Creep crack growth in udimet 700

Crack growth behavior in Udimet 700 was studied at 850°C (1560°F) and the crack growth rates are correlated with various fracture mechanics parameters,viz the stress intensity factor,J andC* integrals as well as with nominal stress. While there is considerable spread in the data in terms of all of these parameters, the crack growth rates seems to correlate better with the stress intensity factor than with the other three parameters. The crack growth behavior in the Udimet alloy is shown to differ significantly from that in previously studied Alloy 718 and these differences are attributed to the possible difference in the mechanisms of crack growth in the two alloys.     

Creep crack growth in alloy 718

Subcritical crack growth behavior in Alloy 718 was studied under creep conditions at 538, 649, and 760°C (1000, 1200, 1400°F) and crack growth rates were correlated using both linear and nonlinear elastic fracture mechanics. The results show that for a given stress intensity value crack growth rate increases significantly with increase in temperature from 538 to 649°C but either decreases or increases slightly with further increase in temperature to 760°C. On the basis of these results it is concluded that creep crack growth results from a balance of two competing processes, diffusion of point defects which contributes to crack growth, and creep deformation process that causes retardation of crack growth and even its arrest. Significance of these concepts in relation to enhancing the resistance of a given material to creep crack growth is discussed in detail.     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

Visual simulation of fatigue crack growth

An attempt has been made to visually simulate fatigue crack propagation from a precrack. An integrated program was developed for this purpose. The crack-tip shape was determined at four load positions in the first load cycle. The final shape was a blunt front with an “ear” profile at the precrack tip. A more general model, schematically illustrating the mechanism of fatigue crack growth and striation formation in a ductile material, was proposed based on this simulation. According to the present model, fatigue crack growth is an intermittent process; cyclic plastic shear strain is the driving force applied to both state I and II crack growth. No fracture mode transition occurs between the two stages in the present study. The crack growth direction alternates, moving up and down successively, producing fatigue striations. A brief examination has been made of the crack growth path in a ductile two-phase material.     

Creep crack growth of HK40 steel: Microstructural effects

Creep crack growth tests at 871 °C have been performed on compact tension specimens of HK40 steel having different microstructures. The skeleton-shaped carbides on the grain boundaries have a higher resistance to crack growth than the blocky-shaped carbides. The secondary carbide size and distribution explicitly affect crack growth behavior. There exists a critical size of the secondary carbides. With an increase in the secondary carbide size, the resistance to crack growth increases up to the critical size and decreases beyond the critical size.     

Creep deformation and crack growth behavior of a single-crystal nickel-base superalloy

Uniaxial creep deformation and crack growth data are presented on the single-crystal nickel-base superalloy SC16, which is a candidate material for industrial gas turbine applications. All testing was performed at 900 °C. The uniaxial experiments were conducted with the loading direction aligned approximately along the [001] crystallographic axis of the material. Under these conditions, a small primary region followed by mainly tertiary creep was obtained, and failure initiated from cracks at interdendritic pores. The crack growth experiments were performed on single-edge notch tension specimens and compact tension test pieces containing deep side grooves to examine state-of-stress effects. A selection of crystallographic orientations was also examined. Little effect of stress state and orientation was obtained. It has been found that the creep crack growth characteristics of the alloy can be predicted satisfactorily from a model of the accumulation of damage at a crack tip using the creep fracture mechanics parameter C* and assuming plane stress conditions. This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium, as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow and Fracture Committee.     

Subcritical crack growth under sustained load

Compact tension specimens of annealed Ti-4 Al-3 Mo-l V were exposed under sustained load for periods of up to 8 days to determine the effects of initial stress intensity and test environment on subcritical crack growth. Crack growth occurred by a tunneling process with no surface crack extension until just prior to final rapid failure. Crack growth in vacuum or moist air environments occurred at stress intensities as low as 40 pct of the fracture toughness, and there was no evidence of a threshold stress intensity below which crack growth would not occur. Specimens tested in salt water behaved similarly at stress intensities of greater than about 60 pct of the fracture toughness, but showed crack arrest at lower stress intensities. At lower stress intensities, resistance to crack growth in a saltwater environment was superior to that in vacuum or moist air. Subcritical crack growth was readily identified on the fracture surface after exposure in all three environments through the presence of numerous cleavage-like facets. A critical strain concept, with crack growth occurring as a result of creep processes, can be used to explain the results.     

