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.     

Prediction of notched specimen behaviour at ambient and high temperatures in Ti6246

Life prediction methods are essential in the selection of materials for high performance applications. These design criteria allow safe lives to be predicted for areas of geometrical discontinuity where localized increases in stress lead to the early initiation and propagation of fatigue cracks under cyclic loading. This paper explains two methods for predicting the initiation life of a double edged notch specimen (K_t = 1.9), and applies them to the α + β titanium alloy Ti6246 over a range of temperatures. The Coffin–Manson equation is effective for fully reversed cyclic behaviour. However a Walker strain-based parameter was found to be more appropriate when mean stresses are introduced. The analysis encompasses traditional analytical approaches which are limited in relation to the determination of the stresses and strains at the notch root and a finite element analysis based on ABAQUS^®. The FEA is used to characterize loop generation at the notches and to optimize the stress and strain conditions in the critical root positions.The main focus of the paper, however, is high temperature behaviour where creep and environmental damage impact on fatigue crack development. The limitations of the modelling approach under these conditions are discussed.     

Crack growth in discontinuous glass fibre reinforced polypropylene under dynamic and static loading conditions

Crack propagation in single edge notched tensile specimens of isotactic polypropylene reinforced with short E-glass fibres has been investigated under both fatigue and creep loading conditions. Fatigue crack propagation (FCP) experiments have been performed at three different frequencies (0.1, 1, 10 Hz) and at a mean applied tensile load of 1200 N. Isothermal creep crack propagation (CCP) tests have been conducted under a constant tensile applied load of 1200 N at various temperatures in the range from 32 to 60 °C. Analysis of FCP data allowed an estimation of the pure fatigue and pure creep components of the crack velocity under the adopted cyclic loading conditions. Crack growth at low frequencies (0.1 and 1 Hz) is mainly associated with a non-isothermal creep process. At higher frequency (10 Hz), the pure fatigue contribution appeared more pronounced. Finally, the comparison of FCP and CCP as a function of the mean applied stress intensity factor confirmed the major contribution of creep crack growth during FCP process at low frequencies.     

Crack growth mechanisms in an aged superalloy at high temperature

Turbine blades made of Ni-base superalloy IN738 in stationary gas turbine had been removed during routine maintenance procedures after 10,000–20,000 h of operation. Crack growth in this material was studied by performing lab tests with specimens directly cut from the hot area of these blades with testing temperatures of 20 °C and 700 °C. The crack paths in these tests were analyzed and compared to those of service cracks. It was found that even though intergranular crack growth was dominant at elevated temperatures there was still a certain amount of transgranular crack propagation in all cases. Transgranular crack growth under operating conditions led to local recrystallization in the plastic zone ahead of the crack tip. The implications of the recrystallization process on the crack growth rate are discussed in the paper.     

The role of oxidation damage in fatigue crack initiation of an advanced Ni-based superalloy

The effects of prior oxidation on the room temperature fatigue life of coarse-grained Ni-based superalloy, RR1000, have been investigated. High cycle fatigue tests were conducted, on both machined and pre-oxidised testpieces, at room temperature at an R ratio of 0.1. The oxidation damage was produced by pre-exposures at 700 °C for either 100 or 2000 h. Pre-oxidised testpieces tended to fail with shorter fatigue lives than those obtained from the as-machined testpieces although they were also observed to outperform the as-machined test pieces at peak stress levels around 900 MPa. The chromia scale and intergranular alumina intrusions formed during pre-oxidation are prone to crack under fatigue loading leading to early crack nucleation and an associated reduction in fatigue life. This has been confirmed to be the case both below and above a peak stress level of ∼900 MPa. The better fatigue performance of the pre-oxidised specimens around this stress level is attributed to plastic yielding of the weaker γ′ denuded zone, which effectively eases the stress concentration introduced by the cracking of the chromia scale and intergranular internal oxides. This γ′ denuded zone is also a product of pre-oxidation and develops as a result of the selective oxidation of Al and Ti. Over a limited stress range, its presence confers a beneficial effect of oxidation on fatigue life.     

