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.     

Loading frequency and microstructure interactions in intergranular fatigue crack growth in a disk Ni-based superalloy

The paper examines the role of the loading frequency on the dwell fatigue crack growth mechanism in the super-solvus nickel-based superalloy, ME3. This is accomplished by carrying out a set of crack growth experiments in air and vacuum at three temperatures; 650 °C, 704 °C and 760 °C using a dwell loading cycle with hold time periods up to 7200 s imposed at the maximum load level. Results of these tests show that the transitional transgranular/intergranular loading frequency is 0.1 Hz, and are used to determine the apparent activation energy of the time-dependent crack growth process. Analysis of this energy in both air and vacuum showed that the intergranular cracking is governed by a mechanism involving grain boundary sliding. This mechanism is explained in terms of absorption of dissociated lattice dislocations into grain boundary dislocations. The gliding of these dislocations under shear loading is assumed to cause grain boundary sliding. A condition for this mechanism to occur, is that a critical minimum distance exists between slip bands impinging the affected grain boundary. This condition is examined by correlating the slip band spacing (SBS) and loading frequency using a model based on minimum strain energy accumulation within slip bands and that a unique configuration of number and spacing of bands exists for a given plastic strain. The model outcome expressed in terms of SBS as a function of loading frequency is supported by experimental measurements at both high and low loading frequencies. Results of the model show that a saturation of SBS, signifying a condition for intergranualr cracking, is reached at approximately 3 μm which is shown to coincide with the transitional loading frequency of 0.1 Hz.     

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.     

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.     

Microstructure and fatigue crack growth behavior in tungsten inert gas welded DP780 dual-phase steel

The purpose of this study was to evaluate microstructural and mechanical change of DP780 steel after tungsten inert gas (TIG) welding and the influence of notch locations on the fatigue crack growth (FCG) behavior. The tempering of martensite in the sub-critical heat affected zone (HAZ) resulted in a lower hardness (~ 220 HV) compared to the base material (~ 270 HV), failure was found to originate in the soft HAZ during tensile test. The fusion zone (FZ) consisted of martensite and some acicular ferrite. The joint showed a superior tensile strength with a joint efficiency of 94.6%. The crack growth path of HAZ gradually deviated towards BM due to the asymmetrical plastic zone at the crack tip. The FCG rate of the crack transverse to the weld was fluctuant. The Paris model can describe the FCG rate of homogeneous material rather well, but it cannot precisely represent the FCG rate of heterogeneous material. The fatigue fracture surface showed that the stable expanding region was mainly characterized by typical fatigue striations in conjunction with secondary cracks; the rapid expanding region contained quasi-cleavage morphology and dimples. However, ductile fracture mechanism predominated with an increasing stress intensity factor range (ΔK). The final unstable failure fractograph was subtotal dimples.     

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.     

Experimental study on the fatigue failure mechanism of QP-16 type ball-eye under asymmetric load

The ball eye (BE) is a key connecting component between the insulator and transmission tower, whose fatigue characteristics concern the safety of transmission lines. To understand the fatigue mechanism and characteristics of it, the fatigue test was conducted based on the following data: r = 0.25, S = 500 MPa,then plotting of SN and Δε_axis − N, to analyze the fatigue failure of the test specimen from the macro and micro point of views. The research results show that: the life of BE significantly reduces with the increase of the stress amplitude, but the relative reduction in life is not the same; softening and strain amplitude of the specimen change differently before and after the stress amplitude of 300 MPa; when S ≤ 300 MPa, the fracture is more smooth, the fatigue crack propagation is slow; when S > 300 MPa, the rate of fatigue crack growth is faster, and the fatigue crack growth zones are not obvious. The cracks are easily detectable appear at the joint of the BE and insulator cap, and the cracks along the fracture cross section are constantly expanding, showing multiple fatigue sources and fatigue steps. The number of fatigue steps increases as the magnitude of the tensile stress increases. When S = 500 MPa, the yield strength decreases during the lifetime, the decrease rate of the tensile strength and microstructure strength in each stage are different. Axial lengthening and section shrinkage ratio decrease with the development of fatigue, fatigue evolution process is accompanied by phenomenon of crystalline slip, deformation, dislocation, at the same time, dissipation and decomposition of pearlite occur, and carbide precipitates from the matrix, growing and moving to the grain boundaries, the specific phenomenon of grain growth appears.     