Fatigue crack growth and stress redistribution at interfaces

The role of the interface in redistributing stress around cracks in multilayered ceramic/metal composites is investigated. The emphasis is on the different effects of interfacial debonding or of plastic slip in the metal phase adjacent to strongly bonded interfaces. The experiments are conducted on alumina/aluminum multilayered composites. Monotonic loading precracked test pieces causes plastic shear deformation within the aluminum layer at the tip of the notch without debonding. However, interfacial debonding can be induced by cyclic loading, in accordance with a classical fatigue mechanism. Measurements of the stress around the crack demonstrate that debonding is much more effective than slip at reducing the stress ahead of the crack.     

The threshold stress intensity for hydrogen-induced crack growth

The crack growth rates and threshold stress intensities,K _TH, for a 3 1/2 NiCrMoV steel (0.2 pct proof stress 1200 MPa) have been measured in a hydrogen environment at various temperatures and hydrogen pressures. Fractographic evidence and the observation of alternating fast and slow crack growth nearK _TH suggests that the crack advances by the repeated nucleation of microcracks at microstructural features ahead of the main crack. Transient crack growth is observed following load increases just belowK _TH. Using the idea, from unstable cleavage fracture theory, that for fracture a critical stress must be exceeded over a critical distance ahead of the crack, and assuming that this critical stress is reduced in proportion to the local hydrogen concentration (in equilibrium with the external hydrogen atK _TH), a theoretical dependence ofK _TH on hydrogen pressure is derived which compares well with the experimental evidence.     

Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors. S. XU, formerly Graduate Student, Department of Engineering Physics and Materials Engineering, Ecole Polytechnique de Montreal A. K. KOUL, formerly Senior Research Officer, NRC, Ottawa     

Creep crack growth in the absence of grain boundary precipitates in UDIMET 520

The effects of grain size and environment on creep crack growth (CCG) in Ni-base superalloy, UDIMET 520, were studied through experiments at 540 °C. Specially designed solution and aging treatments were used to produce γ′ strengthened microstructures with different grain sizes but without any M₂₃C₆ grain boundary precipitates. Five grain sizes, which fall into three groups (i.e., small, medium, and large), were employed. The creep crack growth rates (CCGRs) in specimens with small grain sizes were approximately 2.5 times lower than those with medium and large grain sizes, as a result of crack branching and the presence of some undissolved primary MC carbides at the grain boundaries. Otherwise, the CCGRs were insensitive to the grain size. Fractographic observations on the fracture surfaces and metallographic examinations on the cross sections of the interrupted CCG specimen revealed intergranular microcracks and a faceted intergranular mode of fracture in both air and argon environments. The test results suggest that the formation and propagation of intergranular cracks by grain boundary sliding (GBS) is the main micromechanism responsible for CCG in both air and argon environments at the relatively low test temperature employed. Grain boundary oxidation attack in the air environment simply accelerates the crack growth process. The present results are in agreement with the theoretical predictions of the GBS-controlled CCG model previously developed by the authors.     

Fatigue crack propagation under varied mean stress conditions

Measurements of fatigue crack growth rates in copper monocrystalline and polycrystalline sheet specimens have been made at 295 K and 77 K to determine mean stress effects on growth rates. When load conditions remained unchanged throughout the period of crack growth, the rate of fatigue crack growth is independent of the level of mean stress and depends only on the cyclic stress amplitude. When the mean stress is changed during the crack growth period, a reduction of mean stress under plane strain conditions causes complete cessation of growth. A similar effect was not observed in plane stress crack growth, presumably due to reduced elastic constraint in narrow specimens containing large cracks. No change in growth rates occurs if the mean load is increased. In the event of crack growth stoppage, either restoration of the full previous mean load or crack re-nucleation under continued cycling at the reduced load levels is sufficient to restore the prior growth rate. A simple model is adapted to explain these observations which emphasizes the interaction of the growth rate with compressive residual stresses generated at the tip of the propagating crack. R. A. Yeske, formerly Research Assistant at Materials Science Department, Northwestern University, Evanston, III. This paper is based on a portion of a thesis submitted by R. A. Yeske in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Northwestern University.     