Fatigue and fracture of a 316 stainless steel metal matrix composite reinforced with 25% titanium diboride

Fatigue and fracture mechanisms have been studied in a steel-based metal matrix composite (MMC), comprising a 316L austenitic matrix reinforced with 25 wt.% particulate titanium diboride (TiB₂). The fracture toughness was determined in the as-HIPped condition as being slightly below 30 MPa√m. Fatigue crack growth rates have been determined, and corrected for the effects of crack closure. The fracture surfaces have been studied to determine the mechanisms of damage during crack advance, which are determined as matrix fatigue, reinforcement particle fracture, and ductile rupture of the matrix. We show that the occurrence of damage mechanisms during fatigue of the material is linked to K_max, rather than to ΔK. This is rationalised in terms of a semi-cohesive process zone within the monotonic plastic zone ahead of the crack tip.     

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C^* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30 mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.     

Time dependent effects in fatigue of titanium and nickel alloys

The paper explores fatigue at both low and high temperature where creep and environmental damage interact with the normal cyclic processes of crack development. This is achieved by studying two contrasting material systems: the titanium alloys (Ti685, Ti834, Ti6246) and the nickel alloys (Udimet 720Li). Particular attention is given to both load and strain control fatigue response and crack development at stress concentration features. In each case there is an interesting balance between the beneficial effects of stress relaxation and the damaging effects of creep and environmental factors. On the crack growth side, the relative contributions of creep and environment are highlighted through measurements made in air and vacuum and by varying R value and dwell time. At the same time, any complications due to closure are removed by careful measurement of closure levels for each condition. The inadequacy of linear damage models for combining cyclic and time dependent effects is highlighted.     

The effects of texture and grain morphology on the fracture toughness and fatigue crack growth resistance of nanocrystalline platinum films

The fracture toughness and fatigue crack growth resistance of nanocrystalline materials are significantly affected by the thickness of the specimen. In this work we relate the mechanical properties of nanocrystalline platinum films to their texture and grain morphology. Tensile, creep and fatigue testing of annealed, ∼1 μm films resulted in mechanical properties similar to the as-received films (yield strength of ∼1.2 GPa, fracture toughness ∼17.8 MPa √m, and a fatigue crack growth power law exponent of ∼4.2). However, the breakdown of the initially columnar grain morphology had a marked effect on the transition point from an intergranular to transgranular fatigue cracking mode. Finite element modeling suggests that cyclic (fatigue) grain coarsening and the transition from inter- to transgranular cracking modes are a result of the relative importance of dislocation slip accommodation on in-plane and through-thickness oriented slip directions.     

Effect of stress ratio on fatigue lifetime of high Nb containing TiAl alloy at elevated temperature

The effect of stress ratio (R) on fatigue lifetime of a cast Ti–45Al–8.0Nb–0.2W–0.2B–0.1Y (at.%) alloy was investigated at 750 °C. Fatigue tests with various stress ratios ranging from 0.1 to 1 were performed using a mini servo-hydraulic fatigue machine inside a chamber of scanning electron microscope (SEM). Fatigue crack initiation and propagation behavior was studied by in situ SEM observation and fatigue fracture mode was examined by fracture surface analysis. It is found that fatigue lifetime shows a reversed S-type curve with the increase of stress ratio. At R ranging from 0.1 to 0.4, creep–fatigue interaction dominates the fatigue lifetime and the fatigue lifetime reaches its minimum value at R = 0.3. At R ranging from 0.4 to 1, creep damage dominates the fatigue lifetime and the fatigue lifetime exhibits inverse proportional relation with R. Meanwhile, with the increase of stress ratio, the fatigue crack initiation sites transform from lamellar interface at R = 0.1, to lamellar interface and colony boundary at R = 0.3, and to lamellar colony boundary at R = 0.5. Accordingly, the fatigue fracture mode transforms from transgranular cracking, to transgranular and intergranular cracking, and to intergranular cracking.     

The interactions between fatigue, creep and environmental damage in Ti 6246 and Udimet 720Li

The paper reports the results of a comprehensive research programme on two different compressor disc alloys: titanium alloy Ti 6246 and the nickel based superalloy Udimet 720Li. Both alloys are used for disc applications in gas turbine engines under conditions where the rims are exposed to fatigue, creep and environmental damage. The titanium alloy was investigated at temperatures of 80, 450, 500 and 550 °C, whilst the nickel at 650 and 700 °C. The paper presents the strain-life response of plain specimens and relates these data to notch fatigue behaviour. It also explores fatigue crack propagation behaviour in air and under hard vacuum (10⁻⁶ Torr), in order to characterise the creep/environment/fatigue interactions that occur at the crack tip. The growth data encompass R values of −3 to 0.5 for cyclic and dwell waveforms. The information obtained allows the effects of environment and creep at these temperatures to be partitioned. The observed R value dependence is related to both empirical models and closure measurements. The experimental data are supported by detailed metallographic and fractographic studies using optical and SEM techniques.     