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

Fatigue crack initiation and propagation behaviour in subsolvus heat treated turbine disc alloy N18 has been assessed in air and vacuum at 650 and 725 °C under three-point loading. Fatigue crack initiation processes have been evaluated using single edge U-notch specimens under a 1-1-1-1 trapezoidal loading waveform along with interrupted tests at 650 °C to allow intermittent observations of the notch surface. The results show apparent grain boundary (GB) oxidation can occur under an oxygen partial pressure of 10⁻²–10⁻³ Pa. Cracks mainly initiate from grain boundaries or γ/γ′ interfaces due to the formation and subsequent cracking of Cr-rich and/or Co-rich oxides, and occasionally initiate from surface pores. Fatigue life in these tests appears to be dominated by this crack initiation process and is significantly reduced by increasing temperature and/or application of an oxidizing environment. Crack growth tests conducted under 1-1-1-1 and 1-20-1-1 loading waveforms indicate that oxidation significantly degrades the crack growth resistance of N18 and is associated with more intergranular fracture surface features. Additional oxidation effects on propagation caused by higher temperature or prolonging dwell time appear limited, whereas a prolonged dwell period seems to instead promote additional creep process, which further enhance crack growth, especially at higher temperature.     

Influence of hot corrosion on low cycle fatigue behavior of nickel base superalloy SU 263

The influence of hot corrosion on low cycle fatigue behavior is studied by conducting fatigue tests at 800 °C in air on bare and salt-coated (90%Na₂SO₄ + 10%NaCl) specimens. This was followed by extensive scanning electron microscopic (SEM) examinations. Significant reduction in fatigue life is observed across all values of Δε_t/2 for the salt-coated specimens in comparison with bare specimens. SEM examination reveals that the fused salt mixture sporadically removes the protective chromium oxide layer and exposes the substrate. Subsequent SEM analysis reveals that severe grain boundary oxidation leads to grain boundary cracking and provides numerous sites for fatigue crack nucleation and growth.     

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.     

Threshold crack growth behavior of shear and tensile cracks

Crack-face interference-free mode I and mode II crack-growth data was combined with smooth axial (λ = ε_xy/ε_xx = 0) and torsional (λ = ∞) endurance limit data to develop unified crack growth models that incorporate both shear and tensile cracking. The crack growth models incorporated growth from a slip band (including short crack behavior) size crack until the final failure of a long crack, and the ability to switch between crack growth on shear planes to growth on tensile planes. The models successfully predicted smooth specimen crack-face interference-free fatigue lives and gave reasonable estimates of the smooth specimen endurance limits of crack-face interference free tubular tests run at intermediate strain ratios (λ = 3/4, 3/2, and 3). The series of Kitigawa–Takahashi (threshold fatigue) diagrams developed from the models help illustrate the competition between shear and tensile cracking at the fatigue limit under crack-face interference-free crack growth.     

Effect of Cr precipitates and He bubbles on the strength of 〈1 1 0〉 tilt grain boundaries in BCC Fe: An atomistic study

Molecular static simulations were carried out to study the fracture process of different 〈1 1 0〉 tilt grain boundaries (Σ19{3 3 1}, Σ9{2 2 1}, Σ3{1 1 1}, Σ3{1 1 2}, Σ11{1 1 3}, Σ9{1 1 4}). The main goal of this work was to investigate variation of the deformation mechanism and fracture stress in the presence of Cr precipitates, voids and He bubbles at the core of the grain boundaries (GBs). The corresponding deformation process was characterized in terms of stress–strain relationship and deformation mechanisms were inspected by visualization tools. Based on the obtained stress–strain curves, the studied GBs can be subdivided into two types, those that exhibit extensive slip and those that do not show slip at all. The presence of Cr precipitates at the GB core increases critical shear stress necessary to initiate the slip, and nucleation of a crack was regularly seen to occur at the precipitate–matrix interface. The effect of voids and He bubbles on the fracture stress is much stronger. It was revealed that the plastic deformation was essentially suppressed. The reason for the suppression was attributed to the emission of the dislocations from voids/bubbles and their pile up.     

Gigacycle fatigue initiation mechanism in Armco iron

Microstructure irreversibility plays a major role in the gigacycle fatigue crack initiation. Surface Persistent Slip Bands (PSB) formation on Copper and its alloy was well studied by Mughrabi et al. as typical fatigue crack nucleation in the very high cycle fatigue regime. In the present paper, Armco iron sheet specimens (1 mm thickness) were tested under ultrasonic frequency fatigue loading in tension–compression (R = −1). The test on the thin sheets has required a new design of specimen and new attachment of specimen. After gigacycle fatigue testing, the surface appearance was observed by optical and Scanning Electron Microscope (SEM). Below about 88 MPa stress, there is no PSBs even after fatigue cycle up to 5 × 10⁹. With a sufficient stress (above 88 MPa), PSBs in the ferrite grain was observed by optic microscope after 10⁸ cycles loading. Investigation with the SEM shows that the PSB can appear in the body-centered cubic crystal in the gigacycle fatigue regime. Because of the grain boundary, however, the local PSB did not continually progress to the grain beside even after 10⁹ cycles when the stress remained at the low level.     

