. Within the framework, the overspeed testing is innovatively combined with stochastic finite element (FE) analysis for quantifying uncertainties in the experimental data, material properties and loads. Meanwhile, two schemes are elaborated based on probabilistic S-N curves and stochastic FE simulation coupling with sampling technique. The stochastic FE simulation incorporates the Chaboche constitutive model with Fatemi–Socie criterion for fatigue behavior modeling and life prediction. Moreover, experimental deformation and numerical FE analysis are conducted with regard to the full-scale bladed disk test with increased step-stress overloading. Reliability sensitivity analysis is performed to provide an importance ranking of random variables for fatigue design of the bladed disk. Results indicate that stochastic FE analysis-based scheme provides more conservative predictions than the probabilistic S-N curves-based one.

In this study, the weld residual stresses (RS) in a 25 mm thick ferrite steel plate with newly developed low-temperature transformation (LTT) welding wire were investigated by finite element method and neutron diffraction (ND) measurement. A thermo-elastic–plastic finite element model coupled with solid-state phase transformation (SSPT) was developed to investigate the distribution and formation mechanism of RS, which has been verified by ND measurement. The results demonstrate that the developed LTT alloy can significantly reduce the RS and even generate compressive RS in the weld zone, due to the interrupted cooling shrinkage caused by the LTT. The higher inter-pass temperatures related to the microstructure evolution result in an increased region of compressive stress within the weldment. Moreover, the longitudinal RS in the weld zone gradually changes to tension as the initial temperature of martensitic transformation increases. Notably, the relaxation effect of transformation-induced plasticity on RS and its influence on model accuracy were discussed. Keywords: Weld residual stress, Low-temperature transformation, Transformation-induced plasticity

The fatigue crack initiation life and damage mechanisms of three heat-treated nickel-based alloy with different volume fractions of δ phase have been investigated based on experiment and crystal plasticity simulation. With the help of fatigue indicator parameters (FIPs), the predicted fatigue lives are agreed well with experimental results, where energy dissipation based FIP shows more accurate life prediction than plastic slip based FIP does. Maximum fatigue damage sites shift from grain boundary to δ phase boundary by increasing δ phase and applied strain level. Furthermore, the existence of δ phase leads to the increase of plastic slip accumulation and decrease of localized stress level.

Recently, there is the serious fracture for two high-temperature steam turbine rotors made of 30Cr1Mo1V steel in different power plants, characterized by brittle fracture. This steel has the disadvantage of low temperature brittleness. Currently, a large number of 30Cr1Mo1V rotors are in service. Meanwhile, these rotors are required to start up and warm up faster due to the demand for operation flexibility of the power system. In order to ensure the safety of the 30Cr1Mo1V rotors under flexible operation, it is necessary to study the low temperature brittleness of the rotors and its influence on the fatigue life in rapid start-up and warm-up processes. In this paper, the fracture causes of the 30Cr1Mo1V rotor were analyzed by the mechanical property tests, microstructure examination and fracture analysis. The test results showed that the toughness of rotor decreased significantly after service. Secondly, the temperature and stress changes of the high-temperature rotors during the rapid start-up and warm-up process were calculated by the finite element method. The simulation results showed that if the speed rose too fast, the thermal stress in the rotor inlet area would increase significantly and the temperature was lower than the fracture appearance transition temperature when reaching the full speed. The high stress will easily lead to a crack initiation under low-cycle fatigue. Then, crack instability propagation will soon occur due to the low temperature and poor toughness. Thirdly, the life of the turbine in the rapid start-up was optimized by controlling the startup time. In addition, the life under different warm-up conditions was quantitatively analyzed. It not only reveals the necessity of the warm-up process for the center hole rotor, but also successfully demonstrates that the warm-up process of the solid rotor can be cancelled, which improves the economy of the power plant.

Creep crack growth behavior of the 9Cr-1Mo steel under multi-axial stress state at 600 °C was investigated by a modified creep damage model. Firstly, a modified creep damage model was proposed and incorporated into the finite element software ABAQUS by the CREEP subroutine. And then the creep and damage behaviors of the 9Cr-1Mo steel were simulated by the notched bar and CT specimen. The results indicate that the multi-axial creep ductility and the life of the high temperature components calculated by the modified model are corresponding well with the experimental data, and the life prediction precision has been greatly improved compared to that from the K-R model. This demonstrates that the proposed model can be used to accurately predict the creep crack growth behaviors of the high temperature materials. The creep crack growth rate a presents a linear relationship with the crack driving force parameter C* in log-log coordinate system. Based on the established relationship between the a and C*, the creep crack growth rate of the 9Cr-1Mo steel components working at high temperatures can be predicted.

