Creep-fatigue Interaction Tests Research Articles

Low cycle fatigue and creep-fatigue interaction behavior of 2.25CrMoV steel at high temperature

This paper presents a comprehensive investigation into the low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of 2.25CrMoV steel at an elevated temperature of 455 °C. The LCF tests are conducted using a triangular waveform with varying strain amplitudes ranging from 0.4 % to 0.7 %. Furthermore, the CFI tests are conducted using a trapezoidal waveform with different hold times and hold directions. To gain further insights, the microstructure and fracture behavior of the steel are characterized using optical, scanning, and transmission electron microscopy techniques. The results reveal that the softening behavior and fatigue life degradation of the steel are dependent on the applied strain amplitude, hold time, and hold direction. Higher strain amplitudes in the LCF tests lead to an increased number of crack initiation sources. During the CFI tests, fatigue fracture is identified as the primary failure mechanism under tensile hold conditions, and an increase in hold time promotes crack propagation. The interaction between fatigue and creep is found to be more significant in compressive hold and tensile compressive combination hold conditions. Microstructure analysis demonstrates that the lath structure recovers during cyclic loading, resulting in cyclic softening. Finally, a modified energy-based model is proposed to distinguish the different roles of tensile energy and compressive energy in the fatigue process. The proposed model accurately predicts the fatigue life of the 2.25CrMoV steel, as demonstrated by the good agreement between the experimental and predicted results.

A New Creep-Fatigue Interaction Model for Predicting Deformation of Coarse-Grained Soil.

Studying the creep–fatigue interaction of the coarse-grained soil (CGS) is very important for safety assessment and disaster prevention in subgrade engineering. Current research work is mainly focused on single creep or fatigue deformation. In this paper, a new creep–fatigue interaction model is established to predict the creep–fatigue interaction deformation of different gradation CGS based on the rheological mechanics and the interactive relationship between creep and fatigue complex compliance method. Triaxial creep–fatigue interaction tests of different gradations CGS under different average stresses and frequencies were conducted to verify the new creep–fatigue interaction model. Research results show that for the creep–fatigue and fatigue–creep interaction, the fatigue deformation is always larger than the creep deformation under the same stress level. For the creep–fatigue multi-interaction, the second creep and fatigue deformation are always smaller than the first creep and fatigue deformation. The results of the triaxial creep–fatigue interaction tests verify the validity of this new model.

Characterization of the cyclic softening and remaining creep behaviour of P92 steel weldment

Different micro-regions of weldment are expected to present various cyclic softening behaviours during creep–fatigue interaction (CFI), which has a significant influence on the remaining mechanical properties. The present work aims to investigate the cyclic softening and remaining creep behaviour of different micro-regions of P92 steel weldment using electron backscattered diffraction (EBSD) and nanoindentation technology. CFI tests were performed and interrupted at various pre-set fatigue cycles at elevated temperature of 650 °C and hold time of 180 s, and thereafter EBSD analyses, nanoindentation tests and high-temperature creep tests were conducted on these interrupted specimens. Experimental results reveal that the microstructure of the parent metal and fine austenite grain heat-affected zone, which is adjacent to the intercritical heat affected zone, are sensitive to CFI loading. Notably, the microstructural evolution in intercritical heat affected zone almost reaches saturated after fatigue cycles of 20%Nf. Nanoindentation results uncover that the intercritical heat affected zone exhibits the lowest elastic modulus and microhardness, and with increasing the number of fatigue cycles, the weakest region changes. Accordingly, creep fracture location transits and creep life was degraded. Finally, a fatigue damage parameter based on the microhardness was developed to capture the effects of CFI on remaining creep life.

An improved unified viscoplastic model for modelling low cycle fatigue and creep fatigue interaction loadings of 9–12%Cr steel

An accurate constitutive model is one of the basic aspects to ensure a precise simulation. This study presents an improved unified viscoplastic model to simulate the various behavior of P92 steel under low cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings. In the proposed model, an accumulated inelastic strain dependent parameter is introduced into the nonlinear kinematic hardening rule to represent the evolutionary behavior of strain-stress hysteresis loops and the varied relaxation behavior during CFI loadings. The traditional isotropic hardening rule is modified as well to capture the accelerated cyclic softening phenomena observed in the prolonged hold time of CFI tests. To validate the accuracy and the predictive capability of the proposed model, LCF tests at various strain amplitudes and CFI tests at different hold time are conducted at elevated temperature of 650 °C. Good agreement between the experimental and simulated results verifies the robustness of the proposed model. In addition, the proposed model is distinguished from published models by few determined material parameters.

