Tensile Dwell Research Articles

Increase in residual lifetime due to low amplitude cycles and dwell times at room temperature: Observations and suspected mechanisms

With the onset of VHCF research, material behavior inconsistent with conventional versions of the Miner rule started occurring repeatedly: instead of decreasing the residual lifetime of a specimen, load cycles just below the so-called endurance limit increase lifetime compared to CA tests even if only the cycles of the highest load step are counted; the low load cycles contribute a negative amount of damage in damage accumulation computations. We give an overview over related observations on carbon steel, martensitic precipitation–hardening steel and aluminum and discuss conditions of occurrence and possible mechanisms. We show experimentally that tensile dwell stresses enhances static strain aging compared to dwelling at zero stress.

Influence of Creep Damage on the Fatigue Life of P91 Steel.

The following paper presents the results of tests on samples made of P91 steel under the conditions of simultaneously occurring fatigue and creep at a temperature of 600 °C. The load program consisted of symmetrical fatigue cycles with tensile dwell times to introduce creep. Static load (creep) was carried out by stopping the alternating load at the maximum value of the alternating stress. The tests were carried out for two load dwell times, 5 s and 30 s. A comparative analysis of the test results of fatigue load with a dwell time on each cycle confirmed that creep accompanying the variable load causes a significant reduction in sample durability. It was shown in the paper that regarding the creep influence in the linear fatigue damage summation approach, it is possible to improve the compliance of the fatigue life predictions with the experimental results.

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.

Effects of dwell time on the isothermal and thermomechanical fatigue behavior of 316LN stainless steel

Isothermal fatigue (IF) and thermomechanical fatigue (TMF) tests with and without tensile dwell time at various strain amplitudes were conducted on 316LN stainless steel. The results showed that the inelastic relaxation strain rate is more appropriate to characterize creep damage than the amount of stress relaxation. The introduction of tensile strain hold imparted a more deleterious effect on the TMF than the IF performance based on a statistical analysis of the number of secondary cracks and their size distribution, which was primarily attributed to the more severe oxidation-assisted cracking at the interface between the oxide scale and substrate. Most importantly, the lowest fatigue life occurred in the TMF test with tensile dwell time at the low strain amplitude of 0.4% rather than in the creep-fatigue test at the isothermal maximum temperature, indicating that it was not always conservative to predict the life of engineering components based on the IF data.

Creep-Fatigue Behavior of a Newly Developed Ultra-Supercritical Steam Turbine Grade Nickel-Based Superalloy, HAYNES 282

Abstract Most engineering components used in gas/steam turbines are exposed to a range of complex loading conditions resulting from startup and shutdown procedures. These loading conditions involve superimposition of time-dependent creep on cyclic fatigue and can be simulated by properly designed high-temperature creep-fatigue tests. Creep-fatigue interaction is a function of duration and position of dwell in the loading waveform, and the material microstructure. The objective of this work is to investigate the creep-fatigue interaction response of a newly developed γ′-strengthened wrought nickel-based superalloy (HAYNES 282), which has a potential application in advanced ultra-supercritical steam turbines. Creep-fatigue tests are conducted at 760°C with strain dwell either at tensile peak or compressive peak or at both tensile and compressive peak positions for different dwell times of 100 and 1,000 s. The test results are analyzed with respect to evolutions of peak stress, stress amplitude, stress relaxation, hysteresis loop, inelastic strain energy density, and degree of softening. Degree of softening is found to increase with dwell position at tensile, compressive, and both peaks in that order. Tests with dwell at both tensile and compressive peak positions are found to be the most damaging, showing the least life. Between tensile dwell and compressive dwell tests, interestingly, those with compressive dwell show a significantly reduced life. Increasing dwell time aggravates the damaging effect manifold. The mechanism of fracture at the end of life is illustrated with fractographic characterization.

