Creep-fatigue Interaction Life Research Articles

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.

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.

Experimental Investigation on the LCF Behavior Affected by Manufacturing Defects and Creep Damage of One Selective Laser Melting Nickel-Based Superalloy at 815 °C

Uniaxial tensile tests and stress-controlled low-cycle fatigue (LCF) and creep–fatigue interaction (CFI) tests of Inconel 625 alloy manufactured by selective laser melting (SLM) were performed at 815 °C in air environments. The microstructure was characterized by optical microscopy and scanning electron microscopy after testing. The results confirmed that significant embrittlement and large scatter in LCF life are resulted from manufacturing defects. The CFI life is decreased sharply to approximately dozens of cycles with the accumulated creep strain; however, the selected dwell time (i.e., 60 s and 300 s) exhibits low sensitivity to the fracture time and elongation to failure. The embrittlement of SLM Inconel 625 was proposed to be due to the low grain uniformity and precipitation of carbides at the grain boundaries. Due to the quality of the SLM process, the accelerated initiation and propagation of fatigue crack are caused by the present unmelted powder particles, which result in the large dispersion of LCF life. Meanwhile, due to the accumulation of creep damage, cracks in the CFI test are initiated along the grain boundaries and then linked together, contributing to a significant decline in fatigue life.

Influence of W and effect of loading mode on the substructural evolution of reduced activation ferritic/martensitic (RAFM) steels

Influence of tungsten (W) on the evolution of substructure during creep-fatigue interaction (CFI) and effect of loading mode in terms of continuous cycling (CC) and CFI at 823 K of reduced activation ferritic/martensitic (RAFM) steels has been investigated. Increase in W from 1.4 to 2 wt% resulted in an improvement of 1.6 times of CFI life. Electron back scatter diffraction (EBSD) analysis of CFI deformed specimens demonstrated that the subgrain size was 1.8 times smaller and boundary dislocation density was 3 times higher in case of 2 wt% W steel compared to 1.4 wt% W. It can be deduced that W addition resulted in decrease in the recovery rate and enhancing the CFI life. In context to loading mode, the subgrain size was 1.5 times higher and dislocation density was 2 times lower in case of CC tested specimen with respect to CFI tested. Under CFI, the number of cycles to failure were drastically reduced due to the synergistic damage of cyclic loading and creep deformation.

Creep-Fatigue Interaction Life Consumption of Industrial Gas Turbine Blades

This paper presents the creep-fatigue interaction life consumption of industrial gas turbine blades using the LM2500+ engine operated at Pulrose Power station, Isle of Mann as a case study. The linear damage summation approach where creep damage and fatigue damage are combined was used for the creep-fatigue interaction life consumption of the target blades. The creep damage was modelled with the Larson-Miller parameter method while fatigue damage was assessed with the modified universal slopes method and the damage due to creep-fatigue interaction was obtained from the respective life fractions. Because of the difficulty in predicting the life of engine components accurately, relative life consumption analysis was carried out in the work using the concept of creep-fatigue interaction factor which is the ratio of the creep-fatigue interaction life obtained from any condition of engine operation to a reference creep-fatigue interaction life. The developed creep-fatigue interaction life consumption analysis procedure was applied to 8 most of real engine operation. It was observed that the contribution of creep to creep-fatigue interaction life consumption is greater than that of fatigue at all ambient temperatures. The fatigue contribution is greater at lower ambient temperatures as against higher ambient temperatures. For the case study, the overall equivalent creep-fatigue factor obtained was 1.5 which indicates safe engine operation compared to the reference condition. The developed life analysis algorithm could be applied to other engines and could serve as useful tool in engine life monitoring by engine operators.

Low cycle fatigue, creep and creep-fatigue interaction behavior of a TiAl alloy at high temperatures

The creep-fatigue interaction (CFI) tests of a TiAl alloy were carried out at 750°C and 800°C in air to investigate the effect of dwell time on fatigue life. The low cycle fatigue (LCF) and creep tests were also conducted for comparison. The life to failure decreased with increasing dwell time and decreased with increasing minimum strain rate. Two life models based on dwell time and minimum strain rate were proposed to predict LCF, creep and CFI life, and the predicted life of both life models agrees very well with the tested life of the alloy at high temperatures.

