High-temperature Fatigue Behavior Research Articles

Effect of prestraining on high-temperature fatigue behaviour of two Ni-base superalloys

Abstract Isothermal cyclic deformation tests were carried out on two nickel-base alloys (Nimonic 105 and Nimonic 75) after monotonic tensile predeformation to various strains at room temperature. The effect of predeformation was found to lose importance for the fatigue behaviour with increasing temperature, increasing plastic strain amplitude, decreasing prestrain and decreasing influence of particle strengthening. Appreciable changes in the crack initiation mechanisms and the steady-state dislocation arrangements established during cyclic loading resulted from predeformation. However, the change in the cyclic stress–strain response was identified to be the main reason for the effect of predeformation on fatigue life. Consequently, a predeformation parameter SP_SWT was introduced which combines the Smith-Watson-Topper damage parameters obtained in fatigue tests on predeformed and not predeformed samples. It was shown that SP_SWT is suitable to quantitatively describe the effect of loading conditions on th...

High temperature fatigue behavior and residual stress stability of laser-shock peened and deep rolled austenitic steel AISI 304

In this paper, we investigate how laser-shock peening and deep rolling affect the cyclic deformation and S/N-behavior of austenitic stainless steel AISI 304 at elevated temperatures (up to 600 °C). The results demonstrate that laser shock peening can produce similar amounts of lifetime enhancements as deep rolling. The cycle, stress amplitude and temperature-dependent relaxation of compressive residual stresses is more pronounced than the decrease of near-surface work hardening.

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

Notches were machined in specimens of INCONEL 718 by a broaching process, where differing broaching runs led to differing extents of subsurface deformation and surface roughness. Fatigue tests were carried out at 600 °C with a trapezoidal loading waveform at 0.25 Hz. The broaching process that led to the more severe subsurface deformation (but lower surface roughness) showed the worst fatigue performance. Analysis of total strain amplitude in the notch root with the aid of an elastoplastic finite-element (FE) model showed that the work hardening related to the subsurface deformation caused by the different broaching can account for the difference in fatigue lives. Differences in initiation and growth behavior were seen for the two broached finishes as well as for broached and subsequently polished samples. These differences are discussed in terms of a change in crack growth initiation and growth mechanisms due to the presence of the work-hardened layer.

Isothermal high-temperature fatigue behaviour of a near-γ titanium aluminide alloy

Abstract The high-temperature fatigue behaviour of smooth cylindrical specimens of a near-γ-TiAl alloy with duplex microstructure was studied performing total-strain-controlled low-cycle-fatigue tests in the temperature range 500–750°C. The tests were run in air and high vacuum. Mean stress, environmental degradation (oxygen- and hydrogen-induced embrittlement) and plastic strain range were identified to be the most significant parameters affecting damage evolution. The cyclic stress–strain response was found to depend strongly and characteristically on temperature. Below the brittle-to-ductile-transition temperature (TBD) of about 650°C, cyclic deformation leads to a pronounced initial hardening manifesting itself in an increase of stress amplitude and a decrease of plastic strain range. Conversely, cyclic deformation curves from tests performed above TBD show a marked state of saturation which is established already after few cycles. With increasing temperature, fatigue life decreases in vacuum. However...

OS11(4)-17(OS11W0021) Fatigue Behavior of Thermally Sprayed Materials at High Temperature