The effects of tensile and compressive dwells on creep-fatigue behavior and fracture mechanism in welded joint of P92 steel

The creep-fatigue interactions on the long-term service damage of P92 welded joint were investigated on the tensile and compressive holdings with a series of applied strains, respectively. The local mechanical properties including hardness, elastic modulus and creep resistance of welded joint were uncovered using nanoindentation. The features of fracture morphology and internal defects after CF tests were studied by scanning electron microscope. Based on the characteristic of microstructural evolution and the variation in local mechanical properties, holding type effects of creep-fatigue interaction on the local creep behavior and fracture mechanism of the welds were systematically investigated. • Longer CF life under tensile dwell in comparison to compressive dwell. • Different local creep behaviors were observed for the two dwell modes. • The hardness, elastic modulus and creep resistances near fracture edges were clearly reduced. • Two independent fracture mechanisms were suggested for tensile and compressive dwells.

Multiscale modeling of cruciform dwell tests with the uncertainty-quantified parametrically homogenized constitutive model

This paper presents a novel multiscale approach for analyzing multi-axial stress-strain evolution in Ti-7Al cruciform specimens under dwell loading, through the use of an uncertainty-quantified, parametrically homogenized constitutive model (UQ-PHCM). The thermodynamically-consistent UQ-PHCM is built from rigorous upscaling of crystal plasticity FE models (CPFEM) using machine learning and uncertainty quantification. They explicitly incorporate microstructural information in the form of representative aggregated microstructural parameters (RAMPs). Uncertainty quantification accounts for uncertainty in model reduction, data sparsity and microstructural descriptors. This paper integrates advanced multiscale, multi-axial experiments with computational modeling at multiple scales to establish the UQ-PHCM as an effective tool for bridging the gap between laboratory specimen-scale experimental observations and micro-scale stresses and strains using CPFEM. The CPFEM is calibrated and validated by experimental data from surface strain measurements using digital image correlation (DIC) and grain-by-grain lattice strain measurements with in-situ far field high energy diffraction microscopy (ff-HEDM). A computational method is developed in CPFEM, to incorporate initial residual stresses consistent with measured lattice strains. The UQ-PHCM is validated with biaxial tensile dwell test results performed on the cruciform specimen with satisfactory prediction of gauge strain evolution in DIC measurements. Uncertainty in the strain field due to microstructural variability is also corroborated by the DIC measurements.

Damage‐based low‐cycle fatigue lifetime prediction of nickel‐based single‐crystal superalloy considering anisotropy and dwell types

AbstractBased on the physical phenomenon that the fatigue cracks initiate along specific slip plane, a slip plane damage‐based low‐cycle fatigue (LCF) lifetime model for the nickel‐based single‐crystal superalloy is established. The predicted results indicate that the lifetime model can reflect the orientation effect. In addition, in order to characterize the dwell‐time dependence of the LCF lifetime, creep damage and compression‐creep damage are introduced to the lifetime model. Finally, the lifetime predictions under LCF loading with tensile dwell time, compressive dwell time and tensile‐compressive dwell time are conducted by employing the lifetime model, respectively. The predicted lifetimes show a good agreement with the experimental data, which verifies the accuracy of the developed lifetime model in this paper.

Low cycle fatigue and dwell-fatigue of diffusion coated superalloy Inconel 713LC at 800 °C

The charm of diffusion coatings is mainly in their protective function of the surface of gas turbine components, which are often subjected to fatigue-creep loading in extremely hostile environments. Their impact gains in importance with the increasing time that the components are exposed to high temperatures and aggressive environments. This motivates the authors to study the low cycle fatigue behaviour of Al-Cr coated nickel-based superalloy Inconel 713LC. Polycrystalline diffusion Al-Cr coating was produced by out-of-pack Cr-modified aluminising chemical vapour deposition technique on the surface of cylindrical fatigue specimens manufactured by investment casting. Continuous low cycle fatigue (CLCF) tests and dwell-fatigue (DF) tests where a 10-minute tensile dwell was included in each cycle were conducted in strain control mode with constant total strain amplitude at 800 °C. The fatigue behaviour was characterised by cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves. The Al-Cr coating provided excellent protection against surface oxidation of Inconel 713LC, especially during dwell-fatigue tests. The substrate as well as the Al-Cr coating were examined in as-coated conditions and after fatigue loading through scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Dislocation arrangement in the substrate material was investigated in a transmission electron microscope (TEM). The microstructural observations and the discussion on the degradation mechanisms of surface-treated superalloy have been provided for both regimes of cycling.