High temperature fatigue-creep interactive behavior and life prediction of a TiAl alloy

The strain controlled low cycle fatigue (LCF) and creep-fatigue interaction (CFI) tests of a TiAl alloy were conducted at 750 °C in air. The strain ratio, strain amplitude and strain rate were –1, 0.4% and 1 × 10–3 s–1 respectively. The high temperature LCF and CFI stress-strain stable hysteresis loop, cyclic hardening/softening, mean stress relaxation and life were studied and compared in details. Morrow modified method and unified life model developed in the present study were employed to predict the fatigue life for LCF and CFI life of TiAl alloy. Finally, the failure mechanisms between LCF and CFI specimens were compared, and some interesting conclusions were obtained. The results show that: (1) there are obvious differences on stress-strain stable hysteresis loop, cyclic hardening/softening, mean stress relaxation and life between LCF and CFI tests; (2) Morrow modified method is not able to predict the fatigue life for LCF and CFI life for TiAl alloy, however, the predicted fatigue life by the unified life model is located in the scatter band of ± 1.5; (3) the fatigue cracks are initiated from the surface for all of LCF and CFI specimens, and LCF specimens show more transgranular appearance while CFI specimens show obvious mixed appearance of both transgranular and intergranular.

Experimental characterisation and computational modelling of cyclic viscoplastic behaviour of turbine steel

Fully reversed strain controlled low cycle fatigue and creep-fatigue interaction tests have been performed at ±0.7% strain amplitude and at three different temperatures (400 °C, 500 °C and 600 °C) to investigate the cyclic behaviour of a FV566 martensitic turbine steel. From a material point of view, the hysteresis mechanical responses have demonstrated cyclic hardening at the running-in stage and subsequent, hysteresis cyclic softening during the rest of the material life. The relaxation and energy behaviours have shown a rapid decrease at the very beginning of loading followed by quasi-stabilisation throughout the test. A unified, temperature- and rate-dependent viscoplastic model was then developed and implemented into the Abaqus finite element (FE) code through a user defined subroutine (UMAT). The material parameters in the model were determined via an optimisation procedure based on a genetic solver. The multi-axial form of the constitutive model developed was demonstrated by analysing the thermo-mechanical responses of an industrial gas turbine rotor subjected to in-service conditions. A sub-modelling technique was used to optimise the FEA. A 2D global model of the rotor with a 3D sub-model of the second stage of the low pressure turbine were then analysed in turn. The complex transient stress and accumulated plastic strain fields were investigated under realistic thermo-mechanical fatigue loading (start-up and shut-down power plant loads). The sub-model was then used for local analysis leading to identification of potential crack initiation sites for the presented types of blade roots.

A New Empirical Life Prediction Model for 9–12%Cr Steels under Low Cycle Fatigue and Creep Fatigue Interaction Loadings

Low cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings are the main factors resulting in the failure of many critical components in the infrastructure of power plants and aeronautics. Accurate prediction of life spans under specified loading conditions is significant for the design and maintenance of components. In the present study, various LCF and CFI tests are conducted to investigate the effects of temperature, strain amplitude, hold time and hold direction on the fatigue life of P92 steel. To predict fatigue life under different experimental conditions, various conventional life prediction models are evaluated and discussed. Moreover, a new empirical life prediction model is proposed based on the conventional Manson-Coffin-Basquin (MCB) model. The newly proposed model is able to simultaneously consider the effects of temperature, strain amplitude, hold time and hold direction on predicted life. The main advantage is that only the known input experimental parameters are required to perform the prediction. In addition to the validation made through the experimental data of P92 steel conducted in the present paper, the model is also verified through numerous experimental data reported in the literature for various 9–12% Cr steels.

Creep Fatigue Interaction under Different Test Variables: Mechanics and Mechanisms

Abstract We present detailed analyses of dwell characteristics of various waveforms of creep fatigue interaction tests performed on the nickel-based superalloy IN 718. We discuss the effects of different dwell modes (strain, stress, and mixed) on creep fatigue properties. Strain dwell tests cause relaxation/accumulation of the mean stress, and stress dwell tests cause accumulation/relaxation of mean strain. True interaction of strain-controlled low cycle fatigue and creep takes place when stress-controlled dwell has been introduced within a strain-controlled cycling. In this article, the effects of pure and mixed modes have been compared in terms of the creep strain accumulation rate at dwell positions. It has been found, irrespective of creep and fatigue, that there is also a second competitive process that goes on, like tensile strain accumulation and compressive strain accumulation. These two processes compete with each other and show a difference in evolution with cycles. Mean strain rates for different stress and mixed control tests have also been determined; by comparing the creep strain accumulation rate and plastic strain rate, the competition of creep and fatigue processes has also been demonstrated in this article. The mechanism of cracking and cavity formation has also been illustrated by computed microtomography and subsequent fractography.

