High Temperature Low Cycle Fatigue Research Articles

High temperature low cycle fatigue properties of 24Cr ferritic stainless steel for SOFC applications

The low cycle fatigue (LCF) properties of 24Cr ferritic stainless steel for solid oxide fuel cell interconnects were investigated. The fatigue strength of 24Cr stainless steel decreased with increasing temperature, but the fatigue life increased at 600°C and 700°C. The fatigue behavior at room temperature (RT) was characterized as cyclic hardening followed by saturation and cyclic softening while marginal cyclic hardening was observed at 600°C and 700°C. The superior oxidation resistance of 24Cr stainless steel allows preventing the fatal impact of oxidation on the high-temperature fatigue life. Microstructural analysis showed that persistent slip bands (PSBs) developed prevalently at RT but not at 600°C and 700°C. Such temperature-dependent microstructural difference retarded the crack initiation and prolonged fatigue life at high temperatures.

Microstructure evolution of Al–12Si–CuNiMg alloy under high temperature low cycle fatigue

Microstructure evolution of the Al–12Si–CuNiMg alloy under high temperature low cycle fatigue was investigated with scanning and transmission electron microscopy. The alloy exhibits cyclic softening at diverse total strain amplitudes and loading temperatures. The material fatigue life obviously decreases with the increase of the strain amplitude at the same temperature. However, fatigue life increases and microstructure improves with temperature increase at the same strain amplitude. At certain loading temperatures and strain amplitudes, the microstructure can be refined. The fracture morphology changes gradually from brittle quasi-cleavage fracture, with numerous small cracks, to quasi-cleavage fracture with numerous small dimple gliding fractures.

High temperature low cycle fatigue deformation behaviour of forged IN 718 superalloy turbine disc

In the present investigation, low cycle fatigue deformation behaviour of forged turbine disc of IN 718 superalloy of different sections (rim, hub, and bore) was studied under asymmetrical waveforms (slow–fast and fast–slow) at 650°C. The superalloy exhibited dynamic strain aging (DSA) by showing the manifestation of serrations during the lower strain rate plastic region of the hysteresis loop under asymmetrical waveforms. Irrespective of different sections, the superalloy showed marginally reduced fatigue life under slow–fast waveform as compared to fast–slow waveform. This was attributed to higher accumulation of LCF damage under slow–fast waveform as confirmed from slip band spacing than that of the fast–slow waveform.

遮熱コーティングが基材の高温低サイクル疲労寿命に及ぼす影響

遮熱コーティングが基材の高温低サイクル疲労寿命に及ぼす影響

Fatigue Deformation Behavior of FGH96 Superalloy under Low Cycle Fatigue Loading

Low cycle fatigue failure tests of the powder metallurgical nickel based superalloy FGH96 at 550°C and 720°C were carried out under total strain-controlled mode (R=-1). The fatigue failure behaviors were investigated by analyzing cyclic stress response and observing microstructure after fatigue through scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The results show that FGH96 superalloy exhibits cyclic stability at 550°C, and cyclic softening afterwards at 720°C with Δε/2=0.4% , and of cyclic softening at 720°C with Δε/2=0.7%. After high temperature low cycle fatigue, the γ′ precipitates are cuboidal for all samples. No coarsening of γ′ precipitates was detected at 550°C and at 720°C with Δε/2=0.4%, but small γ′ precipitates get together to be larger precipitates at 720°C with Δε/2=0.7%. EBSD shows that the continuous small angle grain boundaries are produced during the fatigue. The small angle grain boundaries have a significant increase at 720°C, especially that case at Δε/2=0.7%.