Fatigue–Creep Interaction of P92 Steel and Modified Constitutive Modelling for Simulation of the Responses

Fatigue–creep interaction (FCI) responses of P92 steel are investigated experimentally and numerically. A series of isothermal FCI experiments with tensile dwell time ranging from 60 to 600 s were conducted at two temperatures under strain-controlled trapezoidal waveform. The experimental responses demonstrate that the peak stress is influenced by temperature and dwell time. In other words, creep-mechanism-influenced stress relaxation during dwell time influences the peak stress and fatigue life (Nf). In addition, effects of strain range on peak stress and fatigue life under fatigue–creep loading are evaluated. Towards developing a simulation-based design methodology for high temperature components, first a conventional unified constitutive model is evaluated against the P92 steel experimental responses. Based on the simulation deficiency of the conventional model, a modified static recovery term incorporated in the kinematic hardening rule is proposed and satisfactory simulations of the P92 steel FCI responses are demonstrated. The experimental responses of P92 steel and strengths and deficiencies of the conventional and modified Chaboche models are elaborated identifying the important FCI phenomena and progress in constitutive model development for FCI response simulation.

Planar dislocation structure during creep-fatigue interactions of TP347H heat-resistant austenitic steel at 600 °C

The low-cycle fatigue (LCF) and creep-fatigue (CF) interaction tests of TP347H austenitic stainless steel are carried out in air under a fully-reversed strain amplitude of 1.0% at 600 °C. A 10 min tensile dwell is applied in CF interaction testing. The influences of dwell on dislocation structures and fracture behavior have been evaluated. It is found that CF interaction life is lower than LCF one. The fatigue fracture mode changes from transgranular crack under LCF to transgranular plus intergranular fracture with tensile dwell. A lot of dislocation planar slip bands (PSBs) were found under CF interaction condition. PSB interacts with PSB of another slip system, twins, grain boundary (GB), and triangular GB. Strongly PSB-GB interaction causes stress concentration at GB, creating cavities and crack initiation, causing intergranular fracture. The stress concentration within the PSBs will also cause cracks, resulting in transgranular fracture morphology with fatigue striation.

Thermal-mechanical fatigue behaviour and life prediction of P92 steel, including average temperature and dwell effects

This study was devoted to an investigation of the effects of the average temperature and dwell on the thermal-mechanical fatigue (TMF) behaviour of P92 steel at elevated temperatures in the range of 350–650°C. The results revealed that increased average temperature decreased the in-phase TMF (IPTMF), out-of-phase TMF (OPTMF), and isothermal fatigue (IF) life values, and its effect was more significant in the range of 400–450°C. The effect of a short dwell time on the IPTMF life was dependent on the form of dwell, because tensile dwell decreased the IPTMF life and symmetric dwell increased the IPTMF life. Both creep stress relaxation (CSR) and dynamic strain ageing (DSA) were detected under IPTMF cycling. The application of dwell enhanced the CSR phenomenon. In contrast, the DSA effect was restrained, which was beneficial for fatigue resistance. The appearance of crack tip blunting and crack branching because of symmetric dwell indicated the retardation of crack propagation under the combined effects of enhanced CSR and the disappearance of the DSA phenomenon. In all cases, the transformation of lath structures into substructures was the dominant deformation mechanism. In addition, the equiaxial subgrain grew with increases in the strain amplitude and dwell time. Finally, the application of the current life models to P92 steel fatigue life prediction under both IF and TMF cycling were evaluated. Because of the insufficiency of the current models, a modified Coffin–Manson model is proposed by incorporating the mean stress and temperature into power law forms.