Studies on creep-fatigue interaction behaviour of Alloy 617M

In this investigation, response of Alloy 617 M to creep-fatigue interaction loading has been studied. The creep-fatigue interaction tests were conducted in air environment at a constant strain rate of 3 × 10−3 s−1 and strain amplitude of ± 0.4% with hold periods of 1 min, 10 min and 30 min at peak tension or compression at 973 K. The material showed initial hardening to a saturation followed by reduction in stress initiated by crack nucleation, propagation and final failure. The fatigue life decreased in presence of hold period. The decrease in fatigue life was more in tensile hold than in compressive hold. The fatigue life showed saturating trend for higher hold periods. Fractographic studies revealed significant intergranular creep cavitation under tensile hold whereas compressive hold exhibited transgranular fatigue striations in association with intergranular creep cavitation. Significant stress relaxation was found to take place during hold and was more in compressive hold than under tensile hold. The creep and fatigue damage have been assessed based on linear damage summation rule. Creep damage fraction was assessed from the stress relaxation considering the average stress, minimum stress and integrated stress over the relaxation curve. The creep-fatigue damage assessment considering integration of creep damage during stress relaxation showed most conservative compared to the other two approaches.

Fatigue–creep interaction based on continuum damage mechanics for AISI H13 hot work tool steel at elevated temperatures

AISI H13 (4Cr5MoSiV1) is one of the commonly used materials for extrusion tool, and it suffers from fatigue–creep damage during the hot extrusion process. Stress-controlled fatigue and creep–fatigue interaction tests were carried out at 500 °C to investigate its damage evolution. The accumulated plastic strain was selected to define the damage variable due to its clear physical meaning. A new fatigue–creep interaction damage model was proposed on the basis of continuum damage mechanics. A new equivalent impulse density for fatigue–creep tests was proposed to incorporate the holding time effect by transforming creep impulse density into fatigue impulse density. The experimental results indicated that the damage model is able to describe the damage evolution under these working conditions.

Low cycle fatigue and creep fatigue interaction behavior of 9Cr-0.5Mo-1.8W-V-Nb heat-resistant steel at high temperature

In this paper, Low Cycle Fatigue (LCF) and Creep-Fatigue Interaction (CFI) behavior of 9Cr-0.5Mo-1.8 W-V-Nb heat-resistant steel (ASME Grade P92 steel) at elevated temperature of 600 °C are investigated. Strain controlled LCF tests are conducted in fully reversed triangular waveform at different strain amplitudes ranging from 0.2% to 0.8%. CFI tests are conducted at 0.4% strain amplitude in trapezoid waveform with tensile hold time varying from 1 min to 60 min and compressive hold time varying from 1 min to 10 min. During LCF and CFI loadings, the evolution of cyclic stress, mean stress and stress relaxation behavior are investigated. It turns out that the softening behavior and lifetime degradation are dependent on strain amplitude, hold time and hold direction. In addition, the microstructure evolution and fracture behavior are characterized by optical, scanning and transmission electron microscope. The initial rapid softening behavior is attributed to the quick elimination of low angle boundaries, whereas no obvious microstructure alteration appears in the stable stage. Fracture behavior analysis reveals creep voids in long-term CFI tests facilitates the initiation and propagation of secondary cracks. The different responses of outer surface oxidation layer during cycling provides an explanation for severer damage of compressive hold and also accounts for the observed various fracture behavior of failed samples.

Experimental and numerical characterization of low cycle fatigue and creep fatigue behaviour of P92 steel welded joint

AbstractLow cycle fatigue (LCF) and creep fatigue interaction (CFI) behaviour of P92 steel welded joint were investigated experimentally and numerically. Strain‐controlled LCF tests at different strain amplitudes and CFI tests at different peak strain holding time were conducted. Evolutions of cyclic stress response, mean stress, and creep strain during cycling were described, in which the influence of strain amplitude and holding time were investigated. A specific heat treatment process was proposed to get the homogenous simulated material of fine grain region and coarse grain region in the heat affected zone. Material parameters of parent material, fine grain heat affected zone, coarse grain heat affected zone, and weld metal in the unified viscoplasticity model were then determined and validated. To predict the LCF and CFI behaviour of welded joint, 3‐dimensional unified viscoplasticity model with a modified isotropic variable was compiled into ABAQUS UMAT. The comparison between the predicted and experimental result under LCF and CFI loadings showed that the simulation results were reasonable and agreed with the experimental data well.