A phenomenological model to predict the crack growth in single crystal superalloys at high temperature

This paper presents the formulation of a phenomenological model to predict the crack growth in single crystal superalloy at high temperature. The proposed model relies on an extensive experimental study performed on AM1 single crystal superalloy at temperatures ranging from 650 °C to 950 °C. Tests carried out in fatigue and creep–fatigue regimes investigate the effects of time on crack growth rates. The crack growth model follows the framework of classical linear elastic fracture mechanics. Time effects at high temperature are captured by creep–fatigue and oxidation–fatigue interactions. The specific model formulation for nonisothermal conditions is attractive for identifying parameters on a large temperature domain and for predicting complex Thermo-Mechanical Fatigue (TMF) tests. Model predictions are then compared with a large set of experimental results including TMF tests. The application of this model, which accounts for a better understanding and modeling of physical phenomena such as the environmental or creep effects on crack growth rate, should improve the prediction of crack growth regime in single crystal superalloys that are used to design critical components such as turbine blades.     

Numerical investigation of factors affecting creep damage accumulation in ASME P92 steel welded joint

The present study mainly investigated Type IV cracking occurring in the fine grained heat affected zone (FGHAZ) in the welded joint of ASME P92 steel at high temperature and low applied stress by numerical simulation method. Based on the modified Karchanov–Rabotnov constitutive equation, the user defined material subroutine (UMAT) was complied and the creep damage accumulation was carried out by finite element method using ABAQUS codes for the welded joint at 650 °C and 70 MPa. Calculated results revealed that the most severe creep damage and the highest equivalent creep strain occurred in the FGHAZ because of high maximum principle stress and high maximum principle stress. Furthermore, the effect of groove angle and HAZ width on the creep damage accumulation was investigated. It indicated that a small groove angle and a narrow FGHAZ width could deteriorate the creep damage accumulation because of the degradation of maximum principle stress and stress triaxiality in the FGHAZ.     

Role of microstructure on initiation and propagation of fatigue cracks in precipitate strengthened Cu–Ni–Si alloy

Fatigue tests were conducted on round-bar specimens to understand the fatigue behavior of precipitate-strengthened Cu–6Ni–1.5Si alloy. Aging at 500 °C for 0.5 h produced δ-Ni₂Si precipitates in the matrix, homogeneously and heterogeneously precipitated δ-Ni₂Si particles, and a precipitate-free zone around the grain boundaries. The cracks were initiated at the grain boundaries, followed by growth along the crystallographic slip planes in the adjacent grains. Crack propagation from the crack origin along the grain boundaries was occasionally observed. The physical background of fatigue damage is discussed in light of the role of microstructure on the behavior of fatigue cracks.     

The fatigue crack growth rate and crack opening displacement in 18G2A-steel under tension

In the paper, the results of crack tip opening displacement (CTOD) and crack opening displacement (COD) in place of crack initiation as well as the fatigue crack growth rate in higher strength steel are presented. The investigation were carried out on flat specimens with central notch under constant amplitude tensile fatigue loading at stress ratio R = 0.2 and different value of the stress σ_max. The test results showed that with growth of crack length l grew values of the CTOD and COD. In the work, it was proposed calculation of the CTOD value on basis various dependence of plastic zone radius on crack tip.     

A low-cycle fatigue approach to fatigue crack propagation

The damage accumulation hypothesis is used to derive a fatigue crack growth rate equation. The fatigue life of a volume element inside the plastic zone is evaluated by using low-cycle fatigue concepts. Crack growth rate is expressed as a function of cyclic material parameters and plastic zone characteristics. For a given material, crack growth increment, is predicted to be a fraction of the plastic zone size which can be expressed in terms of fracture mechanics parameters,K andJ. Hence, the proposed growth rate equation has a predictive capacity and is not limited to linear elastic conditions.     

