Total Strain Range Research Articles

Orientation dependence of nickel-based single crystal superalloys in VHCF regime

Aimed at research the deformation orientation in VHCF regime under different temperatures, fatigue behavior of [111]-oriented DD6 single crystal superalloy at 1000 °C and 760 °C was studied. Using the stress range criterion, the [111] orientation shows higher fatigue resistance. This difference decreases with lower stress ranges and almost vanishes near 109 cycles. In contrast, the [111] orientation has lower fatigue resistance than the [001] orientation by using the total strain range criterion. Dislocations are restricted from shearing the γ’ phase due to the lower octahedral resolved shear stress in the [111]-oriented, decreasing the deformation in γ’ phase. Combined with distinctive cross-slip behavior, this further increased the deformation along the γ-channel, significantly enhancing deformation inhomogeneity between the γ’ phase and γ-channels. At elevated temperatures, thermally activated dislocation climb and higher climb stress factor lead to a greater reduction in fatigue performance for [111] orientation at lower stress level.

Distributed-collaborative surrogate modeling approach for creep-fatigue reliability assessment of turbine blades considering multi-source uncertainty

This paper proposes a substructure-based distributed-collaborative surrogate modeling approach for improving the accuracy and efficiency in the estimation of the creep-fatigue reliability of gas turbine blades with the integration of the Least Absolute Shrinkage and Selection Operator (LASSO), kriging, Distributed Collaborative (DC) strategy and substructure simulation method, called as the substructure-based DCLKM. Further, the creep-fatigue reliability assessment framework is set by combining the proposed substructure-based DCLKM with some theoretical models, which takes into account the uncertainties of structural sizes, applied loads and material properties and the nonlinearity of creep/fatigue damage interaction. Firstly, the total strain range, mean stress, absolute temperature and maximum stress at the critical location are regarded as the first-level output responses, and their required samples are predicted through the fitted surrogate models. Secondly, the creep life and Low-Cycle Fatigue (LCF) life are regarded as the second-level output responses and predicted. Finally, the creep-fatigue damage and reliability assessment are performed for several given applied cycles, respectively. The proposed substructure-based DCLKM is validated on a case study. The results show that the proposed substructure-based DCLKM is feasible with high accuracy and efficiency in the creep-fatigue reliability assessment of turbine blades.

Isometry of anteromedial reconstructions mimicking the deep medial collateral ligament depends on the femoral insertion.

This study aimed to investigate the length change patterns of the native deep medial collateral ligament (dMCL) and potential anteromedial reconstructions (AMs) that might be added to a reconstruction of the superficial MCL (sMCL)to better understand the control of anteromedial rotatory instability (AMRI). Insertion points of the dMCL and potential AM reconstructions were marked with pins (tibial) and eyelets (femoral) in 11 cadaveric knee specimens. Length changes between the pins and eyelets were then tested using threads in a validated kinematics rig with muscle loading of the quadriceps and iliotibial tract. Between 0° and 100° knee flexion, length change pattern of the anterior, middle and posterior part of the dMCL and simulated AM reconstructions were analysed using a rotary encoder. Isometry was tested using the total strain range (TSR). The tibiofemoral distance of the anterior dMCL part lengthened with flexion (+12.7% at 100°), whereas the posterior part slackened with flexion (-12.9% at 100°). The middle part behaved almost isometrically (maximum length: +2.8% at 100°). Depending on the femoral position within the sMCL footprint, AM reconstructions resulted in an increase in length as the knee flexed when a more centred position was used, irrespective of the tibial attachment position. Femoral positioning in the posterior aspect of the sMCL footprint exhibited <4% length change and was slightly less tight in flexion (min TSR = 3.6 ± 1.5%), irrespective of the tibial attachment position. The length change behaviour of potential AM reconstructions in a functionally intact knee is mainly influenced by the position of the femoral attachment, with different tibial attachments having a minimal effect on length change. Surgeons performing AM reconstructions to control AMRI would be advised to choose a femoral graft position in the posterior part of the native sMCL attachment to optimise graft length change behaviour. Given the high frequency of MCL injuries, sufficient restoration of AMRI is essential in isolated and combined ligamentous knee injuries. There is no level of evidence as thisstudy was an experimental laboratory study.