High temperature fatigue and thermal cycle damage behavior of SUS304 steel thermally sprayed Al_2O_3/NiCr atmospheric plasma was investigated. Surface deformation behavior was measured using an electronic speckle pattern interferometry (ESPI) system. Cracks and delaminations in the thermally sprayed coatings were observed after high temperature fatigue or thermal cycle testing with SEM. The strain distribution across specimen surface was obtained from ESPI measurements. The relationship between crack generation or delamination and strain distribution was discussed. Surface cracks occurred in the outer Al_2O_3 layer but stopped at the inner NiCr layer after one fatigue cycle when tensile stress was lager than 173MPa at 873K. Surface cracks propagated into the NiCr layer but stopped at the substrate, and local delamination occurred along the interface between the NiCr layer and substrate after N=1×(10)^5 cycles, when the fatigue test was carried out for σ_<max>=173 or 202MPa (R=0) at 873K. No cracks or delamination were observed after N=1×(10)^5 cycles for fatigue condition σ_<max>=115MPa (R=0) at 873K. Short cracks but no delamination were observed for fatigue condition σ_<max>=202MPa (R=0) at 573K. No influence of stress velocities (6.7 or 14Hz) was detected. No damage was observed after thermal cycle testing. The strain value obtained from the ESPI system was confirmed to be approximately the same as that from a strain gauge on an un-sprayed specimen at 293K. The thermal expansion or stress deformation at high temperature was easily measured with the ESPI system. The sprayed specimen surface strain was approximately the same as that of an un-sprayed specimen at 873K, showing that the deformation of the sprayed coating always follows substrate interface deformation at high temperature. Surface strain decreased with N=1×(10)^5 cycle fatigue showing evidence of the hardness effect due to fatigue. Strain values along cracks measured with the ESPI system were larger than those in other areas due to crack opening under the tensile load. In this paper, the larger strain zone is called "the strain concentration zone". Comparing crack location with strain concentration zones, a correspondence was found between the strain distribution obtained from the ESPI system and crack distribution in the thermally sprayed coating. Moreover, strain values decreased or disappeared entirely in the area where delamination in sprayed coating can be detected in situ by analyzing strain concentration or decrease with an ESPI system at high temperature.

Influence of Frequency on High-Temperature Fatigue Behavior of 17-4 PH Stainless Steels

The effect of frequency (2 and 20 Hz) on the high-cycle fatigue and fracture behavior was investigated at 573-773 K for 17-4 PH stainless steels in three conditions: Condition A (unaged), H900 (peak-aged) and H1150 (overaged). S-N results indicated that at 573 and 673 K, there was generally no difference in fatigue strength between 2 and 20 Hz, except for H900 at 673 K where the fatigue strength at 2 Hz was lower than that at 20 Hz. At 773 K, the fatigue strength of each condition at 2 Hz was lower than that at 20 Hz due to the occurrence of creep mechanism at this low frequency. At 773 K and 2 Hz, the fatigue fracture mode exhibited a mixed mode involving transgranular and intergranular cracking and the grain boundary cavities were also observed. At a given temperature and frequency, the fatigue strength for the three conditions generally took the following order: H900 > Condition A > H 1150, except for Condition A at 773 K in the long life regime where the fatigue strength was close to that of H 1150 due to a precipitate-coarsening effect. With the exception of Condition A tested at 673 K, the fatigue strength of each condition was decreased with increasing temperature as a result of a reduction in yield strength. At both frequencies, the fatigue strength of Condition A at 673 K was greater than that at 573 K as a result of an in-situ precipitation-hardening effect. Fractography observations indicated that the fatigue crack initiation site, crack propagation path and fracture surface morphology were functions of testing temperature, loading frequency and applied cyclic stress level.

High temperature tensile and fatigue behaviour of the unidirectionally reinforced MMC Ti64/SiC

The material employed in the present work is a Ti6Al4V matrix composite unidirectionally reinforced with eight plies of SiC fibres. This paper studies the tensile behaviour and the fatigue fracture processes at the operation temperatures planned for these materials (550 and 600 °C). The fractographic analysis and microstructural observations of the tested samples at both temperatures are also presented. These are used to determine the mechanisms of nucleation and propagation of damage until fracture.

High-temperature fatigue behavior of a near- γ titanium aluminide alloy under isothermal and thermo-mechanical conditions

Fatigue tests were performed on smooth cylindrical specimens of a near- γ titanium aluminide alloy with a duplex microstructure. The test temperature ranged from 500 to 750 °C in the isothermal tests and these temperatures were also used as minimum and maximum temperature of in-phase and out-of-phase thermo-mechanical fatigue (TMF) tests. Environmental degradation was studied running the experiments in air and high-vacuum, respectively, and the microstructural changes were characterized by transmission electron microscopy. Fatigue behavior was found to be strongly affected by temperature as a consequence of a change in the cyclic stress–strain response at ≈650 °C. At temperatures <650 °C, initial cyclic hardening occurred, whereas >650 °C a cyclic saturation state was observed from the beginning. With increasing temperature, fatigue life decreased in vacuum, but increased in air indicating an abnormal (inverse) environmental effect. The stress–strain behavior under TMF conditions resembles that of the corresponding isothermal experiments. Out-of-phase thermo-mechanical fatigue was found to be more detrimental than in-phase, because of the positive (tensile) mean stress arising in out-of-phase testing in combination with the pronounced mean stress sensitivity of the relatively brittle alloy.