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.

Long term creep-fatigue interaction studies on India-specific reduced activation ferritic-martensitic (IN-RAFM) steel

Deterioration in fatigue life occurs due to introduction of hold in peak tensile or peak compressive or in both peak tensile and compressive strains in a triangular waveform. The hold period of a creep fatigue interaction (CFI) cycle denote the uninterrupted service period during which creep damage is predominant. CFI experiments comprising of long hold periods are relatively scarce but are indispensable for the understanding of long term materials behavior under CFI. Keeping this in view, CFI experiments have been performed at 823K on a commercial heat of reduced activation ferritic-martensitic (RAFM) steel using different directions of hold (tensile hold alone, compressive hold alone and simultaneous tensile and compressive holds) for different durations and by employing two strain amplitudes, ±0.25% and ±0.6%. The extent of deterioration in fatigue life in IN-RAFM steel is found to be dependent upon the direction of application of hold. Factors such as oxidation, plastic strain accumulation, mean stresses and stress relaxation during the application of the hold played a significant role in affecting CFI life of RAFM steel. The importance of stress relaxation (SR) in assessing fatigue damage under CFI has been described. A universal relationship between CFI life and SR has been arrived at and which is found to be independent of the type of the waveform, strain amplitude or the duration of hold. Though the relationship between stress relaxation and fatigue life is non-linear, the log–log plot between SR and CFI life value depict an inverse linear relationship. The sum of least squares of the offset from the linear fit, R2 is 0.93 which is a reasonably good approximation. The log–log plot of SR versus CFI life denotes the creep-fatigue interaction line and can be used to predict CFI life for a given amount of SR occurring in the material. The proposed relationship is applicable only for long duration CFI experiments and depicts microstructural creep-fatigue interaction in true sense. Test conditions that do not prominently show the effects of creep-fatigue interaction, do not fall on the CFI interaction line.

Influence of Varying W and Ta on Low Cycle Fatigue and Creep–Fatigue Interaction Behavior of Reduced Activation Ferritic/Martensitic Steels

The objective of the present paper is to provide first hand experimental data and analysis on the low cycle fatigue (LCF) and creep–fatigue interaction (CFI) performance of different chemical compositions of reduced activation ferritic–martensitic (RAFM) steels. As a part of the development programme, LCF and CFI experiments were conducted on different RAFM steels with varying W and Ta obtained in the normalized and tempered condition. Effect of varying Ta and W upon the resultant LCF and CFI life seemed to be interrelated and an optimum combination of both W and Ta worked out to be the best for CFI life. Stress relaxation that occurs under the application of hold has unique information of the underlying deformation and damage in the RAFM steels. It has been observed that stress relaxation is directly related to the amount of plastic strain accumulated, inversely related to CFI life and strongly dependent on chemical composition. Hence it can be used to explain the deformation and damage accumulation in ferritic–martensitic steels. The plastic strain accumulated during stress relaxation is accommodated as microstructural coarsening. EBSD based microstructural characterization reveals that the recovery process is closely related with the chemical composition and various test parameters. Breakage of protective chromia layer accounts for the more damaging effects of oxides under compressive hold.

Creep–fatigue interaction behavior of 316LN austenitic stainless steel with varying nitrogen content

Creep–fatigue interaction (CFI) behavior of 316LN stainless steel with varying nitrogen content (0.07, 0.14 and 0.22wt.%) is investigated at temperature of 873K under strain-controlled fatigue tests with a tensile-hold period of 1, 13.3 and 30min. Dynamic strain aging, thermal-recovery and/or dislocation–precipitate interactions are found to strongly control CFI deformation. At all the nitrogen contents in 316LN SS, the amount of stress relaxation and the associated relaxation strain rate decreased with increase in hold duration. Irrespective of the hold period, wavy-slip dislocation structures (cells, tangles, walls) are noticed at 0.07wt.% N, whereas dislocation structures such as planar slip bands impinging on grain boundary are noticed, in addition, at nitrogen contents above 0.07wt.%. Nitrogen content played a dual-role in influencing the CFI life. At 1-min hold where the failure is by transgranular plus intergranular fracture, increase in nitrogen from 0.07 to 0.14wt.% caused an improvement in CFI life. On the other hand, CFI life decreased with increasing nitrogen content at 13.3 and 30-min hold durations that induced significant intergranular damage. Intergranular damage is evidenced in the form of triple point cracks and interconnected networks of grain boundary decohesion.