The aim of the present work is to propose a generalized strain energy density exhaustion model to predict the creep-fatigue lives. The oxidation damage, which is described by a correction factor representing the non-linear oxidation damage mechanism, is considered in the proposed model. All the 77 experimental data sets of GH4169 superalloy at 650 °C, Inconel 738 superalloy at 850 °C and P91 steel at 550 °C in both tension-hold-only and compression-hold-only tests are used to validate the prediction capabilities of the model. The fatigue, creep and oxidation damages per cycle are separately calculated. Then a three-dimensional damage interaction diagram combined with a suitable enveloping surface is proposed.

A series of strain-controlled creep-fatigue tests with various dwell times and strain ratios are carried out on a nickel-based superalloy at 650 °C. Aiming at the mechanical understanding of creep-fatigue behavior, a non-unified constitutive framework is used to describe macroscopic cyclic deformation process to understand creep-fatigue behavior. Afterward, the quantitative evaluation of creep-fatigue damage is quantitatively determined by the cycle-by-cycle strain energy density exhaustion (SEDE) approach. The effects of strain ratio and dwell time on creep-fatigue life endurances can be well revealed by the proposed life model, where all the data points are predicted within ± 1.5 error band. Particularly, life prediction capacities with long dwell times are further improved based on three-regime creep mechanism.

Understanding the evolution of microstructure and micro-region properties of weldments during cyclic loading is a challenge for the reliability assessment of high-temperature components. This work is devoted to quantitatively evaluating the variation of micro-region properties and corresponding responsible microstructural features of P92 steel weldments under interrupted fatigue tests and subsequent creep fracture. To achieve this target, high-resolution characterization techniques, including electron backscatter diffraction (EBSD) and nanoindentation tester were used. The results revealed that the microstructures in the inter-critical heat affected zone (ICHAZ) and adjacent regions, fine-grained HAZ (FGHAZ) and parent metal (PM), show significant sensitivity to fatigue loading. Notably, the grain size in the ICHAZ reaches saturation after fatigue cycles of 10% fatigue lifetime, and the weld metal (WM) keeps unchanged throughout the fatigue loading. After subsequent creep fracture, only the WM presents an increasing trend in the average grain size. Nanoindentation tests uncover that the reductions of the elastic modulus and microhardness in each region also present three cyclic softening behaviour during the fatigue process due to the evolution of martensite lath structure. However, due to nucleation and growth of cavities accelerated by carbide precipitation, relatively low elastic modulus and microhardness were observed after subsequent creep loading.

A series of continuous and interrupted cyclic tests in Inconel 718 with the dwell times ranging from 0 s to 10,800 s at 650 °C were carried out in order to study the multi-stage dwell fatigue crack growth (DFCG) behavior. Replica technique was used to measure the intermittent DFCG rates from short to long periods and to retrospect specific damage mechanisms based on microstructural characterizations. From the macroscopic viewpoint, the DFCG curves were classified into short, moderate and long dwell conditions according to the tendencies in DFCG rates. Correspondingly, the dwell fatigue life distribution was divided into three periods, which exhibit different sensitivities in the length of dwell time on dwell fatigue life. From the microscopic viewpoint, the high-scattered points reflected in the DFCG curves were elaborately discussed with the combinations of EBSD mapping and high-resolution TEM analysis. Σ3 twin boundaries (Σ3 TBs) were regarded as the barriers to resist the DFCG due to their special performances. In addition, with the increase of dwell times, the main crack morphologies changed from transgranular to intergranular mode. Simultaneously, local crack growth modes at high-scattered points changed from across to along the Σ3 TBs with extreme low DFCG rates. Also, high-angle crack deflection was another factor that was responsible for the decrease of the DFCG rates.

A large number of strain-controlled creep-fatigue tests under wide loading waveforms are conducted at 650 ℃ in nickel-based forged GH4169 superalloy. Comprehensive characterizations, including scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscope (TEM), are observed from the post-test examinations. Particular focus is brought to the physical understanding of damage mechanisms under wide creep-fatigue loading conditions using EBSD analysis. The representative misorientation parameters are calculated for constructing diffraction-based misorientation mapping. Semi-quantitative analysis of longitudinal EBSD observations is conducted to prove that strain ratio has little influence on creep-fatigue damage degrees, while dwell time causes noticeable changes to damage progressions. In particular for geometrically necessary dislocation (GND) map explored in this work, more fundamental information based on failure physics is obtained to analyze the creep-fatigue crack initiation mechanism.