Cyclic strain rate dependent behaviour of Alloy 617

This paper investigates the effect of strain rate on the properties of Alloy 617 under low-cycle fatigue (LCF) in room temperature (RT). The LCF life properties and the damage mechanism are studied for this Alloy over a range of strain rate (5E-4 ∼ 1E-2 s−1) under fully-reversed controlled total strain range of 1.2%. It is found that LCF life of Alloy 617 is strongly influenced by the time-dependent mechanism, in terms of slow strain rate test (SSRT). The relationship between total strain, plastic strain, and time to failure with strain rates are established and expressed by means of power law function to describe the fatigue life. Metallography examination is carried out on the fractured specimens with the help of electron microscope and the fractography is discussed to distinguish the effect of SSRT on the physical damage characteristic under LCF loadings.

Investigation of the High-Temperature Low-Cycle fatigue failure characteristics of P91 steel weld joints and their fatigue strength reduction factors under various load control regimes

Low cycle fatigue tests are conducted on the base metal, weld metal, a welded joint under the condition that the strain control region is the joint (WJWJ) and a welded joint under the condition that the strain control region is the base metal (WJBM) of P91 steel at 500℃ using different total strain ranges. The cyclic stress response is highest for the weld metal, followed by the base metal, and is relatively similar for WJWJ and WJBM. At 2.0 % total strain range conditions, the materials generally exhibit non-Masing behavior and dynamic strain hardening aging. At all total strain ranges, WJWJ failed in the heat-affected zone, and WJBM broke in the base metal. For the difference in fatigue life between base metal and welded joints, the fatigue strength reduction factor of P91 steel welded joints at 500 °C is given as a suggested value. The fatigue behavior of P91 steel welded joints is verified based on a hybrid hardening model and a modified damage model. This study presents a theoretical framework for analyzing the fatigue strength reduction factors of weldments and comparatively analyzes the effects of different geometrical features.

Cyclic softening behavior of TC17 titanium alloy fabricated by laser directly energy deposition

In aero-engines, cyclic softening can cause premature fracture of components, which is regarded as a challenge for the security of titanium alloy parts in service. In this work, the cyclic deformation behavior of LDEDed and β-forging TC17 titanium alloys with different morphology of α phase was explored, cyclic softening mechanism was investigated by using transmission electron microscopy (TEM) observation of dislocation morphology. The experimental results demonstrate that cyclic softening occurs when the total strain range increases to 2.0 %. The LDEDed specimen has a smaller stress amplitude and a larger plastic strain during cyclic softening than wrought specimen. In the softened deformation microstructure of LDEDed specimen, the size of αp decreases with the increase of strain, but the size and shape of αp and αs in wrought specimen do not change much. The main dislocation forms that dominate the cyclic softening in the α phase and the β matrix are slip bands and dislocation tangles, respectively, and the slip band in the α phase cannot pass through the secondary phase in the β matrix for slip transfer. Under the same strain, the lower dislocation density in the LDEDed microstructure is due to the larger directional back stress produced by the forward loading, which causes the dislocation rearrangement and annihilation under the reverse loading.

Investigation on microstructure evolution of hybrid manufactured TC17 titanium alloy during cyclic deformation

In this work, laser direct energy deposition technology was used to produce hybrid manufactured TC17 titanium alloy with wrought substrate. The cyclic deformation behavior in the total strain range of 1.5%, 2.0%, 2.5%, and 3.0% at ambient temperature and dislocation morphology was investigated. The experimental results show that the three specimens represent cyclic softening when the total strain range is large than 1.5%. All the hybrid manufactured specimens are fractured in the deposition zone and the size of the deformated α phase decreases with increased strain. Slip transfer between α phase and β matrix and a small amount of dislocation tanglement in β matrix was observed in the specimen with strain of 1.5% and cyclic hardening. The dislocation density in β phase increased abruptly when the strain range is 2.5%, which promotes cyclic softening. The heterogeneous deformation between the high and low density dislocation zones formed in the α phase of the cyclic softening samples is the cause of the deformation and fracture of the α phase. The tension-compression asymmetry under forward and reverse loading revealed the Bauschinger effect in specimens with total strain exceeding 1.5%. Dislocation tanglement promoted cyclic softening by increasing the local back stress, at the same time, the back stress helped easier dislocation movement during reverse loading. Meanwhile, dislocation rearrangement and annihilation reduced the friction stress and further accelerate cyclic softening.