Thermomechanical fatigue in 9-12Cr steels: Life prediction models and the effect of tensile dwell periods

This paper is concerned with the assessment of life prediction models for thermomechanical fatigue (TMF), with specific application to P91 steel. A program of TMF tests, including dwell periods, are performed to determine the role of thermomechanical loading on fatigue life. As expected, fatigue life under conventional TMF testing (no dwells) is governed by maximum applied stress and inelastic strain-range. However, with the introduction of dwell periods, at maximum tensile stress during TMF loading, in-phase loading becomes the life-limiting case. This is attributed here to increased microstructural degradation and oxidation, associated with the dwell at peak temperature. Analysis of commonly used TMF life prediction models shows that the effect of dwell periods currently cannot be predicted for in-phase loading. Thus, it is concluded that physically-motivated approaches are required to successfully predict fatigue life under more complex (service) thermomechanical loading histories.

Revealing the creep-fatigue deformation mechanism for a directionally-solidified Ni-based superalloy DZ445 at 900 °C

The total strain-controlled creep-fatigue behavior of a first generation directionally-solidified Ni-based superalloy DZ445 at 900 °C in air was reported in the previous investigation. The deformation mechanisms with different dwell times for this superalloy were further expounded from the reduction in area, surface cracks, internal voids, stability of γ′ strengthening phase, and dislocation characteristics in this investigation. The results demonstrated the reduction in area increased, the number of the surface cracks on the vertical section decreased and the number and area fraction of internal voids increased with the increase of tensile dwell time. The non-directional coarsening trend of the secondary γ′ precipitate increases with increasing the dwell time. These microstructure changes affirmed that the deformation of DZ445 goes through transformations from the typical fatigue mode into the mixture of creep and fatigue, and further into the compound of creep and ductile when the dwell time is initially applied and continuously increased.

Influence of dwell time on the creep–fatigue behavior of a directionally solidified Ni-based superalloy DZ445 at 850 °C

Total strain-controlled creep-fatigue tests were performed for a directionally solidified Ni-based superalloy at 850 °C in air. Dwell times of 0, 1, 2, 3, 5, and 8 min were introduced at the tensile peak for each cycle. The results demonstrated that the fatigue life firstly decreased and then slightly increased with increasing dwell time. The fatigue life showed a minimum at a dwell time of 3 min. The initial softening rate was faster than the subsequent softening rate when dwell times of 1 and 2 min were used compared to the case with no dwell time. A compressive mean stress was produced by stress relaxation in the tensile dwell period. The reduction in the fatigue life was caused by a mixture of fatigue and creep deformation behaviors. However, the initial softening rate increased and a small amount of subsequent hardening occurred when 5 and 8 min of dwell times were applied. The higher compressive mean stress and a slight increase in the fatigue life for these samples of 5 and 8 min dwell times were attributed to a mixture of creep and plastic deformation. The change in the fatigue life with increasing dwell time was explained on the basis of the mechanical response, deformation mechanism, and fracture mechanism of the DZ445 superalloy.

Effect of tensile dwell on high-temperature low-cycle fatigue and fracture behaviour of cast superalloy MAR-M247

Hot isostatically-pressed, nickel-based superalloy MAR-M247 was tested under fatigue and creep-fatigue conditions. Continuous cycling and cycling with 10min tensile dwells in each cycle were applied in symmetrical strain control cycle with constant total strain amplitude at 900°C in air. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were obtained. The introduction of tensile dwells resulted in lower fatigue life in total strain representation. Surface relief, specimen sections and fracture surface observations revealed crack initiation sites, crack density and crack paths, and help to clarify the effect of dwells on fatigue behaviour.

Experimental study of the cyclic mechanical behaviour of two Ni-based superalloys at evaluated temperature

The fatigue tests of two Ni-base superalloys, powder metallurgy (PM) FGH95 and GH4169, were performed at 650°C with four different dwells to study their cyclic mechanical behaviour, particularly the evaluation law of cyclic mean stress and cyclic hardening/softening. The cyclic mean stress of the two alloys decreases with the number of fatigue cycles under tensile dwell, while it increases with the number of fatigue cycles under compressive dwell. PM FGH95 is found to undergo cyclic hardening, and its stress range -logarithmic cycle curve can be described by a quadratic polynomial. GH4169 exhibits cyclic softening, and its stress range -logarithmic cycle curve in the stable softening phase can be described by a linear equation. Furthermore, the hardening amplitude factor (HAF), the hardening velocity factor (HVF), the softening amplitude factor (SAF) and the softening velocity factor (SVF) are defined, based on the features of cyclic stress range, to correlate the total strain range, which are used to determine a fatigue damage parameter that introduces the effect of loading history and consequently improve the accuracy of low cycle fatigue (LCF) life prediction.