Effects of microstructure in high temperature fatigue: Lifetime to crack initiation of a single crystal superalloy in high temperature low cycle fatigue

Single crystal superalloys are now the best alloys to make blades and vanes in aero-engines and gas turbines due to their temperature capability, their creep and fatigue resistance. Their composition and microstructure are optimised by heat treatments but low cycle fatigue resistance is mostly controlled by the initiation and early growth of micro-cracks at casting pores. An enriched engineering damage model is proposed to describe micro-crack growth from pores with a process zone concept. Damage equations use summation of contributions on all slip systems as the constitutive model. Oxidation can play a significant role that can be described through embrittled material ahead of micro-crack tip. To investigate the behaviour under small scale yielding at finer scale, experiments were carried out using sharp notches. Early growth of cracks in the notch vicinity was studied at two temperatures 650°C and 950°C. While at the lower temperature the notch can be analysed as a crack, anomalous crack growth rates are observed at the higher temperature. Using finite element computations and a damage model, this behaviour is attributed to local viscoplastic strain concentrations, and oxidation effects. Effects of strain concentration are attenuated since only a small volume of material is highly strained at the notch vicinity.

High temperature fatigue behaviors of a cast ferritic stainless steel

Abstract This work investigated the high temperature low cycle fatigue (LCF) behavior of ACI HB20-type, cast ferritic stainless steel. In addition to the preliminary results from tension test, isothermal strain controlled fatigue tests were carried out at 600 °C and 800 °C. The total strain amplitudes applied to the samples were from ±0.25% to ±0.65% with a strain rate of 10 −3 /s. The fatigue lifetime curves at each temperature were obtained based on Basquin and Coffin–Manson equations. Also, these temperature dependent plots were re-constructed into a general superposition curve and compared to other heat resistant stainless steels. Additionally, microstructure and fractured surface after LCF test were analyzed using a scanning electron microscopy suggesting that the crack initiated at the grain boundary of the surface and propagated intergranularly as well as transgranularly. This is attributed by the formation of Cr and Nb carbide along the grain boundaries.

Discussion of the Evaluating Criterion of 400°C Low Cycle Fatigue Life under Multiaxial Stress States

By the investigation of the 400°C low cycle fatigue behavior of 4 kinds of notched cylindrical specimens made 2.25Cr-1Mo stee1. The high temperature low cycle fatigue life was evaluated by the criterons of equivalent range. The results show that equivalent criterion for evaluating high temperature low cycle fatigue life has its experimental and theoretical foundation, but further investigation is still needed．

Surplus Fatigue Life Prediction of Diesel Engine Piston by the Equivalent J-Integral Approach

Three dimension temperature fields and stress-strain fields of the 6110-type diesel engine piston were analyzed by using the program ANSYS. The simulating tests of crack propagation under high temperature low cycle fatigue were carried out with pre-crack notched specimens. According to the theory of high temperature strength and fracture, the equivalent J-integral range ΔJf were induced to predict the surplus fatigue life of diesel engine piston. It was concluded that this approach is suitable and provides valuable reference for designers of diesel engine.

Research on High Temperature Low Cycle Fatigue Crack Propagation of 2.25Cr-lMo Steel under Complex Stress State

The crack propagation law of 2.25Cr-1Mo steel with notched cylinder was researched under high temperature low cycle fatigue. The crack propagation life was viewed by fatigue experiment and the equivalent stress-strain on the crack tip was calculated by the ANSYS. The equivalent J-integral range which was computed by equivalent elastic and plastic strain ranges were employed to denote the fatigue crack propagation rate. The results showed that crack propagation law of this material under complex stress state can be characterized by equivalent J-integral ranges and the relation between da/dN and ΔJf is not influenced by the type of notch and the load strain range.

Influence of Al-Si Diffusion Coating on Low Cycle Fatigue Properties of Cast Superalloy Inconel 738LC at 800 °C

High temperature low cycle fatigue behaviour of cast nickel-based superalloy Inconel 738LC in as-received condition and coated with an Al-Si diffusion layer was studied. The Al-Si protective layer was deposited on the gauge section of cylindrical specimens using the slurry technique. Fatigue tests were performed on cylindrical specimens under total strain control in symmetrical cycle at 800 °C in air. The coating has a beneficial effect on fatigue life in the low amplitude domain. The stress response of the coated material is higher for high amplitudes in comparison with the uncoated one. Fracture surfaces and sections parallel to the specimen axis have been examined to study fatigue damage mechanisms.

