Compressive Dwell Research Articles

High-temperature low-cycle fatigue and fatigue–creep behaviour of Inconel 718 superalloy: Damage and deformation mechanisms

In this article, strain-controlled Low-Cycle Fatigue (LCF) and fatigue–creep tests were performed on Inconel 718 nickel-based superalloy at temperatures of 650 °C and 730 °C. LCF tests at elevated temperatures were performed with a mechanical strain rate of 1×10−3/s, while fatigue–creep tests involved either tensile or compressive strain dwell. Both the LCF and fatigue–creep tests revealed cyclic softening, with the mean stress evolving oppositely to the applied strain dwell in the fatigue–creep tests. Investigations into the damage mechanisms identified intergranular cracking as the predominant failure mode. Fatigue–creep loading with a compressive dwell resulted in multiple crack initiations from transgranular oxide intrusions, along with multiple creep cavities during loading at 730 °C. Deformation features such as persistent slip bands and deformation nanotwins were observed during cycling at 650 °C. In addition, fatigue–creep tests at 730 °C exhibited δ phase precipitation and a coarsening of strengthening precipitates, contributing to additional softening that increased over prolonged test durations. Finally, the observed lifetime during LCF tests decreased with increasing temperatures, and fatigue–creep loading was observed to be more damaging than LCF. On the other hand, fatigue–creep loading with a tensile strain dwell demonstrated a higher lifetime compared to LCF at 730 °C.

Creep-fatigue properties of austenitic cast iron D5S with tension and compression dwell: A dislocation density-based crystal plasticity study

To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800∘C, a physics-based crystal plasticity model that describes the complex rate- and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.

Creep-Fatigue Response, failure mode and deformation mechanism of HAYNES 282 Ni based superalloy: Effect of dwell position and time

Creep-fatigue (C-F) response of HAYNES-282 as a function of dwell position and time, along with corresponding deformation micro-mechanisms and failure modes are presented here. An inertia effect in inelastic strain beyond the total-strain reversal point is reported for the first time. Damage (DC-F) in the material is found to decrease in the order of both-dwell, compressive-dwell and tensile-dwell. DC-F, an effective stress relaxation per cycle and strain energy density are found to correlate well with life. A change in deformation mechanism from shearing to Orowan-looping explains the difference in damage as a function of dwell position. HAYNES 282 shows an inherent anisotropy in strain hardening behavior, being higher under compressive loading. We postulate that compressive dwell would be more damaging in such material, with dwell-time-softening nullifying the beneficial effect of strain hardening of the same segment. Failure occurs through a mixed-mode, with dominance of either transgranular or intergranular fracture, depending on dwell position and time.

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.

Studies on creep-fatigue interaction behavior of Grade 92 steel and its weld joints

Creep-fatigue interaction (CFI) behavior of 9Cr-1.8W-0.5Mo-VNb steel (Grade 92 steel) base metal and its weld joint has been investigated at 823 K using a constant strain rate of 3 × 10–3 s−1. Both the base metal and the weld joint exhibited continuous softening with an almost equal cyclic stress response. The fatigue life decreased with increasing hold time. The hold applied in the compressive peak strain proved more deleterious compared to the tensile hold, indicating a compressive dwell sensitivity. Besides, the weld joints showed a greater reduction in fatigue life compared to the base metal. Fracture surfaces of the tested specimens revealed the prevalence of secondary cracks and extensive oxidation in the material. Weld joint specimens failed in the base metal region in all the tests. Evidence of dynamic strain aging in the form of serrations in the stress-strain hysteresis loops was also noted.

Influence of tension and compression dwell on the creep-fatigue properties of the austenitic cast iron Ni-resist D5S

Abstract Creep-fatigue tests with either tension or compression dwell and reference low-cycle fatigue test without dwell were conducted on the austenitic cast iron D5S in atmospheric air at 800oC to investigate the influence of dwell time on lifetime and the corresponding failure mechanisms. The addition of tension or compression dwell reduces the lifetime, by up to 50% and 80% in the tested range, respectively. Compared with tension dwell, compression dwell is found to be more detrimental and could further reduce the lifetime by up to 60%. Tension and compression dwell are seen to cause a thicker and thinner gauge length, respectively. Tension dwell causes the formation of creep pinholes at the graphite/matrix interface and subgrain boundaries, leading to the formation of internal microcracks. Compression dwell results in forming large cavities at the graphite/matrix interface. The intermetallics in an untested sample are found to contain G phase, Ni31Si12, α-Fe, and M7C3. The intermetallics in an LCF-tested sample are found to contain G phase, Ni31Si12, M7C3, and χ phase. Cracks inside intermetallics are found to form at the interfaces between Ni31Si12/χ phase, M7C3/χ phase, and Ni31Si12/M7C3.