Effect of Tensile Dwell on Low Cycle Fatigue of Cast Superalloy Inconel 792-5A at 800°C

Effect of tensile dwell on low cycle fatigue of cast Inconel 792-5A is studied in symmetrical strain cycling at 800°C. Cyclic hardening/softening curves, cyclic stress-strain curves (CSSC) and fatigue life curves were obtained in continuous cycling and in cycling with tensile dwells. Dwells have slight effect on hardening/softening curves at high strain amplitudes. CSSC in cycling with dwells is shifted to lower stress amplitudes. No significant effect of dwells on Basquin curves is observed. Density of slip markings in continuous cycling is significantly higher in comparison with cycling with dwells. Samples cycled with dwells are typical of high density of secondary cracks, although sporadic slip markings were also found.

The Effects of Dwell on the LCF Behavior of IN617

Abstract Much has been studied on the individual effects of creep and fatigue on alloy life. However, not much is known of the combined effects of these two mechanisms. Therefore, a study was put into place to determine the effects of dwell on fatigue life and deformation mechanisms, for IN617, a solid-solution strengthened Ni-base alloy used widely in the power generation and aerospace industries. Low cycle fatigue (LCF) tests were conducted from 649–982°C with either tensile or compressive dwell. Fracture surfaces of the test specimens as well as longitudinal and transverse sections were examined via scanning electron microscopy to determine the damage and failure mechanisms. Test results confirmed tensile dwell lives that were significantly lower than those seen in compressive dwell. The mechanics for the reduction in cyclic life for tensile dwell was attributed to creep damage accumulation at grain boundaries that led to widespread intergranular cracking and failure. Tensile dwell life reductions were largest in tests at moderate (649–760°C) temperatures. The failed specimens for this temperature range showed the most evidence of grain boundary cavitation and intergranular cracking. At higher test temperatures, the tensile dwell sensitivity for IN617 was significantly reduced or almost entirely eliminated at high temperatures (871–982°C). This was attributed to the lower stresses that developed at these temperatures for a given strain range. The LCF testing and subsequent analysis indicated that a substantial tensile stress during dwell time coupled with moderate temperatures, to allow for diffusion creep, lead to grain boundary damage that can reduce cyclic life.

Cyclic behavior modeling of a tempered martensitic hot work tool steel

In this paper, a non-unified elasto-viscoplastic behavior model based on internal state variables, is investigated in order to describe the thermo-mechanical stress–strain fatigue response of 55NiCrMoV7 tempered martensitic steels (AISI L6). This model considers a quadratic yield criterion to define the elasticity domain. It allows the determination of two inelastic strain mechanisms resulting from two stress states which can be related to the typical continuous cyclic softening of the tempered martensitic steels. This cyclic softening is reproduced through an isotropic component (drag stress). A memory effect is also introduced to take into account the influence of the plastic strain range on the amount of the cyclic softening. The kinematic component (back stress) of the model allows the description of complex load conditions to which tool steels are subjected. Strain recovery (Baushinger effect), time recovery terms (cyclic behavior including tensile dwell times) and ratcheting effects are considered. The numerical implementation is addressed and two integration methods (explicit and implicit) of the constitutive equations are presented. Moreover, the identification methodology of the model parameters from only two sets of experimental data is presented; the coefficients of the kinematic and isotropic parts are determined successively. The main difficulty consists in reaching a good description both of the cyclic behavior for different strain rates and the ratcheting effect. Last, a validation stage of the three dimensional model is investigated from low cycle fatigue tests performed on different notched specimens.