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.     

Experimental investigation of R-ratio effects on fatigue crack growth of adhesively-bonded pultruded GFRP DCB joints under CA loading

A series of fatigue experiments was performed in order to investigate the effect of the R-ratio on the fatigue/fracture behavior of adhesively-bonded pultruded GFRP double cantilever beam joints. Constant amplitude fatigue experiments were carried out under displacement control with a frequency of 5 Hz in ambient laboratory conditions. Three different R-ratios were applied: R = 0.1, R = 0.5 and R = 0.8. The crack length was determined by means of crack gages and a dynamic compliance method. The dominant failure mode was a fiber-tear failure that occurred in the mat layers of the pultruded laminates. The depth of the crack location significantly affected the energy dissipated for the fracture under cyclic loading. Short-fiber and roving bridging increased the fracture resistance during crack propagation. Fatigue crack growth curves were derived for each R-ratio and each observed crack path location. The fatigue threshold and slope of the fatigue crack growth curve significantly increased with increased R-ratio.     

Crack growth behavior of 7075-T6 under biaxial tension–tension fatigue

Crack growth behavior of aluminum alloy 7075-T6 was investigated under in-plane biaxial tension–tension fatigue with stress ratio of 0.5. Two biaxiality ratios, λ (=1 and 1.5) were used. Cruciform specimens with a center hole, having a notch at 45° to the specimen’s arms, were tested in a biaxial fatigue test machine. Crack initiated and propagated coplanar with the notch for λ = 1 in L–T orientation, while it was non-coplanar for λ = 1.5 between L–T and T–L orientations. Uniaxial fatigue crack growth tests in L–T and T–L orientations were also conducted. Crack growth rate in region II was practically the same for biaxial fatigue with λ = 1 in L–T orientation and for the uniaxial fatigue in L–T or T–L orientations, while it was faster for biaxial fatigue with λ = 1.5 at a given crack driving force. However, fatigue damage mechanisms were quite different in each case. In region I, crack driving force at a given crack growth rate was smallest for biaxial fatigue with λ = 1.5 and for uniaxial fatigue in T–L orientation, followed by biaxial fatigue with λ = 1 and uniaxial fatigue in L–T orientation in ascending order at a given crack growth rate.     

Fatigue crack growth characteristics of a new Ni–Co-base superalloy TMW-4M3: effects of temperature and load ratio

The fatigue crack growth (FCG) behavior of a newly developed Ni–Co-base superalloy TMW-4M3 has been investigated in this study. Experiment was carried out in laboratory air with compact-tension specimen. The effects of temperature (400, 650, and 725 °C) and load ratio (0.05 and 0.5) on crack growth rate were studied in the Paris regime. The results revealed that the crack growth rate increased with increasing temperature under a given load ratio. The influence of load ratio on crack growth rate was pronounced, especially at higher temperature and low stress intensity factor range. Fractographic observations showed that fracture mode was transgranular at 400 °C, mixed transgranular and intergranular at 650 °C, and predominantly intergranular at 725 °C. The possible explanations for the crack growth behavior were discussed based on the degradation of mechanical properties and the oxidation assisted crack growth, as well as the crack closure effect. A comparison of FCG rate was also made between TMW-4M3 and the commercial superalloy U720Li.     

Localized microtwin formation and failure during out-of-phase thermomechanical fatigue of a single crystal nickel-based superalloy

The deformation and damage mechanisms of a single crystal nickel-based superalloy CMSX-4 have been investigated under out-of-phase thermomechanical fatigue (OP TMF) condition. The deformation was highly localized to the area near the crack tip, where multiple groups of parallel twin plates on {1 1 1} planes formed during the high temperature-compressive half cycle. The atomistic a/6 〈1 1 2〉 twinning shear-based approach is presented which explains the origin of twinning. The localized twins provided a preferential path for crack propagation. OP TMF deformation was dominated by partial dislocation movement with {1 1 1}〈1 1 2〉 slip system, resulting in the formation and propagation of deformation twins.     

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.