Dwell effects on high temperature fatigue behavior: Part I

Effects of dwell times on creep–fatigue behavior of the following materials: (a) pure metals; (b) solder alloys; (c) copper alloys; (d) low-alloy steels; (e) stainless steels; (f) titanium alloys; (g) tantalum alloys; and (h) superalloys have been examined from the published data. The melting temperatures of these materials varied from low (over room temperature) to very high temperatures (above 1600°C). The temperature at which creep–fatigue tests were conducted varied from 0.5 to as high as 0.8 of the homologous temperature. Within this temperature range creep and fatigue processes interact and failure occurs in the low-cycle regime. The creep–fatigue data were compiled from the published literature. Dwell sensitivity, the beneficial or detrimental effect of dwell cycles, can be presented in a dwell-sensitivity map. Normalized cycle ratio (NCR), and total strain range were used to construct such maps. A high NCR (≥1) at a given strain range demonstrates high fatigue resistance and vice versa. Generally, the NCR increased with an increase in strain range, dwell time and test temperature. However, as the strain range decreased and dwell time increased, the NCR decreased. Mean stress evolution in either tension or compression directions was correlated with dwell-sensitivity behavior. Materials were sensitive to a particular test condition depending on temperature, strain range, strain rate, as well as time and direction in which a dwell was applied. In general, dwell cycles decreased the cyclic life. However, a beneficial effect was observed for a few copper alloys and superalloys. The mechanisms controlling deformation and failure for all these materials are discussed in an accompanying paper (Paper II) separately.

High temperature fatigue behaviour of TZM molybdenum alloy under mechanical and thermomechanical cyclic loads

High temperature isothermal mechanical fatigue and in-phase thermomechanical fatigue (TMF) tests in load control were carried out on a molybdenum-based alloy, one of the best known of the refractory alloys, TZM. The stress–strain response and the cyclic life of the material were measured during the tests. The fatigue lives obtained in the in-phase TMF tests are lower than those obtained in the isothermal mechanical tests at the same load amplitude. It appears that an additional damage is produced by the reaction of mechanical stress cycles and temperature cycles in TMF situation. Ratcheting phenomenon occurred during the tests with an increasing creep rate and it was dependent on temperature and load amplitude. A model of lifetime prediction, based on the Woehler–Miner law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method the lifetime prediction gives results corresponding well to experimental data.

High temperature fatigue behavior of wrought nickel-base superailoy GH4049

Push-pull total strain-controlled fatigue tests without and with a hold period were carried at elevated temperatures for wrought nickel base superailoy GH4049. The influence of the testing temperature and strain hold period on fatigue behavior was determined. The alloy would exhibit either cyclic strain hardening, softening or stability during cyclic straining. Fatigue life depends strongly on the testing temperature and the introduction of the strain hold period. Observations on fatigue specimens using transmission electron microscopy (TEM) showed that the dislocations were distributed mostly in the γ matrix. It was observed by scanning electron microscopy (SEM) that cracks initiated always in a transgranular mode, but their propagation mode was closely related to the testing temperature. In addition, the fatigue life was predicted by linear damage summation (LDS), strain range partitioning (SRP) and the strain energy partitioning (SEP) method. The results of life prediction indicated that the SRP and SEP methods were in a good agreement as to the measured and predicted life at lower temperatures, while the LDS method showed better predictability at higher temperature as compared to the SRP and SEP methods.

High-temperature fatigue damage mechanisms in near-α titanium alloy IMI 834

The high-temperature fatigue and creep behaviour of near-α titanium alloy IMI 834 was studied for temperatures up to 650°C. In order to identify the relevant damage mechanisms, the test program involved continuous cycling, dwell tests with hold periods in tension and compression, and asymmetrical strain-time cycles. The dominant damage mechanism was found to change at a temperature of about 600°C. At low test temperatures fatigue life is largely dependent on the maximum stress of the fatigue cycle. At high temperatures a brittle oxygen-enriched subsurface layer forms, and thus, environmental degradation governs fatigue life in high-temperature low frequency tests. Lamella boundaries present in the bimodal microstructure were seen to deflect small fatigue cracks, and thus, this microstructure has superior fatigue properties as compared to an equiaxed one. During long-term high-temperature exposure, however, significant degradation of the lamella boundaries was observed, and then both microstructures display similar fatigue properties.