Effect of W and Ta on creep–fatigue interaction behavior of reduced activation ferritic–martensitic (RAFM) steels

The aim of this work is to understand the effect of varying tungsten and tantalum contents on creep–fatigue interaction (CFI) behavior of reduced activation ferritic–martensitic (RAFM) steels. Increase in W improved CFI life. Effect of changing Ta and W upon the resultant CFI life seems to be interrelated and an optimum combination of both W and Ta works out to be the best for CFI life. Stress relaxation obtained during application of hold can be a useful parameter to relate deformation and damage in the RAFM steels.

Low Cycle Fatigue and Creep-Fatigue Interaction Behaviour of Reduced Activation Ferritic Martensitic (RAFM) Steels with Varying W and Ta Contents

Reduced activation ferritic/martensitic (RAFM) steels are candidate materials for the test blanket modules of ITER. Several degradation mechanisms such as thermal fatigue, low cycle fatigue, creep fatigue interaction, creep, irradiation hardening, swelling and phase instability associated irradiation embrittlement must be understood to estimate the component lifetime. The current work focuses on the effect of tungsten and tantalum on low cycle fatigue (LCF) and creep-fatigue interaction (CFI) behavior of four RAFM steels with varying W and Ta contents. Total strain controlled LCF experiments were performed under various strain amplitudes in the range +0.25% to +1% and temperatures (300 K to 873 K) in air at a constant strain rate of 3×10-3s-1 using a servo hydraulic fatigue testing system. CFI experiments were carried out at total strain amplitude of +0.6% and by applying strain hold of different durations (10 min and 30 min) in peak tension and peak compression. Both LCF and CFI life of the RAFM steels improved with the increase in tungsten and tantalum contents. Based on the amount of softening during continuous cycling, tungsten content was optimized at 1.4 wt. % and the tantalum content at 0.06 wt%. Stress relaxation obtained during creep-fatigue interaction studies showed close relation with the chemical composition of the RAFM steels. Other damaging parameters influencing fatigue life were dynamic strain ageing (DSA) occurring in the intermediate temperature regime and oxidation at elevated temperatures. Keywords: RAFM steel, low cycle fatigue, dynamic strain ageing, creep-fatigue interaction, oxidation

Influence of dynamic sodium environment on the creep–fatigue behaviour of Modified 9Cr–1Mo ferritic–martensitic steel

The use of liquid sodium as a heat transfer medium for sodium-cooled fast reactors (SFRs) necessitates a clear understanding of the effects of dynamic sodium on low cycle fatigue (LCF), creep and creep–fatigue interaction (CFI) behaviour of reactor structural materials. Mod. 9Cr–1Mo ferritic steel is the material of current interest for the steam generator components of sodium cooled fast reactors. The steam generator has a design life of 30–40 years. The effects of dynamic sodium on the LCF and CFI behaviour of Mod. 9Cr–1Mo steel have been investigated at 823 and 873 K. The CFI life of the steel showed marginal increase under flowing sodium environment when compared to air environment. Hence, the design rules for creep–fatigue interaction based on air tests can be safely applied for components operating in sodium environment. This paper attempts to explain the observed LCF and CFI results based on the detailed metallography and fractography conducted on the failed samples.