A Continuum Damage Mechanics-Based Viscoplastic Model of Adapted Complexity for High-Temperature Creep–Fatigue Loading

A continuum damage mechanics (CDM) based viscoplastic constitutive model is established in this study to describe the fully coupling of creep and fatigue behavior. The most significant improvement is the introduction of a continuum damage variable into the constitutive equations, instead of considering creep damage and fatigue damage separately. The CDM-based viscoplastic constitutive material model is implemented using a user-defined subroutine (UMAT). A standard specimen is used for carrying out uniaxial creep, fatigue, and creep–fatigue interaction tests to validate the material model. In addition, to further demonstrate the capability of the material model to predict the complex material behavior, a complex strain-control loading test is performed to validate the material model. The simulated and measured results are in good agreement at different temperatures and loadings, in particular for rapid cyclic softening behavior following crack initiation and propagation.

Micromechanism of cyclic plastic deformation of alloy IN 718 at 600 °C

AbstractIn the present investigation, an attempt was made to understand the cyclic deformation micromechanism of gas turbine alloy Inconel 718 at 600 °C (i) by conducting low cycle fatigue and creep–fatigue interaction tests and (ii) by studying the microstructure evolution in the material during fatigue tests through extensive electron microscopy. Bilinear slope was obtained in the Coffin–Manson plot for all low cycle fatigue tests, and it was confirmed through transmission electron microscopic examination that microtwinning was the predominant mode of deformation at low plastic strain values, whereas slip and shearing of γ″ precipitates were the predominant mode of deformation at higher plastic strain values. Fatigue life was adversely affected when hold time was introduced at peak tensile strain during creep–fatigue interaction tests. Formation of stepped interface at microtwin boundaries and coarsening of niobium carbide precipitates were observed to be the major microsturctural changes during creep–fatigue interaction tests.

A Comparative Evaluation of the Effect of Low Cycle Fatigue and Creep–Fatigue Interaction on Surface Morphology and Tensile Properties of 316L(N) Stainless Steel

In the present work, the deformation and damage evolution in 316L(N) stainless steel during low cycle fatigue (LCF) and creep–fatigue interaction (CFI) loadings have been compared by evaluating the residual tensile properties. Towards this, LCF and CFI experiments were carried out at constant strain amplitude of ±0.6 pct, strain rate of 3 × 10−3 s−1 and temperature of 873 K (600 °C). During CFI tests, 30 minutes hold period was introduced at peak tensile strain. Experiments were interrupted up to various levels of fatigue life viz. 5, 10, 30, 50, and 60 pct of the total fatigue life (N f) under both LCF and CFI conditions. The specimens subjected to interrupted fatigue loadings were subsequently monotonically strained at the same strain rate and temperature up to fracture. Optical and scanning electron microscopy and profilometry were conducted on the untested and tested samples to elucidate the damage evolution during the fatigue cycling under both LCF and CFI conditions. The yield strength (YS) increased sharply with the progress of fatigue damage and attained saturation within 10 pct of N f under LCF condition. On the contrary, under CFI loading condition, the YS continuously increased up to 50 pct of N f, with a sharp increase of YS up to 5 pct of N f followed by a more gradual increase up to 50 pct of N f. The difference in the evolution of remnant tensile properties was correlated with the synergistic effects of the underlying deformation and damage processes such as cyclic hardening/softening, oxidation, and creep. The evolution of tensile properties with prior fatigue damage has been correlated with the change in surface roughness and other surface features estimated by surface replica technique and fractography.

Low cycle fatigue and creep–fatigue interaction behaviour of 2.25Cr1MoV steel at elevated temperature

This study reports the fatigue behaviour of 2.25Cr1MoV steel under low cycle fatigue (LCF) loading and creep-fatigue interaction (CFI) loading at 355, 455 and 555 °C. Various hold durations up to 600 s were introduced in the CFI tests at the peak/valley strain under strain or stress control. In LCF tests, the steel exhibited remarkable strengthening at 455 °C, which can be ascribed to the effect of dynamic strain aging. In CFI tests, tensile holds were found more damaging than compressive holds but considerably less harmful than the combined tensile-compressive holds. A modified plastic strain energy approach based on the damage mechanisms was proposed to predict fatigue life under LCF and CFI conditions. The predictions obtained compared very favourably with the experimental results.