Use of instrumented micro-indentation to study the mesoscopic elasto-plastic behavior of GH4145/SQ superalloy during high-temperature cyclic straining

In this paper the instrumented micro-indentation testing (IIT) was used to study the mesoscopic elasto-plastic behavior of nickel-base superalloy GH4145/SQ subjected to cyclic straining at an elevated-temperature. For this purpose, a series of experiments including high-temperature low-cycle fatigue tests, instrumented micro-indentation measurements and OM (TEM) examinations were carried out. The characteristic parameters of indentation (C, hmax, S, hr, We and WP) during high-temperature fatigue failure process were determined from the experimental indentation load–depth (P–h) curves. The fatigue mesoscopic elasto-plastic properties such as E, σy and n were estimated using Dao et al.’s analysis algorithm [19], combined with the indentation characteristic parameters, and their distribution patterns were verified in a statistical sense. Microstructural examinations using both OM and TEM were performed to provide the micromechanisms for the fatigue mesoscopic elasto-plastic behavior of the superalloy during high-temperature low-cycle fatigue.

Specimen-size dependency and modelling of energy evolution during high-temperature low-cycle fatigue of pressure vessel steel

High-temperature low-cycle fatigue testing was conducted on pressure vessel steel using standard and miniature specimen sizes and the fatigue toughness required for macrocrack propagation was investigated. A definite specimen-size dependency was observed for both the threshold cumulative plastic strain energy and cycles required for macrocrack propagation, which was explained to be influenced by geometric conditions. An analytical modelling prediction was developed that accounted for specimen-size dependency and was successfully applied to predict fatigue toughness to macrocrack propagation.

High-temperature low cycle fatigue damage assessment in near alpha IMI-834 titanium alloy

In the present work, evolution of damage under high-temperature (823 K) low cycle fatigue loading condition in near α IMI-834 titanium alloy has been studied. The in situ damage has been experimentally measured during cyclic deformation using the alternating current potential drop (ACPD) technique. The measured damage curve has been compared with the damage curves calculated through mechanical variables such as cyclic modulus and stress amplitude. The ACPD damage curve has been found most sensitive towards high-temperature low cycle fatigue damage evolution.

A New Ductility Exhaustion Model for High Temperature Low Cycle Fatigue Life Prediction of Turbine Disk Alloys

Based on ductility exhaustion theory and the generalized energy-based damage parameter, a new viscosity-based life prediction model is introduced to account for the mean strain/stress effects in the low cycle fatigue regime. The loading waveform parameters and cyclic hardening effects are also incorporated within this model. It is assumed that damage accrues by means of viscous flow and ductility consumption is only related to plastic strain and creep strain under high temperature low cycle fatigue conditions. In the developed model, dynamic viscosity is used to describe the flow behavior. This model provides a better prediction of Superalloy GH4133's fatigue behavior when compared to Goswami's ductility model and the generalized damage parameter. Under non-zero mean strain conditions, moreover, the proposed model provides more accurate predictions of Superalloy GH4133's fatigue behavior than that with zero mean strains.

High temperature low cycle fatigue behaviour of hot isostatically pressed superalloy Udimet 720 LI

Nickel base superalloy Udimet 720 LI was processed through a powder metallurgy (P/M) hot isostatic pressing (HIP) route. The effects of this consolidation technique (HIPing) on low cycle fatigue (LCF) properties were evaluated and are reported in this paper. Total strain controlled LCF tests have been conducted in air at 550°C at a nominal strain rate of 6.67 × 10−3 s−1. The as-HIPed Udimet 720 Li exhibits stable behaviour followed by very mild cyclic hardening at total strain amplitudes ± 0.4–0.5%. However, cyclic hardening has been observed at total strain amplitudes ± 0.6–1.2%. The cyclic hardening is attributed to dynamic strain aging (DSA). The dual slope behaviour in a Coffin-Manson plot is attributed to a change in fracture mechanism.