Fatigue crack initiation and growth behavior within varying notch geometries in the low‐cycle fatigue regime for FV566 turbine blade material

AbstractPlain bend bars made from FV566 martensitic stainless steel were extracted from the root of ex‐service power plant turbine blades and several industry‐relevant notch geometries were introduced. Some of the samples were shot peened. The notched bend bars were loaded plastically in the low‐cycle fatigue regime and finite element (FE) modeling carried out to investigate the effects of changing notch geometry, combined with shot peening, on fatigue behaviors such as crack initiation, short crack growth, and coalescence. Shot peening damaged the notch surface, accelerating initiation behaviors, but had a lifetime‐extending effect by retarding short crack growth in all tested notch geometries. At a total strain range higher than 1.2%, the lifetime extension benefit from shot peening was diminished due to compressive residual stress relaxation in the notch stress field. Notch geometry (and the associated varying constraint levels and stress/strain gradients) was found to have no notable difference on fatigue life when tested at identical notch‐root strain ranges.

Two-fold improvement of low cycle fatigue resistance of steel by bidirectional transformation-induced plasticity

A new deformation mechanism via bidirectional transformation between γ-austenite and ε-martensite [B-transformation-induced plasticity (TRIP)] provides a great possibility to develop fatigue-resistant steel due to its high reversibility in dislocation motion. The Fe-18Mn-11Cr-7.5Ni-4Si B-TRIP steel was designed by modifying the previously developed Fe-15Mn-11Cr-7.5Ni-4Si steel. Strain-controlled fatigue test was conducted at the total strain range of 1%, and newly developed steel demonstrated two-fold fatigue life (21,994 cycles in average) compared with the previous one. Microstructural analysis after fatigue fracture showed that the enhanced irreversible α’-martensitic transformation at the crack tip in the previous steel negatively affected fatigue durability by locally invaliding B-TRIP.

Crystal Plasticity Finite Element Modeling on High Temperature Low Cycle Fatigue of Ti2AlNb Alloy

Ti2AlNb alloy is a three-phase alloy, which consists of O phase, β phase and α2 phase. Because of the difference in the mechanical characteristics between phases, Ti2AlNb alloy often exhibits deformation heterogeneity. Based on EBSD images of the Ti2AlNb alloy, a crystal plasticity finite element model (CPFEM) was built to study the effects of O phase and β phase (dominant phases) on stress and strain distribution. Four types of fatigue experiments, and the Chaboche model with 1.2%~1.6% total strain range were conducted to verify the CPFEM. The simulation results showed that the phase boundary was the important position of stress concentration. The main reason for the stress concentration was the inconsistency deformation of grains which resulted from the different deformation abilities of the O and β phases.

Unified fatigue life modelling and uncertainty estimation of Ni-based superalloy family with a supervised machine learning approach

Machine learning (ML) approaches, especially the supervised learning methods, show enormous advantages in fatigue investigation, whereas few works follow the interest in the generalization and uncertainty estimation of these data-drive approaches. Within this content, a supervised ML method based on the support vector regression (SVR) for key features identification of fatigue life is presented for the Ni-based superalloy family. The unified SVR model is effective at predicting lives for the Ni-based superalloy family under wide loading conditions and fatigue regimes compared with the classical fatigue life models. In addition, a model fusion method is employed to estimate the uncertainty and data dependency of the SVR model, with which the training strategy produces an important effect on the accuracy and stability of the predicted results. It is found that the coefficient of the uncertainty achieves the optimum where the training percentage is 70% of the modelling samples. After input variable selection by the pairwise Pearson correlation and key feature reorganization, the dimensionality reduced SVR model remains an acceptable accuracy. Predictably, the total strain range and the test frequency are recognized as the highly correlated variables with the fatigue life investigating from the perspective of datasets. The trained ML model and uncertainty estimation approach provide potential tools for fatigue investigation under complex loading conditions. Going forwards, it would be beneficial to generalization ability and uncertainty estimation of a ML model for unified fatigue life modelling.

Vertical Graphene Canal Mesh for Strain Sensing with a Supereminent Resolution.