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.

On the role of crack tip creep deformation in hot compressive dwell fatigue crack growth acceleration in aluminum and nickel engine alloys

Ambient and Hot Compressive Dwell (HCD) fatigue experiments were carried out to understand the effects of HCD on the fatigue crack growth behavior of cast 319 Al and Inconel 718 engine alloys. HCD was found to reduce endurance and increase rates of crack growth at engine service temperatures, while far less crack growth acceleration was found at room temperature. A fracture mechanics-based model was developed, considering tensile residual stress contributions due to compressive creep in order to predict crack growth with HCD. Good agreement was found between predictions and experimental results.

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.

Processing Parameter Control of Lifetime-Limiting Failure Mechanisms in Al-Si Cast Alloys at Room and Elevated Temperatures

Aluminum-silicon cast alloys widely used throughout the transportation sector, A356, 319, and A390, have been studied with respect to chemical compositions and processing parameters that control the resultant mechanical behavior. First, the development of strengthening mechanisms is discussed in terms of the role that alloying elements, solidification behavior, and thermal treatments play. Based on this understanding, an extensive matrix of material conditions was developed and characterized in order to provide practical guidelines for alloy development. In order to provide an understanding of lifetime-limiting failure mechanisms, fatigue crack growth and hot compressive dwell behaviors were further investigated. Fatigue crack growth tests were conducted for all alloys at R = 0.1 and room temperature, and creep–fatigue of 319 was studied at R = − 1.5 and 250 °C. The role of processing parameters in controlling the mechanical properties is identified and discussed, and recommendations for optimized design are made.

Effect of dwell condition on fatigue behavior of a high-Nb TiAl alloy at 750 °C

The strain-controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of a newly developed Ti-45Al-8Nb-0.2W-0.2B-0.02Y (at.%) alloy were carried out at 750 °C in air. The hysteresis loop, cyclic stress response and life modeling as well as failure mechanism of the alloy were investigated in detail. It was revealed that the tensile and compressive mean stresses would generate when the dwell condition was introduced at minimum and maximum strain, respectively. In addition, the dwell condition, especially for the compressive dwell condition, would significantly decrease the fatigue life. The typical continuum damage accumulation(CDA) and modified CDA life models proposed in the present study were employed to predict both LCF and CFI life of the alloy, which showed that the modified CDA life model had a higher accuracy than the typical CDA life one. Moreover, only single crack initiation source was observed at 92% (i.e. 11/12) of LCF fracture while multiple crack initiation sources at 84% (i.e. 31/37) of CFI fracture. Apparently different from LCF specimen showing more transgranular appearance, CFI specimen shows more intergranular appearance.

A physics-based model for evaluating hot compressive dwell effects on fatigue crack growth in 319 cast aluminum alloys

Fatigue crack growth under hot compressive dwell (HCD) conditions, a case of creep–fatigue occurring under compressive loading, is an important failure mode in high temperature environments. Creep-induced tensile residual stresses gradually built up in the vicinity of the crack are considered to be a key factor contributing to crack growth under HCD conditions. To understand and quantify this effect, a simple physics-based model has been developed, in which the residual stress contributions associated with creep and plasticity are added to the elastic response of the material to predict crack growth under HCD conditions using Linear Elastic Fracture Mechanics (LEFM). Test of a cast 319 Al alloy has been conducted both for material characterization and under baseline and HCD conditions to evaluate the model. With HCD, the crack growth rate was increased on average by a factor of about 6, which is consistent with model predictions.

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.