Low cycle fatigue and creep–fatigue interaction in short fibre reinforced aluminium alloy composite

The high temperature low cycle fatigue resistance and the creep–fatigue interaction (CFI) behaviour in terms of the effects of prior fatigue exposure on the subsequent creep behaviour are evaluated and reported for a short alumina fibre (Saffil) reinforced aluminium alloy (Al–12Si–CuMgNi) matrix composite at 623 K. The prior fatigue to study the CFI behaviour was imparted in the form of low cycle fatigue loading in a fully reversed, total strain controlled loading up to a quarter of fatigue life at a total strain amplitude of 0·006 (the plastic strain amplitude at half-life is 0·004), corresponding to a plastic strain energy per cycle value of 0·46 MJ m–3. Subsequently, isothermal tensile creep tests were conducted at 623 K to evaluate the minimum creep rate, rupture time and strain to failure as a function of applied creep stress. Also examined were the fracture features as well as the nature and extent of damage that occurs during low cycle fatigue and creep–fatigue loading. The results obtained on the composite material are compared with those of the matrix aluminium alloy to bring out the effects of reinforcement. The results showed that the reinforcement causes significant loss in high temperature low cycle fatigue resistance in terms of fatigue ductility and cyclic energy parameters. Prior fatigue loading was seen to cause a small but consistent decrease in the creep resistance, which is attributed to the combined effects of mechanical loading and microstructural damage from prior fatigue loading.

Effects of Hot Isostatic Pressing on the Microstructure of the Ni-Base Super-Alloy

Applied gas turbine blade material(Ni base Super-alloy) for aircraft as principal parts material of existing electricity generating gas turbine blade. Studied microstructure change after do heat treatment, Hot Isostatic Press(HIP) processing to nickel-based super-alloy. Emphasize in properties of matter refreshing effect confirmation by HIP processing, microstructure observed by scanning electron microscope. HIP processing effect caused positive effect in high temperature Creep and low cycle fatigue failure by microscopic air hole exclusion. Sheep of Eutectic region decreased solid solution being done B, and also precipitated size of cuboidal shape great. Heat treatment effect was seen modification by M23C6 carbide creation that Script MC carbide is microscopic and does Discrete through solution treatment in high temperature state. Precipitation that grow after HIP processing over solution processing and aging processing in heat treatment process solid solution after is done again precipitate being done size decrescent.

High temperature fatigue resistance of an ACI HH50-type cast austenitic stainless steel

A study was undertaken to examine the high temperature low cycle fatigue behavior of an ACI HH50-type cast austenitic stainless steel. The samples were cast and machined into cylindrical specimens and then subjected to isothermal strain controlled fatigue tests at 500, 650, and 800 °C with total strain amplitudes from ±0.30% to ±0.80% at a strain rate of 3.3 × 10 −4/s. The cyclic stress responses and fatigue resistances were analyzed by obtaining the fatigue parameters of both Basquin and Coffin-Manson equations. Additionally, a general superposition curve containing all of test temperatures was obtained using a modified fatigue equation based on the constitutive equation for the energy criterion of fatigue failure in terms of the thermally activated dislocation motions. The temperature dependent fatigue parameters that were obtained from this equation enables compare high temperature fatigue resistances of alloys. The ACI HH50-type alloy showed superior fatigue properties in comparison with the ACI HF30 and AISI 304 stainless stills.

Detecting degradation in Ni-based superalloy Udimet520 with scanning SQUID microscopy

In order to develop a new method for detecting degradation nondestructively in Udimet520 (U520), which is used for blades of aircraft jet-engine components and land-based gas turbines, fractured and interrupted samples in low-cycle fatigue (LCF) and creep tests were studied using scanning SQUID microscopy (SSM). High temperature LCF and creep experiments on U520 were conducted to obtain various samples with different damage level. Simultaneously, we also examined the artificially degraded samples by optical microscopy, electron backscattering diffraction, and hardness measurements to ensure the damage level. On the basis of the magnetic permeability data, the virgin sample is nonmagnetic. However, for the artificially damaged samples, the observation of magnetic signals on the field maps indicates the potential of SSM to evaluate the degradation in U520 superalloy nondestructively.