The development of microstrain sensors offers significant prospects in diverse applications, such as microrobots, intelligent human-computer interaction, health monitoring, and medical rehabilitation. Among strain sensor materials, vertical graphene (VG) has demonstrated considerable potential as a resistive material; however, VG-based strain sensors with high resolution are yet to be developed. In addition, the detection mechanism of VG has not been extensively investigated. Herein, we developed a VG canal mesh (VGCM) to fabricate a flexible strain sensor for ultralow strain sensing, achieving an accurate response to strains as low as 0.1‰ within a total strain range of 0%-4%. The detection of such low strains is due to the rigorous structural design and strain concentration effect of the three-dimensional micronano structure of the VGCM. Through experimental results and theoretical simulation, the evolution of microcracks in VG and the sensing mechanism of VG and VGCM are elaborated, and the unique advantages of VGCM are revealed. Finally, the VGCM-based strain sensors are proposed as portable breathing test equipment for rapid breathing detection.

Saturation effect of creep-fatigue cyclic-life for Nickel-based superalloy DZ445 under long-term tensile dwell periods at 900 °C

This investigation determined the creep-fatigue cyclic-life (Nf) saturation effect for the DZ445 superalloy with 16–128 min dwell time at 900 °C and total strain range of 1.6%. When the dwell time exceeds 8 min, the mechanical response including maximum stress response, hysteresis loop, plastic strain range, and stress relaxation amount at characteristic cycle are also saturated with dwell time gradually. The Nf saturation effect is also reflected in the creep- and ductile-dominated fracture modes. This critical phenomenon is closely related to the dynamic equilibrium reached between the superdislocations of a/2[101] in γˊ precipitation and the formation of the dislocation networks. While the rupture time is increased continuously with dwell time, which could be better used as an evaluation index when the cyclic life is saturated. The Nf saturation effect can not only reduce the cost of tests with long-dwell time, but also provide the guidance for creep-fatigue safety design.

Comparison of the creep-fatigue cyclic life saturation effect for three different superalloys

This investigation reports the results of the strain-controlled creep-fatigue behavior of precipitation-strengthened directionally-solidified Nickel-based superalloy DZ445. The results refer to the processes performed at 850 °C in the total strain range of 1.0% with 0, 3, 10, and 30 min tensile-dwell time. It was found that, compared to cyclic life that decreases rapidly after the initial application of dwell time, the reduced cyclic life slows down gradually when the dwell time exceed 30 min (i.e., the “generalized saturation effect”). As the publicly reported creep-fatigue results of the solid-solution-strengthened superalloy Haynes 230 and Alloy 617 under the same loading conditions, the cyclic life continues to decrease even 30 min dwell time is reached, which is hard to reach the case of Nf saturation. The reasons to reach the cyclic life (Nf) saturation are systematically elaborated from two aspects, namely, mechanical response (maximum stress, plastic strain, hysteresis loop, stress relaxation, etc.), and damage mechanism (cracks, voids, dislocation structures, etc.). This research provides theoretical guidelines for achieving the safety design of the superalloy components in the creep-fatigue deformation.

A modified energy model including mean stress and creep threshold stress effect for creep–fatigue life prediction

AbstractCreep–fatigue interaction plays an important role in the failure of high‐temperature components. However, most of the existing life prediction limits materials and testing conditions. The present study was conducted to modify the strain energy density exhaustion model systematically to predict the creep–fatigue life under a wide range of materials, hold times, and total strain ranges. Mean stress effect was considered by the creep–fatigue net tensile hysteretic energy to eliminate the overestimation of fatigue damage in the traditional method, while creep threshold stress was introduced to estimate the creep damage. In addition, a novel failure strain energy model was proposed to describe the correlation among failure strain energy, creep failure mechanisms, and temperatures. Based on the tension‐hold creep–fatigue data of Grade 91 steel, 304 SS, Alloy 617, and 1Cr1Mo0.25V steel, the modified model predicted reasonable lives within a ±1.5 scatter band.

Impact of Temperature on Low-Cycle Fatigue Characteristics of the HR6W Alloy.

This paper presents the results of tests conducted on the HR6W (23Cr-45Ni-6W-Nb-Ti-B) alloy under low-cycle fatigue at room temperature and at 650 °C. Fatigue tests were carried out at constant values of the total strain ranges. The alloy under low-cycle fatigue showed cyclic strengthening both at room temperature and at 650 °C. The degree of HR6W strengthening described by coefficient n’ was higher at higher temperatures. At the same time, its fatigue life Nf at room temperature was, depending on the range of total strain adopted in the tests, several times higher than observed at 650 °C.