Evaluation of Low Cycle Fatigue Damage in Grade 91 Steel Weld Joints for High Temperature Applications

Abstract Grade 91 steel is a heat treatable steel and hence its microstructure is very sensitive to temperature. The weld joint consisting of a heterogeneous microstructure exhibits a lower fatigue life than that of the base metal. This is due to the presence of soft zone in the heat affected zone (HAZ). The failure location in the weld joint is very sensitive to the test parameters such as temperature and strain amplitude and application of hold. At high temperatures and under hold application, strain localization occurs in the soft intercritical HAZ (ICHAZ) which results in the failure in the HAZ. Sub- surface creep cavity formation in the soft region and their linkage cause enhanced crack propagation and that translates into lower fatigue life of the weld joint at high temperatures. Occurrence of compression dwell sensitivity in the material is attributed to the presence of surface oxides.

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.

Creep–fatigue-oxidation interaction in Grade 91 steel weld joints for high temperature applications

Abstract The weld joint consisting of a heterogeneous microstructure exhibits a lower fatigue life than that of the base metal. This is due to the presence of soft zone in the heat affected zone (HAZ). At high temperatures and under hold application, strain localization occurs in the soft intercritical HAZ (ICHAZ). Sub-surface creep cavity formation in the soft region and their linkage causes enhanced crack propagation and that translates into lower fatigue life of the weld joint at high temperatures. Occurrence of compression dwell sensitivity in the material is attributed to the presence of surface oxides.

Advanced Ductility Exhaustion Methods for the Calculation of Creep Damage During Creep-Fatigue Cycling

Abstract The thermal mechanical cycling of high temperature components can result in creep-fatigue cycles in which the creep dwell has a wide variety of positions within the cycle. During in-phase cycling the creep dwell is placed at the maximum strain in the cycle. Out of phase cycling gives compressive dwells and phase cycling between 0° and 180° gives a cycle in which the dwell is placed before the maximum strain is reached, i.e., at an intermediate position within the cycle. Furthermore, components often experience cycles with a variety of different strain amplitudes, which can result in cycle sequencing effects. The effects of these different types of creep dwells have been investigated as part of the development programme for the R5 High Temperature Life Assessment Procedure. The materials included in the R5 programme were two low alloy ferritic steels; a cast 1CrMoV and a cast 1/2 CrMoV and three austenitic stainless steels; a type 316H steel, a cast type 304L, and a type 347 weld metal. The analysis of these tests has resulted in the proposal of a new method to calculate creep damage. This has been shown to give better predictions for the creep damage at failure in laboratory tests compared with both the current R5 ductility exhaustion approach and the time fraction approach, which is used in typical design codes such as ASME III and RCC-MR. In particular, the new method gives significantly improved predictions of creep damage at failure for creep-fatigue cycles with intermediate dwells and for cycles with low strain ranges, which are of particular relevance to the service cycles in real plant. This paper reviews the findings of the work on the new method, the effects of multiaxial states of stress and the effects of compressive dwells.

Effect of compressive dwell on low cycle fatigue behaviour of nickel based single crystal alloy at 1100°C

In this study, the low cycle fatigue behaviour of CMSX-2 oriented with the loading axis along [001] was investigated under zero compression (Rϵ = −∞) strain controlled loading at 1100°C. The experiments were performed with or without a 2 min dwell (hold) in compression at strain ranges of 0·7, 0·6 and 0·5%. Stress relaxation was observed to occur resulting in the development of tensile mean stress under dwell fatigue loading and zero mean stress under non-dwell fatigue loading conditions. Fatigue cracks were observed to originate from localised regions of oxide attack at the specimen surface. The surface fatigue cracks initially propagated perpendicular to the stress axis but transitioned to other crystallographic planes at larger crack sizes and after the coalescence of multiple fatigue cracks. Non-dwell fatigue loading promoted isotropic coarsening of the γ′ precipitates while compressive dwell fatigue loading resulted in γ′ rafting parallel to the loading direction. It was observed that CMSX-2 was compressive dwell sensitive and exhibited fatigue lives 3–6 times shorter than non-dwell tests under the same total strain ranges.Dans cette étude, on a étudié le comportement de fatigue oligocyclique de CMSX-2 orienté avec l’axe de chargement le long de [001] avec chargement contrôlé par la déformation sous compression zéro (Rϵ = −∞) à 1100°C. On a effectué les essais avec et sans un temps d’arrêt (pause) de deux minutes en compression à des gammes de déformation de 0·7%, 0·6% et 0·5%. On a observé qu’il se produisait une relaxation de la contrainte, résultant dans le développement d’une contrainte moyenne de traction sous le chargement de pause fatigue et d’une contrainte moyenne de zéro sous des conditions de chargement de fatigue sans pause. On a observé que les fissures de fatigue avaient pour origine des régions localisées d’attaque d’oxyde à la surface de l’échantillon. Initialement, les fissures de fatigue de la surface se propageaient perpendiculairement à l’axe de la contrainte, mais se déplaçaient vers d’autres plans cristallographiques à des tailles de fissures plus grandes et après la coalescence de multiples fissures de fatigue. Le chargement de fatigue sans pause favorisait le grossissement isotrope des précipités γ′ alors que le chargement en compression de pause fatigue résultait en flottage de γ′ parallèle à la direction de chargement. On a observé que CMSX-2 était sensible à la pause en compression et exhibait une endurance de 3 à 6 fois plus courte que les essais sans pause sous les mêmes gammes de déformation totale.