Fatigue crack growth and damage characteristics of high-speed rail at low ambient temperature

Low temperature can be a significant problem affecting safety and maintenance of railway. In this study, the fatigue crack growth rate and rolling contact fatigue damage behaviors of high-speed rail material under different temperature conditions were investigated by a series of experiments. The results indicate that the stress and strength of rail material increase with the decrease of ambient temperature. The crack growth rate at 0 °C and − 20 °C is similar with that at 20 °C. While, when the temperature decreases to − 60 °C, the growth rate of crack increases sharply. The promotion of rail embrittlement at low temperature accompanied with the action of high stress causes the rapid failure and increase of surface crack length and subsurface crack damage. Meanwhile, three crack growth mechanism models at different temperatures can be inferred. The brittle fracture mode is increasingly apparent with the temperature decreasing.     

Experimental investigation on low cycle fatigue of DZ125 with film cooling holes in different processes of laser drilling

Due to the different low cycle fatigue (LCF) properties and fatigue fracture behavior around film cooling holes on DZ125, the LCF tests are carried out using tension cycling under stress control conditions (stress ratio R = 0.1) at 900 °C. The specimens were designed as thin-wall plate with single hole and multi holes under picosecond and nanosecond laser drilling processes. Comparative analyses of the differences between fatigue life and microscopic fracture morphology are conducted. It is shown that under the same stress condition, the relationship between fatigue life is as follows: picosecond laser single-holed specimen > nanosecond laser single-holed specimens > picosecond laser multi-holed specimens > nanosecond laser multi-holed specimens. Scanning electron microscope (SEM) analyses of the fracture revealed that the crack initiates from the film cooling holes where fatigue source zone, fatigue crack propagation zone and fatigue fracture zone can be found. However, the different processes lead to slightly different fracture morphology: radial-type ridge centering on the fatigue source zone is more apparent and uniform in picosecond laser drilling specimens than in the nanosecond laser drilling ones. On the other hand, the radial-type ridge is biased toward large-aperture side with nanosecond laser drilling.     

A cohesive zone model for fatigue and creep–fatigue crack growth in single crystal superalloys

A numerical analysis using cohesive zone model under cyclic loading is proposed to develop a coupled predictive approach of crack growth in single crystal. The process of material damage during fatigue crack growth is described using an irreversible cohesive zone model, which governs the separation of the crack flanks and eventually leads to the formation of free surfaces. The cohesive zone element is modeled to accumulate fatigue damage during loadings and no damage during unloadings. This paper presents the damage model and its application in the study of the crack growth for precracked specimens. The use of cohesive zone approach is validated through a convergence study. Then, a general procedure of parameters calibration is presented in pure fatigue crack growth. In the last section, an extension of the cohesive zone model is presented in the case of creep–fatigue regime at high temperature. The model showed its capability to predict with a good agreement the crack growth in the case of complex loading and complex specimen geometries.     

Diffraction characterization of microstructure scale fatigue crack growth in a modern Al–Zn–Mg–Cu alloy

SEM-based electron backscattered diffraction (EBSD) measurements characterize constituent-particle nucleated fatigue crack path relative to local grain orientation and crack wake defect distribution for Al–Zn–Mg–Cu alloy 7050-T7451 stressed in moist air. Crack propagation is primarily transgranular; consisting of facets parallel to {1 0 0}, {1 1 0} and high-index planes with no evidence of {1 1 1} slip-based cracking; and is also inter-subgranular involving pre-existing or fatigue process zone generated subgrain boundaries. Dislocation substructure develops close to the fatigue crack surface due to dynamic recovery of crack tip cyclic plasticity. Crack growth through subgrain structure explains the broad occurrence of crack features without a low-index orientation and is justified based on trapped-hydrogen embrittlement. A failure criterion for environmental fatigue modeling must capture a failure mechanism based on: (a) formation of localized defect structure from cumulative cyclic plasticity (perhaps H sensitive), and (b) subsequent embrittlement due to interaction of H trapped at this defect structure with microstructure-sensitive local tensile stresses normal to this weakened interface. Crack interaction with subgrain (and grain) boundaries produces local deflections and branches that arrest over a short distance. Such features should cause a distribution of microstructure-sensitive growth rates.