An investigation of a new near-beta forging process for titanium alloys and its application in aviation components

In this paper, the authors present a new near-beta forging process, in which materials are heated at about 15 °C below the beta transus, to improve the combined properties of titanium alloys. Materials processed by the near-beta forging process, followed by rapid water-cooling, then high temperature toughening and low temperature strengthening heat treatments, produce a new kind of microstructure for titanium alloys. This new tri-modal microstructure consists of about 15% equiaxed alpha, 50–60% lamellar alpha and transformed beta matrix. Materials with tri-modal microstructure show a high low cycle fatigue property, high creep-fatigue interaction life, high fracture toughness and a high service temperature without decreasing ductility and thermal stability. The experimental fundamentals of the new process and the strengthening and toughening mechanism have been discussed. A critical issue in the practical application of the near-beta forging process is the control of temperature. A new metallographic inspection method was proposed to solve this problem. This new near-beta forging process has been reliably applied to produce several aeroengine compressor disks, rotators, and other airplane components.

A review of fatigue crack growth in steels under mixed mode I and II loading: Bold, P. E., Brown, M. W. and Allen, R. J. Fatigue Fract. Eng. Mater. Struct. (Oct. 1992 15 (10) 965–977

The effects of hold condition (tension-only, compression-only and tension-plus-compression holds of 1 min duration) and hold time (up to 10 min in tension) on the low-cycle fatigue behaviour of type 304 stainless steel base metal and type 308 stainless steel all-weld metal were investigated at 923 K. All the tests were performed in total axial strain control in air, employing a strain range of 1.0%. The welds were prepared by a submerged metal arc welding process. The microstructure of the base metal and the weld consisted of gamma phase and duplex gamma-delta ferrite respectively. The results clearly indicated a reduction in continuous cycling as well as creep-fatigue interaction life of type 308 SS weld metal compared with type 304 SS base metal under identical testing conditions. Hold times displayed an effect on life that was dependent not only on material condition but also on the position of the hold in the cycle. Fatigue lives recorded for type 308 SS weld in the 1 min hold-time tests were in the order: compression hold → continuous cycling → tension-plus-compression hold → tension hold. Fatique lives of 304 SS were in the order: continuous cycling → compression hold → tension-plus-compression hold → tension hold. A significant reduction in the life of the weld metal was noted on increasing the duration of tension hold time to 10 min. 308 SS weld metal exhibited cyclic softening whereas 304 SS base metal showed rapid initial hardening followed by a saturation stage. The observed variations in life are explained on the basis of crack initiation and propagation modes and microstructural changes that occured during low-cycle fatigue and creep-fatigue interaction testing.

Creep-fatigue interaction behaviour of type 308 stainless steel weld metal and type 304 stainless steel base metal

The effects of hold condition (tension-only, compression-only and tension-plus-compression holds of 1 min duration) and hold time (up to 10 min in tension) on the low-cycle fatigue behaviour of type 304 stainless steel base metal and type 308 stainless steel all-weld metal were investigated at 923 K. All the tests were performed in total axial strain control in air, employing a strain range of 1.0%. The welds were prepared by a submerged metal arc welding process. The microstructure of the base metal and the weld consisted of gamma phase and duplex gamma-delta ferrite respectively. The results clearly indicated a reduction in continuous cycling as well as creep-fatigue interaction life of type 308 SS weld metal compared with type 304 SS base metal under identical testing conditions. Hold times displayed an effect on life that was dependent not only on material condition but also on the position of the hold in the cycle. Fatigue lives recorded for type 308 SS weld in the 1 min hold-time tests were in the order: compression hold → continuous cycling → tension-plus-compression hold → tension hold. Fatique lives of 304 SS were in the order: continuous cycling → compression hold → tension-plus-compression hold → tension hold. A significant reduction in the life of the weld metal was noted on increasing the duration of tension hold time to 10 min. 308 SS weld metal exhibited cyclic softening whereas 304 SS base metal showed rapid initial hardening followed by a saturation stage. The observed variations in life are explained on the basis of crack initiation and propagation modes and microstructural changes that occured during low-cycle fatigue and creep-fatigue interaction testing.

'European Pharmacopeia'

The aim of this work is to understand the effect of varying tungsten and tantalum contents on creep–fatigue interaction (CFI) behavior of reduced activation ferritic–martensitic (RAFM) steels. Increase in W improved CFI life. Effect of changing Ta and W upon the resultant CFI life seems to be interrelated and an optimum combination of both W and Ta works out to be the best for CFI life. Stress relaxation obtained during application of hold can be a useful parameter to relate deformation and damage in the RAFM steels.