High Temperature Fatigue Behavior in Tensile hold LCF of Near-alpha Ti-1100 with Lamellar Structure

The effect of various tensile hold times on high temperature LCF behavior of Ti-1100 with a lamellar structure was investigated at 600°C. It was found that fatigue lives for 10 and 30 min hold times were lower than those in continuous cycle fatigue due to additional creep resulting from stress relaxation during the tensile hold. Creep deformation during hold time resulted in a change of dislocation structure from a planar form to an homogeneous distribution within the α lamellae. An apparent activation energy for creep deformation of about 520 kJ/mol was obtained from the stress relaxation curve and was consistent with results of other near-α titanium alloys. In all cases failures were fatigue-dominated, although creep cavity formation was observed in hold time spcimens.

The influence of loading frequency on the high-temperature fatigue behavior of a Nicalon-fabric-reinforced polymer-derived ceramic-matrix composite

Fiber-reinforced ceramic-matrix composites are under development for high-temperature structural applications. These applications involve fatigue loading under a wide range of frequencies. To date, high-temperature fatigue experiments have typically been performed at loading frequencies of 10 Hz or lower. At higher frequencies, a strong effect of loading frequency on fatigue life has been demonstrated for certain CMC`s tested at room temperature. The fatigue life of CMC`s with weak fiber-matrix interfaces typically decreases as the loading frequency increases. This decrease is attributed to frictional heating and frequency dependent interface and fiber damage. More recently, it has been shown that the room temperature fatigue life of a Nicalon-fabric-reinforced composite with a strong interface (SYLRAMIC{trademark}) appears to be independent of loading frequency. The high-temperature low-frequency fatigue behavior of the SYLRAMIC composite has also been investigated. For a fatigue peak stress {sigma}{sub peak} above a proportional limit stress of 70 MPa, the number of cycles to failure N{sub f} decreased with an increase in {sigma}{sub peak}. The material endured more than 10{sup 6} cycles for {sigma}{sub peak} below 70 MPa. In this paper, the influence of loading frequency on the high-temperature fatigue behavior of the SYLRAMIC composite is reported. It will be shown that themore » fatigue limit is unaffected by the loading frequency, that the number of fatigue cycles to failure N{sub f} increases with an increase in frequency, and that the time to failure t{sub f} decreases with an increase in frequency.« less

High-temperature fatigue of titanium alloys

The development of high-temperature titanium alloys has been dominated by the requirements of aerospace industries. Initially, improvements in tensile strength and creep properties were the main interest. Gas inlet temperatures in aero-engines have increased continuously and, nowadays certain components in gas turbine engines that are made out of titanium alloys, such as compressor discs and rotor blades, approach operating temperatures as high as 550°C. As a result of alloy development, near-a alloys capable of operating temperatures up to 600°C are commercially available. These alloys are intended to partially replace heavier nickel-base superalloys, e.g. in the compressor of gas turbine engines and much work has been directed towards understanding high-temperature fatigue, creep-fatigue interaction and oxidation behaviour of titanium alloys. It has been recognized that alloy composition, heat treatment and microstructure all influence strongly the high-temperature properties of titanium alloys. This review focusses on the relationship between microstructure and high-temperature fatigue behaviour of titanium alloys. It will cover mainly conventional (α + β) alloys and near-α alloys which have been optimized with respect to both creep and high-temperature fatigue properties.

Mechanical behavior and damage development during cyclic fatigue at high-temperature of a 2 · 5D [formula omitted] composite

This article describes part of a project to investigate the high-temperature fatigue behavior of a 2 · 5D C SiC multilayer composite. This material is particularly interesting since it has been found that damage exists in the as-supplied material owing to the residual stresses produced during cooling of the composite. This section of work describes the effect of the number of cycles and the temperature on the composite, and attempts to explain the behavior during testing. The parameters controlling damage development and therefore fatigue life, are believed to be both thermal constraint in the composite, and the damage produced. The nature of the damage is such that the material is best considered in terms of damage occuring on the scale of the yarns, rather than at the smaller fiber level.