Life Prediction Method of CC and DS Ni Base Superalloys Under High Temperature Biaxial Fatigue Loading

Polycrystalline conventional casting (CC) and directionally solidified (DS) Ni base superalloys are widely used as gas turbine blade materials. It was reported that the surface of a gas turbine blade is subjected to a biaxial tensile-compressive fatigue loading during a start-stop operation, based on finite element stress analysis results. It is necessary to establish the life prediction method of these superalloys under biaxial fatigue loading for reliable operations. In this study, the in-plane biaxial fatigue tests with different phases of x and y directional strain cycles were conducted on both CC and DS Ni base superalloys (IN738LC and GTD111DS) at high temperatures. The strain ratio ϕ was defined as the ratio between the x and y directional strains at 1/4 cycle and was varied from 1 to −1. In ϕ=1 and −1. The main cracks propagated in both the x and y directions in the CC superalloy. On the other hand, the main cracks of the DS superalloy propagated only in the x direction, indicating that the failure resistance in the solidified direction is weaker than that in the direction normal to the solidified direction. Although the biaxial fatigue life of the CC superalloy was correlated with the conventional Mises equivalent strain range, that of the DS superalloy depended on ϕ. The new biaxial fatigue life criterion, equivalent normal strain range for the DS superalloy was derived from the iso-fatigue life curve on a principal strain plane defined in this study. Fatigue life of the DS superalloy was correlated with the equivalent normal strain range. Fatigue life of the DS superalloy under equibiaxial fatigue loading was significantly reduced by introducing compressive strain hold dwell. Life prediction under equibiaxial fatigue loading with the compressive strain hold was successfully made by the nonlinear damage accumulation model. This suggests that the proposed method can be applied to life prediction of the gas turbine DS blades, which are subjected to biaxial fatigue loading during operation.

Low cycle fatigue and creep-fatigue interaction behavior of modified 9Cr-1Mo ferritic steel and its weld joint

The aim of the present paper is to study the low cycle fatigue and creep-fatigue interaction behavior of modified 9Cr-1Mo ferritic steel weld joint. Total axial strain controlled continuous cycling tests were conducted between 773 K and 873 K and at strain amplitudes ±0.25%, ±0.4%, ±0.6% and ±1%. Hold tests were also conducted at +0.6% and 823 K and 873 K temperatures to study the creep-fatigue interaction behavior of the weld joint. The alloy exhibited cyclic softening from first cycle onwards irrespective of the loading conditions. Failure location in the weld joint was correlated to the test parameters. Detailed replica study conducted on all the failed specimens revealed that most of the failures occurred in one side of the heat affected zone (HAZ) of the weld joint. Strain localization in the soft zone of the HAZ and subsurface creep cavity formation in this region and their linkage had caused enhanced crack propagation that translated into lower fatigue life of the weld joint at high temperatures. Type IV mode of failure was identified to be operative under tensile hold and high temperatures. The alloy was also found to be compressive dwell sensitive and it was ascertained that the lower life under compression hold compared to tension hold was due to the deleterious effect of oxidation.