The Effect of Nitrogen Alloying on the Low Cycle Fatigue and Creep-Fatigue Interaction Behavior of 316LN Stainless Steel

Low cycle fatigue (LCF) and Creep-fatigue interaction (CFI) behavior of 316LN austenitic stainless steel alloyed with 0.07, 0.11, 0.14, .22 wt.% nitrogen is briefly discussed in this paper. The strain-life fatigue behavior of these steels is found to be dictated by not only cyclic plasticity but also by dynamic strain aging (DSA) and secondary cyclic hardening (SCH). The influence of the above phenomenon on cyclic stress response and fatigue life is evaluated in the present study. The above mentioned steels exhibited both single-and dual-slope strain-life fatigue behavior depending on the test temperatures. Concomitant dislocation substructural evolution has revealed transition in substructures from planar to cell structures justifying the change in slope. The beneficial effect of nitrogen on LCF life is observed to be maximum for 316LN with nitrogen in the range 0.11 - 0.14 wt.%, for the tests conducted over a range of temperatures (773-873 K) and at ±0.4 and 0.6 % strain amplitudes at a strain rate of 3*10-3 s-1. A decrease in the applied strain rate from 3*10-3 s-1 to 3*10-5 s-1 or increase in the test temperature from 773 to 873 K led to a peak in the LCF life at a nitrogen content of 0.07 wt.%. Similar results are obtained in CFI tests conducted with tensile hold periods of 13 and 30 minutes. Fractography studies of low strain rate and hold time tested specimens revealed extensive intergranular cracking.

Study of Life Prediction and Damage Mechanism for Modified 9Cr-1Mo Steel Under Creep-Fatigue Interaction

Creep-fatigue interaction is a principal cause of failures of many engineering components under high temperature and cyclic loading. In this work, stress controlled creep-fatigue interaction tests are carried out for modified 9Cr-1Mo (P91) steel. In order to study the damage mechanism of P91 steel under creep-fatigue interaction, Scanning Electron Microscopy (SEM) of specimen fracture morphology and in-situ observation experiments were conducted. Based on the ductility exhaustion theory and creep-fatigue interaction tests data, the modified ductility exhaustion life prediction model was developed. The predicted results are in a good agreement with the experiment. By comparison with frequency separation model, the life predicted by ductility exhaustion model is better than frequency separation model obviously. The results show that different stress amplitude and mean stress have great effect on the fracture damage mechanism when the hold time is invariable. By the SEM analysis of fracture morphology, the damage characters of creep, creep-fatigue interaction and fatigue can be partitioned. The specimen crack initiation source is the modified 9Cr-1Mo steel inclusion. Therefore, this work can provide a reference of life prediction and design for high temperature materials and components.

Low cycle fatigue behaviour of Inconel® alloy 783

Low cycle fatigue (LCF) tests were performed on Inconel® Alloy 783 at a strain rate of 3×10−3s−1 and a strain amplitude of ±0.6%. The tests were conducted in the temperature range, 573 to 923 K besides room temperature. Further, influence of strain amplitude on LCF behaviour was studied at a constant temperature of 923 K using strain amplitudes in the range, ±0.3 to ±0.8%. Creep-fatigue interaction tests were conducted at a strain amplitude of ±0.4%, at 923 K. The material generally showed a stable stress response followed by a region of continuous softening upto failure. Also, the alloy was seen to exhibit dynamic strain ageing (DSA) in the temperature range, 573 to 723 K as evidenced by the observed peaks in the half-life stress in the above temperature range. Dynamic strain ageing was seen to offset the softening exhibited by the alloy outside the above temperature range. Fractography/EDS analysis of the failed samples showed numerous brittle Nb-rich precipitates that were seen to influence crack propagation. A continuous reduction in the LCF life was noticed with increase in the test temperature. The observed material behaviour has been correlated with the damage mechanisms through microstructural observations.

Finite element-based life prediction for high-temperature cyclic loading of a large superplastic forming die

The thermomechanical, time-dependent, elastic-plastic creep behaviour of a representative tool in a high-temperature superplastic forming press has been simulated using a sequential finite element-based heat transfer and mechanical stress analysis approach. The heating and cooling aspects of the heat transfer model have been previously validated against laboratory measurements. This paper focuses on the prediction of die life based on this model. The determination and modelling of the high-temperature mechanical properties of the materials and die are described for both so-called (a) major cycles, relating to die heat-up and cool-down from and to ambient temperature, and (b) minor cycles, relating to die heat-up and cool-down associated with part removal and blank insertion. A creep-fatigue life prediction method using non-linear damage summation is developed and applied to the die-life problem. Isothermal fatigue and creep-fatigue interaction tests at 700 and 900 °C on 40%Ni-20%Cr die material XN40F are carried out to estimate the required strain-life relationships.