The effect of mean stress on the high-temperature fatigue behaviour of SAE 1045 steel

Specimens of the mild steel SAE 1045 in normalized condition were subjected to stress-controlled push-pull fatigue tests at temperatures ranging from 20 °C to 375 °C. Up to about 250 °C, the steel shows a decrease in cyclic strength and in the number of cycles to failure N f with increasing temperature. A further increase in temperature leads to an increase in strength due to dynamic strain ageing (DSA) which reaches its maximum effect at a temperature T DSA,max of about 325 °C. As a consequence of the strongly reduced plastic strain amplitude, a maximum of the cyclic lifetime could be observed at a temperature close to T DSA,max. Tests with superimposed mean stresses varying from σ m = − 80 MPa to σ m = +60 MPa showed that a positive (tensile) mean stress generally increases the plastic strain amplitude and reduces the fatigue life and vice versa. The study of the microstructure revealed dislocation arrangements in the form of debris/bundle structures at T DSA,max and cell structures below and above the temperature range of DSA. At a given temperature in this range, positive mean stresses promote the formation of a cell structure because of the superimposed cyclic creep deformation taking place.

High temperature low cycle fatigue

Fatigue at high temperature is a complex phenomenon as it is influenced by a number of time-dependent processes which become important at elevated temperatures. These processes include creep, oxidation, phase instabilities and dynamic strain ageing (DSA), acting either independently or synergistically influence fatigue behaviour, often lowering the fatigue life. Current design approaches employ linear summation of fatigue and creep damage with suitable factors on permissible damage to take care of uncertainties in interaction between cyclic and time-dependent processes. It is, therefore, important to develop a deeper understanding of the processes that occur during high temperature fatigue so that realistic life predictions could be made. Results on the high temperature fatigue behaviour of austenitic stainless steels, ferritic steels and nickel base alloys are presented here. The important mechanisms of interaction of high temperature time-dependent processes with fatigue under various conditions are discussed in detail. Emphasis is placed on cyclic stress response, fatigue life, deformation substructure and fracture behaviour. This is followed by a review of important life prediction techniques under combined creep-fatigue loading conditions. Life prediction techniques considered here include linear damage summation, strain range partitioning, ductility exhaustion approach, frequency modified and frequency separation methods, techniques based on hysteresis energy and damage rate models, and methods based on crack-cavitation interation models.

Assessment of Microalloying Effects on the High Temperature Fatigue Behavior of NiAl

AbstractBinary NiAl suffers from a lack of strength and poor creep properties at and above 1000 K. Poor creep resistance in turn affects low cycle fatigue (LCF) lives at low strain ranges due to the additional interactions of creep damage. One approach for improving these properties involves microalloying with either Zr or N. As an integral part of a much larger alloying program the low cycle fatigue behavior of Zr and N doped nickel aluminides produced by extrusion of prealloyed powders has been investigated. Strain controlled LCF tests were performed in air at 1000 K. The influence of these microalloying additions on the fatigue life and cyclic stress response of polycrystalline NiAl are discussed.

High-temperature low-cycle fatigue behavior of superalloy MA 760

The high-temperature cyclic behavior of oxide dispersion-strengthened superalloy MA 760 was studied at different temperatures and specimen orientations. The cyclic hardening curves at different strain am-plitudes were determined at 650, 900, and 950 °C, and the stress-strain response of the material was sub-stantiated by detailed fractographical scanning electron microscopy (SEM) and microstructuralJoptical microscopy, STEM) observations. During cycling, MA 760 did not exhibit marked hardening or soften-ing, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the material. The “brittle” nature of deformation behavior of MA 760, especially at lower test tempera-tures, as well as oxidation-induced secondary cracking along grain boundaries, was clearly revealed by microscopical studies of the fracture surfaces. At 650 °C, evidence of dislocation cutting of γ was ob-served in transmission micrographs, but at higher temperatures, dislocation climb and by-passing of γ particles were found to become more prominent features. At all test temperatures, dislocation climb over yttria particles, as well as the departure side pinning effect of dislocations at nonshearable particles, was frequently observed.