Limitations of potentiostatic control in stress corrosion crack growth measurements

The electrode potential distribution along a crack in a potentiostatically polarized specimen has been derived analytically by including polarization behavior and solution conductance considerations. The analysis has been applied to the stress corrosion cracks within low alloy steels in an 8M sodium hydroxide solution at 373 K and shows that the electrode potential at the tip falls to the normal equilibrium corrosion potential as the crack length increases. These results show that potentiostatic control at the tip of a stress corrosion crack is subject to large varying systematic errors. Consequently the validity of stress corrosion mechanisms based on potentiostatically controlled crack growth measurements which do not take into account such errors should be reexamined. List of Symbolsα the Tafel constant for the anodic dissolution reaction on a natural logarithm scale, V,β the Tafel constant for the cathodic reduction reaction on a natural logarithm scale, V,C the specific conductance of a solution, Cl'1 mδ the crack opening displacement, m,E _c the free corrosion potential, V,E _app the potentiostatically applied potential, V,E _x the potential at position x within a crack, VE _y Young's modulus of elasticity, MNm^-22,i _c the corrosion current density at the free corrosion potential, Am^-2,i _app the net anodic current density supplied to polarize the specimen to a potential E _app , Am^-2,i ₁ the cathodic current density at potential E_app, Am^-2,i ₂ the anodic current density at potential E, Am-2,i _x the net anodic current density at potential Ex, Am-2,i f the current flow per unit length along the crack at position x, Am-2,ρ _y the yield stress, MNm-2,K the stress intensity, MNm-3/2, w the width of the stress corrosion crack, m, and x the length of the stress corrosion crack, m.     

An analysis of stress corrosion crack growth by anodic dissolution

An analysis of the distribution of electrode potential within a stress corrosion crack which is growing by an anodic dissolution process has been used to define the electrode potential at the tip of the crack. This potential is used to predict the kinetics of crack growth. The influence of the applied stress intensity and the electrochemical properties of the crack tip and surface on the growth rate have been considered for low alloy steels in concentrated hydroxide solution and aluminum alloys in acidic chloride solution. Crack growth rates obtained in high concentration solutions are extrapolated to lower concentration solutions which may be expected in service environments. Predicted crack growth rates are in good agreement with published data.     

Creep cavity growth under interaction between lattice diffusion and grain-boundary diffusion

A model and a method of numerical analysis of the cavity growth in grain boundaries in metals due to lattice diffusion are proposed. The growth behavior simulated is compared to that due to grain-boundary (GB) diffusion. The simulation method is extended to analyze the growth under the interaction between lattice diffusion and GB diffusion. The growth rate calculated under the interaction is approximately equal to the linear sum of those due to the pure lattice diffusion and the pure GB diffusion.     

The stress intensities for slow crack growth in steels containing hydrogen

A test technique has been developed to determine the stress intensity for slow crack growth in hydrogen precharged steels. Measurements on several grades of maraging steel and a 300M steel show that hydrogen contents on the order of 2 ppm reduce the stress intensity for slow crack growth by 50 pct or more of theK _Ic values. At equivalent hydrogen contents the 300M steel was more severely embrittled than the mar aging steels. Comparison of the present results with aqueousK _Iscc data indicates that the amount of hydrogen “picked up by the steels in stress corrosion increases with increasing yield strength. Formerly with International Nickel Co.     

Creep rupture mechanisms in annealed and overheated 7075 Al under multiaxial stress states

The creep deformation and rupture behavior of annealed and overheated 7075 A1 was investigated under uniaxial, biaxial, and triaxial stress states. Examinations of samples prior to and after testing using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were also performed to develop a better understanding of the microstructural mechanisms governing this behavior. These observations combined with analyses of the test data indicate that annealed 7075 A1 under present testing conditions exhibits characteristics of dislocation creep with a concomitant contribution from grain boundary sliding (GBS). By contrast, the results for overheated 7075 A1 suggest that GBS is suppressed. This hypothesis is supported by observations of large particles at grain boundaries in the overheated microstructure and few or no particles at boundaries in the annealed microstructures. Rupture times for the different stress states were also compared with respect to four multiaxial stress parameters, each of which is linked to a particular physical mechanism that can facilitate creep rupture. It was found that creep rupture in annealed 7075 A1 (regardless of sample orientation) is dominated by cavitation coupled with GBS. By contrast, the rupture behavior of overheated 7075 A1 is consistent with a model that describes cavitation constrained by relatively uniform creep deformation in the matrix. Thus, the rupture findings also indicate that GBS is prevented in the overheated microstructure, while it gives rise to significant stress redistribution in the annealed microstructure.     

Investigation of stress corrosion crack growth in Mg alloys using J-integral estimations

Crack growth resistance curves have been determined for the stress corrosion cracking of two magnesium alloys in which theJ-integral is plotted against crack extension. As determined in this way, the resistance to crack growth initially falls with decreasing applied displacement rate but rises again at the slower rates. The effect is in line with previous results obtained on plain specimens and is thought to be due to increasing passivation at the slower testing rates. The results are discussed in terms of the displacement and displacement rate occurring at the tip of the moving crack. Formerly with The University of Newcastle upon Tyne