850℃ 고온에서 Alloy 800H 모재와 용접재의 저사이클 피로 거동

Alloy 800H is one of the candidate materials for key structural components such as control rod systems, hot gas duct, core barrels, core supports and shut-down cooling systems for the very high temperature reactor (VHTR) system. The aim of this paper is to compare the low-cycle fatigue behaviors of the alloy 800H base metal and weldments at a high temperature of 850℃. Low cycle fatigue tests were carried out for four different total strain ranges of 0.6, 0.9, 1.2 and 1.5% under constant strain rate of 10-3/s. In all test conditions, the fatigue life of base metal and weldments decreased inversely proportional to the total strain range. The cyclic stress response behavior showed cyclic softening phenomenon for both materials. The low cycle fatigue life for base metal and weldments were predicted by the Coffin-Manson-Basquin law. The results showed that transition life of the weldments is shorter than that of the base metal.

Correlation between microstructure evolution and mechanical response in a moderately low stacking-fault-energy austenitic Fe–Mn–Si–Al alloy during low-cycle fatigue deformation

Through in-depth microstructural and mechanical characterization, a mechanism for low-cycle fatigue (LCF) deformation of an Fe-30.7Mn-4.3Si-1.8Al (in wt%) austenitic transformation-induced-plasticity (TRIP) steel at a total strain range of 2% was investigated. The LCF deformation of the steel was performed through two main deformation modes which were planar slip of Shockley partials, and ε-martensitic transformation and its reverse transformation. The varying significance of each deformation mode as well as the relevance between the two modes determined the microstructures evolved during fatigue deformation and resulted in a four-stage cyclic hardening behavior. With increasing fatigue cycles, the LCF deformation was dominated sequentially by the multiplication of partial dislocations, development of slip bands, reversible ε-martensitic transformation, and formation of more thick and crossed ε-martensite along with enhancement of planar dislocation slip, thus generating the characteristic deformation stages of primary cyclic hardening, cyclic softening, cyclic saturation, and secondary cyclic hardening correspondingly. In general, the superior LCF resistance of the steel was attributed to appreciably high plasticity reversibility during the entire fatigue deformation which was caused by reversible planar dislocation slip and ε-martensitic transformation.

Modeling of Total Strain Range-Low Cycle Fatigue For Austenitic Stainless Steel.(Dept.M)

Homologous temperature low cycle fatigue data on type 316 austenitic stainless steel have been collected and analyzed using fracture mechanics approach. Average fatigue curves between 10and 106 cycles have been produced over the temperature rang Room Temperature to 766°C (following homologous temperature range of 0.0 165,0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 44, 0,42 and 0.50). A modified ap proach for deriving predicted fatigue life curves has been developed in the p resent work. The develop ed approach successfully modeled the fatigue endurance in terms of total strain range and a family of temperature dependent fatigue curves has been derived. The data curves have been translated into design curves. Furthermore, the derived fatigue curves are then compared with the available fatigue curves.

Comportamiento a fatiga de baja frecuencia del compuesto Rene®80 recubierto de aluminuro de platino cerca y por encima del DBTT

La superaleación Rene®80 a base de Ni se utiliza para fabricar álabes de turbinas de gas en motores a reacción. La vida útil de algunas palas de turbina de motores a reacción está limitada por la fatiga de baja frecuencia y esta propiedad se ha visto fuertemente afectada por los revestimientos. La temperatura de transición de dúctil a frágil (DBTT) es el factor más importante que afecta las propiedades mecánicas de las aleaciones recubiertas. En este estudio, se evaluó el comportamiento de fatiga a alta temperatura y baja frecuencia del compuesto Rene®80 sin recubrimiento y recubierto con aluminuro de platino (Pt-Al) a temperaturas de 871 °C (cerca del DBTT) y 982 °C (por encima del DBTT). Los resultados de las pruebas de fatiga, en condiciones de deformación controlada a 871 °C para R = 0 y una tasa de deformación de 2×10-3 s-1, en un intervalo de deformación total de 0,8%, mostraron una disminución de la resistencia a la fatiga de las muestras recubiertas de aproximadamente un 14%, en comparación con las no recubiertas. Sin embargo, el aumento de la temperatura de prueba de 871 °C a 982 °C, condujo a un aumento en el comportamiento a fatiga de baja frecuencia del compuesto Rene®80 recubierto en aproximadamente un 10% en comparación con las muestras sin recubrir.